Marine Electrical Technology - Elstan a. Fernandez

Preface to the Sixth Edition The opportunity to share my acquired knowledge with thousands of professionals and students across many countries, has given me an immense sense of accomplishment and satisfaction. It has also been a wonderful journey of discovery for me - both while researching for all editions of this book and also teaching fie subject in India and abroad. This 10th Anniversary Edition is a culmination of nearly 15 yearn of research and collaboration with various organisations and specialists in the global maritime industry. The contents of the book have been adequately structured to cater to the syllabi for Marine Engineering cadets and qualified Marine Engineer Officers up to the rank of a Chief Engineer. Hence this book may also be referred to while preparing for competency examinations at the post-sea level. Electro Technical Officers onboard commercial ships and those undergoing training to qualify for the position wiil find it useful too. It contributes to the syllabus for achieving the new competence standards and certification as per STOW 2010 (effective from the 1st of January 2012). Besides technical information, extracts from SOLAS Regulations, IACS Guidelines, Lloyd’s Register, Det Norske Veritas and American Bureau of Shipping Rules, have been included with permission. However, these extracts must be used only for academic purposes. Relevant documents issued to ships and related organisations must be referred to, for the purpose ot complying with Classification Societies' and other Statutory Requirements. The question bank comprises of over 1000 questions, which are framed at the end of each chapter. More than 500 relevant figures also include photographs that have been contributed by leading equipment manufacturers across the world. The feedback received from various quarters has contributed to improvement in the quality of the content in every edition. Though every effort has been made to cater to the needs of the growing shipping industry and various maritime educational institutions, any further suggestions would always be welcome.

Elstan A. Fernandez

Marine Electrical Technology

Acknowledgement i sincerely thank everyone across the world who has bought all editions of this book. Numerous little children now realize their dream of being educated through a scholarship program that is anonymously funded by the royalty that I receive. I am indebted to many distinguished persons who, in spite of their high office, have been kind enough to encourage and guide me. Without any inhibitions, they have permitted me to publish very valuable content for the purpose of education. These articles are relevant to the building, safe operation and conscientious survey of commercial ships. Many world-class organisations and manufacturers have extended their invaluable support too. I am grateful for the updated information from their websites and related literature. These inclusions have undoubtedly enriched the content of all editions. The encouragement from lay people and professionals alike has thus been a stimulus to my enthusiasm; I now host a free website - www.marine-electricity.com. In this context, I have a beautiful quote to share with my readers:

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and, move t/um allmi/M fjejbrau-edfo-r.

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Unquote Regulations also require that earth fault indicators be fitted to the m ain switchboard in order to indicate the presence o f an earth fault on each isolated section o f a distribution system, e.g., on the 440V and 220V sections. Earth fault indicators can either be a set o f lamps or an instrument calibrated in kQ to show the system’s insulation resistance value with respect to the earth. Earth indication lamps are arranged as shown in Figure 5.7. These lamps will glow continuously under both earth fault and normal conditions; if there is an earth fault when the push button is pressed, for instance in the third phase as shown in Figure 5.7, the lamp will not glow as both ends o f the lamp will be at ground potential and hence there is no potential difference across the respective lamp. Earth fault indication lamps have been the most common method used for many years, being inexpensive and simple. Their major disadvantage is that they are not sensitive and will fail to indicate the presence of a high impedance earth fault. This has led to the development o f ‘instrument type’ earth fault indicators, which are now being extensively used.

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One common type o f earth fault instrument applies a small direct (d.c.) voltage into the distribution system, the resulting current being measured to indicate the insulation resistance o f the system. The instrument permits a maximum earth monitoring current o f only 1mA (compared with about 60mA for earth lamps), and indicates insulation resistance directly in kO. It gives both visual and audible indications in the event o f an earth fault. H ie instrument can be set to trigger an alarm at any pre-set value o f insulation resistance or leakage current This type o f arrangement has been developed to meet regulatory requirements, which, demand that on tankers, for circuits in, or passing through hazardous zones, there must be continuous monitoring o f the system’s insulation resistance. Visual and audible alarms are activated i f the insulation resistance fells below a pre-set critical value (Refer Figure 5.8). In some cases, cold insulation (when the system is shut down / de-energised) can also be measured / monitored. In this case the indicated value would be in M£1 Marine Electrical Technology

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Chapter 5

Figure 5.8 - Earth Fault Instrument As mentioned earlier in this chapter, a high voltage system (Ik V -llk V ) will have the generator neutrals earthed via an earthing resistor. The system can have an insulation fault indicator which may simply be an ammeter measuring the neutral current via a current transformer. Alternatively, automatic disconnection o f a circuit can be arranged if an earth fault occurs. This creates a large earth fault current. This is achieved by using an earth fault relay in place o f the ammeter. Measurement o f the earth fault current in an earthed system can be provided by various means; (some methods are shown in Figures 5.9 and 5.10). The core-balanced current transformer (CT) in Figure 5.9 measures the phasor sum o f the 3 line currents supplied to the motor. If the m otor is healthy i.e., no earth faults prevail, the phasor sum o f the currents measured by the CT is zero. In order to obtain stability, the pick­ up must not be set below 15% to 20% o f the nominal current o f the CTs, which is often too high compared to the maximum earth-fault current. I f an earth fault occurs in the motor, an earth fault current flows; thus the phasor sum o f the currents is now greater than zero and this flows through the earth fault relay; the resultant current in the relay causes it to energise and trip the contactor in the starter (by breaking a normally closed contact in the series with the stop push in the starter) to isolate the faulty motor’s circuit.

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isolated and Earthed Neutral Systems R

S

T

C urrent Transformer

| j

Figure 5 .9 - Monitoring of Earth Fault Current in a 3-phase Motor

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1ST

Chapter 5

Li

1-2

1-3

Figure 5.10 - Direct Measurement of AC Leakage at the Neutral 5.9

Detection and Clearance of an Earth Fault

The earth fault indicator on the switchboard shows the presence o f mi earth fault on the distribution system. It is up to the maintenance staff to trace or find the exact location o f the fault and then to clear it. An apparently simple method is to open the circuit breakers feeding the loads one at a time and watch the earth fault indicator. Note which circuit breaker, when tripped helps in clearing the earth fault. The earth fault would obviously be in that particular circuit. In practice, circuits cannot be disconnected at will in this way. Some vital services may have to be interrupted causing the main engines to sometimes stop - perhaps in dangerous, narrow waterways. It would be preferable to start with users such as the galley, laundry and such other places where faults are common. Tracing the earth fault must be co­ ordinated with the operational requirements o f the ship’s electrical services. The method o f fault-finding will be described fully for a lighting circuit (Refer Figures 5.11 and 5.12). 158

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Figure 5.11 - Detection Mid Clearance of an Earth Fault - The First Step Suppose the earth fault indicator on the lighting distribution board (db) indicates the presence o f an earth fault Switches A, B, C, are opened and closed in turn until, when a switch is opened, the earth fault indicator confirms the fault; suppose this is switch B: H

Circuit B supplies a distribution fuse-board (dfb) located near its lighting circuits. Here there is no earth fault indicator so an insulation resistance (IR) tester must be used.

S

At this dfb, fuse-pair N o.l is removed. One lead o f the IR tester (megger) is now connected to the earth (hull) and the other lead to ‘b ’ the outgoing terminal as shown, and a reading is taken.

S

If it is healthy (i.e., IR>1MQ), connect the lead to ‘a’ and repeat the test. I f both ‘a ’ and ‘b ’ are healthy, circuit 1 is healthy and fuse-pair 1 can be replaced.

S

Fuse-pair 2 is removed and tested at ‘a ’ and ‘b \ I f an earth fault is indicated (IR is low) then the faulty circuit has been located.

•S All fuse pairs are checked in turn to confirm whether they are healthy or not. •S When the fault is located, the respective fuses should be removed, all switches should be opened, and all lamps removed. This breaks the circuit into several isolated conductor sections. Marine Electrical Technology

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Chapter 5 S

A t the distribution board, test at ‘a ’ and then at ‘b ’. I f both are clear (IR>1MQ), then die conductors connected to ‘a ’ and ‘b ’ are healthy.

S

Close the switch and retest at ‘a ’. I f the earth fault is now indicated (IR is low) then die earth fault lies in the conductors between the switch and lamp 1 and between lamp 1 and lamp 2.

'S A t lamp 1 remove the fitting and disconnect the conductors as shown in Figure 5.13 in order to further break down the circuit Use the IR tester to test each o f these disconnected leads. S

I f 1 conductor is found to have an earth fault (suppose it is die conductor between Li and L2) then the earth fault lies in lamp 1 or lamp 2 or on the conductor. Both lamp fittings m ust be opened now and visually inspected to trace the exact location o f the earth fault. The method o f tracing the fault is essentially that o f continually breaking down the circuit into smaller sub-sections until the earth fault is finally located. W hen located, the conductor insulation must be repaired i f it is damaged. The method o f repairing the earth fault depends upon the cause o f the earth fault and this is determined by visual examination. A lamp-fitting that is damaged must be replaced.

^

Dampness in insulation must be dried out by gentle heat and then some precaution must be taken to prevent the future ingress o f moisture. Insulation that has been mechanically damaged or damaged by overheating must be made good again. If surface dirt is the cause, a thorough cleaning will probably clear the fault.

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Figure 5.12 - Detection and Clearance of an Earth Fault - The Second Step Remember... Tofind ‘earths’ —create ‘breaks’ and......... Tofind ‘breaks' —create ‘earths’!! Marine Electrical Technology

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Chapter 5 J5 .IO

R elevant Rules Extractfrom ABS Rulesfor Building and Classing Steel Vessels -2012 Part 4 Vessel Systems andMachinery - Chapter 8 Electrical Systems Section 2 SystemDesign

Quote 73 H ull R etu rn Systems 7.3.1 General A hull return system is not to be used, with the exception as stated below: * Impressed current cathodic protection systems; * Limited locally earthed system, provided that any possible resulting current does not flow through any hazardous locations; * Insulation level monitoring devices, provided the circulation current does not exceed 30 mA under all possible conditions. 7.3.2 Final Sub circuits and Earth Wires Where the hull return system is used, all final sub circuits, i.e., all circuits fitted after the last protective device, are to consist o f two insulated wires, the hull return being achieved by connecting to the hull one o f the busbars o f the distribution board from which they originate. The earth wires are to be in accessible locations to permit their ready examination and to enable their disconnection for testing o f insulation. 7.5

E arth ed AC D istribution System

7.5.1

General Earthing Arrangement For earthed distribution systems, regardless o f the number o f power sources, file neutral o f each power source, including that o f the emergency generator where applicable, is to be connected in parallel and earthed at a single point Reference should be made to manufacturer-specified allowable circulating currents for neutralearthed generators.

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System Earthing Conductor System earthing conductors are to be independent o f conductors used for earthing o f non-current carrying parts o f electrical equipment. See 4-8-4/23.3 for installation details and earth conductor sizing. Four-wire three-phase AC systems having an earthed neutral are not to have protective devices fitted in the neutral conductors. Multipole switches or circuit breakers which simultaneously open all conductors, including neutral, are allowed. In multiple-generator installations, each generator’s neutral connection to earth is to be provided with a disconnecting link for maintenance purpose.

Unquote 5.10.1 Summary ofSOLAS Regulations 1)

To ensure adequate indication and protection the regulations specify that all exposed metal o f the electrical installation other than the current carrying parts should be effectively earthed, except for the following exemptions: a) Lamp caps, shades, reflectors, fixing screws, cable clips, fluorescent lamp clips etc., which are adequately shrouded or so placed that they cannot become alive or come in contact with earthed metal. b) Equipment with an ‘all-insulated’ construction in which the insulation enclosing it is durable and substantially continuous. e) Portable appliances with double insulation or reinforced insulation in accordance with the approved British Standards. d) Equipment supplied at low voltage.

2)

All other metal frames or enclosures o f electrical equipment should be connected to the hull either directly or via the earthing terminal o f a socket outlet. A metallic cable sheath should not be solely relied upon for this purpose.

3)

Metal sheaths o f cables should be electrically connected together (bonded) and earthed either by means o f glands designed for this purpose or by clips or clamps o f corrosion resistant material, making effective contact with the sheath and the earthed metal.

4)

All joints in metal conduits or ducts used for earth continuity should be soundly made and protected against corrosion.

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Chapter 5 5)

All exposed metal work secured to insulated bulkheads, deck heads, etc., should be effectively bonded and earthed (to prevent any differences o f potential from existing).

6) -

All metal parts o f portable appliances, other than the current carrying parts should be effectively connected to the ship’s hull. This is achieved by means o f an earthcontinuity conductor in the flexible cable or cord through the associated plug and socket outlet (sometimes the body o f a machine may be connected to the deck/bulkhead via a naked, flexible, multi-stranded, galvanised wire).

7)

Choice o f Voltages for Portable Appliances Conditions in ships vary extensively and are difficult to define. Precise rules governing the application o f appropriate voltages for portable equipment, in various locations in ships are therefore not practicable. a) The susceptibility o f individuals to shock also varies considerably. Evidence shows that under particular conditions any a.c. voltage above 55 volts can be fatal but with proper selection o f supplies and equipment, risks can be reduced to acceptable limits. b) The risk o f injury owing to a fall after being thrown off balance by a shock should also be considered. Chapter 2 explains the risks o f shocks with portable appliances. c) The worst conditions arise in engine and boiler rooms, in double bottoms, under engine-room floor plates, in crankcases and other conditions particularly those o f moist heat, leading to low skin-contact resistance. Under these conditions the supply should preferably not exceed 55 volts for portable tools and hand-lamps. If portable tools are not used, then 24 volts for hand-lamps is recommended. I f the supply to each appliance is derived from a separate isolating transformer; the safety factor is further improved. d) In some cases a supply not exceeding 115 Volts obtained from a transformer, the mid point o f the secondary being earthed may be used, this limits the shock risk to earth to a maximum o f 60 volts. e) In accommodation spaces where the risk is not abnormal, normal supplies are satisfactory e.g. cabin fans, table lamps, e tc , provided that the equipment is satisfactorily earthed. If necessary, double wound isolating transformers may be used.

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Isolated and Earthed Neutral Systems

1)

Marine electrical equipment is normally tested t o _______ volts.

2)

The earth-fault indicator on the main switchboard is activated. The best procedure for locating the grounded circuit is t o ________ .

3)

If all o f the ground detection lamps bum with equal brilliance, whether die test button is depressed or released, th en ________ .

4)

In a three-phase electrical system, three earth-fault lamps are provided. One lamp goes dark and the others glow brightly when the test button is pressed. W hen the push button is released, all lamps glow with equal brilliance. You should conclude th a t________ .

5)

On a tanker, as per SOLAS Regulations the neutral point o f a medium voltage alternator should b e ________ the earth.

6)

The main purpose o f using a high voltage system on board ship is t o ________

7)

What is the meaning o f earthing in a ship?

8)

What consideration is used to choose the value o f an earthing resistor?

9)

Describe the neutral earthing system in a main power distribution system

10)

Describe the isolated neutral and earthed neutral systems in 3 phase distribution and significance o f each

11)

What is die difference between the neutral and earth?

12)

W hat is the role o f a NER in High-voltage System Earthing?

13)

W hat is the maximum voltage o f electrical systems in a tanker?

14)

What is the significance o f earth faults in an earthed neutral system? Explain with a suitable diagram.

15)

What is the purpose o f the earth fault indication on the switch board?

16)

What happens with one earth fault in an earthed distribution system?

17)

Which distribution system is more efficient in maintaining supply?

18)

W hat is the significance o f earth faults in an isolated neutral system? Explain with a suitable diagram. Marine Electrical Technology

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Chapter 5 19)

20)

What role does an earth-fault indicator play onboard? Explain any type with a suitable diagram. With a suitable diagram explain the method o f locating and eliminating an earth-fault in a simple distribution system.

21)

Why must an earth return path have low impedance?

22)

W ith the help o f a simple circuit, explain how lamps are used to detect an earth fault?

23)

What indications does an earth fault instrument give? How does it work?

24)

With suitable diagrams differentiate between an isolated and earthed neutral system

25)

How can earth faults affect an Earthed Neutral System?

26)

How can earth faults affect an Isolated Neutral System?

27)

How is an earth-fault detected and overcome?

28)

List the causes o f earth faults and briefly explain each cause. How can they be prevented?

29)

With the help o f a diagram explain how an earth-fault alarm works

30)

What are the simple maintenance procedures to be carried out in order to prevent an earth fault?

31)

If you get an earth fault alarm, what will you do?

32)

Explain the procedure for identifying the low insulation on lighting circuits

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*

C A

h l t e

a

p r n

t e a

r t o

6

~

r s

At the end of this chapter you should be abie to:

★

State the operating principle of an alternator

★

Explain the functions of the major components of an alternator

ft*

Distinguish between conventional and shaft-driven alternators

★ ■ ★ (6,1

Explain different applications of alternators onboard ships Comply with regulations governing alternators T he Basic C oncept A generator is a machine that converts mechanical energy into electrical energy using the

principle o f electromagnetic induction. This principle is based on the fact that whenever a conductor is moved within a magnetic field so that the conductor cuts across magnetic lines o f force, an electromotive force is generated in the conductor. The generator uses these essential conditions to separate the valence electron from the atom. Once this is done and a suitable negative election potential is at one terminal and a suitable positive ion potential is at the other terminal, an external circuit can be connected to use this subatomic imbalance. The electrons from the negative terminal will seek out the positive ions at the positive terminal and return to an equilibrium In the process, the negative electron gives us an electrical current flow through the circuit. The circuit is the way the electron’s magnetic charge is directed to operate motors, solenoids, and illuminate lamps. The conventional way o f understanding the operation o f a generator is that it relies on the principle that whenever there is mutual cutting between a conductor and a magneticfield, a resulting electro motiveforce will be induced in the conductor. Theflow of induced EMF is not at random; it is governed by the direction ofmotion of the conductor(s) with respect to the field and can befoundfrom Fleming's Right Hand Rule (Refer Figure 6.1).

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The magnitude o f the induced voltage depends on the strength o f the magnetic field, rate o f cutting and length o f the conductor. To substantiate this statement, the EMF Equation is as follows: e = 2BNlv sin 0 volts Where; e is the EM F generated as mentioned, 2 - since each coil has 2 sides, B is the flux density in wb/m2 N is the number o f turns (conductors) l is the active length o f the conductor (i.e. the length beneath the poles) v is the peripheral velocity in metres / second Sind - when the coil side (conductor) has turned through an angle 0, only Sin0 is effective since it is perpendicular to the direction o f both the magnetic flux and the conductor in reckoning. The Cos 6 component is ignored as it isfound to be a parallel one and hence ineffective.

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Alternators Now when e is a t its maximum, the angle 8 = 90° and as we know that Sin90 = 1, we can re-write the equation as Em = 2BN/v volts. I f h is the w idth o f the coil in metres, / the frequency o f rotation in hertz, then, the peripheral velocity v = FI.b.f So, Em = 2BN//Z&./volts (knowing that / is the active length and b is the breadth, then die area o f the coil in square metres can be represented by A) E m = Ul.fNBA volts

16,2

The Elementary Alternator

An elementary revolving armature AG generator, otherwise commonly called an alternator, consists o f a wire loop that can be rotated in a stationary magnetic field. This will produce an induced EMF in the loop. Sliding contacts (brushes and slip rings) connect the loop to an external circuit (Refer Figure 6.2). The pole pieces (marked N and S) provide the magnetic field. They are shaped and positioned to concentrate the magnetic field as close as possible to the wire loop. The loop o f wire that rotates through the field is called the rotor. The ends o f the rotor are connected to slip rings, which rotate with the rotor. The stationary brushes, usually made o f carbon, maintain contact with the revolving slip rings. Additives like graphite and copper may also be used, based upon the current-rating (or grade) o f the brushes. The brushes are connected to the external circuit via copper conductors commonly known as pigtails.

Figure 6.2 - The Elementary Alternator Marine Electrical Technology

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Chapter 6

The elementary generator produces a voltage in the following manner (Refer Figure 6.3). The rotor (or armature in this example) is rotated in a clockwise direction. Figure 6.3, position A shows its initial or starting position. This will be considered the 0° or initial position. At 0°, the armature loop is perpendicular to the magnetic field. The black and white conductors o f the loop are moving parallel to the field. At the instant when the conductors are moving parallel to the magnetic field, they do not cut any lines o f force. There is no relative motion between the magnetic lines o f force and the conductor when they move in the same direction. As a result, no EMF is induced in the conductors, and the meter in position A indicates ‘O’. As the armature loop rotates from position A to B, the conductors cut through more and more lines o f flux at a continually increasing angle. At 90° (B), they are cutting through a maximum number o f magnetic lines o f flux and at a maximum angle. The result is that between 0 and 90°, the induced EMF in the conductors builds up from 0 to a maximum value. Observe that from 0 to 90°, the black conductor cuts down through the magnetic field (or flux). At the same time, the white conductor cuts up through the magnetic field. The induced EMF in the conductors is series-aiding. This means the resultant voltage across the brushes (the terminal voltage) is the sum o f the two induced voltages. The meter at position B reads maximum value.

0°

90°

180 °

270°

360°

Figure 6.3 - The Elementary Generator’s Sine Wave Output (Rotating Armature) 170

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Alternators As the armature loop continues rotating from position B (90°) to position C (180°), the conductors that were cutting through a maximum number o f lines o f flux at position B now cut through fewer lines o f flux. At C, they are again moving parallel to the magnetic field. They no longer cut through any lines o f flux. As the armature rotates from 90 to 180°, the induced voltage will decrease to 0 in the same manner as it increased from 0 to 90°. The meter again reads 0. From 0 to 180°, the conductors o f the rotor armature loop have been moving in the same direction through the magnetic field. Therefore, the polarity o f the induced voltage has remained the same. This is shown by A through C on the graph. As the loop starts rotating beyond 180°, from C through D to A, the direction o f the cutting action o f the conductors (of the loop) through the magnetic field reverses. Now the black conductor cuts up through the field. The white conductor cuts down through the field. As a result, the polarity o f the inducted voltage reverses. Following the sequence shown in C through D and back to A, the voltage will be in the direction opposite to that shown from positions A, B, and C. The terminal voltage will be die same as it was from A to C except for its reversed polarity, as shown by meter deflection in D. The graph in Figure 6.3 shows the voltage output wave form for the complete revolution o f the loop.

16.3

Rotor and Stator

1....... .’............ §j|j£......... .......... ............. ............................--------------- ......---- ----------—

j

.................. ........................—--- -----

An alternator has two separate coils (or windings) o f wire. One coil will carry d.c. and produce a magnetic field for use inside the generator. This coil o f wire is wrapped around an iron core (pole piece) so as to concentrate its magnetic effects. This coil is always called the field and is supplied only with d.c. This is usually the rotor as it is smaller than the armature and thus a lighter mechanical load for the prime mover. The other coil usually the stator and always called the armature, will have an EMF induced into it, when the rotor’s field or magnetic lines o f force are cut by its conductors. Alternating current flows through the electrical system connected to it and work can be done. This will always be star-wound so that higher line voltages cm be generated. The overall savings are manifold namely lower output currents for a given power rating and thus lower I2R losses, reduction in weight and size due to the smaller cross-sectional area o f the conductors and most o f all the cost. Two o f the three requirements for producing an EMF in an alternator have now been identified. Some form o f relative motion between the magnetic field and the conductor is still necessary. By rotating one o f these coils, an EMF can be developed. The coil o f wire that is rotating can be called the rotor. The coil o f wire that is permanently fixed to the alternator housing can be called the stator. The item that moves a generator coil is called a prime mover. Marine Electrical Technology

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Chapter 6 The prime mover should be selected on the basis o f an economic evaluation o f first cost, operating costs, and the demand for man-hours for maintenance. The prime m over can be a diesel engine or turbine and the basic sources o f energy for such prime movers can be as follows: •

Fossil Fuels namely Diesel, Petrol and Kerosene;

•

Conventional namely Gas, Steam, and W ater Turbines;

•

Non-conventional namely Wind, W ave and Tidal Energies;

•

Futuristic such as Nuclear, Solar or Biomass Energy.

Where diesel engines as prime movers are considered, diesel engines are classified as: 1. Slow Speed - 74 to 150 r.p.m. 2.

Medium Speed - 300 to 850 r.p.m.

3.

High Speed - 850 to 3000 r.p.m.

Note: The speeds mentioned above are based on available information andfrom practical experience. However there may be slight variations based on evolving design standards. Turbines can even rotate at speeds as high as 5000 revolutions per minute or more but generally the speed o f the turbo alternator may be reduced to 3000 or 1500 r.p jbi. - depending upon whether a 2 or 4-pole alternator is used, the latter being a common type.

6.4

Armature and Field

In an alternator, the armature coil does not always have to rotate. Often it is the field (the coil that is supplied witn d.c. to create a magnetic field) that rotates. As long as relative motion exists between the magnetic field and the armature, an EMF will be produced. Either the rotor or the stator can be the armature or the field. Articles 6.5 and 6.6 explain both types,

16.5

Rotating Armature Alternators

The explanation in article 6.2 describes the rotating armature type (Refer Figure 6.2). This is common only to small generators. A s die output is manageably low, the output can be harnessed from the rotor. Even an alternator delivering an output o f say, 415V, 150A i.e., about 62.5kVA o f output power, can be o f this type. A rotating armature alternator requires slip rings and brushes to connect die high output voltage and current from the armature to the load. The armature, brushes, and slip rings are difficult to insulate. Arc-over and short circuits can result at high voltages and maintenance is costly in the long run for these types.

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Alternators 6.6

Rotating Field Alternators The current produced in the armature, for use by the electrical system onboard a ship, can

be enormously high. Power output from main alternators can normally range from about 350kVA to lOOOkVA at 450V and deliver as high as 14.5MVA at 6.6kV on m odem passenger ships; specialised applications elsewhere in the marine industry may demand higher outputs. It is in the best interests o f die electrical system to have a high current connection that is fixed; hence the need arises for such alternators. It must be remembered that die alternator output purely depends on the load analysis as mentioned in article 1.7, emergency alternators will be rated as low as about 20 or 25% o f the main alternator. The rotating field alternator has a stationary armature winding and a rotating field winding. This is the most common type o f small generator in use today. The advantage of having a stationary armature is that the generated EMF can be connected directly and permanently to the load. There are no sliding connections (slip rings and brushes) to carry the heavy output current and hence maintenance is reduced to a great extent. The arrangement used in a majority o f alternators to exploit the principle o f generation is shown in Figure 6.4. Rotating poles, the magnetic fields o f which move through fixed conductors, produce mutual cutting between conductors and magnetic fields.

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Chapter 6

It may be noted that the maximum voltage required to create a field current is about 90V. So, even i f brushes have to be used, the currents are very low - somewhere in the range of about 50 to 100 times less than the armature current. Modem generators avoid the use o f brushes totally, thus minimising the man-hours involved in maintenance. This will be explained later on in article 6.7 under the sub-title 'The BrushlessAlternator’. The rotor in Figure 6.4 has a pair o f poles so that output is generated simultaneously in two conductors. Here, the pole pitch X - 180 based on the formula X = 36Q/P where P is the number o f poles, which in this case is = 2. The pole pitch is thus an arc measured in degrees or radians. Reference to Fleming’s Right Hand Rule will confirm the instantaneous direction o f conventional current indicated by the arrows in Figure 6.1. The two conductors are connected in series so that the electromotive forces generated in them are summed-up to deliver current to the switchboard. The rotating fields, although moving at constant speed, will cut the conductors at a varying rate because o f the circular movement. Voltage induced at any instant is proportional to the sine o f the angle o f the rotating vector (Refer article 6.1.1), Voltage and current are generated in each o f the pairs o f conductors in turn - first in one direction and then in the other % to produce three-phase alternating current. The effect in conductors Y and B is also shown. The following paragraphs explain the basic construction o f a rotating field alternator. 6.6.1

The S tator

The stator is a tubular-shaped casing that normally houses 3.5% silicon steel or carbon steel laminations with axial slots along the inner surface in which the conductors are laid. It serves as a magnetic field concentrator and thus confines the magnetic field within the machine (Refer Figure 6.5). The thickness o f an individual lamination in a large machine is about 0.37 to 0.64mm. The importance o f the stator’s permeability lies in its ability to strengthen the rotor magnetic fields, which cut the conductors. Obviously the iron stator is also a conductor and will have, like the conductors, voltage and current induced in it by the rotating fields. It is to prevent circulation o f unwelcome eddy currents that the stator core is made up as a laminated structure o f steel stampings. For assembly, the slotted laminations (each insulated on one side) are built into a pack with a number o f distance pieces. Substantial steel endplates welded to external axial bars serve to hold the laminations firmly. The distance pieces are inserted to provide radial ventilation ducts for cooling air as shown in Figure 6.5. 174

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Alternators

Figure 6.6 - Cross-section of a Semi-enclosed Stator Slot and Winding High conductivity copper in the form o f round wire, rectangular wire or flat bars is used for the conductors. Round or rectangular wire used in smaller machines, is coiled into semienclosed and insulated slots (Refer Figure 6.6). This type o f slot improves the magnetic field by reducing the air gap (a smaller air gap means lower values o f reluctance). Marine Electrical Technology

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Chapter 6

Form-wound coils are used for medium or high voltage stators. High voltage coils include a semi conducting layer to decrease the electrical field at the slot surface. Rectangularsectioned copper bar conductors in large alternators are laid in open slots (Refer Figure 6.7). Medium and high voltage windings up to 15000 V are made o f form-wound rectangular copper wire insulated with multiple layers o f glass-fiber reinforced mica tape. The bars are insulated from each other and from the metal slot surfaces by a mica-based paper and tape cladding. Wedges are fitted to close the slots and retain the windings. In some machines the wedges are made o f magnetic material which helps to make the field more uniform, hence reducing pulsation and losses. Bonded fabric wedges are used in some alternators. Slots may be skewed to reduce pulsation and wave form ripple.

P re -fo rm e d S ta to r W in d in g (H a irp in -ty p e )

\ \ \ \ \ \ \ \ \ \ \ \ \t3

Image Courtesy: ABB AS - Oslo, Norway - www.abb,com

Figure 6>7 - Bar-type Stator Conductors in Open Slots 176

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Alternators 6.6.1.1

TheS eating Procedure

Insulating material used in the slots and around the conductors is porous. Hence a method o f sealing is necessary to exclude moisture, which would cause insulation breakdown. It starts with diying the stator with its assembled conductors. The Vacuum Pressure Impregnation (VPI) process by ABB as quoted in the company literature features the following cycle: Quote •

•

High vacuum cycle - removing air and moisture from the voids and pores o f the insulation. Highly stable epoxy resin - ensuring superior protection under the most difficult environmental conditions (against lubricants, oil, moisture, common solvents, chemically aggressive gases, abrasive dust, tropical climate etc.).

•

High pressure cycle - forcing the resin into even the smallest pores.

•

Oven curing - after the VPI process has been completed, stators and rotors are cured in an oven at high temperature. This produces very strong and stable insulation, exhibiting high mechanical and electrical strength. This is especially important in order to resist inadvertent high stresses from out-of-phase synchronization, transients and short circuits. Even the largest wound stators are impregnated as complete units. This ensures that both the insulation and the mechanical properties o f the windings are excellent - which means that they can withstand prime mover induced vibrations and the mechanical stresses

caused by transients such as short circuits. Unquote 6.6.2

The R otor

The rotor comprises a shaft, a rotor centre for larger machines, and salient poles. The shaft is manufactured o f forged or rolled steel and machined to exact specifications. An alternator rotor has one or more pairs o f magnetic poles. The rotating field consists o f wire wound m any times around its core. It depends upon the field strength ‘H ’ required; H = N x I where ‘N ’ is the number o f turns and ‘I’ is the current. This wire terminates at two slip rings where d.c. is applied through brushes. W ith reference to Figure 6.4, direct current is necessary to produce alternating current because o f the need to maintain a constant magnetic field similar to that o f a revolving bar magnet, if it were rotated by its centre.

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Chapter 6 The small value o f d.c. needed for the magnetic field can be supplied by a battery or a part o f the alternator’s own rectified output; this is known as “excitation”. Existing rules have made it mandatory that excitation current for generators is to be provided by attached rotating exciters or by static exciters deriving their source o f power from the machines being controlled. Figure 6.8 shows how the direction o f current flow is reversed when the magnetic field changes. Residual magnetism in the iron core is boosted by flux from direct current in the windings around them and this current from the excitation system has to be adjusted to maintain a constant output voltage through load changes. With the exception o f brushless alternators, the direct excitation current for the rotor in conventional machines is supplied through brushes and slip rings on the shaft. The coppernickel alloy rings must be insulated from each other and from the shaft. They are shrunk on to a mica-insulated hub, which is keyed, to the shaft. Brushes held in brush-holders are o f an appropriate material and pressure is applied to them by springs. Rotor and exciter windings are made to m atch the insulation class o f the stator. This ensures high reliability and a long service life even with asymmetric loads and exceptional conditions. 0°

178

90°

180°

270°

360°

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Alternators 6.6.3

Cylindrical Rotor Construction

H ie cylindrical rotor is constructed with axial slots to carry the winding, which forms a solenoid although not o f the usual shape (Refer Figure 6.9). Direct current from the excitation system produces a magnetic field in the winding and the rotor so that N - S poles are formed on the areas_withoutslots. One rotation o f the- omgie pafr^of poles will induce one o^-cie or. outptrr~*R me stator windings (conductors). An alternator wTtrrohe pair o f poles has to rotate at 60 times per second to develop a frequency ‘ f’o f 60 cycles per second. In terms o f revolutions per minute (r.p.m.), the alternator speed must be 3600 r.p.m.

Figure 6.9 - A Cylindrical or Turbo Alternator We are aware that Frequency ' f is in cycles per second or hertz (Hz) so the figure is multiplied by 60 to make it cycles per minute in the calculation (as we know that the speed is

in revolutions per minute); frequency / h a s been derived in article 4.4.2). However, P » 11x601 or P = 120£ or f = Px_N 2 N N 120 Note: In a 50 Hz system, a full cycle’s time period is 20 milliseconds. Each of the three 20 phases then lag behind theprevious one by / j r 6.66 ms. Similarly in a 60 Hz system the time period is 16.66 milliseconds and each of the threephases 1 && then lag behind theprevious one by /3 - 5.55 ms. Marine Electrical Technology

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Chapter S The table below serves as a ready-reckoner while establishing the relationship between the number o f poles (P) and the r.p.m. (N), for SO and 60 Hertz machines respectively: 1

p

2

SO Hz

\

N

3000

6

K

0

3600

1 4 !

6

12

24

1500

1000

500

250

1800

1200

600

30£l

Table 6,1 - Relation between the Nm nberot Foies and the Revolutions per Minute Projecting (salient) poles bolted to the periphery o f a high-speed rotor would be subject to severe stress as a result o f centrifugal force. The rotors are thus made from steel forging or in some cases from thick steel discs bolted together. Using a cylindrical type o f construction with the poles being built into the rotor minimises the effects o f centrifugal force. Small diameter is compensated for by length; this has the advantage o f great strength and stiffness. The overall area o f the coil also increases to maintain the desired output (e = 2BANTT f volts). Alternators with one pair o f rotor poles are designed for a steam or gas turbine drive, through reduction gears. For this reason they are sometimes referred to as turbo-alternators. Cylindrical rotors can also be wound with two pairs o f poles. In this case, if the rotor speed is 1500 r.p.m Construction o f the core is similar in principle to that o f the stator. It is built up o f steel laminations about 0.35mm to 0.5mm thick. They are pressed together and riveted between clamping rings and then keyed to the steel shaft. The net axial length o f magnetic steel that the flux uses is generally less than the measured stacked length by a factor o f 0.90 to 0.95. This is known as the stacking factor, which is caused by the various layers and air spaces between the laminations due to uneven plate thickness and imperfect consolidation. Spacers help to create ventilating ducts that are vital for heat dissipation. The semiendosed slots are lined with mica-based insulation and phosphor-bronze wedges retain the insulated copper windings. The end turns o f the winding are protected from the effects o f centrifugal force by rings o f insulated steel wire or other material. After winding is completed, the rotors are immersed in resin / varnish and oven-cured; modem methods adopt the VPI process as mentioned in article 6.6.1.1. This ensures that all the air gaps are filled thereby preventing any moisture from accumulating within.

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Alternators 6.6.4

Salient Pole Rotor Construction

Salient (i.e. projecting and noticeable) field poles are those which are secured to the periphery o f the alternator rotor. In the case o f high-speed salient pole machines, the rotor diameter is kept to a minimum; the mild steel hub and shaft are forged in one piece. The micanite-insulated pole is integrated or secured to the shaft or rotor centre by means o f dovetails or by bolts from above or below. Laminated sheets about 2mm thick are pressed together with inserted steel bars, which are welded to the end plates. A rotational speed o f 1800 r.p.m. in an alternator with two pairs o f poles, designed for a supply frequency o f 60 Hz (cycles per second) produces severe stress as the result o f centrifugal force. The poles are therefore keyed to flat, machined faces on the hub as shown in Figure 6.10.

Figure 6.10 - High Speed Salient Pole Rotors Alternators designed for rotation at lower speeds w ith slower prime movers have a greater number o f poles that are either dove-tailed or firmly bolted to the machined surface on the hub as shown in Figure 6.11. Rotor diameter is larger on slow-speed machines in order to accommodate the large number o f poles, but low rotational speed produces less stress from centrifugal force. The flywheel effect is beneficial and this, together with good balancing techniques, ensures smooth running.

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Chapter 6 The rotor (main field) coils are wound from enamelled copper wire or copper strips that are interleaved with insulating material* After manufacture, the coils are mounted between flanges o f hard insulating board on the poles. Output distortion is reduced when the laminated poles have damper windings consisting o f copper bars in the pole faces; these are joined together by sectionalised copper end rings. Proper supports between adjacent windings ensure stability up to the rated overspeed. The windings are also vacuum pressure impregnated. After impregnation, the complete rotor assembly is dynamically balanced. Exciter windings which are on the same shaft as the main field are made o f enameled copper wire too. Proper glass fiber supports are used in the exciter rotor to ensure stability up to the rated overspeed. The windings are also vacuum pressure impregnated.

Figure 6.11 - Slow Speed Salient Pole Rotor 182

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Alternators 6.7

The Brushless Alternator

6.7.1

The Unique Features

In this machine, slip rings and brushes are eliminated and excitation is provided not by a conventional direct current exciter but by a small alternator within the set itself. There are no direct electrical connections between the rotating and stationary windings o f the generator (Refer Figure 6.12). The exciter has the unusual arrangement o f three-phase output windings on the rotor and magnetic poles fixed in the casing. The casing pole-coils are supplied with direct current from a static automatic voltage regulator. Three-phase current generated in the windings on the exciter rotor passes through a rectifier assembly on the shaft and then to the main alternator poles. No slip rings are needed. Output to Switchboard

Figure 6.12 - A Brushless Alternator’s Circuit Diagram The silicon rectifiers fitted in the housing at the end o f the shaft are accessible for replacement and their rotation assists cooling. The six rectifiers facilitate full-wave rectification o f the three-phase supply.

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Chapter 6

Main Frame

Main Rotating Field

Figure 6.13 - Construction of a Brushless Alternator

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Alternators The major components are briefly explained below: 6.7.1.1

The Exciter

The exciter portion houses a mini generator that develops the power necessary to develop the magnetic field in the main generator portion. It consists o f the following main components: 6.7.1.1.1

Exciter Field

The exciter field is a stationary direct current-energized winding. This is the winding where the d.c. magnetic field is initially developed. Even before any voltage regulation takes place, a residual magnetic field* exists in the poles. During voltage regulation, d.c. in the exciter’s field induces an EMF, resulting in current flow in its armature. *

Residual Magnetism

(a) Now we know that residual magnetism exists in all ferrous metals that have had a current carried around it. In many generators, there is not enough material to provide a substantial residual magnetic field to use in creating an EMF. The ship’s service generator has a lot o f metal. The material mass maintains adequate residual magnetism in the exciter's field that in turn helps to induce an EMF in the exciter’s armature when there is motion. It will also be found that the properties o f the metal involved will cater to a wider hysteresis loop or “B/H” curve. (b) Residual magnetism in the generator’s exciter field allows the generator to build up voltage while starting. This magnetism is sometimes lost due to shelf time or improper operation, among other reasons. Restoring this residual magnetism is possible and is sometimes referred to as ‘flashing the exciter field’. It is also possible that initially, self-excited ship service generators may need to have the fields flashed to establish the residual magnetism, which is necessary to start the exciter’s induction process. Note: Read the manufacturer’s recommendations carefully. Damage to the generator or voltage regulator will result ifproperprocedures are not correctlyfollowed.

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Chapter 6 In order to restore the small amount o f residual magnetism necessary to begin a voltage build-up, connect a 12-volt battery to the exciter field while the generator is at re s t This is done as follows: 1. Remove the exciter field leads e.g., F+ and F- from the voltage regulator. CAUTION! Failure to remove the field leads from the regulator during flashing procedures may destroy the regulator. 2.

Measure the exciter field resistance across the F+ to the F- ends. You should be able to read some value o f resistance as you are measuring a continuous winding. An infinite resistance reading would indicate an open circuit in the exciter field. Also ensure that there is no grounding in the circuit.

3. Connect F+ to the positive pole o f the battery. 4. Hold the F- lead by the insulated portion o f the lead wire, touch F- to the negative pole o f the battery for about 5 to 10 seconds and then remove it. 5. Reconnect F+ and F- to the regulator. 6.

Repeat the procedure if die generator fails to build voltage.

6.7.1.1.2

Exciter Armature

The exciter armature ,is a three-conductor, three-phase rotating winding. The exciter armature is located directly inside a tabular stator. A three-phase EMF is induced in the exciter armature as it rotates inside the fixed magnetic field o f the exciter. Together, the exciter’s field and armature develop a three-phase &c. output. In effect, this is a rotating armature generator. This portion o f the generator is used to provide die excitation necessary for the main field portion as mentioned earlier. Since current is induced into the armature without the aid o f wires, brushes and slip rings are eliminated. 6.7.1.2

Rotating Rectifier

The output derived from the exciter portion o f the generator is in the form o f an alternating current. In order to produce the enhanced three-phase output from the main armature o f the generator (necessary for the large power requirements o f the distribution system), the main field must be provided with a direct current source. To convert (or rectify) the exciter’s output from a.c. to d.c., the rotating rectifier is used. This rectifier provides the same conversion (from a.c. to d.c.) as is depicted in Figure 6.12. 186

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Alternators 6. 7.1.2.1

Effects o f Diode Failure

1. I f an open circuit occurs in any diode, then the remaining healthy diodes would continue to supply the main field. In the manual control mode, the total field current and hence the generator voltage will be slightly reduced. But when the AVR controls the exciter field, the current would be automatically boosted to maintain the generator’s output voltage while the diode failure m ay probably go undetected; but this will gradually overheat the exciter. 2.

If the diode short circuits, it is far more serious as it leads to a short-circuited exciter. Rapid overheating o f the exciter will occur.

6.7.1.3

Main Rotating Field

The main rotating field can consist o f four to eight individual coils or pole pieces keyed to the rotor shaft. The coils are connected in series and consist o f only one wire. The direction that the wire is wound around the pole piece determines the polarity o f each field coil. The d.c. output from the rotating rectifier develops the revolving magnetic field inside the main field generator portion, providing alternate fixed field polarities. 6.7.1.4

Amortisseur or Damper Winding

Embedded in the face o f each main field pole piece is the Amortisseur or damper winding. These are necessary for generators that operate in parallel. These become very important when dealing with frequency. The frequency o f an alternator must not change. These damper windings prevent hunting during parallel operation. Damper windings are copper or aluminium conductors embedded just below die surface o f the rotor. They are short-circuited at each end to allow currents to circulate so that a magnetic field can be produced to oppose any change in prime mover motion. 6.7.1.5

Main Armature

The main armature consists o f six individual windings. Two windings, as a pair, are connected to each other in series or parallel. Each armature winding pair is then connected to the other two armature winding pairs to form the common Wye or Delta combination, each o f which are spaced 120 mechanical (geometrical) and electrical degrees apart. The actual connection between each winding is completed outside o f the generator's Sam e in the attached terminal connector box. In this manner, the user can connect the individual armature winding pairs in series or parallel and then the pairs in the Delta or Wye configuration. Marine Electrical Technology

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Chapter 6 The configuration is selected for the type o f voltage and current requirements that suit the application. Only a single-phase EMF (voltage) can be induced (produced) in a single pair o f the armature windings. Since there are three such pairs o f windings, three separate single­ phase EMF values are induced. It is the development o f each o f the three single-phase values that together produce the three-phase output from the armature windings. The main armature windings are connected directly to the electrical system through the switchboard. It is from here that the automatic voltage regulator or AVR receives its input which in turn controls the input to the exciter’s stationary field. 6.7.1.6 Flange-mounted Sleeve Bearing Sleeve bearings are flange-mounted on the end-shield and withstand a high level o f vibration. (Refer Figure 6.15). In order to prevent bearing damage from circulating shaft currents, all NDE bearing housings (on the excitation side) are electrically insulated by means o f a non-conducting Polytetrafluoroethylene (PTFE) film that is commonly known as Teflon™ (created by DuPont, USA). The verification o f the insulation is usually done by measuring the resistance o f the intermediate insulated ring with respect to the ground. Most common insulated bearings are single layer types. An insulation layer is fitted in the bearing pedestal so as to insulate the babbitt-side from the ground. All necessary bolts and nuts required for mechanical assembly must also be insulated to avoid any insulation shunting. In this design, the loop path that shaft current could use is cut and thus the babbitt* and journal bearings are safe. However, this simple design has a drawback; the insulation resistance o f the bearing’s insulated part with respect to the ground cannot be easily checked. * Babbitt — it is any of the several soft, silvery, antifriction alloys applied to the class of white metalsfor bearings. It is composed of tin with small amounts of antimony, copper and traces ofother metals; in some cases, lead is substitutedfor tin. Meggering it is not possible as which ever way the megger is connected, a path to ground exists! Reliability o f such a bearing depends on the mounting procedure, checks and craftsmanship. To overcome this drawback, double-insulated bearings have been designed. Two insulated layers are fitted in series between the babbitt and the ground. The intermediate piece being sandwiched between two insulated layers, its insulation resistance can then be measured using a megger. However, experience in the field has shown that this is not a 100% proof design. The effectiveness o f the pedestal insulation can be checked by measuring its voltage with respect to the earth.

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Alternators I f there is no insulation* unbalanced (stray) end-winding magnetic fields induce an unwanted electromotive force along the steel shaft. This will cause a current to circulate through the shaft, bearings and bedplate to produce arcing across the bearing’s races and subsequent degradation o f the oil IayerT SHaftrcartgnt flowing through a bearing also leads to erosion o f the white metal and over-heating in the caSe^ctfa sleeve bearing. When such a critical situation is not handled rapidly, irreversible bearing damage can occur.

F igure 6.15 - Flange-m ounted Sleeve B earing

F igure 6.16 - Integral P edestal Sleeve B earing Images Courtesy A B B A S - Oslo, Norway - www.abb.com 6.7.1.7

Integral Pedestal Sleeve Bearing

These bearings have the same fundamental features as flange bearings, except that they are mounted on a pedestal that is integrated into the stator frame. Generators with integral pedestal bearings are as easy to mount and align as generators with flange-mounted bearings. The bearings are lubricated by an oil ring. An external cooling circuit is typically necessary, unless the oil comes directly from the engine-cooled lubricating oil system. Marine Electrical Technology

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Chapter 6 6.7.2 6.7.2.1

Possible Damages Due to Shaft Current Frosting

By far, it is the most common type o f shaft current damageV r a m ^ffcotod are bearings, seals, thrust collars, journals, and to a lesser extent, gears. The appearance is that oF a-sztrtd-,^ blasted surface, characterized by overlapping, molten, shiny pit marks. I f the entire available surface is affected, the damage may not be noticeable to the naked eye due to its satin-like appearance. When viewed microscopically, however, the frosted surface is seen as very small and individual “craters.” The bottoms of the craters are round and shiny, indicative o f the melting that had occurred. This frosting occurs during voltage discharge and it is commonly referred to as “Electrical Discharge Machining' (EDM), or Electrolysis. As EDM occurs, material is removed. Sometimes chemical attack gives a similar appearance to frosting; however, the marks are smaller, not as deep, and appear dull. 6J.2.2

Spark Tracks

The initial appearance o f these tracks is that o f scratches in the babbitted surface resulting from foreign particles in the lubrication or seal oil. However, a closer examination reveals a continuous, meandering, narrow track depleted o f babbitt. They may run askew or concentric to the direction o f rotation. Under magnification, the track is shiny and molten, ranging in depth from 2 mils (he.

2

/10 0 0

i

o f an inch) to lif, o f an inch. Spark tracks are usually

associated with an electromagnetic source as a large amount o f power is needed to develop the continuous voltage discharge. This damage is often misdiagnosed as mechanical scouring. 6.7.2.3

Pitting

This damage is listed separately from frosting as it is generally much larger (from lyi o f an inch to V4 o f an inch) since its source is extremely powerful. It often occurs in gear teeth, on the backs o f bearings or seals, and sometimes between frame splits. As opposed to frosting where the entire surface might be affected, pitting occurs more randomly and it is sometimes possible to count the number o f discharges, which is impossible to do with frosting. The appearance o f the pits is similar to the individual frosted craters; that is, they often have round shiny bottoms. Sometimes pitting is confused with frosting-type corrosion. In this case, a qualified metallurgist is needed to distinguish the difference. Electrical pitting usually stems from an electromagnetic source, as a much more concentrated amount o f energy is needed to form the larger pits. However, very high voltage electrostatic sources have been known to cause pitting.

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Alternators 6.7.2.4

Welding

Welding o f parts such as frame splits, bearing pads and seals, have occurred due to a great amount o f current (hundreds o f amperes) passing through them. They are easily evident to the naked eye as spot welded marks and quite often have to be separated by sledge hammers or other mechanical means. W elding can only occur from an electromagnetic source, as an extremely large current is generated, causing fusion between two components. In turbinedriven machinery, this damage is usually the result o f an upset in the process, allowing a rotor to momentarily contact the stator, thereby producing the large flow o f current. 16.8

High-voltage Brushless A ltern ato r

The text of this article is reproduced with reference to the company literature for "Brushless Constant-Voltage Synchronous-Alternators TNC9 High - Voltage for industrial andshipboard use” by Uljanik TESU d.d., Croatia (www.uljanik.tel.hr) The basic construction is similar to a medium voltage alternator. The alternators are available for 50 Hz or 60 Hz. The rated voltages o f these alternators are generally between 3.0 kV and 13.8 kV. 6.8.1

Frame and Stator Core

The stator consists o f the housing with the core and windings for the main machine, exciter, end shields and bearings. The alternators are fitted with a top-mounted excitation control unit or it is mounted separately in a box. The housing is made o f steel plating. The mounting feet, which are an integral part o f the housing, are designed depending on the type o f construction. A high-voltage terminal box may be fitted on the side o f the housing. It is designed for IP54 degree o f protection. The stator core o f the main machine is made o f high quality electrical sheet steel insulated on both sides. It is subdivided into packets by means o f ventilating ducts and spacers thus ensuring effective cooling. The complete core with the windings is pressed into the housing. The exciter housed in the same alternator housing is mounted on the end-shield at the non-drive end. The end shields o f the welded structure carry the roller or sleeve bearings assembly, 6.8.2

Main Stator Winding

High voltage stator windings utilise several different insulation systems depending on the voltage rating. This insulation is based on a high percentage o f mica. The conductors are insulated with varnish and a double layer o f glass fibre with a high dielectric strength. Micabased coil insulation is used. The insulation is baked under pressure so as to completely eliminate air and to obtain a homogenous layer. Marine Electrical Technology

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Chapter 6

The winding end-turns are insulated with polymerised m ica tape, which simultaneously gives both dielectric strength and elasticity. The final layer protects the insulation against detrimental atmospheric and chemical influences. Selected material and the method o f binding result in a strong and unified winding assembly that will withstand the large mechanical forces which appear during different generator transient fault conditions (e.g., sudden short-circuits or loss o f synchronism). 6.8.3

Cylindrical Rotor

The rotor is laminated and cylindrical. The windings are placed in the semi-enclosed slots. They are impregnated with solvent-free resin. Such a design offers excellent mechanical properties. The rotor is fitted with a complete cage that serves as a damper winding. The rotor consists o f the shaft, the rotor core, the field and damper windings. The shaft also carries, on the non-drive end, the rotating rectifier and the rotor core o f the exciter with a three-phase winding. The shaft is also fitted with a fan. The rotor core o f the main machine is made up o f electrical non-oriented sheet steels. Ventilating ducts, which are formed by spacers, subdivide the core into packets and ensure effective cooling. The winding is arranged around the core’s periphery. The winding is distributed over slots per pole, which ensures that the rotating masses are uniformly distributed over the rotor’s circumference. Therefore the mechanical stresses due to the centrifugal forces are considerably reduced. In addition to this, a uniform temperature distribution is also obtained, which increases the life o f the winding. In order to make the end turns resistant to the centrifugal forces, rings o f fiberglass are fitted on the overhang. For the slot insulation o f the rotor winding, special materials are used. All the connections o f the winding are hard-soldered. The rotor core assembly along with the winding is impregnated with resin. The damper winding consists o f bars, which are accommodated in equally spaced slots. At the core ends, the bars are bent and welded to end rings, thus forming a damper cage. With synchronous machines, the damper winding reduces distortion o f the voltage and current waves to a minimum, even under conditions o f unbalanced loading. Owing to the good magnetic coupling between the stator and damper winding, the effects o f inducing disturbances, such as torque pulsation, which occur in some driving machines, electrical and mechanical shock loads and harmonics are effectively suppressed.

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Alternators The rotating rectifier comprises a full-wave three-phase rectifying bridge. The rectifier design and selected high-quality silicon elements ensure high operation safety and protect the diodes. The thermal endurance o f the winding insulation o f the main machine and the exciter satisfies the requirements for class F insulation (155°C). The insulation is resistant to moisture, oil vapours and sea air. \6.9 O

O utline o f O peration of a Brushless A ltern ato r Theprime mover starts The prime mover crankshaft revolves, and the generator shaft rotates. This turns the exciter armature, the main field, and the rotating rectifier.

O

The exciter initiates an EMF The rotating exciter armature cuts the residual magnetic field (left over in the exciter field pole pieces). A weak EMF is induced in the Wye-wound (star-connected) rotating exciter armature windings. The exciter portion o f the machine operates as a rotating armature alternator.

O Exciter a.c is rectified to d.c The exciter’s three-phase a.c is directed to the rotating poly-phase rectifier. The diodes convert the a.c. into a pulsating d.c. Five wires are connected to the rotating rectifier. Three wires are from the three-phase exciter armature to the rectifier, and two wires direct the d.c. output to file main field winding. O

The mainfield induces an EMF into the main armature Direct current enters the rotating main field As the rotor shaft turns the main field, the alternating polarities induce an EMF o f alternating potentials in the main armature windings.

O

Three-phase a.c. is producedfrom the main armature The main armature has three windings producing three-phase a.c.. The main portion o f the generator is operating as a revolving field generator. Initially, only a low threephase EMF is produced.

Marine Electrical Technology

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Chapter 6 Voltage control takes over

O

The voltage regulator senses an under-voltage condition and diverts the current flow back to the stationary exciter field. In this case, the exciter field winding is used for the initial voltage build-up. The current flow through the exciter field winding increases its magnetic field. The exciter armature conductors now cut through a greater magnetic field, and the induced EMF in the exciter armature is increased. Q

The process is repeated until satisfactory voltage is achieved The increased exciter armature current is rectified by the rotating rectifier and directed again to the rotating main field. The increased magnetic field (o f the rotating main field) sweeps past the conductors in the stationary main armature. This produces a greater three-phase EMF. Normal voltage control is maintained by the regulator controlling current to the exciter field.

16.10

G en erato r Cooling

I

Power losses (typically 10% to 15 % o f fire generator rating) comprise o f copper losses that is equal to 3IA2RA for the armature and IF2RF for the field, iron losses such as hysteresis losses and mechanical losses such as windage due to rotor design, friction o f rotating parts, etc; these cause internal heating in the windings and cores o f both the stator and rotor. This heat must be continuously dissipated elsewhere in order to prevent it from causing a breakdown o f the windings’ insulation that could lead to reduction in life, short-circuits, and even fires! 6.10.1 Air-to-Water Closed Circuit Cooling Generators with a large power rating may also use this system. Temperature detectors (resistance types or thermistors) are used to monitor the temperature o f stator windings, bearings and cooling air/water o f the generator (Refer Chapter 13 for further information on temperature monitoring). Single or grouped temperature alarms can be remotely activated. The cooling air circulates in a closed circuit through the active parts o f the generator and then through an air-to-water heat exchanger. This configuration passes hardly any heat to the surrounding environment and ideal for installations in engine rooms with limited ventilation, such as ships. It represents an ideal solution for situations where closed circuit cooling is required due to installation in a hazardous area. It is also applicable whenever the quality o f the surrounding air is not otherwise suitable for direct cooling (Refer Figure 6.17). 194

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Alternators Note: In case of a failure of the cooling system, the load on the generator must be reduced and the temperature continuously monitored so as to prevent any possibility of excessive heating and subsequentfailure of the generator and its components. The normally closed air circuit of the generator may now be open to the Image Courtesy: www.abb.com

engine room’s atmosphere if safe to do so.

Figure 6.17 - Air-to-Water Closed Circuit Cooling 6.10.2 Air-to-Air Closed Circuit Cooling Forced-air-circulation in a closed circuit (to prevent ingress o f dirt) via an air cooler is made possible by a fan on the rotor shaft. Here, the cooling air is forced through ventilation ducts in the stator core, between rotor poles and through the air gap (a few millimetres wide) between the stator and rotor. The cooling air circulates in a closed circuit through the active parts o f the generator and through an air-to-air heat exchanger. This solution is generally used in situations where a closed circuit cooling system - such as air-to-water cooling - is required but water is not readily available. This cooling arrangement requires an additional shaft mounted or electric fan to ensure sufficient air flow through the cooler (Refer Figure 6.18).

Image Courtesy: M ’ww.abb.com

Figure 6.18 - Air-to-Air Closed Circuit Cooling Marine Electrical Technology

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Chapter 6

i6 .ll

ill

G en erato r H eating

£

'

7

.; "

j

Insulating materials in large machines and equipment are expected to remain serviceable for a period o f 20 years or more and that too with minimum maintenance. Prolonging its life is exceedingly difficult primarily because o f the inability to anticipate and control the corona activity - the effect o f heat and moisture. While the generator is stopped during standby or maintenance periods, low power electric heaters within the machine - generally in the lower portion, prevent internal condensation from forming on the winding insulation and eventually destroying it; this is also the case for large motors. These “space-heaters” may be switched-on manually or automatically from auxiliary contacts on the generator circuit-breaker when the breaker is only in the “Open” position; obviously this interlock prevents the heaters from coming on when the breaker is made. Heater power supplies are normally 220V a.c. single-phase supplied from a distribution box in the vicinity o f the generator. 16.12

S haft-drives A lternators

Shaft-driven alternators on board ships are alternators driven by the main engine to supply power to the mains. They are also known as ‘Shaft Alternators’ or just ‘Shaft Generators’. The mains m ust be supplied with constant voltage and frequency by the shaft alternator even at changing speeds o f the main engine. Shaft alternators on board ships as system for especially economic power generation are provided for decades on modem ships due to their many advantages: 1. Lowering o f fiiel costs by lower heavy diesel oil cost and better efficiency o f main engine 2. Reduction o f maintenance and lubricant cost by reduction o f operating time o f auxiliary alternator sets 3.

Saving o f operating personnel

4.

Low noise level in the engine room

5.

Smaller or/and less diesel alternator sets

Even in ships with controllable pitch propellers, it is more economical to use a shaft alternator system with a frequency converter to compensate for variable speed. Auxiliary diesel-driven generators, which run continuously for twenty-four hours a day both at sea and in a port, can be expensive in terms o f the fuel cost and maintenance requirements. 196

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Alternators Maintenance is usually based on running hours which, with continuous operation, will add up to 720 per month o r over 8000 in a year. Even where economy is achieved by the use o f a blend o f cheap residue with the more expensive distillate fuel, the accumulation of running hours still envisages a maintenance requirement and it is possible that the workload will be heavier due to problems with the fuel, A generator drive taken from the main propulsion system reduces maintenance by avoiding the use o f an auxiliary diesel at sea. It also furnishes a method o f obtaining electrical power from the cheapest fuel. The installation o f a shaft generator also means that fewer diesel generator sets are needed. The shaft-driven machine can be o f a large enough capacity to take the full at-sea electrical load. While manoeuvring, power for the bow thruster is provided by the mainengine-driven alternator on some ships, with power for auxiliaries being provided at that time by two diesel sets. At sea, when the bow thruster is shut down and auxiliary load is transferred to the main-engine-driven alternator, the diesel engines are stopped. They are classified into the conventional type and the constant frequency type: Shaft G enerator System

Conventional

Fixed Pitch Propeller 6.12.1

Constant Frequency

Controllable Pitch

Thyristor Inverter

Propeller

Motor-Generator

System

System

Conventional Shaft Generator System

In this system the output from the shaft generator is directly coupled to the main busbars which means that the output frequency is directly influenced by the main engine speed. For controllable-pitch-propellers (CPP) the main engine speed and thus the frequency are almost constant. Typically, the frequency range would be 59 to 61 Hz with a rough sea tolerance of^|2 Hz. This frequency range is wider for the fixed pitch propeller jjfro m about 55 to 61 Hz (Refer Figure 6.19). The frequency variation prevents parallel operation with auxiliary generators and changeover during a blackout situation is necessary. The range o f engine speeds within which the shaft generator can be operated is generally 90% to 100% o f the normal value. It is positioned directly in the shaft line, between the main engine and propeller. Marine Electrical Technology

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Chapter 6 It can be built so that its shaft is flange-coupled as part o f the intermediate shaft system or the rotor can be based on a split hub which is clamped to a section o f the main shaft. The frame o f the shaft generator is sometimes supported on the tank top. The problem with this arrangement is that the air gap will vary due to hull flexure. Thus an excessive clearance o f perhaps 6mm may be required to be maintained.

FPP or CPP

F igure 6.19 - Conventional Shaft G en erato r System 6.12,2 Static Frequency Converterfor a Shaft Generator M ost ships have fixed-pitch propellers so that the ship’s speed variation necessitates changing the engine’s revolutions. To accommodate changes o f engine and shaft alternator speed, a constant-speed power take-off system may be installed or, more usually, output from the alternator is delivered to the electrical system through a static converter. The converter accepts a range o f generated frequencies but delivers a supply at the frequency required by the system. Static frequency converters have been developed for use with shaft alternators where the speed range extends from 40% to 100% o f the rated speed o f the main engine. The converter system shown in Figure 6.20 serves the shaft generator o f a ship, with a fixed-pitch propeller and a large main-engine speed range. The shaft generator must supply full output over the permitted speed range, and to achieve this at the lower end (i.e. down to 40% o f the rated speed J i t is over-rated for higher speeds. The a.c. shaft generator itself is a synchronous machine, which produces alternating current with a frequency that is dictated by variations in engine speed. At maximum r.p.m., the frequency may match that o f the electrical system. The output is delivered to the static converter, which has two main parts. The first part is a 3-phase rectifier bridge to change the shaft generator output from alternating to direct current The second part is an inverter to change the d.c. back to alternating current, at the correct frequency. 198

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Alternators

Bus bars of Main

Alternator and Synchronous Compensator

Frequency Controller

Auxiliary Engine

inverter —P + —

SCR Controller -td —

Current r 4” Lim iter [

h K W

< h

Smoothing Reactor 3-phase Rectifier

-Kf-rW Excitation I Controller

Shaft Generator

Main Engine

Figure 6.20 - S haft G en erato r w ith a Static Frequency C onverter Alternating current from the shaft generator, when delivered to the three-phase rectifier bridge, passes through the diodes in the forward direction only, as a direct current (Refer Figure 6.21). Marine Electrical Technology

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Chapter 6

DC

Output

ggn

Figure 6.21 - Three Phase Rectifier and its Associated Wave-forms The smoothing reactor reduces ripples caused by commutation o f the thyristors. It will also limit short circuit current delivered by the shaft generator system (the filter shown, is only conceptual; it normally comes in many variants). The original frequency (within limits) is unimportant once the rectifier has converted the supply to d.c. The inverter for transposition o f the temporary direct current back to alternating current is a bridge made up o f six thyristors. Direct current, available to the thyristor bridge, is blocked unless the thyristors are triggered or fired by a gate signal. Gate signals are controlled to switch each thyristor in sequence, in order to pass a pulse o f current. The pattern o f alternate current flow and break constitutes an approximation to a three-phase alternating current. 200

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Alternators Voltage and frequency o f the inverter supply to the a.c. system must he kept constant within limits. These characteristics are controlled for a normal alternator by the automatic; voltage regulator and die governor o f the prime mover, respectively. They could be controlled for the shaft alternator inverter by a separate diesel-driven synchronous alternator running in parallel. The extra alternator could also supply other effects necessary for the proper functioning o f an inverter, but the objective o f gaining fuel and maintenance economy with a shaft alternator would be lost. Fortunately the benefits can be obtained from a synchronous compensator (sometimes termed a synchronous condenser), which does not require a prime mover or driving motor except for starting. The compensator may be an exclusive device with its own starter motor or it may be an ordinary alternator with a clutch on the drive shaft from the prime mover. The alternator set that fulfils the role o f a synchronous compensator for the system is shown in Figure 6.20. The diesel prime mover for the compensator is started and used to bring it up to the adequate speed to facilitate connection to the switchboard. The excitation is then set to provide the reactive power, and finally the clutch is opened, the diesel shuts down and the synchronous machine then continues to rotate independently like a synchronous motor, at a speed corresponding to the frequency o f the a.c. system. A synchronous compensator is used with the monitoring and controlling system in order to dictate or define the frequency. It also maintains constant a.c, system voltage, damps any harmonics and meets the reactive power requirements o f the system and converter, as w ell as supplying, in the event o f a short circuit, the current necessary to operate trips. It is run at no load, at a leading power factor and operates like a power factor correction capacitor (Refer to the text under the subheading Harmonics at the end o f this chapter). The shaft generator provides the system’s active power (kW) at unity power factor and the synchronous condenser provides the kVAr. If the synchronous condenser is a dedicated motor, it requires a run-up motor to bring it up to the speed required for synchronising. As seen in Figure 6.20, a diesel engine is used for the purpose. The cooling arrangements for static frequency converters include the provision o f fans as well as the necessary heat sinks for thyristors. M ost systems can provide full power output even when the rated speed is reduced to 75% & 80%, and reduced power at approximately 50% o f the rated speed. Marine Electrical Technology

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Chapter 6 6.12.3 Power Factor Correction using a Synchronous Motor The use o f induction motors onboard a ship is mainly responsible for reducing the system’s power factor. W e must remember that more current is required for supplying a given power at a low power factor than at a high power factor. A n increase in the system current caused by low power factor leads to increased power losses thus reducing the efficiency o f the supply system as a whole and a large voltage drop makes the voltage regulation o f generators, transformers and transmission lines poor. Now, a synchronous motor when over-excited, operates at a leading power factor. Because o f this unique property, it can be used to improve toe power factor o f a system that is working at low (lagging) power factor. Consider a general case o f a system with a three-phase inductive load (a number o f induction motors generally onboard) drawing a current o f L amperes at a voltage o f V volts. Let toe power factor o f toe system be Cos«J>t lagging. If now an over-excited synchronous motor drawing current per phase o f Is amperes at a leading power factor o f Coss and supplying toe mechanical power to a load coupled to it, requiring 3VIsCoss watts, is connected in parallel w ith such a system, toe new current drawn (L) from toe supply will then be given by the phasor addition o f I}and Is and will lag toe voltage V by an angle

------VAAAV---S hunt-w ound

C om pound-w ound

S eries-w o u n d

S e p ara tely -ex cite d

Figure 7.21 - Schematic Diagrams of DC Motors 17.17

Principle of DC Motor Operation

The operation o f a d.c. motor depends on the attraction and repulsion principles o f magnetism. W hen current is supplied to the field poles o f a motor, the field poles turn into electromagnets. If a two-pole machine is used, north and south polarities are established toward the centre o f the machine. While a generator follows Fleming’s Right-hand Rule, the m otor here, follows Fleming’s Left-hand Rule. Figure 7.22 shows how the two field poles are wound to produce the opposite magnetic effect. Marine Electrical Technology

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Chapter 7 The magnetic lines o f force, between these two unlike magnetic poles, establish a direction o f movement from the north polarity to the south polarity. By themselves, these lines o f force from the field poles cannot do anything to force the motor's armature to rotate. Field P o le

F ield P o le

Figure 7.22 - Lines of Force in a Magnetic Field If current is supplied from the generator through the motor's brushes and commutator to the armature windings, a magnetic field results around the armature windings. The d.c. motor torque depends on the principle that a current-carrying armature conductor has a magnetic force encircling it. The current entering the m otor’s armature windings and the magnetic lines o f force that result around the armature windings, interact with the magnetic lines o f force from the field poles. Torque is produced in proportion to the current in the armature windings. The greater the armature current, the greater the m otor torque. Additionally, the direction o f current flow through the armature and the polarity o f the field poles determine the direction that the armature will revolve. Figure 7.23 depicts the lines o f force established around the armature coils. The cross signifies the current from the generator's negative terminal (moving away from the viewer into the motor armature). The dot represents the current moving towards the viewer (and toward the positive terminal) in the motor armature. The left-hand rule establishes the lines o f force around these armature conductors. 250

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Direct Current Machines

Figure 7.23 - Field and Armature Magnetic Lines of Force (Combined) The two field poles show their magnetic lines o f force establishing a direction from north to south (left to right). The armature conductor magnetic lines o f force are circular and are determined by the current direction. The following outline describes the combining o f the current-carrying armature’s magnetic lines o f force with the field pole’s magnetic lines o f force: •

The circular lines o f force in the ‘+’conductor (the *+’ is to indicate that current is flowing away from the viewer) and the magnetic lines o f force from the field poles effectively cancel out each other directly above the *+’ conductor.

•

The circular lines o f force below the *+’conductor work with or add to each other's magnetic lines o f force. In this way, the additive force below the ‘+ ’ conductor forces the conductor up through the cancelled lines o f force directly above it.

•

The circular lines o f force developed from the ‘dot’ conductor (the dot is to indicate that current is flowing towards the viewer) effectively cancel the magnetic lines o f force from toe field poles directly below the ‘dot’ conductor.

•

The circular lines o f force directly above the ‘dot’ conductor add to the magnetic lines o f force from the field poles. In this manner, the ‘dot’ portion o f the armature is moved down.

Since both the ‘+ ’ and the ‘dot’ conductors are connected together (on the same shaft) and rotate in opposite directions - one upwards and the other downwards, a ‘couple’ is formed and the armature starts to turn. This turning moment developed from the magnetic lines o f force is known as ‘torque’. The amount o f torque developed depends primarily on the current through the armature and o f course the (magnetic) field strength. Marine Electrical Technology

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Chapter 7 7.17.1 Back EMF (Eb) or Counter Electromotive Force Referring to Faraday’s Laws o f Electromagnetic Induction, when a current-carrying conductor is moved in a magnetic field, an electromotive force is produced. W hen this occurs in a motor as a by-product o f m otor torque, the electromotive force is called “Back electromotive force” - in short “Back EMF”. This is because the electromotive force produced in the motor opposes the electromotive force o f the generator. To distinguish between the two electromotive forces, the term “Back electromotive force” is applied to every component that is not a direct product o f a prime distribution system power-generating device. The ship’s service generators, battery systems, and the emergency generator are electromotive force-designated devices. Back electromotive force is directly proportional to the speed o f the armature and the field strength. That is, the back electromotive force is increased or decreased if the speed is increased or decreased, respectively. The same is true if the field strength is increased or decreased. Back electromotive force is a form o f resistance. Any resistance opposes and reduces die overall current. The greater the back electromotive force, the less current delivered to the motor armature. When the motor is first started, when the armature has not yet begun to turn, armature back electromotive force is at zero. Maximum current is available from the generator to the motor armature because the only resistance is in the motor winding. To summarise the above, back electromotive force is produced in the m otor armature as it begins to turn. The faster the armature turns, the more back electromotive force is generated. This back electromotive force reduces the effective current from the ship’s service generator. Table 7.1 is a comparison o f the armature speed, back electromotive force, m otor armature current, and resulting motor torque for normal motor operations. The back electromotive force restricts the current flow. W hen current in the motor armature is reduced so is die m otor’s torque. Since back electromotive force is proportional to the speed o f a motor and current is indirectly proportional to back electromotive force, a motor automatically adjusts its speed to corresponding changes in load. When the m otor’s speed decreases because o f an increase in load, the back electromotive force is reduced and current increases. The increased current produces greater torque, and the motor increases its RPM. All d.c. motors conform to Table 7.L They will deviate only in the specific characteristics o f that m otor’s individual design. For example, all torque is increased when the armature moves slowly. In the series motor, however, its design produces an unusually high value o f m otor torque. This becomes the characteristic o f the series motor. 252

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Direct Current Machines ...................................... ) Increasing Load j

j

Starting

j

Normal Operation

i Motor Armature Speed r.p.m

|

Zero

1

Highest

Decreasing

| Back electromotive force

|

Zero

|

Highest

Decreasing

j Motor Armature Current

I

Highest

|

Lowest

Increasing

i Motor Torque

|

Highest

j

Lowest

Increasing

Parameters

____ _ __ _

J

j

Table 7.1 - Effects of Back EMF and Current on a DC Motor A motor is not designed to operate at the excessive current levels exhibited when it is first started. I f the motor were unable to increase in speed because it was too heavily loaded, sufficient back electromotive force would be unavailable to reduce the generator’s current. This excessive current would quickly bum out die motor. A motor must be allowed to come up to its rated speed rapidly. 7.17,2 Armature Reaction There are individual magnetic lines o f force from the field poles and the armature. Magnetic fields tend to combine. Additionally, the magnetic lines o f force are distorted (or concentrated) by an iron core. Figure 7.24 shows the field flux (Figure 7.24(a)) and the armature flux (Figure 7.24(b)) individually. Figure 7.24(c) shows the distortion caused by the interaction o f the two fields and the armature core movement. This distortion is known as armature reaction. The armature current in a generator flows in the same direction as the generated electromotive force, but the armature current in a m otor is forced to flow in fire opposite direction to that o f the back electromotive force.

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Chapter 7

FIELD FLUX

(A)

sis ARMATURE FLUX

(B)

(C ) F igure 7.24 - A rm atu re R eaction in a M otor

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Marine Electrical Technology

Direct Current Machines In a motor, the main field flux is always distorted in the direction that is opposite to the armature’s direction o f rotation (Figure 7.24(c)); whereas in a generator, the main field flux is always distorted in the same direction as die armature’s rotation. The resultant field in the m otor (view C) is strengthened at the leading pole tips and weakened at the trailing pole tips. This action causes the neutral plane to shift to A'B*. The armature reaction is overcome in a m otor by the same methods used in the generator; that is, by the use o f laminated pole tips with slotted ends, interpoles, and compensating windings. In each case, the effect produced is die same as the results produced in the generator, but it is in the opposite direction. To further ensure successful commutation, small slots on the brush rigging permit a slight adjustment o f the brush position. By placing a tachometer on the m otor shaft, an indication o f motor efficiency may be obtained. Adjust the brush position for die fastest armature rotation in the absence o f sparking.

7.18

Shunt \ \ ound Motor

The shunt wound motor is used where uniform speed, regardless o f load, is desired. It has reasonably good starting torque but is not suited for starting very heavy loads. It is therefore used where the starting load is not too heavy, as in blowers, or where the mechanical load is not applied until the motor has achieved its speed. It is essentially a constant speed machine. The shunt motor is electrically identical with a shunt generator and is depicted in Figure 7.25. It is considered to be a constant speed machine because speed does not ordinarily change more than 10 to 15% within the load limits. The field pole circuit o f a shunt m otor is connected across the line and is thus in parallel with the motor armature; both the motor armature and the shunt field are in parallel with the switchboard bus. If the supply voltage is constant, the current through the field pole coils and consequently the magnetic field will remain constant. The resistance in the field pole coils will change little. Hence, the current in the field poles will remain virtually constant. On the other hand, the resistance in the armature will change as the back electromotive force increases and decreases. This means that the current in the armature will vary inversely with the back electromotive force. W hen there is no load on a shunt motor, the only torque necessary is that which is required to overcome friction and windage (windage is a mechanical loss due to the friction between the moving armature and the surrounding air). The rotation o f the armature coils through the field pole flux develops a back electromotive force. The back electromotive force limits the armature current to the relatively small value required to maintain the necessary torque to run the m otor at no load. Marine Electrical Technology

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Chapter 7

Figure 7.25 - A Shunt M o to r’s C ircuit When an external load is applied to the shunt motor, it tends to slow down slightly. The slight decrease in speed causes a corresponding decrease in back electromotive force. If the armature resistance is low, the resulting increase in armature current and torque will be relatively large. Therefore, the torque is increased until it matches the resisting torque o f the load. The speed o f the motor will then remain constant at the new value as long as the load is constant. Conversely, if the load on the shunt motor is reduced, the motor tends to speed up slightly. The increased speed causes a corresponding increase in back electromotive force and a relatively large decrease in armature current and torque. The amount o f current flowing through the armature o f a shunt motor depends on the load on die motor. The larger the load, the larger the current and conversely, the smaller the load, the smaller will the current be. The change in speed causes a change in back electromotive force and armature current in each case. 7.18.1

No-Field Condition

W e already know that in order for a d.c. motor to turn, there must be the magnetic lines o f force from the armature and the magnetic lines o f force from the field poles. As shunt motors age and corrosion becomes a problem, a runaway condition may present itself. When the shunt field is opened and current is available only to the armature, the motor speed will increase dangerously.

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Direct Current Machines It would seem that without the shunt field the motor would stop. However, the large metal pole shoes o f the d.c, machine support a fairly substantial residual magnetic field. This residual magnetism is ju st enough to ensure that the magnetic principles that sustain the armature movement are present The residual magnetic field is not, however, substantial enough to develop a suitable back electromotive force in the armature. Without the proper proportion o f back electromotive force, current flow to the armature increases. The more current flowing to the armature, the greater the torque and the faster the damaged shunt motor rotates. A no-field trip is employed by shunt motors to prevent such a casualty. When the shunt field is de-energized, the no-field trip circuit disconnects the motor from the circuit 7.18.2 Speed Control The magnetic field from the shunt motor field poles is necessary to maintain an adequate back electromotive force in the motor armature. As long as the back electromotive force is maintained, the current to the armature is restricted, and the motor operates at its rated speed. 7.18.2.1

Above Normal Speed Control

DC motors with shunt fields (both shunt and compound motors) can control the speed above a certain operating (or base) point. This is called speed control above normal speed. Figure 7.26 shows a shunt motor with full field resistance.

F igure 7.26 - A S hunt M otor with Full Field Resistance Marine Electrical Technology

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Chapter 7 A rheostat in series with the shunt field will determine the amount o f resistance in the shunt field. The greater the resistance in the shunt field, the less will the current passing through it be; the reduced current in the shunt field means that the magnetic field has been reduced. With a reduction in magnetic field, there is a reduction in armature back electromotive force. When the back electromotive force is reduced, the motor armature receives more current. The more current in the armature, the greater will be the torque developed. Therefore, motor speed increases. 7.18.2.2

Below Normal Speed Control

In order to reduce the speed o f the shunt or any d.c. motor, it is necessary to reduce the current to the armature. A rheostat in series with the armature will increase the resistance in the armature circuit or decrease the resistance in the armature circuit. A s armature resistance is increased, current to the armature is decreased. The decrease in armature current decreases the torque and armature speed. Control o f the armature circuit in this manner does not substantially affect the back electromotive force created from the rotating armature conductors within the field poles’ strong magnetic field. 7.18.2.3

Use o f Shunt Motors

The speed o f a shunt motor remains nearly constant for a given field current. The constant speed characteristic makes the use o f shunt motors desirable for driving machine tools or any other device that requires a constant speed driving source.

7.19

Series Wound Motor

j

Where there is a wide variation in load or where the motor must start under a heavy load, series motors possess desirable features that are not found in shunt motors. The series wound m otor is used where high starting torque and varying speed is desired. The armature and the series field are connected in series. With high armature and field currents, it has a very high starting torque and is well suited for starting heavy loads such as diesel engines. Figure 7.27 depicts the basic circuit diagram o f a series motor. Notice that the seriesfield is in series with the armature windings. When the motor is first started, with the negligible effects o f the back electromotive force, the magnitude o f current flowing through the armature is high. Since the armature and the series field are in series, the current in. the armature is the same as the current through the series winding. A large current develops a very strong magnetic field and results in an extremely high torque. Conversely, i f the motor is operating at its rated speed, the back electromotive force will be very high, and the current in the series field winding and armature is reduced proportionally. This means that the series motor can develop a very high torque and respond to increases in loading (reductions in armature speed) rapidly. 258

Marine Electrical Technology

Direct Current Machines

Figure 7.27 - A Series-wound Motor 7.19.1

Series Motor Speed

The series motor will continue to increase in speed as long as there is more torque developed than is necessary to turn the load. This additional torque is called acceleration torque. When a series motor is heavily loaded, it slows and produces more torque. As the load is removed, the motor increases in speed. If the load is suddenly removed from the series motor, the accelerating torque is ju st enough to continue to increase the m otor’s speed. The continuously increasing speed can destroy the motor. 7.19.2 No-Load Operation With the load removed and armature speed increasing, back electromotive force should also increase. However, hack electromotive force is a by-product o f a conductor moving in a magnetic field. The series motor field varies with armature current, and back electromotive force decreases as the field decreases. There is sufficient back electromotive force to reduce current to the armature, but in doing so, back electromotive force also limits the current to the series field pole windings. The series field still passes enough current to overcome windage and friction and develop an accelerating torque. Marine Electrical Technology

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Chapter 7 However, at a reduced current flow, there is not enough o f a magnetic field established to generate a proportional back electromotive force at these reduced current levels. Even though back electromotive force increases as speed increases, the overall reduction o f current through the series field winding makes it impossible for a magnetic field to produce the back electromotive force necessary to eliminate the acceleration torque. Due to internal losses, the back electromotive force will always be overcome by the electromotive force in a branch circuit. After all, the electromotive force from the power supply was essential to file creation o f the back electromotive force. The difference between the shunt field and the series field is that the shunt field current is not changed by the armature current. When the load is removed from the series motor, enough current and accelerating torque is available to exceed the feeble back electromotive force the armature r.p.m. increases endlessly. To prevent the series motor from over-speeding and destroying itself, many series motors are provided with a small shunt field to maintain adequate back electromotive force if the load is accidentally removed from the motor. 7.20

C om pound M otors

J

Compound motors, like compound generators, have both a shunt and a series field. In m ost cases, the series winding is connected so that its magnetic field aids that o f the shunt winding5? magnetic field (Refer Figure 7.28 (a)). The current entering both the series field and the shunt field is moving in the same direction. Both fields produce the same magnetic field and aid each other. Motors o f this type are called cumulative compound motors. In the cumulative motor, the speed decreases (when a load is applied) more rapidly than it does in a shunt motor, but less rapidly than in a series motor. The cumulative compound m otor is used where reasonably uniform speed combined with good starting torque is needed. The differential compound motor is used only for low power work. Figure 7.28(b) shows the opposing magnetic fields o f the differential compound motor. Notice that the series winding’s magnetic field is connected to oppose the shunt winding’s magnetic field. The differential compound motor maintains even better constant speed, within its load limit, than the shunt motor. But it has very poor starting torque and is unable to handle serious overloads. 260

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(a) - C um ulative C om pound

(b) - D ifferential C om pound

Figure 7.28 - Types o f C om pound M otors 7.20.1

Separately Excited Motor

Figure 7.29 shows the separately excited d.c. motor. This circuit diagram shows an individual armature circuit and an individual field circuit. A d.c. power source that is not armature-connected supplies power to the field poles. Notice the variable resistors for speed control. The armature rheostat controls speeds below the normal base speed, and the rheostat in the separately excited field controls speeds above the rated base speed.

Above normal speed adjustment

Below normal speed adjustment

Figure 7,29 - Separately Excited M otor

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Chapter 7 7.21

DC M otor R otation Reversal

The direction in which the d.c. motor armature will rotate depends on two conditions: 1.

The direction o f the magnetic lines o f force from the field poles.

2.

The direction of the current through the armature windings and the resulting armature lines o f force.

Article 7.17 explained the principle o f d.c. motor operation and how the lines o f force from the field poles and the current-carrying armature conductors interacted to produce torque. In order to change the direction o f armature rotation, it is necessary only to change the two fields’ relationship. In practice, it is unimportant what magnetic field is changed as long as their relationship is changed. Figure 7.30(a) shows an armature turning in a clockwise direction. By changing the direction o f current through the armature alone (Figure 7.30(b)), the magnetic lines o f force from the armature react differently to the field poleTines o f force. The armature now moves in the counter clockwise direction. Any interposes or compensating windings m ust also maintain the same current direction as the armature windings to effectively eliminate the armature reaction caused by armature current. However, the shunt and/or series fields must not be changed.

F igure 7 JO - R eversing DC M otors The motor rotation can also be changed by reversing the current through the field poles alone. I f the motor is a compound motor, then both the series and shunt fields must have their current flow reversed.

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Direct Current Machines The current flow in the armature m ust be maintained in the original direction. The m otor direction cannot be changed by reversing the polarity o f the incoming power lines. Figure 7.30(c) shows the armature rotating in a clockwise direction. When the incoming power line polarities are reversed, the m otor still rotates in the same direction. Although the field pole polarity and the armature conductor current flows have reversed, tire relationship between the fields in Figure 7.30(a) and 7.30(c) have not changed. As long as the relationship between the field pole magnetic lines o f force and the armature magnetic lines o f force remain unchanged, the direction o f rotation will not change. 7.22

M o to r B raking

7.22.1 Electromechanical Braking Hoists are generally equipped with ordinary friction brakes so that cargo loads can be stopped exactly when and where desired. Friction brakes, like those found on the automobile, are an asbestos and metallic material that is pressed against a metal drum connected to the motor armature or winch drum. The friction between the brake pads and the drum bring the motor armature speed rapidly undo* control. Since the point where braking is to take place is usually remote from the operator, the brakes are usually mechanically applied and electrically released. When electrical power is not applied to the brake system, springs hold the friction brake and drum securely. Energizing a solenoid provides a magnetic field that overcomes die spring pressure, and the brake is then released. This arrangement follows a fail-safe principle employed on winches and capstans and some lifts. I f a power failure should occur with a load hoisted, the load could otherwise drop, damaging the cargo in the case o f a winch and endangering anyone working nearby. Instead, the power failure would de-energize the solenoid, and the spring pressure would again be applied to the brake drum. A friction brake is very effective at moderate and slow speeds too. 7.22.2 Dynamic Braking Depending on the motor application, either friction braking alone o r friction cum dynamic braking can be used. There are only minor differences between generators and motors. A voltage applied to a generator will produce torque. Similarly, when a motor is mechanically turned, it will produce an electromotive force. Dynamic braking takes advantage o f these similarities (Figure 7.31). Marine Electrical Technology

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Chapter 7

+

Figure 7.31 - Dynamic Braking Circuit Any motor will stop eventually when power is disconnected. In order to decrease the armature speed rapidly, the m otor is reconnected as a generator. The field poles maintain their excitation from the normal line voltage. W hen the ‘Stop’ button is pressed, the friction brake is applied. At high armature speeds, the friction brake is inefficient and would bum out after a tew applications. To prevent this, only the armature o f the motor is disconnected from the line voltage. The armature conductors are rapidly turning in the magnetic field o f the field poles. Through external switches, a complete path has been provided through the armature and brush assemblies and connected to a braking resistor. As the armature conductors cut the lines o f force from the magnetic field poles, the armature produces an electromotive force. Since there is a completed electrical circuit, a current flow exists in the armature. The magnetic lines o f force from the armature current interact with the lines o f force from the field poles in a way that opposes the rotation o f the armature. The faster the armature moves, the greater the generated electromotive force and resulting opposing armature magnetic field. The greater the armature speed, therefore, the greater the slowing ability o f the motor. As armature speed reduces, so does the generated electromotive force. A motor cannot be stopped with dynamic braking; it can only be slowed down. Dynamic braking is exceptionally well-suited for rapidly slowing fast-moving armatures. Together, dynamic braking and the friction brake provide an effective way to manage motor armature and thus winch speeds in some cases. Note: The current developed in the armature during dynamic braking is applied to a resistor bank (braking resistor), and thepower is consumedas heat. 264

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1)

Explain the operation o f a basic d.c. generator?

2)

W hat are the methods adopted to reduce armature reaction?

3)

List the types o f direct current generators.

4)

W ith simple sketches explain the series wound generator.

5)

What is a shunt wound generator? Explain the same w ith suitable sketches.

6)

Briefly explain a compound wound generator.

7)

W hy is flashing required? How is it done?

8)

Compare series and shunt fields in a few sentences.

9)

List the types o f direct current motors.

10)

With simple sketches explain the series wound motor.

11)

What is a shunt wound motor? Explain the same with suitable sketches.

12)

How is the speed o f a DC m otor varied?

13)

How is the direction o f a DC m otor varied?

14)

Briefly explain a compound wound generator.

15)

W hat are over-, flat-, and under-compounding?

16)

What is critical field resistance?

17)

H o w is a direct current generator controlled?

18)

How can you take a generator on load?

19)

Explain parallel operation o f d.c. generators.

20)

W hat is a diverter and how does it work?

21)

With the help o f a suitable diagram, explain the significance o f dynamic braking.

22)

W hat is the significance o f the Field poles in a DC generator? Compare the same with the Field o f a DC motor,

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Chapter 7 23)

What is the role o f the Armature in a DC generator? Compare it with an armature o f a DC motor.

24)

Why is a commutator needed in a DC machine? With a suitable diagram explain i)s function.

25)

With suitable sketches explain armature reaction.

26)

Why are compensating windings needed in a DC machine? Where are they located?

27)

What would happen if commutating poles or interpoles are not used in a DC generator?

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Chapter 8 ^ ^ Automatic Voltage Regulators |At the end of this chapter you should be able to: |★

S ta te th e o p e ra tin g p rin c ip le o f a n A V R a n d its m a in c o m p o n e n ts

j

C o m p a re v a rio u s e x c ita tio n s y s te m s

|

|*

E x p la in th e s ig n ific a n c e o f v o lta g e d ip a n d a lte rn a to r re s p o n s e

I

j★

L is t th e fe a tu re s o f a m o d e rn A V R

|

m

C o m p ly w ith re g u la tio n s g o v e rn in g A V R s

I

*

(8.1

Performance Requirements of Alternators A generator’s excitation system is designed to maintain dynamic and transient stability in

the power supply system. The former is nothing but the ability to take care o f small load changes while the latter is the ability to maintain synchronous operation in the event o f system faults. Now when a fault occurs, the synchronous torque, a result o f active power, must be boosted to maintain synchronism. It is done by increasing the field current. The standard condition for generator performance is based on the starting kVA o f the largest motor, or a group o f motors which can be started simultaneously and this kVA should not exceed 60% o f the generator’s capacity. On an unregulated alternator, the voltage may fall by as much as 30% in the event o f a sudden increase in load, especially when heavy-duty equipment like compressors and cargo­ handling systems are started and stopped. H ie AVR must be adequately tuned to ensure stability under various operating conditions. Voltage should not fall below 85% or rise above 120% i.e., -15% to +20% o f the rated voltage when such a load with a power factor from zero to 0.4 is connected to or disconnected from the switchboard. The recovery time is to be within 1.5 seconds. Voltage must be restored to within 3% o f the rated voltage within 1.5 seconds. For emergency generators, the voltage with a tolerance o f 4% in 5 seconds is allowed (some classification societies may permit the permanent variation to remain within +6% and -10%).

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Chapter 8 The transient effect when a load is suddenly connected causes a voltage dip. This dip may be made less i f the generator is designed to have a lower reactance during transient conditions. However, too low a reactance with a smaller voltage dip may involve high shortcircuit currents in excess o f capabilities o f the available protective devices. The designer m ust consider the opposing conditions o f low transient voltage dip and low short circuit currents and balance these conditions against possible increase in machine size, weight and cost. Functional systems generally operate fester than error-operated systems. Most functional systems use an A.V.R. for trimming purposes because o f practical difficulties o f maintaining normal voltage within narrow limits. Methods normally applied will maintain voltages within + 2M> % with many attaining + VA%. I f alternators and their excitation systems undergo steady short-circuit conditions they should be capable o f maintaining a current o f at least three times its rated value for 2 seconds unless requirements are made for a shorter duration. The safety o f the installations must always be ensured.

8.2

i

Operating Principle of an AYR

The way in which an AVR controls the excitation o f a generator or exciter varies, however all o f them fall under two basic categories namely: 8.2.1

The E xcitation Supply AVR

Here, the AVR supplies the whole o f the required excitation current under normal operating conditions. 8.2.2

The Com pounding C ontrol A VR

Here, the generator is compounded to produce an excessive excitation current at all times. The function o f the AVR is to trim down the current to the correct value.

8.3

Excitation System.*

"]

The excitation system has to both supply and control the direct current for the rotor’s pole windings in the case o f a rotating field, stationary armature alternator. The level o f the excitation current and resulting field strength is automatically adjusted by the voltage component. The basic types o f systems are: a)

Direct Self-excitation System (the conventional type)

b)

Indirect Self-excitation System (the brushless alternator type)

c)

Separately Excited System (the permanent magnet generator type)

The brushless alternator has been explained in article 6.7 and hence will not be repeated; the other two types are explained in the paragraphs that follow. 268

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Automatic Voltage Regulators 8.3.1

S elf-excited A VR C ontrolled Generator

H ie main stator provides the input for excitation via the AVR, which governs the level o f excitation provided to die exciter field. The AVR responds to a voltage sensing signal derived from the main stator winding. By controlling the low power o f the exciter field, control o f the high power requirement o f the main field is achieved through the rectified output o f the exciter armature. The AVR senses the average voltage on two phases thereby ensuring effective regulation. In addition it detects the engine speed and provides voltage fall-off (i.e. reduction) with speed, below a pre-selected speed (frequency) setting, which could be + 5% under permanent conditions and + 10% under transient conditions, preventing over-excitation at low engine speeds and softening the effect o f load switching in order to relieve the burden on the engine. The recovery time may be as high as 5 seconds. The AVR may also possess features like three-phase root mean square sensing, also providing for over-voltage protection when used in conjunction with an external circuit breaker that is switchboard mounted. A block diagram o f the same is depicted in Figure 8.1.

Figure 8.1 - Self-excitation System

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Chapter 8 8.3.2

Perm anent M agnet Generator (E xcite d —A VR C ontrolled Generators)

N ew s' generators employ six separate windings. The additional two windings are identical in operation to any pair o f field and armature windings. These extra windings provide external excitation for the generator in the same way the four-winding generator provided for its own self-excitation. The magnet is mounted on the rotor and is located inside the permanent magnet generator’s armature. When the generator is running, the magnet generates an EMF in the armature, providing current directly for control o f the exciter field. A block diagram o f the system is depicted in Figure 8,2.

Figure 8.2 - PMG-excited AVR Controlled Generator 270

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Automatic Voltage Regulators The permanent magnet provides power for excitation o f the exciter’s field (while starting) via the AVR which governs the excitation provided to the exciter’s field. It facilitates greater voltage control under extreme load conditions. The AVR responds to a voltage­ sensing signal derived via an isolating transformer connected to the main stator winding. By controlling the low power o f the exciter field, control o f the high power requirement o f the main field is achieved through the rectified output o f the exciter armature. The PMG system provides a constant source o f excitation power irrespective o f the main stator’s load. It ensures a high motor-starting capability as well as immunity to waveform distortion on the main stator output that is created by non-linear loads, e.g., thyristor controlled d.c. motors. The AVR senses average voltage on two phases ensuring close regulation. In addition it detects engine speed and provides an adjustable voltage fall-off with speed, below a pre­ selected speed (frequency) setting, preventing over-excitation at low engine speeds and softening the effect o f load switching to relieve the burden on the engine. It also provides over-excitation protection which acts following a time-delay, to de-excite the generator in the event o f excessive exciter field voltage.

18.4

Thyristor-based Static Automatic Voltage Regulator

1

Older AVRs e.g., the carbon pile regulator used a magnetic coil powered from the alternator output. The strength o f the field varied with alternator voltage and this strength was tested against springs that act as a reference voltage. The moving contact regulator employed a similar matching o f alternator output effect through a magnetic coil against springs. The availability o f transformed, rectified and smoothed low power supply from the alternator output makes it possible to directly match it against an electronic reference in the static automatic voltage regulator. 8.4.1

Main Components

The AVR circuit is divided into the following basic blocks: 1. A voltage comparison circuit that detects any discrepancy between the generated voltage and the required value. 2. An amplifier and conditioning circuit which converts the signal output from the voltage comparison circuit into a control signal for actuating the control elem ent 3. A control element that varies the excitation current as demanded by the amplifier. 4. Miscellaneous sub-circuits not essential to the basic operation but could improve performance. Marine Electrical Technology

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Chapter 8 8.4.1.1

The Voltage Com parison C irc u it

The voltage reference element in all modem AVRs is die zener diode. This is subjected to a slightly higher voltage than it can withstand. The input is received from a smoothed, rectified supply, fed either from a three-phase rectifier or a simple full-wave bridge rectifier as shown in Figure 8.3(a). To Switchboard

Figure 8.3(a) - Static Automatic Voltage Regulator 8.4.1,2

Role of the Zener Diodes in the Circuit

As long as reverse current is flowing, the voltage appearing across the zener diode is almost independent o f the current and the ambient temperature. This voltage forms the standard with which a known fraction o f the generated voltage is compared by the AVR. The bridge is arranged to be balanced when the generator is producing its correct voltage. There is no output signal at this point o f time. I f the generator voltage falls, the current flowing through the arms o f the bridge will also fall, and so will the voltages across the resistors. The voltages across the zeners remain unchanged. A rise in voltage will produce the opposite effect. The zeners operate in the reverse breakdown mode, having been manufactured with a zener breakdown voltage o f a very low value. (Refer Figure 8.3(b), The error signal can be amplified and is used to control the alternator’s excitation in a number o f different ways.

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Vz (Zener break-down voltage)

|

A.

y ->

v

Izmin (minimum current to sustain breakdown)

!z max (maximum current limited by maximum power dissipation)

Figure 8.3(b) - Characteristic of a Zener Diode Thus it can control the firing angle o f the thyristors through a triggering circuit to give the desired voltage especially in a brushless alternator. It can be used in a statically excited alternator to correct small errors through a magnetic amplifier arrangement (as in the case o f the trimming AVR depicted in Figure 8.6). 8.4.1.3

A m p lifie r and C onditioning C irc u it

The error voltage produced by the voltage comparison circuit is amplified by a transistor amplifier as shown in Figure 8.4(b) and converted if necessary, into a form suitable for the excitation control elem ent If this is a thyristor as in the case o f Figure 8.4(a) the output would normally consist o f a train o f pulses, synchronised with die excitation supply voltage, maintaining a phase relationship which is controlled by the amplifier. If the excitation control element is a saturable reactor or transformer, the output would consist o f a direct current o f variable magnitude as shown in Figure 8.4(b).

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Chapter 8 Feedback is normally applied across the amplifier to avoid instability due to time delays in the generator and exciter. The amount o f feedback is often varied in order to achieve an optimum voltage response to a sudden change in load. Too little feedback will cause the voltage to overshoot, and perhaps to oscillate several times before settling to its steady state value. Too much will cause an unnecessarily slow recovery. Though designs may vary, the functions are similar. Switchboard

Figure 8.4(a) - A Thyristor-controlled Static Excitation System 8.4.1.4

E xcitation C ontrol E lem ent

The final stage o f the AVR which controls the excitation current may have to handle a current o f few amperes if the controlled field winding is that o f a n exciter with a rotating rectifier as in the case o f a brushless alternator, but i f it controls the main field directly, the current can even be in the range o f a few hundred amperes.

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Automatic Voltage Regulators The most common element is a thyristor. We know that it is designed to prevent the flow o f current until a gate signal is fed to it. If this occurs when the voltage is in the forward direction, it will conduct until the forward current falls to a preset value wherein it will revert to the blocked state. The simplest application o f a thyristor is therefore to produce direct current from an alternating source. Again, Figures 8.4(a) and 8.4(h) are good examples. The latter is a detailed version. As the input voltage goes negative, the current dies away, and the thyristor regains its cut­ o ff or blocked state. The magnitude o f the excitation current is controlled by varying the point in the positive half cycle, at which the thyristor is turned on. This point is called the firing angle, which may be advanced or retarded to ultimately control the output. Alternatively, power transistors and saturable transformers or reactors may be used as in the case o f Figure 8.4(b).

Figure 8.4(b) - Static Excitation AVR Circuit with a Thyristor and Saturable Reactor Marine Electrical Technology

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Chapter 8 The direct current derived from the alternator output is applied to a bridge (Refer Figure 8.3(a)) which has fixed resistances on two arms and variable resistances (zener diodes that serve as voltage references) on the other two. As is already known, in zener diodes, voltage remains constant once breakdown occurs despite the change in current. This implies however, that Changes in the applied voltage, while not affecting voltage across the diode, will cause a change in resistance, which permits a change in current. It is similar to a Wheatstone bridge wherein the imbalance o f resistance in opposite arms changes the flow pattern and produces an error signal in the voltage measuring bridge. 8.4.1.5

R ole o f the S ilicon C on trolled R ectifiers in th e C ircu it

A Silicon Controlled Rectifier (Thyristor) is a four-layer, three-terminal, solid state device with the ability to block the flow o f current, even i f it is forward biased, until the gate signal is applied. It is a basic rectifier with a control element. In fact it consists o f three diodes (or two transistors connected back-to-back with a gate connection (Refer Figure 8.4(c)). In the absence o f a control signal, it prevents the flow o f current but, when triggered, will permit current to flow in one direction only.

Anode Anode

T

JL

p N P N :

Cathode

Output wave form (to one term inal of the Exciter)

Figure 8.4(c) - Role of the Thyristor 276

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Automatic Voltage Regulators Once turned on it stays on until the current has fallen to a very low value when it reverts to its blocking state until a trigger or gate signal is re-applied. The thyristor can be turned on in microseconds and requires very little control power, hence it is suitable for use as a synchronous switch forced every cycle at a point on the waveform selected to control the power flow. Typically, thyristors are rated from a few amps to hundreds o f amps. It can be seen that the unit has a limiting voltage in both directions and an excess voltage in the reverse direction will destroy i t In the forward direction it is possible that the device will turn on if it is subjected to over-voltage but a number o f factors m ust be taken into account It may be destroyed by local overheating in the semiconductor junction. In practice, thyristors are not subjected to a working voltage o f more than 50% o f the peak value that they can withstand. In addition, protective devices are added to the circuit to trap or divert spurious pulses and transients which may be present in the supply due to the operation and conductors elsewhere. The signal applied to the control terminal or “gate” of the thyristor is normally only a few volts although this may rise for very high-current devices. The signal may be a d.c. voltage applied for the duration o f the required conduction period. The gate signal could come from a Zener diode voltage reference bridge. The gate signal will switch-on the forward biased S.C.R. and current flows through the exciter field. W hen it is reverse biased, the S.C.R. will again block current flow. When fast trigging is important (for example when thyristors are operating in series or parallel in the same conduction path) the signal may be boosted to give a strong pulse initially with a lower sustained value. It can control loads by switching on and off even up to many thousand times a second. It can also switch on for variable lengths o f time, thereby delivering selected amount o f power to the load, Hence it possesses the advantages o f a switch and also a rheostat with none o f their disadvantages. In Figure 8.4(a), two thyristors are used in a full-wave bridge rectifier. The amplified error signal from the zener-resistor bridge will serve to regulate the d.c output connected to the exciter’s field winding by controlling the firing angle o f the thyristor. Hence the net power output can be controlled. The diode is just a blocking diode that protects the gate from negative voltage, i f any. Only one thyristor and its associated waveforms are shown. Full-wave rectification will be finally achieved in the actual circuit as there are two thyristors and o f course a bridge rectifier.

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Chapter 8 8.5

Alternative Thyristor-based AVR Circuits

An alternative to the circuit depicted in Figures 8.4(a) and 8.4(b) is the one in Figure 8.5. Now due to inductance o f the field winding, the S.C.R. yrould continue to pass current for a part o f the negative cycle. By fitting a ‘free-wheeling* diode the current through the thyristor falls quickly at the end o f the positive cycle. In some circuits the excitation current is designed to exceed the requirements so that the gate signal reduces flow. Here, the voltage across each zener diode remains constant and is almost independent o f the current variations. Filtered d.c. output voltage is applied to the voltage reference bridge. This bridge is balanced when the pre-determined generator output voltage is achieved and no potential difference exists between ‘A* and H Three-phase R ectifier

Smoothing Circuit

Gate Control Circuit

Three-phase a.c. supply from generator

Figure 8.5 - An alternative circuit of a Thyristor-controlled Static Excitation System

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Automatic Voltage Regulators c

If the generator voltage foils, the current through the bridge arm falls and current flows from ‘A* to ‘B ’ through the amplifier.

°

If the generator voltage fells, the current through die bridge arm falls and current flows from ‘B ’ to ‘A ’ through the amplifier.

°

I f the generator voltage rises, the current through the bridge arm rises and current flows from ‘A ’ to ‘B ’ through the amplifier.

0

The signal from the amplifier will automatically vary the field excitation current, usually through the silicon controlled rectifier control element.

A simplified diagram o f a typical “direct-feed” thyristor AVR is shown in Figure 8.6. The generator voltage is stepped down, rectified and then applied to a reference circuit.

Figure 8.6 - Typical Direct-feed Thyristor AVR Any difference between the generator voltage and the desired voltage produces an error voltage. This error is amplified and fed to a blocking oscillator, which controls the firing angle o f the thyristor. The magnitude o f the excitation current depends upon the time during each cycle for which the thyristor is conducting.

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Chapter 8 If the generator voltage fells* the increased error voltage increases the conduction time. This results in increased excitation current and rotor flux, which brings fee generator voltage back to fee desired value. Short-circuit excitation current transformers (CTs) are used to prevent a complete collapse o f fee generator excitation under short-circuit conditions. These CTs provide all the excitation currents under short-circuit conditions and enable a sufficiently large generator current to be maintained in order to ensure circuit breaker tripping. For parallel operation, fee AV R must have “droop” and a quadrature current compensation (QCC) circuit consisting o f a CT and resistor. The CT detects lagging load current and causes fee AVR to reduce the output voltage; this is depicted in Figure 8.4(b).

8.6

Transformer-based Static Excitation System Direct on-line started induction motors take six to eight times fee normal full load current

as they are started. A large motor therefore puts a heavy current demand on fee a.c. system, causing fee applied voltage to dip, where recovery from the dip is slow. This results in a momentary dimming o f lights and similar effects on other equipment. There is a limit to the size o f a motor feat can be started direct-online, but the ability o f alternators to recover from large starting currents has been enhanced by fee development o f the static excitation system. The direct current required for fee production o f the rotor pole magnetic field is derived from fee alternator output without fee necessity for a rotating exciter as described for fee carbon-pile,/ d.c. exciter system or for fee brushless machine. The principle o f the static or self-excitation system is feat a three-phase transformer with two primaries, one in shunt and the other in series w ith fee alternator output, feeds current from its secondaiy windings through a 3-phase rectifier for the excitation o f the main alternator rotor (Refer Figure 8.7). Excitation for the no-load condition is provided by fee shunt-connected primary, which is designed to give sufficient m ain rotor field current for normal alternator voltage at no load. The reactor coils create an inductive effect so that fee current in fee shunt winding lags the main output voltage by 90°. Build up o f voltage is assisted by capacitors, which promote a resonance condition with the reactors (causing voltage magnification) or by means o f a pilot exciter. This is also termed as impedance matching.

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j

Automatic Voltage Regulators The load current in fee series primary coils contributes to fee additional input to fee excitation system to maintain voltage as fee load increases. Variations in load current directly alter excitation and rotor field strength to keep the voltage approximately right. Both shunt and series inputs are vectorially added in the transformer. Diodes in the threephase rectifier change fee alternating current into direct current, which is then smoothed and fed to fee alternator rotor through slip rings. Voltage control within close limits is achieved by trimming fee same wife a static AVR to counteract small deviations due to internal effects and wandering from the ideal load / voltage line. The AVR may be o f fee static type already described with the error signal amplified and fed to coils in the three-phase transformer. Changes in fee coil current brought about by fee AVR alter the transformer’s output enough to trim fee voltage.

Figure 8.7 - Transformer-based Static Excitation System Marine Electrical Technology

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Chapter 8 8.7

Transient Voltage Dip and Alternator Response

A gradual change o f alternator load over the range from no load to M l load would allow the automatic voltage regulator (AYR) and excitation systems described, to maintain terminal voltage to within perhaps 2% o f the nominal figure, thus exceeding the requirements as mentioned in article 8.1. The imposition o f load however is not gradual, particularly when starting large direct on-line squirrel cage induction motors. Starting current for these may be six times the normal value as mentioned earlier, and their power factor is very low, at say Cos = 0.40 while starting. Figure 8.8 shows the pattern o f voltage dip and its recovery when the steady state o f a machine that is running with normal voltage, is interrupted by the impact o f the load while starting a direct on-line induction motor. Steady state error

■Effect of poor prime-mover governor response

Figure 8.8 - Typical Voltage Dip I Recovery Pattern for an Alternator The initial sharp dip in voltage followed by a slower fall to a minimum voltage is mainly the result o f the magnitude and power factor o f the load and reactance characteristics o f the alternator. Recovery to normal voltage is dependent on the alternator, its excitation system, automatic voltage regulator and the prime mover’s governor. Both frequency and voltage are affected by changes o f the electrical load on the generator. The governor basically ensures that the frequency is maintained, but in doing so, it also sustains a major part o f the voltage output, in other words, the active power or kW output. However to keep the voltage constant within tolerable limits, die AVR is required. Thus they both play a vital role while paralleling. Both, th*e ‘conventional’ alternator with a d.c.-exciter-carbon-pile-regulator combination, which is not included in this chapter due to obsolescence and the brushless machine explained in articles 6.12 and 6.13, have an error-operated AVR and excitation system.

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Automatic Voltage Regulators The voltage has to change for the AVR to register the deviation from normal and to then adjust the excitation for correction. The initial voltage dip (specifically due to transient reactance) is such that the response from the error-operated system cannot come until the dip is in the second slower stage. Thus neither machine can prevent the rapid vertical voltage dip due to the transient reactance. The faster acting voltage regulator o f the brushless machine will arrest the voltage drop sooner on the secondary (slower) part o f its descent. The older carbon pile regulator was slow as compared with die present static type. The brushless machine also achieves good recovery by the process o f field-forcing i.e. excitation boosting thereby ensuring a quick build-up o f voltage. (Refer Figure 8.9).

Figure 8.9 - Comparison of Voltage Dip / Recovery Pattern for Different Excitation Systems Static excitation systems make good use o f the load current from the alternator to supply that component o f excitation current which is needed to maintain the voltage as the load increases. This component o f excitation is a ‘function’, therefore, o f the load. Field current is thus forced to adjust rapidly as die load changes. Voltage disturbances accompanying the application or removal o f load are greatly reduced. Statically excited alternators have better recovery from voltage disturbance and permit the use o f large, direct on-line starting induction motors. As seen in Figure 8.9, the recovery time is just 0.2 seconds for a statically excited system while the brushless generator needs 0.6 seconds and the obsolete carbon pile regulator, 1.6 seconds respectively. The graphs in Figures 8.10 and 8.11 will also help to clarify the above.

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Chapter 8

%

E xcitation

Figure 8.10 - Variation of Excitation at Constant Voltage % Nominal Voltage

Figure 8 . 1 1 Variation of Voltage at Constant Excitation

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Effect of kW Loading

When the generator is on no-load die governor set point is manually adjusted until the frequency is correct. The AVR trimmer (if fitted) is adjusted until die voltage is correct. The prime mover does not require much fuel to run the generator on no-load, so the governor opens the fuel throttle valve only by a small amount If a kW load such as the galley heaters is switched on, then as we know by now, the energy is drawn from the generator and converted into heat Increasing the rate of fuel supply to the prime mover will result in providing this energy. This happens automatically in the following way: *

When the load is applied the load draws current from the stator windings.

■ This current flowing in the stator windings provides a rotating magnetic field. This field rotates at the same speed as the rotor. ■ The stator field lies across the rotor field and exerts a magnetic puli or torque on the rotor that tries to pull the rotor backwards (Refer Figure 8.12). *

The magnetic torque exerted on the rotor causes the rotor to slow down.

*

The governor detects this reduction of speed and opens up the throttle to increase the fuel supply.

*

The throttle is opened up until the frequency returns to normal (in fact slightly less).

■ Now the prime mover develops enough power to drive the alternator at the correct speed and meet the kW load demand. In short, the governor responds to changes of kW load to keep the system frequency constant. The graph of frequency against kW for the governor shows how closely it maintains constant frequency. For perfect accuracy the characteristic must be horizontal. This implies that the systems frequency is constant at every change in kW load and is referred to as being isochronous. In reality, most governors exhibit a droop of up to 5% . This is so that the generator can be run in parallel with other generators (Refer Figure 8.13). Some modem electronic governors may provide a selector switch where isochronous operation is selected when the generator is running alone and droop injected when running in parallel. Marine Electrical Technology

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Figure 8.12 - Exertion of Torque on the Rotor Due to the Stator Field

Frequency {or Speed)

No Load

kW Load

Figure 8.13 - Governor Characteristic

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100% Load

Automatic Voltage Regulators 8.9

Effect of kVAr Loading When a kVAr load is applied to an alternator, there is no power demand on the prime

mover. This is because the energy flow with kVAr loading is backwards and forwards between the generator and the load, the prime mover is not involved. The stator current again produces a rotating magnetic field, but unlike kW loading, it does not exert a magnetic torque on the rotor. This time the stator field is in line with the rotor field and so no torque is produced. With reference to Figure 8.14 we see that the stator field is acting in the opposite direction to the rotor field which results in a large reduction o f flux in the machine and reduced flux means reduced output voltage. The AVR responds to the fall o f the output voltage and boosts the excitation current to the rotor to increase the flux. The excitation is increased until the voltage is back to normal (in practice slightly less than normal). Thus the AVR responds to changes o f kVAr load to keep the system voltage constant The AVR characteristic, which is a graph o f volts / kVAr, like the governor, exhibits a droop, which is required for stable operation.

Figure 8.14 - Opposition of the Stator & Rotor Fields With regard to kVAr load sharing, also explained in Chapter 10, it is achieved automatically by the AVR units which adjust the excitation after synchronisation so that each machine shares the reactive power (kVAr) and generates the correct voltage. Marine Electrical Technology

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Chapter 8 8.10

Additional (Important) Features in a Modern AVR

1) Voltage regulation with droop-type reactive load sharing and power factor control during a parallel operation with is an important feature; the latest AVRs follow the three-phase root mean square sensing method for superior voltage regulation and form part o f an excitation system for brushless alternators. 2) Voltage matching and Over-voltage protection 3) Excitation for sustained short circuits 4) Adjustable Soft start circuits provide a smooth controlled build-up o f voltage in a time span o f about 0.4 to 4 seconds. 5) Under-frequency protection; a frequency measuring circuit continually monitors the shaft speed o f the generator and provides under-speed protection o f the excitation system by reducing the generator output voltage proportionally with speed below a pre-set threshold. 6) Under/over-excitation protection 7) Rotating diode condition monitoring 8) Field winding thermal overload protection 9) Back-up arrangements with either manual control or redundant double AVRs with automatic or manual change-over. 10) The permanent magnet generator in the circuit isolates die AVR control circuits from the effects o f non-linear loads and alleviates the effects o f radio frequency interference. 11) Connection to an external potentiometer for trimming the output voltage o f the AVR. 12) Connection to an auxiliary (winding on the generator) input - generally 220 to 240V to help boost the output o f the AVR especially when starting the generator set. Some o f the above features and others are explained in the following paragraphs: 8.10.1

S ta b ility A dju stm en t

This feature comprises o f a damping circuit to provide good steady state and transient performance o f the generator.

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Automatic Voltage Regulators The correct setting o f the Stability adjustment can be found by running the generator at no load mid slowly turning the stability control anti-clockwise until the generator voltage starts to become unstable. The optimum or critically damped position is slightly clockwise from this point i.e. where the generator’s voltage is stable but close to the unstable region. 8.10.2

U nder F requency R o ll O ff

The AVR incorporates an under-speed protection circuit which gives a voltage/speed (frequency) characteristic when the generator speed falls below a preset threshold known as the “knee point” as shown in Figure 8.15. The knee point is set by the underfrequency roll off control potentiometer. Knee Point

% Speed (Hz)

—

>

Figure 8.15 - The UFRO Control Potentiometer Sets the Knee Point The symptoms for incorrect setting are: a) A light emitting diode glows permanently when the generator is on load. b) Poor voltage regulation while bearing load i.e., operation on the sloping part o f the characteristic.

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Chapter 8 The UFRO adjustment is generally preset and sealed and only requires the selection o f 50 or 60 Hz. The closest setting should be such that die light emitting diode glows when the frequency falls just below the rated frequency e.g., by about 3Hz i.e.,57 H z in a 60Hz system and 47 Hz in a 50 H z system. 8.10.3

Excitation Trip

An AVR supplied from a PMG inherently delivers maximum excitation power on a lineto-line or line-to-neutral short circuit. In order to protect the generator windings, the AVR incorporates an over-excitation protection circuit which detects high excitation and removes it after a pre-determined time o f about 8 to 10 seconds. A symptom o f incorrect setting is indicated by a collapse o f the generator’s output while on load or when small overloads occur. The light emitting diode glows permanently to indicate this situation. The correct setting may range from 70 to 90 volts with a tolerance o f + 5%. 8.10.4

Over Voltage Protection

Over voltage protection circuitry is included in the AVR to remove the generator’s excitation supply in the event o f a loss o f AVR (sensing) input. Both internal electronic de­ excitation and the provision o f a signal to operate an external circuit breaker can be available, the latter being a must i f over voltage protection is required. Incorrect setting would cause the generator’s output voltage to collapse under no-load conditions or on removal o f load. The light emitting diode glows to indicate this condition. 8.10.5

Transient Load Switching Adjustments

The additional function controls o f Dip and Dwell are incoiporated in order to optimise the load acceptance capability o f the generating set. The overall performance o f the generator depends upon the engine’s capacity and the governor’s response, in conjunction with the generator’s characteristics. It is not possible to adjust the level o f dip or recovery independently from the engine performance. There w ill always be a relationship between frequency and voltage dips. 8.10.5.1 Dip This feature is used when the generator is coupled to turbo-charged engines with limited block load acceptance. The dip function potentiometer helps to adjust the voltage / speed (frequency) characteristic below the knee point (Refer Figure 8.16).

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Figure 8.16 - Dip Function Potentiometer Adjustment With the Dip control fully anti-clockwise, the generator voltage will follow the normal V/Hz line as the speed falls below normal. Turning the Dip control clockwise provides greater voltage roll o ff aiding engine recovery i.e., greater roll o ff in proportion to speed. 8.10.5.2 Dwell The dwell function caters to providing a time delay between the recovery o f voltage and recovery o f speed; it also allows a greater Dip setting without instability. The purpose o f the time delay is to maintain the generator’s active power output (kW) below the engine’s rated capacity, during die recovery period, thus permitting better speed recovery. This control is also functional only below knee point, i.e. if the speed stays above the knee point during load switching there is no effect from the dwell function setting (Refer Figure 8.17). With the Dwell control fully anti-clockwise, the generator voltage will follow the V/Hz line. Turning the Dwell control clockwise increases the time delay between speed recovery and voltage recovery. Marine Electrical Technology

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Chapter 8 Adjustable Slope

instant of load application

Figure 8.17 - Dwell Function 8.10,6

Ramp

The ramp potentiometer enables a soft start or adjustment o f die time taken for the generator’s initial build-up to its rated (normal) voltage during each start and run-up to its rated speed. The potentiometer is pre-set, to give a ramp o f about 3 seconds, which is considered adequate for most applications; die limits lying between 1 and 8 seconds respectively. 8.10.7

Droop

The most common method o f kVAr sharing is to create a generator voltage characteristic which falls with a decreasing power factor (increasing kVAr). This is achieved with a current transformer, connected to the blue (or 3rd) phase which provides a signal that is dependent upon the current’s phase angle (Cos) to the AVR. The current transformer has a burden resistor on the AVR board and a percentage o f the burden resistor’s voltage is added into the AVR’s circuit (Refer Figure 8.18).

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(a) No. 1 Droop > No. 2 Droop

(b) No. 1 Droop = No. 2 Droop

(c) No. 1 Droop < No. 2 Droop

Figure 8,18 ~ Effect o f D roop In a T w o-generator System Depending upon available load the following settings should be used to ensure kVAr sharing: •

5% droop setting - at full load current, at zero power factor load lag.

•

3% droop setting - at full load current, at 0.8 power factor load lag

Accuracy is achieved when the droop is set with a low power factor load.

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Chapter 8 8.10.8

Over-voltage De-excitation Breaker

This provides positive interruption o f the excitation power in the event o f an over-voltage condition that occurs due to the loss o f signal sensing or internal AVR faults including the output power device. The generator must be stopped to reset an over-voltage trip. 8.10.9

The Block Diagram

Referring to Figure 8.19, the functions o f the blocks are explained briefly: 8.10.9.1

The Power Supply

It provides the required voltages for the AVR circuitry. 8.10.9.2

The Potential Divider and Rectifier

It takes a proportion o f the generator output voltage and attenuates if. The potential divider is adjustable by the AVR Volts potentiometer and external hand trimmer (when fitted). The output from the droop CT is also added to this signal. A rectifier converts the a.c. input signal into a d.c. signal representing generator voltage. 8.10.9.3

The DC Mixer

It adds the Analogue input signal the generator voltage signal. 8.10.9.4

The 3-Phase Rectifier

It converts the output o f the current limiting CTs into a dc signal representing generator current 8.10.9.5

The Amplifier (Amp)

It compares the generator voltage or current signals to the Reference Voltage and amplifies the difference (error) to provide a controlling signal for the power devices. The Ramp Generator and Level Detector and Driver infinitely control the conduction period o f the Power Control Devices and hence provides the excitation system with the required power to maintain the generator voltage within specified limits; 8.10.9.6

The Stability Circuit

It provides adjustable negative a.e. feedback to ensure good steady state and transient performance o f the control system.

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Stator

Voltage Sensing

Droop

Hand Trim m er

Analog Input

Current

Limit Input

Figure 8.19 - Block Diagram of a Modern AVR 8.10.9.7

The Low Hz Detector

It measures the period o f each electrical cycle and causes the reference voltage to be reduced approximately linearly with speed below a presettable threshold. The Dip and Dwell circuits provide adjustments for greater voltage toll o ff and recovery time. A Light Emitting Diode gives indication o f under-speed running. 8.10.9.8

The Synchronising Circuit

It is used to keep the Ramp Generator and Low H z Detector locked to the Permanent Magnet Generator waveform period. 8.10.9.9 Power Control Devices They vary the amount o f exciter field current in response to the error signal produced by the Amplifier. 8.10.9.10

The Circuit Breaker

It provides circuit isolation o f the control system in the event o f an over excitation or over voltage condition. Marine Electrical Technology

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Chapters 8.10.9.11

The O ver E xcitation D etecto r

It continuously monitors the exciter field voltage and turns o ff the power device if this rises above the reference level for greater than the stated tim e period. An external signal is also provided to trip the Circuit Breaker. 8.10.9.12

The O ver V oltage D etector

It continuously monitors the generator stator voltage and turns o ff the power device if this rises above the reference level, for greater than the stated time period. An external signal is also provided to trip the Circuit Breaker.

1)

Automatic voltage regulators provided on switchboards function t o _________ ,

2)

The output voltage o f a 440 volt, 60 Hz, 4-pole, 1500 r.p.m. alternator is controlled by the

3)

The AVR is capable o f maintaining a steady output when the alternator's output varies betw een________ .

4)

The output voltage from the main alternator must be w ithin__ _______in 1.5 seconds.

5)

The output voltage from the emergency alternator must be w ithin______ in 5 seconds.

6)

When large DOL squirrel cage motors are started, the power factor may fall to______ .

7)

An AVR helps to maintain the output Voltage within ________ %.

8)

The input to the AVR is taken from th e _________ .

9)

The output o f the AVR is connected t o _________ .

10)

The AVR is meant to control_________ .

11)

A static excitation system is capable o f recovery (to the steady-state value) w ithin____ second(s).

12)

The instantaneous reduction in voltage o f an alternator, resulting from an increase in load, and prior to the AVR correcting the situation, is called_____________.

13)

W hat is the meaning o f excitation in an alternator? How is it supplied?

14)

Draw and explain the transformer-based static excitation system.

15)

Briefly describe the operation o f an automatic voltage regulator.

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Automatic Voltage Regulators 16)

Explain die significance o f the pick-up point o f the generator’s output and the routing o f the control circuit’s input in an AVR.

17)

How does a thyristor-controlled static AVR work? Explain the same with a suitable diagram,

18)

W ith a suitable graph explain what you know about alternator response for different excitation systems.

19)

What are the effects o f kW loading?

20)

What are the effects o f kVAr loading?

21)

Explain Alternator Response in relation to different types o f AVRs.

22)

Explain the 3-phase transformer-based static excitation system.

23)

Explain why the setting o f an automatic voltage regulator for an a.c. generator should not be altered while the machine is operating in parallel with another machine.

24)

State the various safety devices on an Automatic Voltage Regulator, explaining the conditions under which they operate.

25)

How does a Zener diode work?

26)

Explain the basic role o f a Zener diode in an AVR.

27)

How does a thyristor work?

28)

Explain the role o f a thyristor in an AVR.

29)

Explain the operation o f an automatic voltage regulator o f the Zener bridge type.

30)

What are the effects o f kW loading?

31)

What are the effects o f kVAr loading?

32)

What is the significance o f UFRO in a modern AVR?

33)

Where is an excitation trip used? W hat is the symptom o f incorrect setting?

34)

Explain over voltage protection in about 5 lines.

35)

Write short notes on Dip and Dwell.

36)

What is the role o f the ramp potentiometer? Marine Electrical Technology

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Chapter 8 37)

Why is droop important?

38)

W hat are the droop settings to ensure kVAr load sharing?

39)

What is the role o f the over-voltage de-excitation breaker?

40)

What is the significance o f over voltage protection in a generator’s system?

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Chapter 9 ^ ^ Panel Instrumentation |[A t the end of this chapter you should be able to: | ★

C o r r e la te b a s ic m e a s u rin g s y s te m te rm in o lo g y w ith in s tru m e n ta tio n

★

E x p la in t h e c o n s tru c tio n a l fe a tu r e s o f p a n e l in s tru m e n ts

I ★

E x p la in th e th e o r y a n d o p e ra tin g p rin c ip le s o f p a n e l in s tru m e n ts

| ★

Id e n tify p a n e l in s tru m e n ts o n b o a rd a s h ip a n d u tilis e th e m a d e q u a te ly

I

★

C o m p ly w ith re g u la tio n s g o v e rn in g in s tru m e n ta tio n fo r s w itc h b o a rd s

| 9.1

The Basics The high cost involved in the repair o f equipment and replacement o f spare parts, demand

that the marine engineer correctly diagnoses and efficiently repairs defects in electrical equipment, wherever humanly possible. With the correct choice o f meters, it is possible to determine parameters needed to troubleshoot the electrical system even without shutting down systems. More often than not, accurate predictions can also be made thus preventing heavier losses. Instrumentation is in fact a subject by itself. As the title o f this chapter suggests, only panel instrumentation or rather those measuring instruments dealt-with by the marine engineer in the Engine (or Machinery) Control Room and elsewhere on the vessel have been explained in near detail. In order to recapitulate the fundamentals o f measurement, a few basic terms are also explained.

| 9.2

Measuring System Terminology Accurate measurement o f the various parameters in a control system is the foremost

requirement. This will necessitate knowledge o f the parameter to be measured e.g., pressure, temperature, flow, and the standards used as a basis for the measurement. The functional elements, which enable the measurement to be taken, must be examined, i.e., the measurement system, and their actions understood. Then it will be necessary to determine how the actual measuring system performs as compared to an ideal measuring system. Marine Electrical Technology

Chapter 9 When all of these factors are established and in some way specified, then suitable instruments can be selected for particular measuring duties. The actual measuring instrument may then act either as an indicating instrument or a recording instrument. An indicating instrument will visually display the measured value (e.g. a voltmeter). A recording instrument will provide some form o f permanent record o f measurements taken over a period o f time. A few common terms are briefly explained under the following subheading: 9.2.1

Information is the basis o f ail control.

9.2.2

Instrumentation is the use o f instruments to obtain this information.

9.2.3

Measurement is a comparison with a basis or standard and is the key to satisfactory instrumentation.

9.2.4

Standard - Any reading given on a measuring instrument must be related to some accepted standard in order to be meaningful. All scales, graduations and markings are in some unit o f measurement which can be related to a standard basis for the parameter under consideration.

9.2.5

Calibration - Where an instrument has its readings compared with some standard or known value, this is known as calibration. The actual process o f calibration may be achieved by the use o f a primary or secondary standard. It is usual for the primary standard to be used to calibrate a secondary or working standard. The working standard is then used for instrument calibration. The accuracy o f the instrument can thus be traced back to the primary standard.

9.2.6

Static Sensitivity - As a result o f static calibration o f a measuring system, a calibration curve cm be drawn. The slope or gradient o f this line will be the static sensitivity. Where the curve is not linear the sensitivity will vary according to the input value. A correct measurement o f sensitivity would give the relationship between the actual physical output (angular deflection o f a pointer) and the input; although often the scale reading provided is used. When several elements in a system have static sensitivities, and are connected in series, then the static sensitivity is the product o f the individual values. It is assumed that the loading effects between the elements are taken into account. Where the input and output to an element are in the same physical form then the term ‘gain’ can be used, which is similar to ‘amplification’ and ‘magnification’.

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9 .2 .7

L in ea rity is a term associated with sensitivity and is a measure o f the maximum

deviation from a linear input / output relationship, usually expressed as a percentage o f the full scale value. 9 .2.8

True Value is that value wherein the average o f the deviations from the desired output

tends to zero or is zero itself. This is hypothetical and in most cases unachievable. 9.2.9

A ccu racy is an indication o f the nearness to which the true value is measured.

9.2.10 P recision while associated with accuracy does not mean the same thing where, for the

same input that is applied on a number o f occasions, an instrument provides readings which are very close in value, it is said to have high precision. If however a zero error offset existed, i.e. for a zero o f the measured value, then die instrument gave a reading either above or below zero, then the instrument could not be said to be accurate, although o f high precision. 9.2.11 R eprodu cibility is a general term used with regard to precision and provides a

measure o f the closeness o f readings given for a constant input. 9.2.12 R ep ea ta b ility refers to reproducibility when a constant input is repeatedly applied for

short-time intervals under fixed conditions. 9.2.13 S ta b ility concerns repeatability when the constant input is applied for a long time

compared with the time required to take a reading under fixed conditions. 9.2 .1 4 C onstancy refers to reproducibility when a constant input is provided continuously

but the prevailing conditions during measurement are permitted to vary within specified limits. 9.2 .1 5 E rrors exist in all measurement units or systems and are the differences between the

indicated and the true values, often expressed as a percentage o f the full-scale deflection. Errors can be the result o f incorrect observation (parallax error), the incorrect position or graduation o f a scale, an indication which is found to be incorrect following calibration, or a zero offset error. 9.2 .1 6 T olerance is the term used for maximum possible error and may be both positive and

negative and is expressed either in % or units itself.

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Chapter 9 9 .2 .1 7 H ysteresis is a result o f loading o f the measuring instrument and subsequent less-

than-complete unloading. This is also a difference in reading for input as a result o f the direction o f approach. The numerical value o f hysteresis may be given in terms o f output or input and is usually expressed as a percentage o f the full scale. 9 .2.18

T hreshold is the maximum value o f input, below which no output change can be

detected. 9.2.19 D ea d Z on e is the zone between zero (input) and the threshold value. 9 .2.20 R esolu tion is the smallest change in input at any other point, which can be definitely

detected. 9.2.21 Span is the extent o f input values (variable between upper and lower limits o f

measurement). 9 .2 .2 2 R an ge is the difference between the upper and lower limits o f the instrument’s

displayed measurements or output.

| 9.3

Some Useful Fundamentals To many, it may seem paltry to discuss fundamentals at this stage in die book. However,

since instruments are discussed here, it will also be apt to recall some o f them - at least those related to this chapter. 9.3.1

The R elation sh ip betw een V, I, W a n d R

y

V (volts)

y

I (amperes) =

(a) V/R (b) W /V and (c) V(W/R)

y

W (watts) =

(a) VI (b) I2R and (c) V2/R

y

R (ohm s)

(a) V/I (b) W/I2 and (c) V2/W

9 .3.2

=

=

(a) W/T (b) IR and (c) V(WR)

W hy D o M odern V essels C hoose to G enerate H igh V oltages?

Conventionally, electricity is generated at 440 Volts (ship’s manuals mention the designed voltage to be 450V also) and then delivered to the systems through conductors. Assuming that there are 10 consumers that need 1500 Watts each; It means that 10 x 1500 = 15,000 Watts must be supplied.

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We know that Power W = V x I and I = W/V .*.1 = 15,000/440 = 34 amperes And we also know that electrical power is dissipated as heat according to the known formula W= I2R. Assuming that R = 1; we now have heat dissipation = 342 x 1 = 1156 watts. H eat dissipation is energy lo st by the system . This lo ss is unavoidable!

Hence in order to deliver the 15,000 watts that the consumers need we m ust generate 16,156 watts and then have an overall efficiency o f 15,000/16,156 = 92.8% For convenience o f calculations, if the generated voltage V is increased by a factor o f 10, then the current I will be reduced by a factor o f 10 (at constant power output) and the power dissipated as heat lowers by a factor o f 102 for the same value o f resistance - this is just 11.56 watts. The efficiency would now be improved to about 99.99%! Hence at 4400 Volts we have an energy efficient delivery system. However, this is not the real case; power factor must also be considered, as we deal with not ju st resistance in the system but reactance in the form o f motors and other similar equipment. 9.3.3 Power Factor Power factor (PF) measures how effectively your a.c. electrical system operates. A high PF (considered to be above 0.95) means the apparent power requirements for an electrical system, as expressed in kilovolt-amperes (kVA), are very close to the actual power requirements, as expressed in kilowatts (kW). The actual power requirement represents the power needed for useful work as well as normal line losses. The apparent power relates to the total electrical system requirements, including reactive power (which in itself performs no useful work). The reactive power requirements o f an electrical system are the volt-amperes necessary to provide the magnetizing power for inductive loads. In an ideal situation, voltage and current (amperes) are in phase, and the voltage and current cross the neutral horizontal line concurrently. When dealing with realistic a.c. electrical systems, the current usually crosses the neutral after the voltage (current lags voltage causing phase displacement); a condition caused by inductivetype loads. The amount o f phase displacement relates to the power factor. I f an electrical system has a low PF, it means the apparent power requirements are somewhat larger than the actual power requirements. A system with low power factor can result in losses to the supplier. Marine Electrical Technology

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Chapter 9 The power factor o f an electric motor reaches its maximum rated value when it is under a full load, and the power factor decreases rapidly as the load decreases. The power factor associated with a transformer reacts to load variation in a similar manner. The reactive power used by a transformer at full load may be as high as 8% to 12% o f the rated power o f the transformer. When unloaded, the amount o f reactive power may remain as high as 4% to 6% o f the rated power. Thus, a transformer consumes power even though it may not serve a load. As mentioned in article 6.9.4, PF can be improved by using synchronous motors, usually in very large sizes, to drive mechanical loads. The motors also provide a leading PF to help overcome an electrical system with a lagging PF (which is almost always the case). Sometimes, people use synchronous motors (that aren’t driving any loads) solely to provide leading kvars into an electrical system to counteract inductive loads. Automatic controls can adjust the field excitation in the rotor windings o f a synchronous motor, which in turn determines the amount o f leading kvars produced. These motors are usually referred to as synchronous condensers. Adding power factor correction capacitors to compensate for the consumption o f reactive energy, is another method used to improve PF.

9.4

In-Circuit Meters

_______ _____________ ;_________ __ ________ ________ ___________ _________ ______________________________j

Some electrical devices have meters built into them. These are in-circuit meters, which monitor the operation o f the circuit in which they are installed. Some examples o f in-circuit meters are the voltage, current, and frequency meters on ships’ switchboards; and the electrical energy meter that records the amount o f energy consumed in a building. It is not practicable to install an in-circuit meter in every circuit. However, it is possible to install an in-circuit meter in each critical or representative circuit so as to monitor the operation o f a piece o f equipm ent A mere glance at an in-circuit meter on a control panel is often sufficient to monitor the working o f equipment. It is important to become familiar with in-circuit meter values during all regimes / modes o f the system’s operation. Only after observing familiar ‘normal’ readings an engineer can readily notice the abnormal operation o f a system. As an in-circuit meter is a mere indicator, the cause o f the malfunction is determined by troubleshooting. This process involves locating and repairing faults in equipment after they have occurred. Chapter 26 will help one to gain insight in this regard.

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Out-of-Circuit Meters In troubleshooting, it is usually necessary to use an out-of-circuit m eter that can be

connected to the electrical equipment at various testing points. Out-of-circuit meters may be moved from one piece o f equipment to another. They are generally portable and selfcontained in the sense that they have their own power supply and other accessories to facilitate convenient logging o f data e.g., a multimeter.

9.6

Permanent Magnet Moving Coil Meter A permanent magnet moving coil m eter’s movement is based upon a permanent magnetic

field and a coil o f wire that can deflect about an axis, as shown in Figure 9-1. When the switch is closed, current flows through the coil; the coil will develop a magnetic field that reacts with the magnetic field o f the permanent magnet. Applying the right-hand gripping rule to the solenoid, we will find that the bottom portion o f the coil in Figure 9-1 will be the north pole o f this electromagnet. Since opposite poles attract, the coil will rotate counter clockwise and attain the position shown in Figure 9-2.

A Moving Coil in a Magnetic Field Figure 9.1 - No C urrent Flow

Figure 9.2 - Current Flowing

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Chapter 9 The hobbffflssupported by jewelled bearings that allow it to deflect freely. This type o f bearing reduces friction and wear and tear in the long run due to the hardness o f the jewel and minimum area o f contact between the pivot and the jew elled bearing. There are spring loaded jewel bearings too, wherein a constant pressure is maintained on the pivots o f the moving coil (Refer Figure 9.3).

Iron Core (Magnetic Field Concentrator)

F igure 9.3 - Basic A rran g em en t in a PM M C M eter To use this permanent magnet moving coil device as a meter, two problems m ust be solved. First, a w ay must be found to return the coil to its original position when there is no current through the coil. Second, a method is needed to indicate the amount o f coil movement which is otherwise known as ‘deflection’. The first problem is solved by attaching phosphor-bronze hairsprings to either end o f the coil. These hairsprings can also be used to make the electrical connections to the coil. By using hairsprings, the coil will return to its initial position when there is no current. This is because o f ‘Restoring Torque’ that is applied to the coil. 306

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The springs will also tend to resist the movement o f the coil when there is current through the coil (this happens because o f ‘control torque’). W hen the attraction between the magnetic fields from the permanent magnet and the coil, in other words the ‘deflecting torque’, exactly equals the force o f the hairsprings, the coil will stop moving toward the magnet. In short, the pointer comes to a standstill when the deflecting torque equals the control torque. The damping torque ensures that the pointer does not vibrate or hunt about its steady state position. This also ensures that the pointer smoothly comes to rest at a point on the scale (depending upon the value o f the measurand). Damping is achieved by adopting numerous methods, the most common in this type o f instrument being eddy current damping which as already mentioned, is achieved by the aluminium former on which the moving coil is wound. As the current through the coil increases, the intensity o f the magnetic field generated around the coil proportionately increases; the greater the intensity o f the magnetic field around the coils, the greater is the angle o f deflection. The second problem is solved using a pointer attached to the coil and extended out to a scale. The pointer will deflect with the coil. The scale can be marked to indicate the amount o f current through the coil.

D’Arsonval Movement

Perman Magne (Field

Moving Coil on Former

Control Spring Field Concentrator Cum Connector for the Moving Coil

Figure 9.4 - D’Arsonval Movement in a PMMC Meter Marine Electrical Technology

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Chapter 9 Two other features are used to increase the accuracy and efficiency o f this meter movement: 1. An iron core is placed inside the coil to concentrate the magnetic fields. This alleviates the loss o f flux due to the large void within the inner boundaries o f the former. 2.

Curved pole pieces are attached to the magnet to ensure the deflecting force on the coil increases steadily as the current increases. This ensures an equal air gap between the magnets and the coil. Similarly, curved pole pieces are found in a motor.

This permanent magnet moving coil meter movement is the basic movement in most analog measuring instruments (meters with pointers that serve as indicators). It is commonly called d’Arsonval movement because it was first employed by the Frenchman d’Arsonval in making electrical measurements. Figure 9.4 depicts the d’Arsonval meter movement used in a meter.

| 9.7

Power Measurement Figures 9.5 and 9.6(a) and (b) depict the simple methods to measure power in d.c. circuits.

This has been explained in order to obtain a clear idea o f power measurement. The shortcomings in both arrangements can be overcome by using a single wattmeter.

P ow er consum ed by the load, VJL = V (I-Iv) = V (I-V /R y) = VJ-V2/ R y P ow er indicated by instruments = po w er consumed by the lo a d + pow er loss in the voltm eter

Figure 9.5 - Power Measurement in DC Circuits (Variant 1) 308

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Power consumed by the load, VJ = (V-Va) I = VI- Va.l = VI- I2Ra Power indicatedby the instruments = power consumedby the load + power loss in the ammeter

Figure 9.6(a) - Power Measurement in DC Circuits (Variant 2)

Figure 9.6(b) - The Magic Circle Marine Electrical Technology

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Chapter 9 Multiply by

Conversely Multiply by

horsepower to kW

0.7457

1.341

horsepower to W

745.700

1341

horsepower (metric) to kW

0.7355

1.360

1.014

0.986

To Convert into Si Units Power

British h.p. (745.7 w) to metric h.p. (735.5w)

T able 9.1 - C orrelation betw een B ritish a n d SI U nits for Pow er Calculation Peak-to-Peak

(V p p >

V p -(-V p ) = 2VP Peak Value (Vp)

2

2

i> = *7Cos

Where V and 1 are the root mean square values o f voltage and current; Cos — 0 and Cos 0 = I). The readings o f two wattmeters are:

=YZ VICos (30°- $) = Yz VICos 30° = (3/2) VI P2=YZ VI Cos (30°+ )= YZ VI Cos 30° = (3/2) VI Pi

Pi +P2 = 3 VI. So, at unity power factor, the total power - P = Z VI Cos = 3 VI Thus at unity power factor, the reading o f the two wattmeters are equal. Each wattmeter reads h alf o f the total power. 2) When thepf. - 0.5, we have = 60°

YZ VI Cos (30° - ) P2 = YZ VI Cos (30° + ) P, =

And

= =

YZ VI Cos (30°- 60°) = (3/2)F7 Yz VICos (30°+ 60°) = 0

P\ + P2 = ( 3/2) VI + 0 = (3/2) VI = Total pow erP = Z F7Cos = ( 3/2) VI Therefore when the power factor is 0.5, one o f the wattmeters reads zero and the other reads the total power. 3) When thepf. — 0, we have —90° Therefore, P , = Yz VICos (30° - ) =V3 F7Cos (30°- 90°) = ( v/3/2) VI And Or

P2 =YZ VIC o s (30° + (j)) = V'3 VICos (30°+ 90°) = - ( % ) VI P]+P 2 = 0; Total power P = 3 VI Cos = 0 Marine Electrical Technology

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Chapter 9 Therefore, with zero power-factor the readings o f the two wattmeters are equal but have opposite signs. It should be noted that when the power factor is below 0.5, one o f the wattmeters would give a negative indication. Under these conditions in order to read the wattmeter, we must either reverse the current coil or the pressure coil connections. The wattmeter will then give a positive reading but this m ust be taken as a negative value while calculating the total power. 9.12.4 Delta Connection Instantaneous reading o f wattmeter P i is: pi = -v,3(i\ - h) Instantaneous reading o f wattmeter P 2is: P2= V2 (/> h) The sum o f instantaneous readings o f wattmeters

= P 3+ P 2 = - v3 (/'i - i3) + v2 (*r *1) = v2 h + v3 h- iifa +v3)

Figure 9.13 - Two Wattmeter Method - Delta Connection From K irchhoff s voltage law, vj + v2 + v3 = 0 .\ vi = - (v2 +v3) .*. The sum o f instantaneous readings o f the two wattmeters - V2 /2+ v3 i3 - h (- vi) —vi ii+ v2 h + v3 h Hence the sum o f the two wattmeters’ readings is equal to the power consumed by the load. This is irrespective o f whether the load is balanced or unbalanced.

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Panel Instrumentation 9.12.5 Three-Phase Wattmeter A dynamometer type three-phase wattmeter consists o f two separate wattmeter movements mounted together in one case (the two moving coils are mounted on the same spindle). The arrangement is shown in Figure 9.14. There are two current coils and two pressure coils. A current coil together with its pressure coil is known as an element. Therefore a three-phase wattmeter has two elements. The connection o f two elements o f a 3-phase wattmeter is the same as that for the two-wattmeter method using two single-phase wattmeters. The torque on each element is proportional to the power being measured by it. The total torque deflecting the moving system is the sum o f deflecting torques o f the two elements i.e. the deflecting torque o f the elements l a P\ and 2 cc P2; and the total deflecting torque oc (P{+ p2) cc P\ hence the total deflecting torque on the moving system is proportional to the total power.

Compensation for mutual effects between two elements o f a three-phase wattmeter

Figure 9.14 - Three-phase Two Element Wattmeter

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Chapter 9 In order that a 3-phase wattmeter reads correctly, there should not be any mutual interference between the two elements. A laminated iron (magnetic) shield may be placed between the two elements to eliminate the mutual effects. The use o f W eston’s method can help compensate for mutual effects. Resistance R' may be adjusted to compensate for errors caused by mutual interference.

s9.13

Power Factor Measurement

9.13.1 Power Factor This plays a vital role in active power calculations in a.c. circuits where, P = VI Cos) Sin (60°+ 0). Torque acting on coil B is: T b = K V u IM ^ C o s (30° - ) Sin (120° + 0) = ^ K V I M ^ C o s (30°- ) Sin (120°+ 0). Torques TA and TB act in the opposite direction and the moving system takes up a position where TA = Te and Cos (30° + (J>) Sin (60° +0) = Cos (30° - ) Sin (120° + 0). Solving the above expression, we have 0 = ; thus the angular deflection o f the pointer from the plane o f reference is equal to the phase angle o f the circuit to which the meter is connected. The three-phase power factor meter gives indications, which are independent o f waveform and frequency o f the supply, since the currents in the two moving coils are equally affected by any change o f frequency. For measurements o f power factor in three-phase unbalanced systems, a two-element power factor m eter where two sets o f fixed coils and two sets o f moving coils mounted on the spindle have to be used. 9.13.2.2 Three-Phase Moving Iron Power Factor Meter In this instrument, there is a fixed coil P connected in series with a high resistance across one pair o f lines. This coil is in the centre o f three fixed coils, each o f which are connected to lines 1, 2 and 3 respectively. A concentric cylinder carrying sector-shaped vanes and a pointer, both o f which are mounted on a spindle passes through this coil. Now the alternating flux produced by coil P interacts with the fluxes produced by the three current coils; this causes the moving system to attain a position determined by the power factor angle o f the load.

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Due to the rotating field produced by the three coils, there is a slight tendency for the instrument to behave like an induction motor. Hence it is essential that the moving iron is made to be highly resistive; this reduces the eddy currents and thus the rotation o f the vanes. Damping is achieved with the help o f air friction (damping vanes). Total deflecting torque Td a [U p Cos (90°- ) Sin (90° +0) + I2IpCos (330°- ) Sin (210° +0) + I3IPCos (210°- ) Sin (330° +0)]. For a steady deflection,, the total torque must be equal to zero. Considering the system’s load to be balanced, I| = I2 =

13 ,

W e now have,

Cos(9Q°-) Sin (9O°+0) + Cos(3300-4>) Sin(21O°+0) + Cos(210°- ) Sin (330° + 0) - 0 Hence 0 = and the deflection o f the iron vane from the reference axis is a direct measure o f the phase angle between each line current and the corresponding phase voltage. The phasor diagram for this type o f meter is depicted in Figure 9.19 and the related schematic diagram is depicted in Figure 9.20.

Figure 9.19 - Phasor Diagram for Figure 9.20

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Supply 1

2

3

Figure 9.20 ~ Three-Phase Moving Iron Power Factor Meter 9.14

Frequency Meters

The different types o f frequency meters are: 1. Mechanical resonance type 2. Electrical resonance type 3.

Electrodynamometer type

4.

Weston type

5. Ratiometer type 6

. Saturable core type.

Frequency can also be measured and compared by other arrangements like electronic counters, frequency bridges, stroboscopic methods and the cathode ray oscilloscope. Some of them are explained in this article.

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Panel Instrumentation 9.14.1 Mechanical Resonance Type Frequency Meter (Vibrating Reed Type) This m eter consists o f a number o f thin steel strips called reeds. These reeds are placed in a row alongside and close to an electromagnet as shown in Figure 9.21. The electromagnet has a laminated iron core and its coil is connected in series with a resistance, across the supply whose frequency is to be measured. The reeds are approximately 4mm wide and 0.5mm thick. Flags

Figure 9.21 - Vibrating Reed Frequency Meter All the reeds are not exactly similar to each other. They have either slightly different dimensions or carry different weights or flags at their tops. The natural frequency o f vibration o f the reeds depends upon their weights and dimensions. Since the reeds have different weight and sizes, their natural frequencies of vibration are obviously different. The reeds are arranged in ascending order o f their natural frequency, the difference usually being 0.5 Hz. Thus the natural frequency o f the first reed may be 47 Hz, o f the second 47.5 Hz, o f the next 48 Hz and m ay stop at 53 Hz. The reeds are fixed at the bottom end and are free at the top end. Since the reeds on a frequency meter are arranged to be viewed end on, they have a portion bent over at the free end to serve as a flag as shown in Figure 9.21. The flags are painted white to afford maximum contrast against their black background. When the frequency meter is connected across the supply whose frequency is to be measured the coil o f the electromagnet carries a current i which alternates at the supply frequency. The force o f attraction between the reeds and the electromagnet is proportional to i2 and therefore this force varies at twice the supply frequency. Marine Electrical Technology

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F igure 9.22 - Frequency versus A m plitude Thus the force that is exerted on the reeds varies every half cycle. All the reeds will tend to vibrate, but the reed whose natural frequency is equal to twice the frequency o f the supply will be in resonance and will vibrate the m ost Normally the vibration o f the other reeds is so slight that they are overlooked. The tuning in these meters is so sharp that as the excitation frequency departs from the resonant frequency the amplitude o f vibration decreases rapidly thus becoming negligible for a frequency perhaps 1 to 2 % away from resonance; this is clear from Figure 9.22. Now Figure 9.23(a) shows the condition o f the reeds when the frequency meter is unexcited i.e., it is not connected to the supply. W hen the 50 Hz reed is vibrating with its maximum amplitude (when it is in resonance), the 49.5 Hz and 50.5 Hz’ reeds may also be observed to vibrate as depicted in Figure 9.23(b) but very little vibration will be observed on the 49 Hz and 51 Hz reeds. For a frequency exactly midway between that o f the reeds, both will vibrate with amplitudes, which are equal in magnitude, but considerably less than the amplitude, which is at resonance. Figure 9.23(c) shows the condition o f vibrating reeds when the frequency is exactly midway between 49.5 Hz and 50 Hz.

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(a)

(b)

(c)

Figure 9.23 - Indications from Vibrating Reeds The usual frequency span o f these meters is six Hz, say 47 Hz to 53 Hz. The frequency range o f a set o f reeds m ay be doubled in a simple manner. In the presence o f an alternating flux alone, the reeds are attracted twice in a cycle and the reed whose frequency is twice the frequency o f supply responds. I f however, the electromagnet is polarized by a direct flux and is equal in magnitude, the fields (d.c. and a.c.) will cancel each other in one half cycle and reinforce during the other half cycle and so the reeds will be attracted only once in a cycle. Thus a reed whose natural frequency is 100 Hz will respond to 50Hz when the electromagnet is unpolarized and to 100 Hz when the electromagnet is polarized. It is clear now that the range o f the frequency meter will be doubled with polarization. The polarization may be accomplished by using a d.c. winding in addition to the a.c. winding or by using a permanent magnet. An advantage o f the reed type o f frequency meter is that the indication is virtually independent o f the waveform o f the supply voltage. The indication is independent o f the magnitude o f the applied voltage also provided that the voltage is not too low, as at a low voltage, the amplitude o f vibrations will not be sufficient and thus the readings will not be reliable. Marine Electrical Technology

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Chapter 9 The disadvantage is that such instruments cannot be read much closer than half the frequency difference between adjacent reeds. Thus they cannot be used for precision measurements. The reliability o f reading also depends upon the accuracy with which the meter reeds have been tuned. 9.14.2 Electrodynamometer Type Frequency Meter The schematic diagram o f this meter is shown in Figure 9.24. The fixed coil is divided into two parts - 1 and 2. The two parts o f the fixed coil form two separate resonant circuits. Fixed coil 1 is in series with an inductance Li and a capacitance Cj, forming a resonant circuit o f frequency fj slightly below the lower end o f the instrument scale. Fixed coil 2 is in series with inductance Lz and capacitance C 2 forming a resonant circuit o f frequency f2 slightly higher than the upper end o f the instrument scale. In the case o f instruments for power frequency measurements, the circuits may be tuned to frequencies o f 40 Hz and 60 Hz respectively with 50 Hz in the middle o f the scale. The two parts o f the fixed-coil are arranged as shown in the diagram, their return circuits being through the movable coil. Input ~

Fixed Coii 2

Fixed Coil 1

VAAAA/

/

\

a a a a

J

Moving Coil

Figure 9.24 - Electrodynamometer-type Frequency Meter

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The torque on the movable element is proportional to die current through the moving coil. This current is the sum o f the currents in the tw o parts o f the fixed coil. For an applied frequency, within the limits o f the frequency range o f the instrument, the circuit o f fixed coil 1 operates above the resonant frequency (as X li>X ci) with current through it lagging the applied voltage. The circuit o f fixed coil 2 operates below the resonant frequency (Xc 2>XL2) with current 12 leading the applied voltage. One fixed coil circuit is inductive and the other is capacitive and therefore the torques produced by the two currents i\ and ii act in opposition (on the moving coil). The resultant torque is a function o f frequency o f the applied voltage and therefore the meter scale can be calibrated in terms o f frequency. The instrument scale spreads over an angle o f about 90°. A small iron vane mounted on the moving system provides the controlling torque. This meter is only used in power systems.

19.15

Synchroscope

A synchroscope is used to determine the correct instant for making the circuit breaker that connects an alternator to the busbars. This process o f connecting at the correct instant or synchronizing is necessary when an unloaded ‘incoming’ machine is to be connected to the busbars in order to share the load. The correct instant o f synchronizing is when the bus bar and the incoming machine voltages: (i)

Are equal in magnitude

(ii)

Are in phase

(iii)

Have the same frequency

The function o f the synchroscope is to simultaneously indicate the difference in phase and frequency o f the voltage o f the bus bar and the incoming machine. Synchroscopes may either be o f the electro-dynamometer type or the moving iron type. Both types are special forms o f their respective power factor meters. 9.15.1

Electrodynam om eter (Weston Type) Synchroscope

Figure 9.25 shows a simple circuit o f a Weston type synchroscope. It consists o f a threelimbed transformer. Marine Electrical Technology

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Chapter 9 The winding on one o f the outer limbs is excited from the busbars and that on the other outer limb by the incoming machine. The winding on the central limb is connected to a lamp. The windings on the outer limbs produce two fluxes, which are forced through die central limb. The resultant flux through the central limb is equal to the phasor sum o f these fluxes. This resultant flux induces an EMF in the winding o f the central limb. The two outer-limb windings are so arranged that when the bus bar and die incoming machine voltages are in phase, the two fluxes through the central limb are additive and thus EMF induced in the central limb winding is maximum. Hence under these conditions the lamp glows with maximum brightness. When the two voltages are 180° out o f phase with each other the resultant flux is zero and hence no EMF is induced in the central limb winding, resulting in the lamp going dark i.e., not glowing at all. I f the frequency o f the incoming machine is different from that o f the busbars, the lamp will be alternately bright and dark or in other words the lamp flickers. The frequency o f flickering is equal to the difference in frequencies o f the bus bar and the incoming machine. The correct instant o f synchronizing is when the lamp is flickering at a very low rate and is at its maximum brightness. One o f the defects o f this simple circuit is that it does not indicate whether the incoming machine is too fast or too slow. This defect can be corrected by introducing an electrodynamometer type instrument into the circuit as shown in Figure 9.26. The Electrodynamometer Instrument, as mentioned earlier, consists o f a fixed coil divided into two parts. The fixed coil is designed to carry a small current and is connected in series with a resistance across the bus bar. The moving coil is connected in series with a capacitor across the terminals o f the incoming machine. The instrument is provided with control springs, which act as current leads for the moving coil. The shadow o f the pointer is silhouetted against an opal glass.

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Main Bus bars

Incoming Machine’s Bus bars

Panel Instrumentation

B

Figure 9.25 - Simple Weston-type Synchroscope

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Chapter 9

v,

Figure 9.26 - An Improved Version of the Weston Synchroscope

Figure 9.27 - Phaser Diagrams for Different Conditions of the Bus bar and Incoming Voltages

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Panel Instrumentation W hen die two voltages are in phase with each other, currents Ii and I2 in the fixed and moving coils respectively will be in quadrature (90°) with each other (Refer Figure 9.27(a)) and therefore there will be no torque on the instrument. The control springs are so arranged that the pointer is vertical under this condition. The lamp is also at its maximum brightness and the pointer is silhouetted against the opal glass. I f the incoming machine voltage V2 is leading the bus bar voltage Vi and the incoming machine is slightly slow, the conditions o f the circuit will slowly change from those shown in Figure 9.27(b) to those shown in Figure 9.27 (c), then the torque will change from KIiI2Cos (90+9 ) to KIiI2Cos (90-0). During this period the lamp will be bright and the pointer will be seen to move from the left-hand side o f the dial through the vertical position to the right hand side o f the dial. The dial can thus be marked with directions Fast and Slow as shown in Figure 9.28. During the period when the voltages Vi and V2 are 180° out o f phase, the pointer will move back. But it will not be visible as under these conditions the lamp is dark. The visible movement o f the pointer is therefore a series o f traverses on the dial in one direction. When the incoming machine is too fast the visible traverses will be in the other direction. The correct instant o f synchronizing is when the pointer is visible at its central position and is moving very slowly at about 15 revolutions per minute in the “fast” direction. There could be a variant to this i.e., light emitting diodes are also used today instead o f a pointer or disc. In this case the light emitting diodes are placed in a circle and a light emitting diodes appears to be rotating in the ‘fast’ or ‘slow’ direction as the case may be. Ideally, the point in time at which the breaker o f the incoming machine must be made is when the light emitting diodes appears to be also rotating slowly in the fast direction and almost comes to a standstill at the 12 O’clock position. Lam p

Figure 9.28 - Dial of a Weston Synchroscope Marine Electrical Technology

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It may be observed that it is possible to have an exact quadrature relationship between currents Ii and h when voltages Vj and V 2 are in phase, only if a small inductance L is introduced in the fixed coil circuit. An alternative arrangement is shown in Figure 9.29.

Figure 9.29 - Alternative Arrangement (Circuit) in a Synchroscope (In stru m en t T ransform ers n o t show n)

|9.16

Phase Sequence Indicators

These instruments are used to determine the phase sequence o f three-phase supplies. There are two types o f phase sequence indicators. (i) (ii) 9.16.1

Rotating-type Static-type. R o ta tin g T ype P hase S equ en ce In d ica to r

The principle o f operation is similar to that o f a 3-phase induction motor. It consists o f three coils mounted 120° apart in space. The three ends o f the coils are brought out and connected to three terminals marked RYB. The coils are star-connected and are excited by the supply whose phase sequence is to be determined.

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An aluminium disc is mounted on the top o f the coils (Refer Figure 9.30). The coils produce a rotating magnetic field and an eddy EMF is induced in the disc. These electromotive forces cause eddy currents to flow in the aluminium disc. A torque is produced due to the interaction o f the eddy currents with the field. The disc revolves because o f the torque and die direction o f rotation depends upon the phase sequence o f the supply. A n arrow indicates the direction o f rotation o f the disc. I f the direction o f rotation is the same as that indicated by the arrowhead, the phase sequence o f the supply is the same as marked on the terminals o f the instrument. I f the disc revolves opposite to the direction indicated by the arrowhead, the sequence o f the supply is opposite to that marked on the terminals.

Figure 930 - Dial of a Rotating-type Phase Sequence Indicator 9.16.2 S ta tic Type P hase S equence In dicator

The simplest type o f phase sequence indicator is depicted in Figure 9.31 (a). It comprises o f a red and blue (or green) lamp o f adequate voltage rating. When the phase sequence is correct i.e.., RYB, the voltage and current vectors o f the three arms are as shown in Figure 9.31 (b). Referring to the vector diagram, it is found that for the assumed phase-sequence the blue (or green) lamp in phase B will be brighter than the red lamp in phase R as the voltage across it is higher. In case the phase sequence is reversed, the red lamp will be brighter. Therefore w e can conclude that the lamp in the phase following the phase containing the capacitor, in sequence, will be brighter. Alternatively, an inductor instead o f the capacitor can also be used, but the situation will be reversed. Marine Electrical Technology

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(a) Static Phase Sequence Indicator

(b) Vector Diagram of (a)

Figure 9.31 - Static-type Phase Sequence Indicator 9.17

Electric Tachometer

This may be an a.c. or d.c. type. It basically consists o f two main components - a tachogenerator driven by the shaft whose speed is to be measured and a tachometer itself. The d.c. type is a generator with its flux provided by permanent magnets. It uses a wound armature with a commutator and like any d.c. generator, is designed to give an output voltage directly proportional to speed, as is generally the case (Refer Figure 9.32). The output is taken to a bi-directional moving coil ammeter calibrated in r.p.m. The pointer obviously indicates the direction o f rotation as well as the r.p.m.

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The a.c. type produces a voltage, with its magnitude proportional to the speed under consideration (at constant frequency). The stator carries two windings with their axes 90° to each other, one being the supply coil and the other being the pick-up or output coil. The rotor consists o f a thin aluminium or copper cup rotating about a soft iron core. W hen the cup is stationary, there is no output, as the coils are at 90°. W hen the cup is being rotated at constant speed, if a d.c. supply is provided to the input coil, then electromotive forces will be induced by generator action; but the effect is to produce an interacting flux behaviour so that there is no output. Acceleration o f shaft r.p.m. (rotational speed) and thus the cup’s rotational speed produces an output voltage in the coils (Refer Figure 9.33).

Figure 933 - The Drag Cup Tachometer (AC) Another type o f a.c. tachogenerator delivers a 10V 50Kz 3i and Cos2 . The total load is the phasor sum o f Ii and I2. This could be shown in the phasor diagram but has been omitted in the interest o f clarity. Marine Electrical Technology

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Chapter 10 Assume that the power input to machine No.2 is increased and the set tries to accelerate. It advances by a small angle a . New load conditions are set up. Now E ’2 and E t produce ER acting round the local circuit. This causes the circulating current, which under no load conditions was designated as the synchronising current

Is ,

lagging Er by almost 90°. This

current Is can be added by phasors to the original currents. Thus it combines with I2 to give the new machine a current I 2.

Is

is received by machine No. 1 and lessens the current output

giving I'i the resultant o f Fj and Is. The increased input to machine No.2 makes it bear a greater load so that its speed settles to that decided by the governor-actuated throttle-valve opening. Meanwhile machine No. 1, having been relieved o f load, accelerates to a new speed (and hence a higher frequency), determined in the final stage by the overall loading o f the system. Therefore Is is a short time circulating current, brought about by the transient conditions resulting from the adjustment o f the controls. Once the overall paralleled system settles down, we have operating conditions similar to those existing originally, except that Ij, I2 and Cos i, Cos 2 would have new values.

19.10 Load Sharing W e will see that increasing the excitation o f a machine produces a wattless circulating current; this means that a change o f generated voltage relative to the bus bars, changes the amount o f reactive kVA which the machine supplies. An overall balance o f load sharing for kW and kVAR can be seen by comparing the power factor meters o f each generator. Varying the power input tends to speed up the machine and power E 2I2 Cos2 would have new values. Load sharing can therefore be considered from two viewpoints: 1) Sharing o f kW. 2) Sharing o f reactive k VA. 10.10.1

kWLoad Sharing

This is an important aspect o f paralleling and more often than not depends upon the skill o f the engineer on watch especially when it is done manually.

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Paralleling o f Alternators 10.10.1.1

Prime-mover Characteristics

In general, we know that for two alternators to operate successfully in parallel, the loadspeed characteristics o f the prime movers should be drooping, that is to say, the speed o f the prime-mover should decrease slightly with increasing loads. The speed droop, also called governor droop or speed regulation, is usually expressed as a percentage o f the full-load speed and is one method o f creating stability in a governor. Droop is used to divide and balance loads during a paralleling operation. Speed droop = Nnl - N il x 100% i.e. No load speed - Full load speed x 100 N/7 Full load speed The percentage o f droop normally varies from 2 to 4 % from no-load to full-load. Usually the speed-load characteristics are linear. Not enough droop can cause hunting, surging or difficulty in response to a load change. Too much droop can result in slow governor response in picking up or dropping o ff a load. The amount o f power generated by a machine is determined by its prime mover. The speed o f the prime mover is fixed, but its torque can be varied. The effect o f changing the governor characteristics is shown in Figure 10.11. Remember that the power output is related to the frequency o f the machine and P =

S p (/i0ioad -./syste m )

where:

SP is the Slope (Kw / Hz or MW / Hz), / noioad is the no load frequency a n d /system is the system frequency

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The speed-load characteristic is shifted to a new position parallel to the initial position. W hen two alternators are operating in parallel, an increase in governor set points in one o f them: a) Increases the system frequency and b) Increases the power supplied by that alternator and reduces the power supplied by the other alternator. When two alternators are operating in parallel, and the field current o f the second alternator is increased, then: a) The system terminal voltage is increased; and b) The reactive power Q supplied by that alternator is increased, while the reactive power supplied by the other alternator is decreased.

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Paralleling o f Alternators 10.10.1.2 Load Sharing by Two Alternators Let us assume that two alternators are running in parallel. The frequency-load characteristics o f the two machines are depicted in Figure 10.12(a).

Figure 10.12(a) - Load Sharing of Two Alternators Let Wj = full load power rating o f machine 1;

W 2 = full load power rating o f machine 2

Pj = Power shared by machine 1;

P 2 = Power shared by machine 2

P = Power supplied by two machines yOi = no load frequency o f machine 1 ;

fh . =

fix = full load frequency o f machine 1 ;

fa = full load frequency o f machine 2

00

load frequency o f machine 2

/ = common operating frequency when the two machines are running in parallel

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Chapter 10 Machine 1 Drop in frequency from no load to full load = J0\ ~ fl\ Drop in frequency per unit rating —/Oi — fl\ Wi Drop in frequency for a load o f P I = JOi —fl\ . P] wi Operating frequency o f machine 1 = no-load frequency - drop in frequency

Wx Machine 2 Similarly for alternator No. 2. the same operating frequency is

w2

p2

W here / i s the common frequency Also, P, + P 2 = P 10.10.1.3

Load Sharing Between Alternators o f Equal Capacities and Different Droop

Characteristics In the following example, the capacity o f generator A is lOOOkW with a droop o f 3% and that o f generator B is 1000 kW with a droop 4%. The two alternators are operating in parallel and have to share a total load o f 800 kW: Pi = load taken by generator A in kW P 2 = load taken by generator B in kW Total power to be shared = P = Pj + P 2 = 800kW Original frequency at no load fO - 62Hz

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Generator A (capacity o f1000 kWand 3% droop) For a max load o f 1000 kW, the drop in frequency = 3% o f/P = 3 .6 2 = 1.86 Hz

100 Now for a load o f 1 kW , the drop is 1.86

1000 For a load o f Pi kW, the drop is therefore = 1.86 . Pi

1000 Operating frequency o f generator A - f A= original frequency - drop in frequency

= 6 2 - 1.86. Pi 1000 Generator B (capacity o f1000 kW and 4% droop) For a max load o f 1000 kW, the drop in frequency = 4% offO —_4_. 62 = 2.48 Hz

100 Now for a load o f 1 kW, the drop is 2.48

1000 For a load o f P 2 kW , the drop is therefore = 2.48 . P±

1000 Operating frequency o f generator B -fa = original frequency - drop in frequency = 6 2 - 2 .4 8 , P,

1000 SincefA=JB 62 - i .8 6 . Pi = 6 2 - 2 .4 8 . P,

1000

1000

I. 8 6 P 1 - 2 .4 8 P 2

1.86 . Pi = P, 2.48 or % PI = P2 Marine Electrical Technology

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Chapter 10 Now Pi + P 2 = 800 kW Substituting for P 2 we get 4P1 + 3P 1 = 8 0 0 4 or 7P1 = 3200 .•.Pt = 3200 = 457.14 kW 7 and P 2 = 8 0 0 -4 5 7 .1 4 = 342.8 kW Therefore we see that generator A with a flatter characteristic (3% droop) is capable of bearing more load as compared to generator B with a steeper characteristic (4% droop). Note: In case the droop characteristics ofthe above generators are the same (say 3%), then: 1.86 Pj = 1.86 P2; this will result in Pj being equal to P2 or 2Pi — 800 = 400 kWper generator. 10.10.1.4

L oad S h arin g B etw een A ltern a to rs w ith U n equ al C apacities a n d S am e D roop

C haracteristics

In the following example, two three-phase alternators operate in parallel; the rating o f A is 1000 kW and B is 500 kW . The droop setting o f each generator is 4%. The load to be shared is 800kW. Pi = load taken by generator A in kW P 2 = load taken by generator B in kW Total power to be shared = P = Pj + P 2 = 800kW Original frequency at no load fO = 62Hz Drop in frequency at full load = 4% o f 62 = 2.48 Hz In the case o f generator A, for a load o f 1 kW the drop in frequency is 2.48

1000 For a load o f Pi kW, the drop is therefore = 2.48 . Pi

1000 In the case o f generator B, for a load o f 1 kW the drop in frequency is 2.48 500

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Paralleling o f Alternators For a load o f P 2 kW, the drop is therefore = 2 .4 8 . P 2 500 Operating frequency o f generator A = / A= original frequency - drop in frequency = 6 2 - 2 .4 8 . P,

1000 Operating frequency o f generator B = fB= original frequency - drop in frequency = 6 2 - 2 ,4 8 . 500 We know th at/^ = f B 6 2 - 2 4 8 . Pi = 6 2 - 2 .4 8 . P? 1000 500

2 A S . Pi = 1000

Z 4 8 . P2 o rP ! = 2P2 500

Substituting for Pi 2P 2 + P2= 800 or P2= 800 = 266.67 kW 3 Pi = 8 0 0 -2 6 6 .6 = 533.33 kW Thus it can be seen that since generator A is twice the capacity o f B, it also bears twice the load. Figure 10.12(b) depicts the load shared with each generator’s characteristic plotted back to back.

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Operating frequency

Figure 10.12(b) - Load Sharing Characteristics Plotted Back-to-Back 10.10.2

kVAr Load Sharing

The relative internal voltages largely govern the way in which machines run in parallel and share the reactive kVA. The voltage regulation characteristics o f two machines are as shown in Figure 10.14. Note that the voltage is plotted against the kVAr load. As for kW load sharing, the characteristics can also be plotted back to back as shown in Figure 10.13. The position o f the characteristics is determined by the amount o f excitation. The role o f the A.V.R. in this regard is explained in Chapter 8 . An increase o f excitation for one machine, such as machine N o .l, will raise the curve to li. Machine No. 1 then takes a larger share o f the kVAr load and the bus bar voltage is raised. Condition 12 shows how machine No. 1 may be operated at a leading power factor even though the total load is lagging. Figure 10.14 depicts that the machine with the flatter characteristics takes the largest share o f the load.

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Paralleling o f Alternators

F igure 10.13 - L oad S haring (B ack to Back) 10.10.3 Manual Load Sharing This is achieved by raising the governor setting o f the ‘incoming’ machine while lowering the setting on the ‘running’ machine. The balance o f power sharing that is dictated by the governor ‘droop’ o f each machine directs the balance o f power sharing. I f the alternator is operating out o f synchronism it will begin to vibrate severely and eventually trip with the help o f the reverse power relay. Current (or kVAr) sharing is set by the generator’s voltage ‘droop’ set by the AVR. For equal load sharing o f kW and kVAr, each machine must have similar ‘droops’- typically 2 to 4% as seen in the examples above. An overall balance o f load sharing kW and kVAr can be seen by comparing the power factor (Cos3>) meters o f each machine. I f two generators are sharing load equally in parallel when a total loss o f excitation occurs on machine No.2, the likely outcome is that Generator No. 2 will run as an induction generator drawing its excitation from N o.l. Both generator currents will rise rapidly with N o.l lagging more, while No.2 runs with a leading power factor (indicated on the power factor meter). A ‘loss o f excitation’ trip (if fitted) or an overcurrent trip should trip No. 2 generator possibly causing an overload on N o .l. Alternatively, N o.l trips on overcurrent that deprives machine No.2 o f excitation and its breaker trips due to an under voltage condition. The result - total power failure! Marine Electrical Technology

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Figure 10.14 - kVAr Load Sharing (Plot Not To Scale) 10.10.4 Automatic Load Sharing Automatic load sharing circuits in a power management system compare the kW loading o f each generator (via CTs and PTs) and any difference is used to provide an error signal to raise or lower the governor setting o f each prime mover as necessary. Such equipment is usually trouble-free, requiring little maintenance other than an occasional visual inspection and clearing / checking the tightness o f connections. Manual load sharing is the obvious alternative i f the auto control equipment fails.

10.11 The Induction Generator Although it is not very important at this juncture, it is desirable that one knows that an induction generator is a special purpose motor that is run slightly above synchronous speed. Induction generators receive their excitation from the grid, or electric utility and they have no means o f producing or generating voltage until such time the generator is connected to the grid. Induction generators are direct-drive types. The frequency and voltage o f the power generated with induction generators are governed by the frequency and voltage o f the incoming electric utility line.

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Paralleling o f Alternators The voltage collapses very rapidly when the generator is supplying inductive loads because the capacitors must supply all the reactive power needed by the load and the generator. Any reactive power diverted to the load moves the generator back along its magnetization curve. This results in a major drop in generator voltage. (Refer Figures 10.15 and 10.16). Induction generators can only be run in parallel with the grid, which means when the electric grid goes down, or there is a blackout, all gensets, cogeneration and trigeneration power plants within the grid that has the blackout, also go down.

Figure 10.15 - Torque-speed Characteristic of an Induction Machine

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Figure 10.16 - The Terminal Voltage-Current Characteristic of an Induction Generator for a Load with a Constant Lagging Power Factor 10.11.1

Comparison between Induction and Synchronous Generators

In practical terms, the advantages and disadvantages o f synchronous and induction generators can be summarized as follows: i

Induction generators

Parallel or stand-alone?

Typical price comparison

Power factor issues

j10.12

Can only run in parallel with the utility.

Synchronous generators

Can run in parallel or standalone modes

Cannot provide back-up power during utility outage

Can provide back-up power.

Less expensive < 700 kW

Less expensive > 700 kW

Should not be used for more than about 33% of total plant electrical load.

Can be used to improve power factor. Can provide up to 100% of plant load, or more.

Speed Droop and Power Generation

Thefollowing text is a reformatted but unedited extractfrom Application Note 01302 by Woodward Governor Company titled ‘Speed Droop and Power GenerationThefigures have been replicatedfor clarity and re-numberedfor continuity. It has been included with due permissionfrom JeffSnowden. Senior Technical Writer, WoodwardIndustrial Controls, Fort Collins, Colorado, USA.

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Paralleling o f Alternators Quote D roop E n gin e C on trol f o r S ta b le O peration

Speed droop is a governor function which reduces the governor reference speed as fuel position (load) increases. All engine controls use the principle o f droop to provide stable operation. The simpler mechanical governors have the droop function built into the control system, and it cannot be changed. More complex hydraulic governors can include temporary droop, returning the speed setting to its original place after the engine has recovered from a change in fuel position. This temporary droop is called compensation. The ability to return to the original speed after a change in load is called isochronous speed control. All electronic controls have circuits which effectively provide a form o f temporary droop by adjusting the amount o f actuator position change according to how much off-speed is sensed. Without some form o f droop, engine-speed regulation would always be unstable. A load increase would cause the engine to slow down. The governor would respond by increasing the fuel position until the reference speed was attained. However, the combined properties of inertia and power lag would cause the speed to recover to a level greater than the reference. The governor would reduce fuel and the off-speed would then occur in the underspeed direction. In most instances the off-speed conditions would build until the unit tripped due to overspeed. With droop, the governor speed setting moves toward the off-speed as the fuel control moves to increase, allowing a stable return to steady state control. The feedback in the governor is from the output position. Since a minimal movement o f the output position can cause major speed changes in an unloaded engine, it is sometimes difficult to gain stability in unloaded conditions. Actuator linkage requiring more movement o f the output to achieve a given amount o f rack movement at the idle settings than at the loaded settings will often help achieve stability in the unloaded position. Setting a greater amount o f droop in the governor is another solution. In the case o f isochronous (temporary droop) control, the governor speed with which the engine returns to the predetermined speed reference is adjustable, allowing greater flexibility in achieving stable operation, even when unloaded.

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Chapter 10 The Droop Curve Droop is a straight-line function, with a certain speed reference for every fuel position. Normally, a droop governor lowers the speed reference from 3 to 5 percent o f the reference speed over the full range o f the governor output. Thus a 3% droop governor with a reference speed o f 1854 rpm at no fuel would have a reference speed o f 1800 rpm at max fUel (61.8 Hz at no fuel and 60 Hz at max fuel). (Notice that the feedback is over the full output-shaft rotation or fuel rod retraction o f the governor. I f only a portion o f the output is used, the amount o f droop will be reduced by the same proportion. Likewise the same governor would only have a droop from 1827 to 1800 if half o f the full output moved the fuel rack from no fuel to full fuel (60.9 Hz droop to 60 Hz; probably not enough droop to provide stability). Figure 10.17 illustrates 3% and 5% droop governor speed curves, assuming the use o f all o f the servo movement. The speed figures given are theoretical since servo position and rack position are seldom absolutely linear. Most complex hydraulic governors have adjustable droop. In these cases, droop may be set between 0% and 5%. Droop is not adjustable in most mechanical governors, although some mechanical governors have provisions for changes in springs which will change the amount o f droop. Five percent droop is common in simple mechanical governors, although 3% and 10% droop is not uncommon.

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Paralleling o f Alternators

F igure 10.17 - 3 % an d 5% D roop C urves Marine Electrical Technology

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Chapter 10 Electric Generation A single engine electrical generator can operate in isochronous, changing speeds only temporarily in response to changes in load. This system can also operate in droop, i f a lower speed is permissible under loaded conditions (see Figure 10.18).

F igure 10.18 - Response C urves o f Isochronous an d D roop G overnors Parallel with a Utility If, however, the single engine generator is connected to a utility bus, the utility will determine the frequency o f the alternator. Should the governor speed reference be less than the utility frequency, power in the utility bus will flow to the alternator and motor the unit. 386

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Paralleling o f Alternators If the governor speed is even fractionally higher than the frequency o f the utility, the governor will go to full load in an attempt to increase the bus speed. Since the definition o f a utility is a frequency which is too strong to influence, the engine will remain at full fuel. Isochronous governor control is impractical when paralleling with a utility because a speed setting above utility frequency, by however small an amount, would call for full rack, since the actual speed could not reach the reference speed. Similarly, if the setting were even slightly below actual speed, the racks would go to fuel-off position. Governors should not be paralleled isochronously with any system so big that the governed unit cannot affect the speed o f the system. Droop provides the solution to this problem. Droop causes the governor speed reference to decrease as load increases. This allows the governor to vary the load since the speed cannot change (see Figure 10.19).

Figure 10.19 - Comparison of 3% and 5% Droop Speed Settings For 50% and 100% Load G overnor S p eed S ettin g D eterm in es L oad

When paralleled with a bus, the load on an engine is determined by the reference speed setting o f the droop governor. Increasing the speed setting cannot cause a change in the speed o f the bus, but it will cause a change in the amount o f load the engine is carrying. The graph shows that the amount o f load is determined by where the droop line intersects the speed o f the bus. I f the location o f this line is moved, either by changing the reference speed or the amount o f droop in the unit, the amount o f load will also be moved. Marine Electrical Technology

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Notice that the amount o f droop set in the governor has little effect on the ability o f the governor reference speed setting to determine the amount o f load the engine will carry. The greater the droop the less sensitive engine load will be to speed setting. However, excessive droop presents the possibility o f overspeed should the engine be removed from the bus, thus becoming unloaded. In most cases, 4% droop is adequate to provide stability and also allow for precise loading o f the engine (see Figure 10.20).

Figure 10.20 - Speed Setting for 3% an d 5% D roop a t 70% L oad Identical engines can show different characteristics if droop settings are not identical. An engine with more droop will require a greater change in the speed setting to accomplish a given change in load than will an engine with less droop in the governor. As explained in the following paragraphs, the amount o f droop is also controlled by the amount o f terminal shaft travel used between no load and full load. Both o f these considerations should be investigated when apparently identical units show different responses to changes in the reference speed. Output Shaft Movement The amount o f droop in a governor is also influenced by the amount o f available output shaft movement used. The governor’s speed reference is changed by feedback from the position o f the governor output. A governor with 4% droop over the full travel o f the output shaft will have an effective droop o f only 2 % i f only half o f the output is used from minimum to maximum fuel.

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Paralleling o f Alternators Two percent droop is probably not enough to provide stability in many operations. Using less than the optimum amount o f terminal shaft movement will require a higher droop adjustment (knob or slider) than other engines, increasing the danger o f overspeed should the generator suddenly become separated from the bus (load). The low amount o f governor travel may also cause the engine to be unstable. Multiple Engine Isolated Bus Droop may also be used to parallel multiple engines on an isolated bus. In this case, the engines are capable o f changing the frequency o f the bus, and i f all engines are operating in droop, the speed o f the bus will change with a change in load. This is satisfactory only in cases where variations in the speed are acceptable. Multiple engines can also be paralleled on an isolated bus with all but one o f the engines in droop and that one engine in isochronous. These systems will be able to maintain a constant speed as long as the isochronous engine is capable o f accommodating any load changes (see Figure 10.21). In these cases, should load decrease below the combined load setting o f the droop engines, the isochronous engine will completely unload, and the system frequency will increase to the point that load equals the combined droop setting o f the droop engines. The isochronous engine would be motored in this instance unless it was automatically removed from the bus. I f the load increases beyond the capacity o f the isochronous unit, the entire system will slow to the point where the combined droop o f the other units meets the droop-speed position. In this case, the isochronous unit would remain overloaded to a point where it was unable to achieve the governor reference speed.

Actual Speed set by Isochronous machine

Figure 10.21 - Use of Isochronous and Droop Units on an Isolated System Marine Electrical Technology

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Chapter 10 Negative Droop As has been stated, all mechanical governors use droop, either constantly or in the case of isochronous governors temporarily, to achieve stable engine control. It is possible to adjust negative droop (speed reference increases as load increases) into some governors. Satisfactory governor control (engine stability) cannot be achieved with negative droop adjusted into a governor. Unquote

1)

The governor control switch o f an alternator is moved to the “raise” position; this will cause_______ .

2)

A n alternator that is being paralleled with another that is on load; the moment the circuit breaker is closed, the frequency o f the incoming alternator will norm ally_____ .

3)

W hile synchronising two alternators, the incoming machine must always b e ________ .

4)

W hen using the 3-lamp method for synchronising, the top lamp must be dark while the bottom two m ust glow with the same brilliancy. Justify this statement.

5)

A voltmeter m ay be used to parallel two alternators (T / F).

6)

In case a synchroscope is faulty, paralleling o f two alternators is impossible (T / F)

7)

The purpose o f the reverse power relay, provided on a ship's service alternator panel, is to trip the circuit in case th e _________ .

8)

The division o f the reactive KVA between paralleled alternators is initiated b y ______ .

9)

KW load sharing between paralleled generators can be changed b y ________ .

10)

A s load is increased for an alternator, the prime mover eventually slows down resulting i n _________ .

11)

A n alternator panel is fitted with both synchronising lamps and a synchroscope. When the synchroscope pointer reaches the 12 O'clock position, the two bottom lamps are bright and the top one is dark. This m eans__________ .

12)

When paralleling two alternators, the synchroscope selector switch should be set so as to monitor the frequency o f th e __________ alternator.

13)

When paralleling two alternators, the frequency o f the incoming generator, just prior to closing the circuit breaker, should b e __________ .

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Paralleling o f Alternators 14)

When two alternators are being paralleled, the breaker should be closed with the synchroscope pointer rotating in th e __________ direction.

15)

While attempting to parallel two alternators if the synchroscope pointer stops at a position other than 0 and the circuit breaker is closed at this moment th en ___________.

16)

An alternator operating in parallel loses its excitation. This will cause_________

17)

After paralleling two alternators the next step is to balance th e __________ .

18)

An alternator operating in parallel begins to vibrate severely and eventually trips with the help o f the reverse power relay. This is due to_________ .

19)

If field excitation is suddenly lost to an alternator operating in parallel, that alternator w ill___________.

20)

If the energy input is significantly reduced to the prime m over o f one alternator operating in parallel with others, that alternator w ill_________ .

21)

If two alternators have ju st been paralleled, the kW load is initially distributed by

22)

You are attempting to parallel two alternators and the synchroscope pointer is revolving slowly in the “fast” direction. You should____________.

23)

The kilowatt load sharing between two alternators that are operating in parallel is controlled by the settings and characteristics o f th e __________ .

24)

To stop an alternator operating in parallel with another, you must first o f all

25)

When paralleled, alternators m ust have the sam e_________ .

26)

Motoring o f an alternator is undesirable because__________ .

27)

The frequency o f an alternator is adjusted by means o f th e _________ .

28)

When paralleling two alternators using three synchronizing lamps, the flickering o f all three lamps becomes progressively slower and slower. This means th e __________ .

29)

Voltage failure o f an alternator may be caused b y _____________.

30)

The output voltage o f a 440 volt, 60 hertz, alternator is controlled by th e _________ .

31)

If the pointer o f the synchroscope is rotating in the slow direction when you are preparing to parallel two alternators, th e ___________.

32)

A change in field excitation o f an alternator operating in parallel will cause a change in its Marine Electrical Technology

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Chapter 10 33)

A constant output voltage from an alternator is maintained by th e __________ .

34)

Before the generator is paralleled with the MSB, if its governor setting is increased th e n _________ a

35)

The division o f kilowatt load between two paralleled alternators is determined by the

36)

An overall balance o f load sharing for kW and kVAR can be seen by comparing

37)

The ideal time for synchronising o f generators is when ________

38)

Generators N o.l and 2 are operating in parallel and prime mover o f generator No.2 suffers a total fuel loss - the consequence is:

39)

W hy is it necessary that incoming alternator frequency is more than bus bar?

40)

W hy do we close the breaker at 12 O’clock and not at 6 O'clock?

41)

Why is it desirable to operate paralleled alternators at the same power factor?

42)

I f the synchroscope is malfunctioning, which instrument is the most essential to parallel an alternator with the bus bars? Justify your answer.

43)

Draw a simple sketch to depict the instrumentation on a Synchronising Panel.

44)

W hen voltages are equal in frequency where does the synchroscope pointer lie? Why is this so?

45)

What is the maximum time a synchroscope should be online for?

46)

Write short notes on auto synchronising

47)

Explain the method to test the “Auto Gut In” o f the stand by generator

48)

Write short notes on manual synchronising

49)

Write short notes on the importance o f synchronising lamps.

50)

Explain paralleling o f alternators using a synchroscope.

51)

Explain the alternate method to parallel alternators, if the synchroscope is faulty

52)

With the help o f a phasor diagram explain Excitation Control while paralleling alternators.

53)

What are the phases in paralleling o f alternators? With relevant diagrams explain either the synchroscope or three-lamp method.

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Paraiteling o f Alternators 54)

How could you monitor the correct instant for synchronising without the aid o f a synchroscope or synchronising lamps?

55)

What are the likely consequences o f attempting to close the incomer's circuit breaker when the generator voltages are not in synchronism?

56)

With a neat Flow Chart, explain Manual and Automatic Synchronising.

57)

What should be done after successful synchronisation?

58)

What is load sharing? With the help o f relevant diagrams explain load sharing between two alternators. What is the effect o f frequency on load bearing o f an alternator?

59)

Explain kW and kVAr load sharing with a suitable graph.

60)

Explain methods to ensure that load sharing between generators is satisfactory

61)

What is used to adjust the power factor?

62)

What happens when there is a loss o f excitation in a parallel alternator system? Which alternator would trip and why?

63)

What do you understand by Paralleling?

64)

Describe the load sharing process o f generators and state its purpose

65)

List the factors that will determine the output frequency with two generators running in parallel

66)

W ith the help o f a phasor diagram explain Throttle Control while paralleling alternators

67)

Differentiate between throttle control and excitation control while paralleling alternators.

68)

Explain droop characteristics while paralleling.

69)

Write short notes on negative droop.

70)

I f an alternator has isochronous characteristics, how will you parallel this machine with another? Justify your action.

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C S

w

i t c

h

b

o

a

h

a

r d

s

p a

t e

r

1 1

n

d

S

w

i t c

h

g

e

a

r

At the end of this chapter you should be able to: ★

S ta te t h e n e e d fo r d iffe re n t ty p e s o f s w itc h b o a rd s

★

id e n tify v a rio u s ty p e s o f s w itc h g e a r

★

S p e c ify th e ro le o f e a c h k e y c o m p o n e n t in t h e s y s te m

★

S a fe ly o p e r a te s w itc h g e a r o n b o a rd s h ip s

★

C o m p ly w ith re g u la tio n s g o v e rn in g s w itc h b o a rd s a n d s w itc h g e a r

11.1

Switchboards

Marine switchboards can be classified as: □

Main Switchboards

□

Section or Sub-switchboards

□

Group Starter Boards (or Panels)

□

Distribution Fuse Boards*

□

Emergency Switchboards

* Smaller versions ofthese are generally referred to as Distribution Boxes orjust "DBs ” Switchboards are also described as “open” or “dead-front”. For the former, all the essential components such as circuit-breakers, switches, links and terminals are exposed on the front o f an insulated base, whilst for the latter, as the name implies, all live parts are concealed behind steel sheeting or built into steel cubicles and only the operating handles, instruments and lenses o f indicating lamps appear on the front. It should be noted that for a.c. installations, open-type panels are not permitted except for voltages o f 55V to earth and below - as part o f the safety requirements for a.c. voltages in excess o f 55 V (article 2.10 emphasises upon this aspect o f safety). With the heavy prevalence o f a.c. systems, modem switchboards are o f the “dead-front” cubicle-type where all the switchgear and equipment are enclosed in sheet steel compartments (or cubicles). Marine Electrical Technology

Chapter 11 Steel partitions are incorporated within the switchboards in order to segregate the subsections and thereby prevent molten metal and arcs due to short circuits from entering the neighbouring sections. A t the main, section, and sub-switchboards throughout a ship, switching and protective devices such as circuit breakers and fused isolators collectively called “switchgear” control the electrical power. The following paragraphs describe a main switchboard’s arrangement together with its protective and control equipment. 11.1.1

The Main Switchboard

A typical layout o f a ship’s main switchboard is shown in Figure I L L This is invariably made up o f panels, which are arranged to accommodate all the electrical components necessary for performing one function. The central section o f the main switchboard is used to house the control gear o f the main generators. Thus a generator panel will accommodate the supply breaker associated with one machine, the instrument relays, speed and voltage controls, together with any indicating lamps and auxiliary circuit switches that may be required for electrical interlocking or sequential switching. Group Starter Pane! [Auxiliary Services Panel) Generator Panel [Auxiliary Services Panel) Group Starter Panel

□ r □ /

r

r

□ / fi r

r

p r □ /

r

t

r □ r □

r r r ?

r

rrs / o

/

r

r □ / □

□□□ □□□

/

Figure 11.1 - Typical Layout of a Main Switchboard Similarly for paralleling o f alternators, a synchronising panel (or panels) will be provided. It carries the necessary equipment i.e., two means o f indicating synchronism, and sometimes additionally a “Check Synchronising” device (the purpose o f this device has been explained in article 10.3). 396

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Switchboards and Switchgear The switchgear cubicles on either side o f the generator panels are used for essential services and flanking these are the group motor starter panels. A separate section controls the 3'phase 220V low power and lighting services. Apart from these features, there will be a link to the emergency switchboard, steering gear supplies (duplicated), and other essential services to the engine-room, bridge supplies and section board feeders. This has also been mentioned in article 3.4. Provision will be made for alarms and earth insulation resistance monitors on both the 440V and the 220V sections. Articles 5.8 and 5.9 can be referred to, for further details on earth fault indicators and insulation monitoring. The ship’s electrical diagrams will include a drawing o f the front, and perhaps the rear o f the main switchboard showing the fitted equipment. The electrical distribution diagrams will follow the physical arrangement o f the main switchboard layout. Studying the schematic electrical diagrams o f a particular ship helps to identify, locate and appreciate the role o f each key component in the system. Efficient faultfinding on a distribution network can only be achieved by a thorough understanding o f the system and its normal operation. This aspect will be further discussed in Chapter 26. The 440V/220V lighting transformers may be mounted inside the main switchboard cubicle, or may be freestanding behind i t The main generator’s supply cables are connected directly to their respective circuit breakers. Operating handles and push buttons are mounted on the hinged doors at the front o f the switchboard. Most front panels and rear doors are hinged in order to facilitate inspection and maintenance. A passageway with a width o f approximately 60-cm is provided at the rear for this reason. Take care when opening ihe doors o f switchboards. Live parts are generally exposed - you are in danger! Feeder panels as designated, provide outlets to the power utilisation or distribution points. The distribution points are usually Sub-switchboards or Section Boards and/or Group-Starter Boards. The latter may be located at any convenient power utilisation centre, such as amidships for a tanker; it is usual to place the engine-room equipment starter boards (port and starboard) at either end o f the M ain Switchboard. For a.c. switchgear, general practice usually favours retractable circuit breakers, which plug into suitable fixed isolating contacts, which in turn are rigidly connected to the busbars.

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Chapter 11 11.1.1.1

AC Switchboards

Figures 11.2(a), (b) and (c) depict different views o f dead-front-type main switchboards used in m ost ships today. Refer Chapter 3 for more information on distribution systems.

Figure 11.2(a) - A Main Switchboard in the Machinery Control Room

Figure 11.2(b) - Close-Up View of an Older Main Switchboard 398

Marine Electrical Technology

Switchboards and Switchgear

Figure 11.2(c) - A Modern Main Switchboard As seen from Figures 11.2(a), (b) and (c), these have panels invariably in the form o f cubicles bolted together to give a flush appearance at the front. As mentioned earlier in this chapter, Regulations require that a passageway be provided at the rear o f the switchboard, the passageway is to have a minimum width of 60cm and to be screened off by sheet-steel or wire-mesh doors. Although, as stated above, the cubicles will carry components related to specific purposes such as circuit breakers, synchronising gear, instrumentation transformers, etc., the busbars being common to all circuits will probably run the whole length o f the board; they will be connected to appropriate panels by rigid connectors. Some merchant-ship operators have preferred a single solid busbar system with all generators operating in parallel. This does not ensure maximum security o f supply, since a fault on the main switchboard may cause a loss o f total power. With the solid busbar system, maintenance can be carried out only when the whole ship is shut down. This concept however is going through a sea change and modular types are being preferred. For passenger vessels, where greater safety is required, the main switchboard is often sectionalised and vital services are provided with dual or standby feeders being fed from different busbar sections; this is rare for cargo ships because o f the complexity involved. Splitting o f the busbars by isolating links is possible. These are the practically the last line o f defence in the distribution system as they have high melting points and also permit isolation o f sections o f the switchboard if so desired (Refer Figure 1.10). Figure 11.3 also depicts an isolating link. Marine Electrical Technology

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Chapter 11

B acking (support) P late Locking Spring C o ntact Block

H p ___Insulated B as e Plate

O perating Screw

C o ntact Bridging C o n tact Locking Spring

Figure 11.3 - An Isolating Link in a Busbar Section of a Switchboard 11.1.1.2

DC Switchboards

The older types, usually o f the open-front type were simpler to design and construct. The circuit breakers were surface-mounted, with threaded studs passing through the main insulating panel. This allowed connections to be made with the busbars, which were usually mounted, in a vertical plane. The need for a third bar - the equaliser bar arose when compound generators were required to operate in parallel. Note: The Institution ofElectrical Engineers' Regulations (worldwide) requires thefollowing: a) This bar and related switches are to have a current rating of not less than half the rated full-load current of the generator i.e. a cross-sectional area of at least 50% of the crosssectional area ofthepositive or negative bars. b) Instrument shunts, mounted in the busbars or connections should also be considered when making arrangementsfor sectionalising. c) Knife-switches are also to be providedfor controllingfeeders to section boards or sub­ switchboards and large current-carryingfuses will be installed. d) Preferential tripping arrangements with surface mounted relays and time-delay elements may be additionalfacilities on the main switchboard and thefunction of these should be studied. The newer versions are similar to the dead-front switchboards used in alternating current systems as they are safer.

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Switchboards and Switchgear 11.L2

Section Switchboards or Sub-switchboards

When these are necessary, the practice is the same for either a.c, or d.c. installations. Several sub-switchboards may be fed from the main switchboard, each being situated as near as possible to the centre o f their respective groups o f loads. Cable length and voltage drop is thus reduced to a minimum. As mentioned earlier it may be convenient to build some o f this equipment into the main switchboard itself. The incoming cable from the main switchboard is usually connected directly to the sub-switchboard busbars - an isolating switch may, however, be deemed necessary. Over-current protection is not provided, as it would duplicate that provided at the main switchboard. The outgoing circuits should be controlled by fuse-switches with H.R.C. / H.B.C. (High Rupturing Capacity or High Breaking Capacity) fuse links, o f the normal full-load current rating. The switches should also be capable o f withstanding stalled motor currents, a condition, which arises with a control-circuit failure or a welded-in contactor. Sub-switchboards are frequently arranged for front access only with the control gear located in individual switchboxes or retractable trays. The covers should be interlocked so that they cannot be opened unless the switch is open. Terminals are frequently shrouded and a further interlock prevents operation with an open cover. This interlock can be broken and great care and precautions must be taken if maintenance or testing is undertaken for this irregular situation. 11.1.3

Group Starter Boards (or Panels)

Except for large machines fed from circuit breakers on the main switchboard, all motor circuits will be protected and controlled by H.R.C. fuses and contactor gear. Earlier vessels had starters mounted adjacent to the motors but the modem tendency is to incorporate the group starter board with its advantage o f easy accessibility for cleaning and maintenance. The above explanation pertains to a.c. equipment; DC ships usually followed the earlier practice o f separately located starters fed from a sub-switchboard and controlled through knife-switches and rewireable fuses. 11.1.4

Distribution Fuse Boards

These are provided at convenient points throughout the ship for heating, lighting and low-power circuits. Lighting boards are usually fed separately to allow maintenance to be carried out on power circuits. The boards can be the single or three-phase type for a.c. systems and two-pole for d.c. systems. Marine Electrical Technology

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Chapter 11 Switches can sometimes be mounted in the boards to provide control o f circuits but it is more common to provide isolation by the removal o f fuses. The incoming cable is usually connected directly to the fuse board busbars as the fuse holders are normally fully shrouded. Hence it is possible to make alterations to outgoing circuits with the busbars ‘alive’. For an a.c. ship the fuses can be o f the H.R.C. (High Rupturing Capacity) or re-wireable types. The appropriate type o f fuse-link must be fitted in each case. Under no circumstances should an H.R.C. fuse be rewired or replaced by a wire-fuse element. The more familiar re-wireable or semi-enclosed fuse is now very largely unsuitable for the following reasons: (a)

Low rupturing capacity - dangerous to operators and the system,

(b)

Subject to deterioration,

(c)

Accurate calibration is not possible and a system o f grading is not feasible,

(d)

The slow speed o f operation m ay result in other protective gear being brought into operation.

[11.2

Busbars

Short copper bars from each generator’s respective circuit breakers connect it to the busbars, which run through the length o f the switchboard. The busbars may be seen i f the rear doors o f the switchboard cubicle are opened. They may be in a specially enclosed busbar duct. Busbars are flat, hard-drawn, high-conductivity, electrolytic-copper bars that are rated to withstand the thermal and electromagnetic forces, which is inevitable in the event o f a short circuit at the busbars, with all the generators in parallel. The busbars will withstand these conditions at least for the length o f time it takes for the alternator circuit breakers to trip or back-up fuse to rupture or “blow”. Generally it is the isolating links that will fail before the busbars breakdown.

Figure 11.4 - Busbars in a Switchboard 402

Marine Electrical Technology

Switchboards and Switchgear The busbars are mounted on insulators, firmly secured to the inside o f the cubicle. The busbars are rigidly mounted in order to maintain clearances under short-circuit conditions (Refer Figure 11.4). All copper joints are bolted and the joining surfaces are tinned. Smaller versions of these busbars are located in power and lighting distribution panels and the motor control centre (MCC) controllers. Certain instruments and controls require to be fed directly from the busbars. Any connection between the busbars and protecting fuses must be capable of withstanding maximum fault currents. The standard practice is to provide a three-phase set o f fuses, known as “back-up” fuses, as near to the busbars as possible. Connections are then led to the racks o f the many instruments fuses fitted. 11.2.1 Skin Effect This phenomenon is being explained to help one to understand the reason for utilising flat, rectangular-cross-sectioned busbars in switch boards and for similar applications. The alternating current carried by a conductor sets up a magnetic flux and the corresponding lines o f force cut through the conductor. The linkages are not uniform across the conductor’s crosssection but are proportionately greater at the centre. As the frequency increases, so does the effect until at very high frequencies the entire current flows in a very narrow skin on the conductor; hence the name. Consequently a higher back EMF is set up at the centre and the resulting effect on the current distribution across the conductor is that the current tends to concentrate where the back EMF is minimum, i.e., at the ‘skin’ o f a round rod or at the com er o f a flat strip. This is generally known as ‘skin effect’. There is an apparent increase in resistance compared with the value when carrying d.c. because the whole o f the copper is not carrying an equal share o f the current; this also increases the losses. The ratio o f a.c. resistance to d c . resistance, ‘the skin effect ratio’, is dependent on the frequency, conductor shape and operating temperature. Rods and tubes o f a size likely to be affected by skin effect are unlikely to be used in modem ships’ installations and attention can be confined to flat copper bars. Skin effect causes concentration o f current at the edges (not the sides) o f the section and the effect will always be less than in a circular rod unless the bar section is square rather than rectangular, in fact for larger cross-sectional areas, the skin effect is greatly dependent on the ratio o f width to thickness. Marine Electrical Technology

403

ill.3

Instrumentation and Controls

Switchboard instruments and controls for particular functions are grouped together. As mentioned earlier in this chapter, the generator synchronising panel has all the instruments, relays and switches necessary for generator paralleling; these are depicted in Figures 10.3(c) and 10.3(d). Figures 11.5(a), 11.5(b) and 11.5(c) depict Generator Control Panels on two different types o f vessels. The detailed working principles o f these instruments have been mentioned in Chapter 9. The instruments on the panels o f outgoing circuits are usually limited to an ammeter, status lamps, function switches (e.g. hand/off7auto) and push buttons. Emergency Stop Indications Start Failure

Generator Control Panel FW High Temp

Over Speed

<

Low Lube Oil Pressure _______ ___ _____ 1

Generator Engine Abnormal Indications Pressure

Temperature ____________ _

||

ACB Non Close

Excitation

Level ACB Abnormal Trip

Main Output

V

A

V

OC I i

PT

Gen Cap - 700kVA, 560 kW Cos f 0.8 O/p 900A max @ 450V Excitation - 75A (max 118A), 65V (max 97V)

O O - © O :0 A

RP

Kw Energy Meter Cos Space Heater

Ammeter Voltmeter Select Select

Pre-excitation

Cable type H7 BYC-125 x 6 1000/899A ACB AC 660V, 3 poles

Figure 11.5(a) - A Generator Control Panel on an Older Vessel The Synchronising Panel is depicted in Figure 10.3(c)

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Marine Electrical Technology

Voltage

Switchboards and Switchgear

BUSBAR VOLTAGE

BUSBAR FREQUENCY

[ 4 4 0 , 2 Vj

6 0 . 0 HZ

STANDBY

8 3 0 KW GENERATOR STATUS

llU tll SSS88 M l ilili

STOPPED

B ill

READY

RUNNING STARTING STOPPING

GENERATOR FAILURE

llS ii 111! f ill im i llllll

STARTING FAILURE SYNCHRONISING FAILURE SHUTDOWN ACB ABNOMAL TRIP ACB NOT CLOSED

IB il BLOCK

CONTROL MODE

OUTPUTS

fe fftli START i l i l i STOP M M SYNCHRO

HU!

GOVERNOR RAISE

ACB OPEN

f t AUTO H AUTO STOP

(M a

AUTO

SYNCHRONISING

MAN

LOAD DEPENDENT SHARING

MAN f f M AUTO

LOAD DEPENDENT STOP

MAN (1

LOAD DEPENDENT START

MAN

118111] GOVERNOR LOWER

START

H VOLT / KW

®

RESET

M

AUTO

mm

AUTO

Figure 11.5 (b) - A Geuerator Status Panel (for One Generator) on a Tanker Low power control and instrument wiring is o f a relatively smaller cross-section, with multi-coloured plastic insulation, which is clearly identified against the larger main power cables. The instrumentation and control wiring is supplied through fuses which are located within the appropriate panel. Green or green-and-yellow-striped, earth wiring from instruments and panel doors, etc, is connected to a common copper earth bar running the length o f the switchboard at its rear. This earth bar is itself fixed to the ship’s hull. W hen the ship is docked, the hull is in tarn connected to an earthing point on the jetty or in the dock itself.

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Chapter 11 Lam ps:

CBO - Circuit Breaker Open PAL - Power Available CBC - Circuit Br. Closed RCL - Remote Control Available HL - Heater On OL - Overload RL - Reverse Power Trip S w itches: AS - Ammeter Switch - L1/L2/L3 VS - Voltm eter Switch - L1/ L2 / L3 HS - Heater Switch (On / Off) ESS - Engine Stop ES - Engine Start

Figure 11.5 (c) - A Generator Control Panel on a Tanker The Synchronising P anel is depicted in F ig u re 10.3(d)

j 11.4

Circuit Breaker (CB)

A Circuit breaker is not a normal switch. It derives its name from the fact that besides completing a circuit, is has protection circuits that perform at least 3 basic safety functions: 1. Sense the occurrence o f an overcurrent. 2.

Measure the amount o f overcurrent.

3. Act by tripping the circuit breaker in a time frame that is necessary to prevent damage to itself and its associated circuits. When selecting a circuit breaker for a particular application the principal factors to consider are: system voltage, rated load current, and fault level at the point o f installation. Note: Terasaki developed the world'sfirst current limiting circuit breaker ini965. Previous to this, fuses were alvmys installed in series with circuit breakers to provide the current limitation. Hence the first name of Terasaki in the UK "The No-Fuse Circuit Breaker Company" Source.'www.terasaki.co.uk

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Marine Electrical Technology

Switchboards and Switchgear 11.4.1

Voltage R ating

At medium voltages the phase to neutral voltage may be 250 volts but the potential difference between any two phases with the neutral insulated would be 440 volts. A t these voltages, no difficulties should aiisc in selecting the circuit breaker equipment. However, on a 3.3kV insulated neutral system the phase to neutral voltage is 3.3kV/V3 = 1.9kV, If an earth fault occurs on one phase, the potential o f the other two phases to earth is 3.3kV. To ensure that the insulation is not subject to excessive stress a circuit breaker designed for a normal system voltage o f 6 .6 kV may be fitted. In insulated neutral systems, high over voltages may be caused by arcing faults. Medium voltage systems’ switch gear insulation should be able to withstand such voltages, but with 3.3kV and above, the margin o f safety is reduced. When a high voltage system is installed both the voltage rating o f the circuit breaker and the method o f earthing must be considered. 11.4.2

C urrent R ating

Three factors are to be considered: 1. Maximum permissible operating temperature o f circuit breaker’s copper-work and contacts 2. Temperature rise o f copper work due to the load current 3. Ambient temperature In industrial use the ambient temperature considered is usually 35°C. In a marine environment, temperatures o f 40°C (restricted areas) and 45°C (unrestricted areas) are considered, therefore the circuit breaker rating m ay be a “free-air” value and this does not consider the degree o f ventilation, the number and position o f the circuit breakers or the layout o f the busbars. The final switchboard arrangement could be only 80 to 90% o f the free air rating. 11.4.3 F a u lt R ating

The four fault ratings o f a circuit breaker are: i.

The symmetrical breaking current: It is the root mean square value of the a.c. component of the breaking current.

ii.

The asymmetrical breaking current: It is the root mean square value of the total breaking current which includes both a.c. and the d.c. components.

Marine Electrical Technology

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Chapter 11 iii.

The making current: It is the peak value of maximum current loop, including the d.c. component in any __ ______ — — pole during thefirst cycle ofcurrent when the circuit is closed.

rv;----- T h e o h o rt- tm iO id. tin g .

It is the root mean square value of the current that a circuit breaker is capable of carryingfor the stated time. N ote: It may also be necessary to consider thefrequency rating of the breaker. 11.4.4

In te rru p tin g R ating

Circuit breakers are also rated according to the maximum level o f current they can interrupt. This is the interrupting rating or ampere interrupting rating (AIR). W hen designing an electrical power distribution system, a main circuit breaker must be selected so that it can interrupt the largest potential fault current that could possibly occur in the selected application. The interrupting ratings for branch circuit breakers must also be taken into consideration. But these interrupting ratings will depend upon whether series ratings can be applied. The interrupting ratings for a circuit breaker are typically specified in symmetrical RMS (effective) amperes for specific rated voltages. The term symmetrical indicates that the alternating current value specified is centered around zero and has equal positive and negative h alf cycles. W ARNING! I f circuit breakers are operated beyond their ratings or without proper maintenance, then the following are imminent: •

Catastrophic failure o f the power system, circuit breaker, or switchgear

•

Serious injury or even death o f personnel working in the area

11.4.5 Im portant Aspects o f a C irc u it B reaker

A circuit breaker should: S

Be rated to accept a breaking current o f about 10 times the full-load current.

S

Be able to make against a fault condition where the making current m ay be 25 times the full load current when the contact first makes.

^

Remain closed for a snort tim e when a fault occurs in order to allow other devices which are nearer to the fault to trip first.

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Marine Electrical Technology

Switchboards and Switchgear S

Be capable o f carrying its breaking current for a specified time o f usually about one second. I f the circuit breaker were rated at less than the expected fault level, the breaker would be liable to explode and cause a fire.

S

Safely make onto and interrupt the prospective short-circuit fault current expected at that point in the circuit. The main contacts m ust open rapidly while the resulting arc is transferred to special arcing contacts above the main contacts. Arc chutes with arc ‘splitters’ quickly stretch and cool the arc until it snaps. The circuit breaker is considered ‘open’ when the arc is quenched.

11.4.6

Contacts

Attention should be paid to all contacts likely to deteriorate due to wear, burning, inadequate pressure, and the formation o f a high resistance film or becoming welded together. Faulty contacts are often indicated by overheating when loaded. Different contact materials may need different treatment. Copper is widely used but is liable to develop a high resistance film due to oxidation, and copper contacts may become welded together i f the contact pressure is low and the contacts have to c an y a high current. Copper is commonly used for contacts which have a wiping action when closing and opening, this action removing the film. Copper contacts are used on knife switches, laminated (brush) contacts o f regulators and other controllers, drum contacts, etc. Carbon and metalised carbon contacts are unsuitable for carrying high currents for long periods but, as they do not weld together, they are used for arcing contacts on some control gear. Pure silver and silver alloy contacts tend to blacken in service but the oxide film has a low resistance. ; Copper-tungsten (sintered compound), which is grey in colour, is used in contact facing. This material has a high surface resistance which resists heavy arcing and does not weld. Silver-tungsten (sintered) has properties similar to copper-tungsten but has a lower contact resistance and is less liable to overheating on continuous load. 11.4.7

C losing M echanism

Rapid closing o f the breaker helps to prevent damage and m ost are power, rather than manually, closed. Various types o f closing mechanisms may be fitted: (a) Independent manual spring -

The spring charge is directly applied by manual

depression o f the closing handle. The last few centimeters o f handle movement releases the spring to close the ‘breaker’. Closing speed is fast and independent o f the operator. Marine Electrical Technology

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Chapter 11 (b) Motor-wound stored-charge spring - The closing spring is charged by a motor / gearbox unit. Spring recharging is automatic, following closure o f the ‘breaker*. Breaker closure is operated by a push button. This may be a direct mechanical release o f the charged spring or it may initiate an electrical release via a solenoid-operated latch. After the operation o f a spring-activated breaker, the motor can usually be heard charging the spring for the next time. (c) Hand-wound stored-charge spring - This is similar to (b) above but has manually-charged closing springs. The emergency hand-tensioning method is arranged for use with a dead switchboard, so that the spring can be wound up and made ready for closing the breaker. (d) Solenoid - The breaker is closed by a d.c. solenoid energised from the generator or bus­ bars via a transformer / rectifier unit, contactor, push button and, sometimes, a timing relay. CAUTION! $

C irc u it breakers store m echanical energy in : (a) Store-charge mechanisms in the closing springs. (b) Contact springs. (c) K ick-offsprings.

Extreme care m ust be exercised when handling circuit breakers with the closing spring charged, or when the circuit breaker is in the “On” position. isolated circuit-breakers when racked out for maintenance should be left with the closing spring discharged and in the “Oft” position. Circuit breakers are held in the “Closed” or “On” position by a mechanical latch. The breaker is tripped by releasing this latch allowing the kick-offspring and contact pressure to force the contacts open. Tripping can be initiated in the following manner: (a)

Manually with the help o f a push button with a mechanical linkage that trips the latch.

(b)

By an under voltage trip coil (trips when de-energised).

(c)

By an over current / short circuit trip device (trips when energised)

(d)

By a solenoid trip coil - when energised by a remote switch or relay (such as an electronic over current relay)

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Marine Electrical Technology

Switchboards and Switchgear !11.5 11.5.1

Circuit Breakers for Alternators A ir C irc u it Breakers (ACBs)

For several reasons, a majority o f circuit breakers used onboard ships are air circuit breakers rather than oil-immersed types which are common ashore. The alternator breaker for three-phase supply has a single unit for each phase. The three units are linked together by an insulated bar for simultaneous operation. This avoids imbalance in toads, single-phasing, etc. The air-break circuit breakers used for m arine installations are frame-mounted and arranged for isolation from the busbars and alternator input cable contacts by being moved horizontally forward to a fixed position. The isolating plugs are not designed for making or breaking contact on load and so the breaker must be open before the assembly is withdrawn. Electrical interlock switches are connected into circuit breaker control circuits to prevent incorrect sequence o f operation, e.g., when a shore supply breaker is closed on to a switchboard. The ship’s generator breakers are usually interlocked (“O ff’) to prevent parallel running o f a ship’s generator and the shore supply. Figure 11.6 depicts a typical air circuit breaker’s basic construction while Figure 11.7 is a sectional view o f the ACB. It comprises o f fixed contacts, arranged in such a manner that the arcing contacts make before and break after the main contacts.

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Chapter 11

Figure 11.6 - A Basic Air-break Circuit Breaker (ACB) for Alternators The main contacts are designed to carry the full-load current without overheating and overload current, until tripped i.e. when a fault occurs. The main contacts are usually silver­ faced copper, copper with silver inserts or silver-cadmium oxide. Interruption o f current flow results in the production o f an arc between contact faces. Arcing is severe with overload current but is not a serious problem during normal operation. In order to prolong the life o f the costly main contacts, arcing contacts are incorporated.

4 12

Marina Electrical Technology

Switchboards and Switchgear The arcing contacts shown in Figures 11.6 and 11.7 have a spring that pushes them forward so as to ensure that they make before the main contacts and hold them in the closed position until after the main contacts have opened. The arcing contacts thus designed to alleviate the effects o f arcing at the main contacts, are usually silver-tungsten or silver-cadmium oxide. These combine to provide minimum contact resistance and carry current with reduced erosion. Now arc control requires that the arc be elongated and removed from the gap between the arcing contacts. Electrodynamic forces associated with the arc produce a funnel effect, which assisted by thermal action, causes it to move up the arc runners to the arc chute. The arc is elongated and finally chopped into sections while also being cooled with the help o f the splitter plates (made o f steel or copper in some breakers, and in others, made o f insulating material) in the chute. These plates have a large surface area. Arc chutes are made o f insulated, arc-resisting material. They serve to confine the arc. Some breakers have horizontal rods fitted to cool and split the arc. Arc runners are generally fixed and o f the moving type in some designs. Never allow a circuit breaker to operate with the arc chutes removed. Interruption o f the arc is assisted by the current dropping to zero during the cycle. However, with three phases the zero points in each phase are staggered (120° apart). Contact opening is therefore followed by the current falling to zero and this means that for the next part o f the cycle an arc has to be struck across a gap. Contact design and other innovations have rendered blow out coils obsolete but they may still be found. Blow-out coils if used, are connected in series with the circuit breaker contacts. They form an electro-magnetic field which reacts with the arc to give a deflecting force that tends to blow the arc upwards and thus outwards. The increase in effective length o f the arc causes it to extinguish more quickly. The blow out coil is protected from the arc by arc resistant material which m ay be in the form o f an air chute.

Marine Electrical Technology

4 13

Chapter 11 Arc Runner Copper Flexible Arc Barrier

Fixed Arcing Contact

Arc Splitter

Elkonite Tips Fixed Arcing Contact Spring

Fed Sparker onnection Leather Buffer Closing Cam Return Spring

Contact Clip Spring & Screws Upper Contact Main Moving Contact" Contact Carrier Casting -

Emergency Manual piosing Handle

r

Lower Contact Main Isolating Contact

Lrp=

Copper Flexible

Mechanism Return Spring

Prop Catch Spring

Fixed Core

Moving Core

Trip Rod \Core Return Return Spring Trip Catch Return Spring Trip Spindle

Figure 11.7 - Sectional View of an Air Circuit Breaker Successful removal o f ionised gas (from the arc, a result o f the contact opening) will increase the resistance in the air gap between the contacts. Hot ionised gases around the arc and contacts are displaced by cold air thereby forming an eddy current air flow. This helps to increase the resistance between contacts.

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Switchboards and Switchgear When gas remains, it provides a path across which the arc can re -strike. The rate at which the gas is removed is such that the arc will not re-strike more than two or three times. Breaking speed is thus made as high as possible by powerful throw -off springs and light construction o f the moving arm assembly. Anti-bounce devices prevent any rebound at the end o f the opening movement. Circuit breakers are also capable o f breaking under die influence o f very large shortcircuit currents. This is achieved by using (electromagnetic) repulsion methods thereby providing a fast break with long travel to hinder arc formation.

Image Courtesy: www.terasaki.com

Figure 11.8 - Pictorial Diagram of an Air Circuit Breaker

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4 15

Chapter 11 Maintenance is facilitated by the draw-out compartments and extending guide rails which allow the breaker to be pulled out completely. Safety interlocks should be fitted to ensure that the circuit breaker assembly cannot be melted out or in when the contacts are closed. The interlocks are usually mechanical in nature to prevent racking-out if the breaker is still in the “On” position. Care must be taken not to exert undue force if the breaker does not move out - otherwise damage may be caused to the interlocks and other mechanical parts. Dangers o f explosion and fire may also result from such actions. I f severe burning or pitting occurs on the main contacts, they need to be filed. The manufacturer's handbook will give instructions to rectify this. It is often caused by misalignment o f the contacts. The arcing contacts are normally subject to burning and can be dressed with a smooth file too. Emery cloth m ust not be used as it has abrasive particles that may get dislodged and embed themselves in the smooth silvered surfaces o f the contacts and cause further problems. 11.5.2

B reakers U sed in a H igh-V oltage S ystem

The breakers used in a high-voltage system are generally oil, SFg gas or vacuum types. 11.5.2.1

Vacuum C ircu it B reakers (I'C B s)

Vacuum types are generally used in HV applications on board ships. This type o f breaker is used on LNG vessels too.The high dielectric strength o f the non-flammable and non-toxic vacuum allows a very short contact separation. The dielectric strength across the contacts builds up at a rate that is thousands o f times higher than that obtained with conventional circuit breakers; rapid re-strike-free interruption o f the arc is thus achieved. It must be understood that when contact separation occurs in air, the ionised molecules carry the electric charge. These charged molecules are thus responsible for the low breakdown value. Thus the absence o f air molecules in a VCB increases the dielectric strength. Its very short contact travel ensures its compact size and quiet operation thus requiring minimum maintenance too. All these properties make the VCB m ore efficient, less bulky and o f course cheaper. It can be used either as a main breaker where the load is connected to the turbine-driven high-voltage alternator by means o f a “VCB Close” signal or as a Feeder Circuit Breaker where the feeder circuits fed from the 6600V feeder panel o f the switchboard are protected by a vacuum circuit breaker. The control power supply is 110V DC from the outside o f the 6.6 kV MSB. They are well-suited for use up to 35-kV and 100MVA applications.

4 16

Marine tiectrical Technology

Switchboards and Switchgear The VCB is electrically operated, and drawn-out for maintenance purposes, which is minimal. The service life o f these breakers is much longer than conventional breakers. Low vacuum pressures o f 10'7 torr can be achieved (1 torr = 1mm o f mercury column). 11.5.2.2

S ulphur H exafluoride ( S F C ir c u it B reakers

This is the most recently developed type o f breaker in the field o f high-voltage switchgear. These circuit breakers are filled with compressed sulphur-hexafluoride gas which acts to open and close the switch contacts. The gas also interrupts the current flow when the contacts are open. SF6 is a gas that is 5 times heavier than air, chemically very stable, odourless, inert, inflammable and nontoxic. The gas chambers are hermetically sealed and do not allow the gas to escape.

Figure 11.9 - Dielectric Strength versus Pressure for Air, Oil and SF6

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4 17

Chapier 11

SF6 gas and its decomposition products are electronegative; free electrons are readily removed from a discharge by negative ions. They capture electrons at relatively high temperatures; this permits the dielectric strength to rise rapidly and enables the breaker to withstand the recovery voltage even under extreme switching conditions. The gas has low contact erosion and negligible decomposition during arcing. Thus the breaker can be operated for several years without maintenance. 111.6

Moulded Case Circuit Breakers (MCCBs)

These are small, compact air circuit breakers fitted in a moulded plastic case. They have a lower current rating than ACBs (usually 50-1500A) and generally a lower breaking capacity. While Figure 11.10 (a) is a pictorial representation o f this type o f breaker,

Figure 11.10 (b)

depicts a sectional view o f the same. MCCBs usually have an adjustable thermal overload setting and an adjustable or fixed magnetic over-current trip for short-circuit protection built into the case. An under voltage trip coil may also be included within the case. A hand-operated lever usually closes MCCBs but a m otor closing arrangement can also be incorporated. MCCBs are claimed to be reliable, trouble free and require negligible maintenance. I f the breaker remains ‘On’ for long periods, it should be tripped and closed a few times in order to free the mechanism and clean the contacts. Terminals should be checked for tightness otherwise damage due to overheating would occur. The front cover o f larger MCCBs (around 400A rating) can usually be removed, interior dust blown out and the contacts dressed with a fiie if required. Following tripping after a short-circuit fault, the breaker should be inspected for damage, checked for correct operation, and its insulation resistance measured. A reading o f at least 4 to 5M H is usually required. Any other faulty operation usually requires replacement or overhaul by the manufacturer. MCCBs can be used for every application on board a ship - from generator breakers to small distribution breakers. The limited breaking capacity may demand that a ‘back-up’ fuse be fitted for very high prospective short-circuit fault levels.

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Switchboards and Switchgear Typical range: 12A - 2500A ; 10kA - 180kA @ 415V

Arc Chutes

Contact Springs

Moving Repulsion Contact Fixed Contact Thermal Overload Trip Magnetic Short Circuit Trip

Image Courtesy: www.terasaki.com

(a) - Pictorial View (b) - Sectional View Figure 11.10 - A Moulded Case Circuit Breaker Some applications are mentioned in the paragraphs that follow: 11.6.1

F eeder P rotection

They are ideally suited for outgoing feeder circuits on distribution boards. The selected MCCB should have adequate interrupting capacity and the MCCB current rating should be equal to that o f the load. 11.6.2

C apacitor C ontrol

They are suitable for controlling LT capacitors for power factor correction. To avoid tripping while switching on the capacitors, the MCCB rating should be around 1 . 5 - 2 times the rated current o f the capacitors so as to cater to initial inrush currents.

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11.6.3

D iesel-G en erator P rotection

They can be used for effective protection and control o f diesei-generating sets (against overloads and short circuits). 11.6.4

H o ist/ E le v a to r/ C rane C ontrol

They can withstand continuous vibrations o f 2g and shocks up to 15g without affecting their performance. As such they offer the best protection for hoist / elevator / crane applications. 11.6.5

F urnace C ontrol

These are suitable for high frequency systems up to 400 Hz. Hence they can be ideally used for control o f high frequency furnaces. A de-rating factor o f 0.8 is to be employed for arriving at the rated current o f the MCCB. However it m ust be noted that fully magnetic MCCBs are not suitable for this application. 1 1.6.6

D C P ow er S u pply C ontrol

Rectifier panels which act as a source o f d.c. supply can be conveniently protected by MCCBs. Moreover, MCCBs being suitable for both - a.c. as well as d.c. systems, control and protection o f input as well as output sides o f the rectifier is also possible. 1 1 .6 .7

M iscellan eou s M arin e A pplication s

They are ideally suitable for a variety o f marine applications. These have cleared the most stringent performance, durability and environmental tests. While the durability tests include a Vibration Test at lOg, Bump Test at 40g and Shock Test at 70g, the environmental tests include the Damp Heat (steady state) Test, Damp Heat (cyclic) Test, Mould Growth Test and the Corrosive Atmosphere (salt spray) Test.

11.7

Miniature Circuit Breakers (MCBs)

MCBs are very small air circuit breakers also fitted in moulded plastic cases. They have current ratings o f 5-100A and generally thermal overload and magnetic short-circuit protection. They have a very limited breaking capacity (300A) and are commonly used in final distribution boards (DBs) instead o f fuses. The distribution board is supplied via a fuse or MCCB with the required breaking capacity. Figure 11.11 below depicts MCBs o f the single and double-pole types respectively, while Figure 11.12 depicts the operation o f a MCB in the event o f a short-circuit fault.

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Switchboards and Switchgear

Typical range: 1A - 63A; 6kA - 10kA @ 415V Image Courtesy: www.terasaki.com

Figure 11.11 - MCBs of the Single and Double-Pole Types 11.7.1

11.7.2

A dvantages o f M CB s

S

They are set to a pre-determined rating at the factory

S

It is easy to check if the breaker has tripped or not

S

The supply to the circuit is easily reinstated

V

Multi-pole units are available

✓

They can discriminate between sustained and transient loads.

D isadvantages o f M C B s

*

They are costly

x

They have mechanical moving parts

x

Tripping heavy overloads causes distortion due to heat

x

Ambient temperature affects their characteristics

x

Regular tests are required to ensure their satisfactory operation

x

MCBs must be replaced if faults develop. N o maintenance is generally possible

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Chapter 11 T Contact Spring

Repulsive Conductor Holder

Repulsive

II

N orm al Load C urrent

W h en S h o rt C ircu it Occurs

A fter Interruption

The best contact pressure is exerted by the contact springs

A n electrodynam ic force occurs between the repulsive conductors and the fixed conductors. The contacts are rapidly opened by th is force. The arc is nearly extinguished a t this stage

The breaker continues the hipping action with m agnetic trip elem ents w hile the repulsive conductors move. W ith th is the repulsive conductors' holder moves from po in t A to B

Final Position

When the repulsive conductors’ holder m oves from p o in t A to B the ro lle r collides w ith the cover. The repulsive conductors stop a t this position

Figure 11.12 - A Miniature Circuit Breaker (MCB) Operating During a Short-Circuit 11.8

Residual Current Circuit Breakers (RCCBs)

Residual current operated circuit breakers are widely used now to provide shock protection from leakage currents. They are also known as Earth Leakage Circuit Breakers. “ELCB” is the common abbreviation for leakage protection devices o f both voltage-operated and current-operated types. These are now mandatory for voltage installations up to 5kW. The common MCBs, fuses, etc., are not designed for protection against leakage currents, which can cause shocks or even trigger a fire. Effective use o f RCCBs involves good wiring and earthing. Generally a 30mA RCCB with an operating time o f 4 milliseconds is recommended for life safety and 100mA or 300mA ones for fire protection. They are also found in the range o f 40 to 50mA. Leakage currents in the range o f a few hundred miliiamperes might lead to insulation failures and result in fires (Refer Figure 11.13).

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Switchboards and Switchgear

Figure 11.13 - Residual Current Circuit Breakers - 4-Pole and 2-Pole In the United States these are called GFCIs (Ground Fault Current Interrupters) and breakers with barely 5mA sensitivity are recommended for personnel protection. In some countries, only breakers with 30, 100 and 300 mA are available with a tripping time o f 30 to 40 ms and are generally accepted in domestic and industrial circles. Few manufacturers offer MCBs and RCCBs rolled into one unit with a delay tripping time o f up to 200 ms. The increased delay enhances discrimination from nuisance tripping. These are now called RCBOs (Residual Current Circuit Breaker with Over-current Protection). The following specifications are furnished from one manufacturer.

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Chapter 11

Trip Coil

Power Supply e.g. 110V

Figure 11.14 - A Ground Fault Current Interrupter and its Associated Circuit 11.8.1

A pplication a n d S co p e o f th e R esid u a l C u rrent C ircu it B reaker

This is in conformity with the standards o f IEC1008, GB16916, VDE 0664 and BS 4293, It can cut o ff the fault circuit immediately in the event o f a shock hazard or earth leakage o f a trunk line. Thus it is suitable to avoid the shock hazard and fire caused by any earth leakage. This is mainly suitable for use in a variety o f plants and enterprises, buildings, constructions, commercial areas, guesthouses and family dwellings. It can be used in single phase 220Vand three phase 380V 50 to 60 Hz circuits. It is not suitable for use in a d.c. pulse system.

11.9

Arc Fault Current Interrupters

Arcing in low voltage circuits can result from old wiring, improper joints, loose connections and faulty appliance leads, poor installation or the breakdown o f insulating materials. These result in a high temperature build-up resulting in fires. Recently, an innovative electrical safety device called the AFCI (Arc Fault Current Interrupter) designed to prevent electrical fires caused by arcing in low voltage circuits has come into the markets o f developed countries. These are designed to detect arcing patterns of serial and parallel arcs or arcs to earth and to trip the circuit.

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Switchboards and Switchgear It is envisaged that this safety device with its ‘arc detection circuitry’ would considerably control electrical fires in future. After the invention o f RCCBs. AFCIsare considered the first major advance in electrical protection.

Figure 11.15 - An Arc Fault Current Interrupter n11.10

Fused Isolators

Handles for opening the doors on switchboard cubicles are usually linked (or interlocked) to an isolating switch. This ensures that supplies to components in the cubicle are switched o ff before the door can be opened. Fused isolators are isolating switches that incorporate fuses. The action o f opening the switch isolates the fuse so that they can be replaced safely. Fused isolators can also be interlocked to the cubicle door handle. Motor starters frequently incorporate this arrangem ent One type o f interlocked fused isolator can be completely withdrawn and removed to ensure complete safety when carrying out maintenance on equipment.

111.11

Effect of Harmonics at Receptacle Load Centres

Harmonics at receptacle load centres cause circuit breakers to trip prematurely. Thermal magnetic breakers may trip prematurely due to excessive heating in the panel. Erratic tripping due to non-linear currents having a peak value greater than the r.m.s. rating o f the circuit breaker may cause it to trip. The possible solutions are: •S Balance the loads so that harmonic currents are reduced S

Re-distribute the loads

S

Add zero sequence filters to reduce neutral current

v" Oversize the neutral i f it is overloaded S

Replace the panel with one which can be used for non-linear loads. Marine Electrical Technology

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Chapter 11 111.12 Corrective and Preventive Actions in Case of Fire in the Main Switchboard Switchboard rooms containing medium-voltage electrical switchgear do not appear to contain large amounts o f combustible material capable o f sustaining a fire long enough to threaten a neighbouring compartment. However, such spaces do contain cables, switchgear, and associated equipment, which m ay routinely be conducting electrical power o f a few hundred kilowatts or even many megawatts. As explained earlier in this chapter, in die event o f a catastrophic failure o f a circuit breaker either directly or consequentially as a result o f the failure o f other electrical switchgear, the resulting arc has the potential to release enough thermal energy to establish a fire in uninsulated contiguous compartments. Structural fire protection is the primary method o f containing heat within a compartment. The failure o f the main breaker could result in a high-energy electrical discharge with characteristics similar to a bolt o f lightning. The associated radiant heat can be rapidly transmitted through the bare steel deckhead, igniting control electrical cables even in the adjacent compartments. The number o f vessels, equipped with diesel-electric propulsion, continues to grow as owners embrace the benefits o f improved operating efficiencies and lower operating costs. Internationally, many medium-voltage circuit breaker failures have occurred in the past, resulting in switchboard fires. Once started, sliipboard fires can spread rapidly and exponentially. Restricting their spread, containing them to their place o f origin, and extinguishing them quickly with the least possible risk to life are critically important considerations in designing safe vessels. The capacity o f structural fire protection and fixed fire-extinguishing systems to adequately restrict and extinguish a fire is vital. Fixed systems (such as CO 2) are the most effective way to extinguish fires contained within an enclosed space by structural fire protection. This is particularly so in enclosed spaces containing electrical equipment, where other fire-extinguishing efforts in the presence o f live, damaged conductors may endanger crew members. Apart from attempting to contain and finally extinguish such a fire, it is advisable to immediately switch to emergency power and trip any generators) connected to the affected switchboard(s); loads connected to the affected panel(s) must also be isolated in order to avoid any further damage such as short-circuits.

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Marine Electrical Technology

Switchboards and Switchgear In case it is impossible to approach the main switchboard, the running generators must be manually tripped from their respective local control panels. This action will cause the switchboard to be dead, subsequently starting the emergency generator. The bus couplers) between the main and emergency switchboards m ust also be dropped in order to protect die circuits and loads connected to the emergency switchboard Emergency lighting must also be available to ensure safe passage o f personnel during such an emergency. All actions are to be coordinated and m ost o f all practiced during fire and emergency drills.

111.13

Relevant Rules

11.13.1

[

Relevant SOLAS Regulations (C hapter I I —1)

Part D - Electrical Installations - Regulation 41 - M ain source o f electrical power and lighting systems 11.13.2

Sum m ary o f Regulations

1) The main switchboard shall be so placed relative to one main generating station that, as far as is practicable, the integrity o f the normal electrical supply may be affected only by a fire or other casualty in one space. An environmental enclosure for the main switchboard, such as may be provided by a machinery control room situated within the main boundaries o f the space, is not to be considered as separating the switchboards from the generators. 2)

W here the total installed electrical power o f the main generating sets is in excess o f

3MW, the main busbars shall be subdivided into at least two parts which shall normally be connected by removable links or other approved means; so far as is practicable, the connection o f generating sets and any other duplicated equipment shall be equally divided between the parts.

1)

A MCB trip is based on th e ________ principle.

2)

Apart from the main contacts in an ACB, there a re _____ contacts too.

3)

Busbars are generally made o f __________________ .

4)

MCBs generally operate with a range up to______________.

5)

RCCBs are also known a s ______ .

6)

The abbreviation RCBO stands for________. Marine Electrical Technology

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Chapter 11 7)

The main panels / sections o f a main switch board are________ .

8)

_______ mA RCCBs are recommended for life safety w h ile __________ are for fire protection.

9)

MCCBs generally operate with a I range fro m _________up to ___________ .

10)

Instead o f fuses,________ breakers are commonly used in final distribution boards.

11)

______ breaker has the least breaking capacity.

12)

Protection against sustained overloads occurring in a MCCB is ensured b y _________ .

13)

The arc resulting from the tripping o f a circuit breaker is prevented from damaging the contacts because o f _________ .

14)

A moulded-case circuit breaker protects equipment against short circuits by using a / an

15)

The operation o f the high voltage bow thruster motor is controlled b y _________ .

16)

W hat is the duty o f a switchboard?

17)

List die main differences between an a.c. and a d.c. switchboard.

18)

Describe the controls provided on MSB (Main Switch Board) for synchronization.

19)

Explain the functions o f the main components o f an air circuit breaker.

20)

With a simple sketch explain the functions o f instrumentation on a Synchronising Panel.

21)

What are repulsive conductors? Which device uses this? Explain the same with the help o f a sketch.

22)

Protection against sustained overloads occurring in moulded-case circuit breakers is provided by a / an_________ .

23)

428

a)

over voltage release

b)

thermal acting trip

c)

thermal overload relay

d)

current overload relay

Where a thermal-acting breaker is required to be used in an area o f unusually high, low, or constantly fluctuating temperatures, an ambient compensating element must be used. This element consists o f a _______________ . a)

cylindrical spring on the contact arm

b) second bimetal element

c)

conical spring on the contact arm

d) second electromagnet

Marine Electrical Technology

Switchboards and Switchgear 24)

The copper flexible plays an important role in a breaker. Explain the same in not more than three lines.

25)

Explain the role o f arcing contacts and the arc chute in an ACB.

26)

Why is a vacuum circuit breaker preferred for H V applications?

27)

What are the properties o f vacuum and SF 6 gas breakers widely used in high voltage systems.

28)

W hat are the principal factors to be considered when selecting a circuit breaker?

29)

What factors determine the current rating o f a circuit breaker?

30)

By what percentage would the rating o f a circuit breaker change from its free air value due to switchboard mounting?

31)

What is the primary function o f a circuit breaker?

32)

What are the four fault ratings o f a circuit breaker?

33)

What would happen if the circuit breaker was rated at less than the expected fault level?

34)

What is the full form o f ACB?

35)

W hat are the salient features o f an ACB?

36)

With a simple sketch explain the layout o f a Main Switchboard.

37)

W ith a simple sketch explain the operation o f an Air Circuit Breaker.

38)

State why, a breaker may not open upon severe and prolonged voltage dip.

39)

What current rating (range) is used with MCCBs?

40)

W hat is the main difference between a relay and a contactor?

41)

With a simple sketch explain the role o f a busbar.

42)

What maintenance can be carried out on a MCB?

43)

W hat is skin effect?

44)

In few lines explain the effect o f harmonics at receptacle load centres.

45)

With a simple sketch explain the functions o f instrumentation on a Generator Panel.

46)

At what value o f earth currents are earth leakage breakers set?

47)

How will a moulded-case circuit breaker react after it has tripped, as a result o f an overloaded circuit?

48)

Describe the actions to be taken in the instance o f fire in the main generator panel.

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Chapter 12-*^ Starters for Alternating Current Motors At the end of this chapter you should be able to: |★

E x p la in t h e b a s ic f e a tu r e s o f 3 -p h a s e a .c , s ta rte rs f o r m o to rs

!★

T r a c e c o m m o n 3 -p h a s e s ta r te r c irc u its im p le m e n te d o n b o a rd

I★

D is tin g u is h b e tw e e n a n e le c tr o n ic o r s o ft s ta r te r a n d c o n v e n tio n a l s ta rte r

!★

E x p la in t h e b a s ic s o f s p e e d c o n tro l o f 3 -p h a s e in d u c tio n m o to rs C o m p ly w ith re g u la tio n s g o v e rn in g th e o p e ra tio n o f s ta rte rs

A 3-phase induction motor is one o f the most common types o f motors used onboard commercial ships today. When the rotor is stationary and the slip is at its highest, the frequency o f the rotor e.m.f. is the same as that o f the (stator) power supply frequency. The value o f e.m.f. that is induced in a stationary rotor is the highest, as the relative speed between the rotor and the flux revolving in the stator is at a maximum value. It is similar to a threephase transformer with a short-circuited secondary.

Power Connections Image Courtesy: Siemens Industry Inc. - www3.sea.siemens.com

Figure 12.1 - Cutaway View of an Induction Motor Marine Electrical Technology

The switch may be a manually operated load- break switch, but more commonly it would be an electromagnetic contactor which can be opened by the thermal overload relay. Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact is used as a retaining contact, i.e., the contactor is electrically latched closed while the motor is operating. When the contactor is closed, the line voltage to the motor windings is applied. So when its windings are switched directly on to a three-phase supply, a surge current is drawn by the motor, which dies away as the motor accelerates up to its rated speed. This starting surge current can be between five to eight times the motor’s rated current and then the current will be limited to the Locked Rotor Current o f the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards “full speed” i.e. 100% speed. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 80 to 85% o f the synchronous speed. The actual starting current curve is a function o f the motor design, and the terminal voltage, and is totally independent o f the motor load. The motor load, under normal operating conditions (and not a jam med rotor!) will affect the time taken for the motor to accelerate to full speed and therefore the duration o f the high starting current, but not the magnitude o f the starting current. Figure 12.2 shows the starting currents’ graphs for standard motors. Now when the rotor starts, the relative speed (in comparison with the rotating stator flux) decreases and may tend to be equal to zero if the rotor speed equals the stator flux’s speed. Hence for a slip ‘s ’, the rotor induced e.m.f. Er will be ‘s ’ times the induced e.m.f. Es at standstill. The frequency o f the induced e.m.f. will also be f x = sfs and due to this the rotor reactance will also fall; Xr = sXs. The subscripts ‘s ’ and *r* refer to standstill and running conditions. Starting a motor by simply connecting it to the supply is called direct-on-line or across-the-line starting and is the simplest, most economical method o f starting. M ost marine motors are direct-on-line (DOL) started. Provided the torque developed by the motor exceeds the load torque at all speeds during the starting cycle, the m otor will reach full speed. If the torque delivered by the motor is less than the torque required by the load at any speed during the starting cycle, the motor will stop accelerating and in fact start stalling.

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Marine Electrical Technology

Starters for Alternating Current Motors I f the starting torque with a DOL starter is insufficient for the load, the m otor must be replaced with a motor which can develop a higher starting torque. The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load-speed torque curve. The starting time is dependant on the acceleration torque and the load inertia. DOL starting results in maximum start current and maximum starting torque; this may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. Special reduced voltage starters (star/delta and autotransformer types) are used when excessively large starting currents m ay cause a severe voltage ‘dip’ on the supply which could affect the operation o f other loads and sometimes the source itself. Large motors and smaller motors intended for connection to the emergency generator, use reduced voltage starters.

1 !

----------

3 ;

j j j j

4 !

j ___

i i

I___ i I

j

i

< i

1 Standard vaiue for 2-pole motors 2 - Standard value for 4-pole motors 3 - Standard value for 6-pole motors 4 - Standard vaiue for 8-pole motors

j \

j j

i 3

O to

>. CO 5CO

CO

0.2

0.4

1

2

4

10

20

40

100

200

Rated Power (kW)

Figure 12.2 - Breakaway Starting C urrent of Standard Motors as a Multiple of Rated Operational Current Marine Electrical Technology

433

Figure 12.3 shows how a motor’s current varies during the run-up period for a 3 kW motor and a 90kW motor.

F igure 12.3 - Typical S tarting C u rre n t C urve (as a M ultiple o f R ated O perating C u rren t) as a F acto r of Rotational Speed of S quirrel C age M otors

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Marine Electrical Technology

Starters for Alternating Current Motors Motor Capacity Horsepower

kW

{ a p p r o x im a te ly )

Approximate Full-load Current (A) 380V

415 V

440V

1

1.3

2.2

2.1

2.0

2

2.7

4.7

4.2

4.0

3

4

6.7

6.0

5.7

5

6

10.8

10

9.6

13.7

13.0

10

15.4

10

13

20.1

17.8

16.8

20

27

41

37.5

34.5

40

60

54.1

51

54

81

74

71

98

91

84

7.5

30 j

50

67

75

101

146

134

126

o o

40

134

213

176

166

300

402

570

519

489

450

603

835

778

733

Note: In case the running currents suddenly fall below normal, it may be implied that the motor is running unloaded e.g„ the pump that it is driving has suddenly lost its suction and thus there is no useful work being done by the machine.

Table 12.1 - Approximate Full-load Currents - 3-phase Motors of Average Efficiency It is important that motors accelerate quickly up to the predetermined speed. This prevents excessively long run-up times that cause the temperature to rise in the stator winding insulation and long duration voltage dips on the supply. The run-up time depends on the starting torque developed by the motor. This time is directly proportional to the square o f the supply voltage (T

oc

V2). Obviously, the load on the motor will also have an effect on the run­

up time. It is worth noting here that the magnitude o f the starting current is not increased when the motor is started against load. The value o f the supply voltage and the standstill impedance o f the stator windings only determine the starting current. Figure 12.4 shows that unloaded motors reach their idling speed very quickly, and even when started against load, most run-up times should not cause problems.

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Chapter 12 Some drives, such as centrifuges and large fans, may require special high torque motors, if acceptable run-up times are to be achieved. M odem standard motors are designed for high efficiency and minimal manufacturing costs. They usually have large starting currents and poor thermal capacity. W hen run-up times are expected to be over 10 seconds, a larger standard motor or special high-torque motor is used.12

02

0.4

1

2

4

10

20

40

100 200

Rated Power (kW) 1 - Starting Under Load (without large rotational masses) 2 - Idling (Motor + Clutch, etc.)

Figure 12.4 - Standard Values for Run-up Times of Standard Motors as a Function of Rated Power 12.2

The Contactor

Before we study starters, it is important to study about the contactor which is at the heart o f every motor starter. It is an electrically operated switch, consisting o f a coil wound on a former, a fixed magnetic core and a moving magnetic armature carrying the contacts. 436

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Starters for Alternating Current Motors When the coil is energized, it magnetises the core and pulls the armature against the tension o f a spring. The armature is mechanically linked to contacts which close and complete that part o f the electrical circuit in which they function. More often than not, auxiliary contacts that are m eant for signalling or control also make or break as the case may be. When the power supply to the coil is interrupted, the coil is de-energised, the core is de­ magnetised and the contacts return to their original position with the help o f the energy stored in the return springs. Considerable changes in design have occurred over the last several decades. Previously contactors were bulky and used blow-out coils and long arc-extinction principles. Though over-sized, the life o f its contacts were short as compared to modem types. This shortened life was attributed to the use o f copper contacts, with rolling or wiping action (an established procedure to keep contacts clean), actually causing a loss o f contact material. The old practice o f filing contacts to keep them smooth has not helped much. This has led to the evolution o f switch-gear with extended service life and more compact, rugged and economical construction. This is now possible because arc lengths have been reduced, thereby minimizing arc power and obtaining current-zero extinction. The main contacts are usually made o f silver cadmium oxide which ensures an extended service life. Unlike copper contacts, these modem types need not be filed or scraped. Blackening or pitting are common and do not adversely affect contact performance as the black silver oxide is almost as good as the silver itself and hence should not be removed. They must be replaced when the bulk o f the material has been eroded. M ost large contacts have wear indicators on their sides. High voltage or low frequency can cause over-currents in the coils, resulting in overheating. Low voltage or high frequency can cause chattering and contact damage. Contactors are designed for 50 or 60 Hertz operations provided that the voltage to frequency ratio is constant. This means that a 50Hz contactor can operate at 60Hz if the voltage is 1.2 times the 50Hz voltage. The optimal ratio alters the size o f the magnet, 1.15 being a typical value. Frequently used pairs can be 42V 50Hz / 48V 60Hz, 110V 50Hz, 380V 50Hz / 440V 60 Hz and so on. Article 4.3.1 and 4.3.2 may be referred to for details on the effects o f change in supply voltage and frequency on the torque and speed respectively.

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Chapter 12 | 12.3 12.3.1

T he Direct-on-line o r D.O.L. S ta rte r Local Control

a) Starting is achieved with the help o f a manual push button b) Stopping is either with the help o f a manual push button or automatically when the level in a tank, pressure in a system or a hoisting / lowering limit is achieved c) A thermal overcurrent relay operates when a surge in current (above a safe value) occurs. A thermal relay utilizes the bending action o f a bimetallic strip to trip the circuit breaker. In normal running conditions the contacts are closed and open only during abnormal conditions i.e. an overload condition. The tim e taken to heat the bimetal gives the necessary time lag. d) Resetting is manually done after the operation o f the thermal over-current relay. 12.3.2

Protection

Protection against prolonged overloads, phase unbalance and phase failure is by a threepole thermal overcurrent relay. Opening o f the enclosure door is only possible when the isolator is in the o ff position. Short-circuit protection must be provided by fuses or a circuit breaker on the supply side o f the starter (Refer Figure 12.5).

Ove

Figure 12.5 - Pow er and C ontrol C ircuits o f a Direct-on-line S ta rte r

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O peration

Push the start button O S

Contactor coil KMi is energised;

S

The main contacts o f KMj close and the motor starts;

S

The auxiliary contact o f KMi closes so that the start button O can now be released (this contact is also called the retaining contact);

S

The motor is now expected to run norm ally...

Now, should the supply voltage fail, KMj de-energises and opens its contacts; the motor stops. When the power supply is restored, the motor can only restart when the start button is pressed. To stop the motor, push button © . W hen an overload, phase unbalance, or phase failure occurs, the over-current relay will open its contacts that are in series with the main contact o f KMi, which results in stopping o f the motor. 12.3.4

R em ote C ontrol

The addition o f a remote control unit (within dotted lines) provided with (parallel) start and (series) stop buttons, facilitates duplication o f control (Refer Figure 12.6).

Figure 12.6 - Remote Control Circuit for a Direct-on-line Starter

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Chapter 12 12.3.5

P um p C on trol

The direct-on-line starter can also be made to automatically operate a pump such as a fuel oil transfer pump for example, or any other similar pump (that automatically operates with the help o f a float switch), when in the automatic mode (Refer Figure 12.7). The circuit diagram is that o f a simple D.O.L. starter, but has been depicted as may be found onboard a ship, along with the equipment supplied by a manufacturer. The operation o f the circuit is explained below: The 3-phases o f the power supply R, S and T are fed to the MCB, the output o f which is connected in parallel to the following: 1.

Transformer Tr. (via a 5A fuse);

2.

Terminals U, V and W o f the motor via the main contacts MC (1, 2 and 3) and the over-current relay contacts OCR (1), OCR (2) and OCR (3).

3. Main Contactor MC (via the normally open contact o f relay X2) A white lamp (WL) in the secondary o f the transformer will glow to indicate that the power supply is available. 12.3.5.1

M an u al M ode (L ocal C ontrol)

The manual / auto switch is left in the “Manual” position as shown in Figure 12.6. The local start push is pressed and auxiliary relay X I energises. The circuit is as follows: S

From the 110V secondary (for example) o f die transformer Tr. -* through the 3 A fuse -* the auxiliary contact o f the over-current relay OCR (4) -* the N/C contact o f the Auto Mode relay X 3(l) -+ the local stop push -* the local start push (pressed) -* the remote stop push -+ the winding o f auxiliary relay X I and back to the secondary o f the transformer Tr.

S

N/O contact X l ( l) makes to energise the main contactor control relay X2.

S

N/O contact X 2 (l) makes to energise the main contactor MC.

S

Contacts MC(1,2,3) make in the circuit o f the motor to start it direct-on-line.

S

Contact MC(4) makes in the circuit o f the green lamp (GL) to indicate that the pump is running.

The start push may be left free as the N/C contact X3(3) is a normally closed contact, made in the manual mode and X I ( 2 ) in series with it, makes to bypass the start pushes thus retaining the supply to X I. Under normal conditions, the pump will run. 440

Marine Electrical Technology

Starters for Alternating Current Motors The pum p can now be stopped locally (by pressing the local stop push) or remotely (by pressing the remote stop push) Note: The remote starting circuit is the same as above as the remote start push is connected inparallel to the local startpush.

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Chapter 12 12.3.5.2 Automatic Mode The manual / auto switch is changed over to “Auto”. This causes Auto M ode Relay X3 to energise and the following occurs: ^

N/C contacts X 3 (l) and X3(3) open to isolate the local and remote control circuits.

S

N/O contacts X3(2) and X3(4) make to introduce the ‘high’ and ‘low’ contacts o f the float switches.

Considering that thepump starts in the initial stages (dry condition), the circuit is asfollows: The high and low contacts are made as the float is at the lowest level and auxiliary relay X I energises. The circuit is as follows: S

From the secondary o f the transformer Tr.-* through the 3A fuse -* auxiliary contact o f the over-current relay OCR(4) -* N/O contact X3(2) (made) -+ high and low contacts o f the float switches (made) -* the remote stop push -» the winding of auxiliary relay XI and back to the secondary o f the transformer Tr.

^

N/O contact X I(1) makes to energise the main contactor control relay X2.

S

N/O contact X 2(l) makes to energise the main contactor MC.

S

Contacts MC(1,2 and 3) make in the circuit o f the motor to start it direct-on-line.

S

Contact MC(4) makes in the circuit o f the green lamp (GL) to indicate that the pump is running.

The pump will continue running even after the low level contact, breaks because the circuit o f X I is retained m>follows: The initial supply circuit remains the same up to N/O contact X3(2); the rest o f the circuit is as follows: S

N/O contact X3(2) (made) -* high level contact -* N/O contact X3(4) (made) -* retaining contact X I ( 2 ) (made) -* the remote stop push -* the winding o f auxiliary relay X I and back to the secondary o f the transformer Tr.

S

N/C contact X3(3) is made in the manual mode and X l(2 ) in series with it, makes to bypass the start pushes thus retaining the supply to XL

The pump will only stop when the float moves up and the high level contact breaks. This occurs when the tank is filled to the desired level. This breaks even the retaining circuit o f X I . 442

Marine Electrical Technology

Starters for Alternating Current Motors The pump will automatically start only when the float moves down and the low level switch operates. The pump will not automatically start at intermediate levels as both the high and low level contacts will remain open. N ote: When switching large capacity machinery connected to the Emergency Switch Board, it

is advisable to start them in no-load conditions and gradually increase load if possible, in order to avoid any unprecedentedtripping. 12.3 .6 E n gine R oom C rane C on trol

The engine room crane is an indispensable piece o f equipment on a ship. There are various types ranging from the simple two-motor-single-speed type to the three motor dual­ speed type, wherein the speed o f hoisting and lowering may be varied using simple contactors coupled with special-purpose motors to electronic cards that facilitate speed control of standard (induction) motors.

Image Courtesy: Siemens Industry Inc. - www3.sea.siemens.com

Figure 12.8 - Overhead Crane Arrangement

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Chapter 12 The following are the most commonly used crane circuits: 12.3.6.1 Dual-motor Single-speed Crane A dual-motor single-speed crane’s power and control circuits are depicted in Figures 12.9 and 12.10. Here the two induction motors are direct-on-line started; interlocked phase reversal contacts are used to achieve up or down and forward or aft movements o f the crane hook. The circuit is so designed that each motor w ill rotate only in one direction at any given time. The basic operation o f the circuit is as follows: o

The main breaker for the power and control circuit is first made. This provides power supply to the power circuit and control circuit transformer.

o

Now, if the “Up” button is pressed, a normally open contact will energise MCI provided the interlocks in the circuit o f M CI are fulfilled; they are: 1) The Control Switch on the controller which must be made. 2) The normally closed contact o f the “Down” push button (which breaks when the “Down” push button is pressed); 3) A limit switch “L.S. Up” (which is made till the crane hook reaches its upper limit); 4) A normally made contact o f MC2 (which is made as long as it is de-energised) and a mandatory thermal overload contact.

o

When MCI energises, normally open contacts M CI (1, 2 and 3) make to complete power supply for uie motor.

o

Contact M C 1(4) makes to energise the solenoid that releases the brake o f the motor.

o

Contact M C I(5) breaks to further ensure that the “Down” circuit is disabled by keeping the contactor MC2 de-energised.

Note: There is no retaining circuit for the contactors; this ensures that the crane operator presses the button continuously and is aware of the crane’s movement This also avoids accidents. The other operations are identical to the hoisting operation mentionedabove.

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Control Supply Transform er 440V /1 1 0 V

Figure 12.9 - An Engine Room Crane’s Basic D.O.L. Power Circuit

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Chapter 12

Figure 12.10 - Control Circuit for the Engine Room Crane in Figure 12.9 12.3.7 Special Features ofthe Dreggen AS Engine Room Crane

Image Courtesy: www.dreggen.com

Figure 12.11 - The Dreggen AS Engine Room Crane in Operation 12.3.7.1 Hoisting Machinery A n electrical chain hoist is used for hoisting. The hoist is equipped with: ■ C A chain collection box ^

Slip clutch for overload protection and for stopping, when the hook is at the top and bottom positions.

^ Two-speed hoisting permitting slow speed operations when accurate positioning is _______ required. Speed control is explained in article 12.11. 446

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Starters for Alternating Current Motors 12.3.7.2

T ravel M achin ery

S

The Crane Bridge and trolley runs on single flanged wheels suitable for the specified runway beam.

S

To secure safe operation against 2° trim and 5° heel, a rack and pinion drive is incorporated. The gear rack is welded to the under side o f the runway beam.

■ / A drive by electrical motors through oil-filled gear reducers and pinions engage the gear racks. 12.3.7.3

S

M otors

A ll motors are designed for crane duty, 25% ED, IP54, insulation class F and equipped with electromagnetic fail-safe brakes.

12.3.7.4

E lectrica l E qu ipm ent

■ S The starters and transformer for the hoist are located on the hoist unit while the starters, transformer, fuses etc. for the trolley and bridge including the main contactor, are located in a separate panel on the crane. S

The panel is made in steel with IP 65 protection.

12.3.7.5

E lec tric a l P ow er S u pply

■ S Power supply along the runway to the crane and along the bridge to the trolley is ensured by flexible cables suspended from wire rope. 12.3.7.6

O peration o f th e C rane

S A ll motions (hoisting, bridge - and trolley travelling) are operated from a push-button controller suspended from a separate track. s

“Start” / (Emergency) “Stop” buttons are included.

1 2 .3 .7 .7

S a fety F eatures

S

Start / (emergency) stop button.

S

Overload slip clutch, also for stop o f hook in top and bottom position.

v' Limit switches for trolley- and bridge travelling. 12.3.7.8

P arking

S

Special parking arrangement is included in the supply.

S

Both trolley and bridge have to be secured at a convenient location during voyage. Marine Electrical Technology

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Chapter 12 12.4

Star-delta Starter

This starter is used to reduce the starting current in

6 -terminal

motors designed for

star-delta operation. The motor is direct-on-line started with the stator windings being star-connected. With reference to Figure 12.12, we will see that;

I iM lI VlzJV 3 vZ} = I Il(A)

V3Vl - Z

3

Figure 12.12 - Variation of Current with Speed in Star and Delta-connected Windings It is therefore the most common and cost-effective method o f reduced voltage starting. The starting torque will also be about 33% o f the values obtained by direct-on-line delta starting. Since the effective voltage is only 57.7% (V l -*■ V3), starting high inertia loads may be a problem; thus a motor with a high starting torque must be used at extra cost (Refer Figure 12.13).

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Figure 12.13 - Variation of Torque with Speed in Star and Delta-connected Windings Figure 12.14 depicts a basic star-delta starter without any interlocks for efficient changeover from start to delta. When the start button is pressed KM? and KM 2 close together, which connect the motor windings in a star configuration to the supply by shorting terminals X, Y and Z and connecting terminals U, V and W to the supply lines R, S and T respectively. A time delay (approximately 8-10 seconds) is set to allow the motor to run up to about 80% o f its speed at which point the relay opens KM? and closes KM 3 so that the motor's winding configuration is converted from a star connection to a delta connection. The switch-over between star and delta is usually automatic, using a time delay relay or delayed auxiliary contacts on the contactors. In addition, a time delay m ust be inserted between switching o ff the star contactor and switching on the delta contactor to ensure that the switching arc in the star contactor has been quenched before the delta contactor is closed. I f the switch-over is too fast, a short circuit occurs as shown in Figure 12.15. Marine Electrical Technology

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Chapter 12

km 2

S

-

Main

1 L___ 7 —

3

5

KM

H nx 7 _ s

-

1

3I

5

KM*

1j

I 3

ssfl---- -/-—481.....-/' B l r -----f 0 / 8

6

A

/

--------- f .

6

A

i_

Thsrmai Cutouts

__

W

£ X

Motor Y

F igure 12.14 - A B a sk S ta r - D elta S ta rte r - w ithout Interlocks Conversely, if the switch-over time delay is too long, the motor speed will fall so that the delta closing current becomes excessively high. Figure 12.16 shows how this makes the stardelta arrangement ineffective. Once time delays have been set in the starter, it is important that they are not altered i f ‘open transition surge’ problems are to be avoided. A further problem can occur due to switch-over i f the m otor’s deita connection is not as shown in Figure 12.17. When switching from star to delta, the stator current stops flowing when the star contactor opens, but the rotor current flows in a closed circuit and they decay gradually from their instantaneous values at the moment o f switching-off. The decaying rotor currents are direct in nature and produce a flux, which is stationary with respect to the rotor conductors. This flux rotates with the rotor, cuts the stator windings and induces an EMF ju st like an alternator. The frequency o f the EMF in the stator falls as the rotor decelerates. I f the supply is reconnected when this EMF is out o f phase with the supply voltage (as with faulty synchronising o f an alternator), then heavy surge currents can lead to severe momentary torques up to 1 0 or 15 times the full-load torque. 450

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T l(% )

Figure 12.15 - Switch-over Pause Too S hort - S hort-circuit Across th e A rc T he Fuse Is T rip p ed and th e System is T u rn ed O ff

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Chapter 12

0

20

40

60

80

100 = %

T1A=% Tl F igure 12.16 - Sw itch-over P ause Too long - Shaft Speed D rops O ff Direct-on-line S ta rte r in D elta A rrangem ent

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Figure 12.17 - Correct Wiring of Motor Phases for Clock-wise Rotation R

S

T

Figure 12.18 - Incorrect Wiring of Motor Phases - also Causes Clock-wise Rotation Marine Electrical Technology

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Chapter 12 This can cause mechanical damage to shafts and keyways and even to the driven machine. Insulation failure due to movement o f the end windings can occur. This synchronising effect will be aggravated if the motor is incorrectly connected in a delta configuration, although the delta connection in Figure 12.18 will still give a clockwise rotation and it could result in damaging surge currents and torques. Always ensure that star-delta connections are as shown in Figure 12.17 for clockwise rotation and Figure 12.19 for counter-clockwise rotation. These connections are shown in the starter system in Figures 12.21(a) and (b). R

T

A - Figure 12.19 - C orrect W iring o f Phases for Counter-clockwise R otation 12.5

A dditional F eatures in a S tar-delta S ta rte r

12.5.1

Overload Relay Setting

The relay scale pointer should be set at 0.6 times the rated motor current, since the relay as connected, carries only the phase current in the delta mode. 12.5.2

Overload Trip Reset

After the motor starter has tripped owing to an overload, the relay has to be reset by operating the stop button. The relay will be reset only after the bimetal strips have cooled sufficiently.

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Starters for Alternating Current Motors 12.5.3

Back-up Fuses

The motor starter must never be used without back-up fuses. While the sensitive thermal relay mechanism is designed and calibrated to provide effective protection against overload, fuses, preferably o f the HRC cartridge type, are essential to protect the installation as well as the thermal relay o f the motor starter, against short circuits. The fuses for a light starting motor and for step-by-step starting motors are listed under ‘minimum’ in the table o f ratings (Refer Table 13.4). For heavy starting motors, it may be necessary to use the next higher fuse ratings. When several motor starters are connected to a common main fuse, a still larger fuse may be needed, but to protect the thermal relays it must not exceed the rating given in the column titled ‘maximum’. It is recommended that the use o f semi-enclosed re-wireable tinnea copper fuses be avoided as far as possible. When there is no alternative, the standard wire gauge (SWG) sizes stated in the table may be used. Table 13.5 is another suitable guide for fuses 12.5.4

Pilot Wire Fuse

The motor starter is normally supplied without fuse holders for pilot wiring. In case remote control is used and it is felt that the back-up fuses do not adequately protect the pilot wiring, the motor starter can be supplied with built-in pilot circuit fuses. The fuse holders are so designed that they can be ordered separately and easily fixed. 12.5.5

Solenoid Coil

Make sure that the coil o f the starter is suitable for the voltage available at the terminals in the connected circuit. Choice o f the proper coil will ensure trouble-free service. 12.5.6

Time Delay

Time delay in a star-defta starter, before changing over from the star to delta connection should be sufficient to allow the motor to come up to its normal running speed. This period may be taken to be 1 0 seconds, but could be less for a lightly loaded motor and greater for a slow starting or heavily loaded motor. In the fully automatic star-delta starter, this delay can be adjusted by rotating a dial / screw on the timer. Clockwise rotation decreases the time and anti-clockwise rotation increases the time. A thermal timer is situated at the bottom right hand corner o f the enclosure; this is only an example o f one such type o f starter. The manuals / equipment onboard must be studied to get a better picture. Marine Electrical Technology

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Chapter 12 The time delay will be reduced as long as the timer is warm from its previous operation. The time delay will be greater i f the supply voltage is lower than the rated voltage. In the semi-automatic star-delta starter, the tim e delay extends over the period the start button is kept pressed. The change over from star to delta takes place at the instant the start button is released. However, pressing the start button again will not have any effect as long as the starter is on. 12.5.7

Single Pole Float Switch, Thermostat or Similar Device

It is not possible to use such devices in conjunction with the semi-automatic star-delta starter since change over from star to delta takes place when the star contact is released. This applies only to the automatic star-delta starter. 12.5.8

Low Voltage Protection

No-volt coils fitted in motor starters, prevent simultaneous and uncontrolled starting o f machinery after a black out or other loss o f power; except in the case o f sequential starting.

| 12.6

Star-delta Starter with Fusible Isolator

12.6.1

Power Circuit Operation (Refer Figure 12.20 (a))

Qi is closed manually; the closing o f KMi results in a star connection while the motor’s power supply circuit is still ‘dead’; the closing o f KM 2 completes the motor supply; after a pre-set time delay, the closing o f KM 3 results in a delta connection and the opening o f KMi opens the star connection; the motor now runs at its nominal speed and draws nominal current. 12.6.2

Features

The voltage permissible across the motor windings, connected in delta must correspond to the main supply voltage. V

The no-fuse breaker (or MCCB) Qi is rated for In (nominal current) o f the motor.

V

The fusible isolator is rated for IN/ V3 o f the motor.

V

K M i, K M 2 & KM 3 are rated fo r I n / V3 o f the motor.

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Figure 12.20(a) - Star-Delta Starter with Fusible Isolator (Power Circuit) 12.6.3

Control-circuit Operation (Refer Figure 12.20(b))

Press the ‘Start’ pushbutton S2; the ‘Star’ contactor KMj energises as follows: S

From the control supply -* Fuse Fi -* manually closed breaker Qi -» F2 (fusible isolator) -+ Stop pushbutton Si -+ ‘Start’ pushbutton S2 Timer contact (N/C contact) -* KM3 (N/C contact)

the coil o f contactor KMi itself -* Fuse F3 and back to the control

supply circuit (in most cases it is the secondary o f a step-down transformer) Marine Electrical Technology

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Chapter 12 KM l(a) (N /0 contact) closes to energise the ‘M ain’ contactor KM 2 as follows: ^

Control supply -» Fuse F[ -» manually closed breaker Qi -* F 2 (fusible iso lato r)-* Stop pushbutton S,

‘Start’ pushbutton S2 -* KM , (a) (N/O contact) -* the coil o f

contactor KM 2 itself -+ Fuse F 3 and back to the control supply circuit. S

Contactors KM , and KM 2 are now retained by KM 2 (a) (N/O contact) and hence the Start pushbutton S2 can be released.

After the pre-set time (~ 10 seconds), S

The timer is activated to changeover its contacts in the circuits o f contactors KM, and KM3.

S

This causes the ‘Star’ contactor KM, to de-energise (by opening its N/C contact in the circuit) while the circuit o f the ‘Delta’ contactor KM 3 is prepared (by closing its N/O contact in the circuit).

^

‘Delta’ contactor KM 3 will energise when the ‘Star’ contactor KM, de-energises to cause its N/C contact KMi(b) to close thereby facilitating a smooth changeover from ‘Star’ to ‘Delta’.

The motor is now capable o f running at its pre-determined optimum speed to drive the load. The motor stops by operating Stop push S,. In this case ‘Delta’ contactors KM 3 and KM 2 are de-energised simultaneously thus isolating the motor’s windings. The N/C contact KM 3(a) in the circuit o f contactor KM, closes once again to prepare the circuit for the next start.

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A

Figure 12.20(b) - Control Circuit - Star-Delta Starter with Fusible Isolator 12.6.4

E le c tric a l In te rlo ck between K M j and K M 3

The tim e delay contact block has a switching tim e o f 40ms between the opening o f the normally closed (N/C) contact and the closing o f the normally open (N/O) contact. This eliminates the risk o f short-circuit on change-over from star to delta.

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Chapter 12 Many star-delta starters are fitted with mechanical interlocks between contactors KMi and KM3, which prevent them both from being closed simultaneously; the danger o f the supply being short-circuited is overcome in this case. The time delays in the system will prevent this under normal operating conditions but an engineer may attempt to press both contactors shut when faultfinding to check a circuit’s operation. Never try to manually make contacts on a live circuit. If there is no mechanical interlock, use extreme caution and never remove electrical safety interlocks from any equipment. Summarising the above, Star-delta starters can only operate effectively when the time delay between ‘star’ opening and ‘delta’ closing has been correctly set. I f the switch over is too fast, two problems may arise. 1.

Short-circuit current will flow if the star contactor has not quenched the arc.

2. I f the arc is quenched but the rotor flux has not had enough tim e to decay then ‘synchronising’ currents and torques can cause mechanical damage when the delta contactor closes. I f the changeover tim e is too long then the rotor decelerates and delta starting current is drawn when it is re-connected; this type o f starter is used for a motor designed for delta operation and where the load torque is low enough to avoid unacceptable run-up timings. 12.6.5 Star-delta C irc u it fo r a B alla st Pum p *s M o to r

Much has been discussed about star-delta starters. But why do we need them? The reason is the demand for high power output requirements. The m otor can be locally started and stopped; stopping can be additionally done from a remote location. The m otor in question is rated at 440V, 60Hz, 3-phase, star-delta (six-terminal), 60kW (80HP). It is assumed that the m otor’s impedance per phase (ZPjj) is 7.5Q at 0.8 lagging power factor. Now in the star mode, VL = 440V, VPh = 440V -W 3 = 254V; IPh = IL = 254V -5- 7.5 = 33.5A. Pstar = V3 • 440 ■33.5 • 0.8 - 20.4 kW. And in the delta mode, V j, = VPh = 440V; Ipf* = 440 % 7.5 = 58.6A and Il = ^3 IPh ~ 101.6 A. % Poelta = ^3 * 440 • 101.6 • 0.8 = 61.9 kW (which may be rounded o ff to 60kW). From the above calculation, it is obvious that the power output is about 3 times when it changes over to the delta mode! The circuit is depicted in Figures 12.21(a) and (b). 460

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Figure 12.21(a) - Star-delta Starter for a Ballast Pump’s Motor U

YU

Figure 12.21(b) - Star-delta Configuration Marine Electrical Technology

461

Chapter 12 The operation of the control circuit is as follows: S

The white lamp WL glows when the supply is made available by closing the MCB.

S

The pump motor can be locally started by pressing the start push. This energises auxiliary relay X I.

S

N/O contact X l ( l) makes to energise auxiliary relay XI N/O contact X 2(!) closes and MCS - the Main Contactor (Star) energises through the N/C contacts o f the Timer Contactor TC(1) and M ain Contactor (Delta) MCD(4) - both these contactors are now in the de-energised state.

^

The N/O contacts MCS (1,2,3) make and terminals X, Y and Z o f the motor are thus short-circuited, bringing about a star-connection when the motor is dead (the safest method).

S

MCS(4) closes and the Main Contactor (for the motor) MCM now energises as TC(2) is already made.

S

MCM(1,2 and 3) make to complete the supply for the motor by providing supply to terminals U, V and W.

S

MCM(4) makes parallel to TC(1); this ensures that MCS will remain energised even after TC(1) breaks. MCM(5) makes to activate the Timer Contactor TC which will energise after about a maximum o f 1 0 seconds after the motor starts, to initiate the change-over from the star mode to the delta mode.

S

As mentioned earlier in this chapter, the tim er contactor has a switching time o f about 40ms between the opening o f the normally closed (N/C) contact and the closing o f the normally open (N/O) contact. This eliminates the risk o f short-circuit while changingover from star to delta.

^

TC(1) breaks in the circuit o f the star contactor MCS; but MCS remains energised with the help o f MCM(4). The importance on this contact is realised only when and if the change-over from star to delta does not occur (i.e., i f MCD does not energise) and the motor is compelled to run in the star mode and eventually stall, overheat, etc. A t this point o f time the motor will stop because there is a similar contact - TC(2) in the main contactor’s circuit which will also break. This will cause MCM to also de-energise, thus isolating the motor completely; all six terminals would be disconnected from the supply and the circuit o f the m otor will be rendered safe.

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Starters for Alternating Current Motors Y N/O contact TC(3) makes to serve as a self-retaining contact in case the above mentioned situation occurs. TC will remain energisedtill the supply is switchedoff Y N/O contact TC(4) makes to prepare the energising circuit o f the M ain Contactor (Delta) MOD. This is vital as change-over from star to delta occurs now. Y The changeover sequence is that once TC(2) br eaks, we already know MCM de-energises to open MCM (1,2, and 3); MCM(4) breaks to de-energise contactor MCS which in turn opens its N/O contacts MCS(1,2 and 3) in the motor circuit (the shorting link between the motor’s terminals X, Y and Z is opened and now all 6 terminals o f the motor are isolated). MCS(4) breaks; MCS(5) makes to energise contactor MCD. MCD(1,2,3,5 and 6 ) make while MCD(4) breaks. Y

M CD(!,2 and 3) connect the m otor’s terminals in the delta mode while the motor is momentarily isolated from the supply.

Y MCD(4) breaks before MCM(4) makes again in order to prevent contactor MCS from energising after MCD has energised. Y MCD(5) makes to energise main contactor MCM once again to ensure that power supply is available to the motor as it runs in the delta mode. Y

MCD(6 ) makes to complete the supply for the green lamp GL to indicate that the pump is running.

12.6.6 Starting Torque o f a Squirrel-cage Type o f Motor Before the next type o f starter is studied, it is advisable to compare the typical characteristics for direct-on-line and star-delta starters. The resistance o f a squirrel cage rotor is always constant and quite low in comparison to its reactance which is very high especially while starting. In the standstill condition, the frequency o f the rotor current equals the frequency o f the power supply. Hence the starting current o f the rotor, though very high, lags by a very large angle behind the rotor e.m.f. with the result that the starting torque per ampere is poor, roughly being 114 times the full-load torque as shown above in Figure 12.22 (a), despite the fact that the starting current is 5 to 7 times as shown in Figure 12.22 (c). This is how it is calculated: J s = la Tf

xSf

If Marine Electrical Technology

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Chapter 12

Where Ts is the starting torque; Tf is th e full-load torque; Isc is (compared to the transformer secondary short-circuit) current while starting; If is the full-load current; Sf is the full-load slip. Let us assume that the starting current 1^ is 7 times the full-load value If. Now if the slip ‘s ’ is equal to 3%, then the starting torque is Is x s = 7 2 x 0.03 = 1.47 (approximately 1.5). Hence these motors are not suitable for starting against heavy loads. Article 12.8 explains the starting o f special high-torque induction motors.

Figure 12,22(a) - Comparison of Direct-on-line and Star-Delta Characteristics (Torque versus Speed)

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Starters for Alternating Current Motors

Image Courtesy: AlfKare Adnanes, Technology Manager, ABB AS Marine, Oslo, Norway From his article titled “Maritime Electrical Installations andDiesel Electric Propulsion ”

Figure 12.22(b) - Load Characteristics for a Direct On Line Asynchronous Motor With Load Curves for a CPP Propeller

Figure 12.22(c) - Comparison of Direct-on-line and Star-Delta Characteristics (Current versus Speed) Marine Electrical Technology

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12.6.7

Asynchronous 3-phase S q u irre l Cage M o to r D ata

Thefollowing table is insertedwith reference to Maryland Metrics Technical Bulletin and withpermissionfrom MarylandMetrics, Maryland, USA (www.mdmeiric.com).

Power

Nominal current ln

Directly Fused

Star Delta Started

Star - Delta contactor in

Circuit Breaker in

KW

HP

A

A

A

A

A

-

16

0,2

0,3

0,7

2

2

0,33

0,5

1,1

2

2

-

16

0,5

0,7

1,4

2

2

-

16

0,8

1,1

2,1

4

4

-

16

1,1

1,5

2,6

4

4

-

16

1,5

2

3,6

6

4

1000%) and deliver a very low torque (typically < 100%). I f a reduced voltage starter is applied under these conditions, the starting torque will thus be very' low and will not start a machine. To apply a reduced voltage starter to a slip ring motor, first ascertain that a reduced torque is going to start the machine, then connect resistors to the rotor circuit, which will give curves similar to a high-starting torque cage motor. These resistors m ust then be bypassed once the machine has reached its rated speed. The value o f the resistance is dependent on the motor and the curve. 12,10.12 Soft-starter Ratings These ratings are approved by the International Electrotechnical Commission (IEC947-42). There are two categories namely AC-53a and AC-53b. AC-53a applies to starters that are not by-passed while running and currents pass through the SCRs continuously. This causes heat to be generated and elevates the operating temperature o f the SCR junction. AC-5 3b types only pass current through the SCRs while starting and the period between starts is effectively a cool-down period for the SCRs. This can result in an increased rating in some situations. As the rating is essentially thermal, there is an established relationship between the start time, current and frequency, ambient temperature, o ff time and the rating o f the starter. Typically the thermal inertia o f the SCR heat-sink assembly is quite long so there is not a large variation in the rating between a 10-second rating and 30-second rating. Semiconductor fuse curves do not follow the rating curves for soft starters and only offer short-circuit protection. Examples o f the name-plate data on AC-53a and 53b types are given below: liSA: AC-53a 3.5-10:75-10 Where. 1 15A is the starter’s current rating (nominal current In); AC-53a is the type (as mentioned in the previous paragraphs); 3.5 is the starting current ratio (versus the nominal current In); 10 is the starting time in seconds; 75 is the on-load duty cycle in %; and 482

Marine Electrical Technology

Starters for Alternating Current Motors 10 is the starts per hour. 115A: AC-53b 3.5-10:1445 Where, 115A is the starter’s current rating (nominal current In); AC-53b is the type (as mentioned in the previous paragraphs); 3.5 is the starting current ratio (versus the nominal current In); 10 is the starting time in seconds; 1445 is the off-time in seconds

i 12.11 12.11.1

Speed Control of Induction Motors Slip

It must be understood that a m otor never really succeeds in catching up with the stator field’s revolving flux. I f it achieves this, then there would be no relative speed, practically no electromotive force in the rotor, leading to no rotor current and torque to sustain rotation. The difference in speeds is governed by the load resulting in the application o f braking torque and subsequent slowing-down o f the motor. Thus slip increases and with it the torque due to an increase in the load current and is basically the difference between the synchronous speed Ns and the actual speed N. It is generally mentioned as a percentage o f Ns. % Slip ‘s’ = Ns - N x 100 Ns N s - N is also called th e ‘slip speed’ a n d rotor speed o f an induction motor N = N s(l-s). 12.11.2

Control ofSpeed

By changing one o f the three parameters the speed o f the motor can be controlled: 1. In the wound rotor type induction motor the slip can be changed by adjusting the resistances o f the rotor windings. The disadvantages o f this type o f speed control are the power wasted and also the speed varies with torque. At full load torque and h alf speed, approximately half o f the energy is lost due to the resistances. In the cage type motor the rotor resistance cannot be varied Marine Electrical Technology

483

Chapter 12 2. The m ost efficient method o f speed control o f an induction motor is by varying the frequency o f the supply; this method requires not only some form o f frequency converter but also a means o f adjusting the voltage. As the frequency varies the inductive reactance o f the machine also varies, so if the frequency is increased the supply voltage should also be increased. In certain methods o f frequency control, using electronic components, the original supply sine wave is cut to give a part o f the sine wave, or reassembled to give a sine wave at a lower frequency. This method facilitates speeds only lower than the original value. 3.

If two or three speeds near to the possible synchronous speeds are required then pole changing may be used. It must be remembered that pole-changing methods are possible only for cage motors as the cage rotor is capable o f automatically developing the number o f poles equal to the poles o f the stator winding.

a) Two speed dual wound type or multiple stator windings: I f the stator o f a cage type induction motor is wound with two separate windings, each designed for a different speed then by means o f a selector switch two pre-selected speeds can be obtained. It may be assumed that the motor has two types o f windings for 12 poles and 8 poles respectively. For a supply o f 50 Hz, the synchronous speeds will be 500 and 750 rpm respectively. The actual speeds achieved during operation would however be marginally reduced based upon the slip, say by 5%. This method is not only costlier but less efficient and is hence not popular. b) The method o f consequent poles or two-speed -2 :1 pole change A single stator winding is generally divided into two parts, which can be connected to give one or two pairs o f poles, will give two speeds with one speed being half that o f the other. By combining the two above methods, motors may be run at three or four fixed speeds c) Two speed pole amplitude modulation (P.A.M.) pole change If the stator windings are divided into conductor groups, say ‘a’ and ‘b ’, then by connecting them in different ways e.g. (a + b) and (a - b) different speeds can be obtained. Therefore speed combinations in ratios other than 2:1 can be obtained. Where close ratio two-speed motors are required, this type o f motor has largely superseded the two speed dual-wound motor. However the two-speed dual motor is used where wider speed (spread) applications are required.

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Marine Electrical Technology

Starters for Alternating Current Motors In order to use pole changing methods o f speed control with slip-ring wound rotor induction motors, the number o f poles in the rotor and the stator m ust be changed by the same ratio. With two sets o f slip rings and a specially wound rotor, the machine is expensive and this method is seldom used. Other methods o f control include using three-phase commutator motors, or two motors arranged in cascade form. However these methods are expensive and are not normally used for marine applications. 12.11.3

D u al-speed C on trol A p p lied in a C rane C ircu it

Hoisting and lowering arrangements may also incorporate speed variation based on the loads being hoisted. Light loads may be quickly hoisted or lowered and likewise heavy loads may be hoisted and lowered at half the speed. All that is ever needed is another pair of contactors and o f course further interlocks to ensure that the speed-control contactors are not energised simultaneously. The question o f speed variation may be addressed by using either a star-connected or a delta-connected motor. In both cases, as depicted in Figures 12.31 and 12.32, the impedances are halved to achieve higher speeds. This is made possible by utilising the low-speed / high­ speed contactors to connect the windings as desired.

R

y 5 Double Star High Speed

Star Low Speed

Figure 12.31 - Star Connections for Speed Variation

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Chapter 12

Figure 12.32 - Delta Connections for Speed Variation

1)

A DOL starter is normally used in a / a n ________ type o f motor.

2)

An induction motor is expected to draw the highest starting current with a _____ starter.

3)

Auto transformer starters are often used for motors above________ kW.

4)

The high starting current in a motor normally drops to normal at ab o u t______ % o f its speed.

5)

Autotransformer starters are used with polyphase induction motors t o _______ .

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Marine Electrical Technology

Starters for Alternating Current Motors 6)

A soft starter can be connected i n ________ with the motor.

7)

M odem soft start controllers u s e ________ devices.

8)

The main part o f a motor starter includes________ .

9)

A solenoid works on th e ________ principle.

10)

The Function o f a Star-Delta starter o f a motor is t o ________ .

11)

A 3-phase induction motor needs frequent starting and operation in forward and reverse directions; a ________ type o f starter will be preferred.

12)

What is a magnetic contactor? Where is it used?

13)

Name die types o f starters used in marine applications.

14)

With the help o f simple sketches explain the most common types o f starters used and why?

15)

What is the significance o f starting torque? What is the approximate value while starting a motor with a DOL starter?

16)

Why is the contactor the heart o f every motor starter? Explain this with a suitable sketch.

17)

Sketch a simple diagram o f a direct on line starter, showing in detail the overload and single phase protection trip.

18)

Sketch a direct on line starter suitable for a three phase a.c. induction motor.

19)

Explain the limitations o f the direct on line starter with respect to length o f starting time and repeated successive starts.

20)

W hat is the starting current o f an induction motor as compared to the full load current?

21)

Describe the function o f thermal overload relay on motors.

22)

A slip ring induction motor is considered to be a. high torque induction motor. Why is this so?

23)

W hat is the meaning o f Star winding/ Delta winding?

24)

How does a basic star-delta circuit work?

25)

List the areas where timers find applications in motor starters.

26)

Draw and explain circuit o f a star-delta starter for a main seawater pump.

27)

W ith the help o f a neat diagram explain the operation o f a D.O.L. Starter - with automatic level control. Marine Electrical Technology

487

Chapter 12 28)

What is the basic circuit behind an engine room crane? Explain its operation with a suitable sketch.

29)

W hen connected in star what is the voltage as a percentage o f foil load voltage?

30)

Why are back-up fuses needed for a motor starter?

31)

What is the most common reduced voltage starter?

32)

What are the voltage tappings within an autotransformer? W hy are they used?

33)

With autotransformer starters what is used to overcome the transition switching problem?

34)

What is the significance o f an auxiliary relay in an autotransformer starter’s circuit?

35)

How is a low voltage situation taken care o f in a starter?

36)

Draw and explain the power and control circuits o f a starter for a slip-ring induction motor.

37)

How is a starter for a slip-ring induction motor superior to a D.O.L. starter?

38)

With the help o f a neat diagram explain the operation o f a basic Auto-transformer Starter.

39)

Draw and explain the power and control circuits o f an Auto-transformer Starter.

40)

Explain the operation o f a soft starter and its advantages.

41)

Write short notes on: a)

Open Loop Soft Starter

b) Closed Loop Soft Starter 42)

How are soft starters rated? Give suitable examples to prove your point.

43)

How is the speed control o f induction motors achieved?

44)

What are the motors employed for different operations o f overhead cranes?

45)

State the basic precautionary measure to protect the generator when switching large capacity machinery connected to the Emergency Switch Board.

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Marine Electrical Technology

Fault Protection Devices At the end of this chapter you should be able to: ★

E x p la in t h e fu n d a m e n ta l re a s o n s fo r fa u lts in e le c tr ic a l e q u ip m e n t R e c o g n is e th e s ig n ific a n c e o f o v e ra ll p ro te c tio n o f c ircu its E x p la in th e o p e ra tio n o f v a rio u s p ro te c tiv e d e v ic e s in a s h ip ’s e le c tr ic a l s y s te m Id e n tify th e n e e d fo r m o to r a n d c a b le p ro te c tio n C o m p ly w ith re g u la tio n s g o v e rn in g t h e p ro te c tio n o f e q u ip m e n t

13.1

Identifying the Need for Circuit Protection

As mentioned in Chapter 1, an electrical system is designed and built with great care to ensure that each electrical circuit is adequately segregated from the other. This is done with the intention that the current will follow its pre-determined path. No matter how well designed and operated, there is always a possibility o f faults developing in electrical equipm ent Once the equipment or system is in service, many things can happen to alter the characteristics (and behaviour) o f the original circuitry. Some o f these changes can cause serious problems if they are not detected and corrected in time. While circuit protection devices cannot correct abnormal current conditions, they can indicate that an abnormal condition exists and protect personnel from electrical shocks and circuits from the adverse effects; this is basically achieved by disconnecting the faulty equipment which also ensures that the power supply to normal equipment is not interrupted. Faults can develop due to natural wear and tear, incorrect operation, accidental / intentional damage and by negligence. The breakdown o f essential equipment may endanger the ship, but probably the m ost serious hazard is fire. We are aware that current flow in a conductor always generates heat. The greater the current flow, the hotter the conductor. Thus over-currents in cables and associated equipment obviously cause overheating (also known as the I2R effect) and possibly fire too. Excess heat is also damaging to electrical components and conductor insulation. For that reason, conductors have a rated continuous current carrying capacity or “ampacity” as some define it. Some common terms that relate to faults and fault protection systems, are explained in the following articles. Marine Electrical Technology

Chapter 13

113.2

Direct Shorts

A user such as an electric motor or lighting system will constitute a ‘circuit’ and will carry a designed load current in accordance with the impedance o f the circuit and the applied voltage. One o f the m ost serious faults that can occur in a circuit is a direct short. Another term for this condition is a short circuit. A ‘short-circuit’ occurs when an alternative path of low or negligible impedance becomes available to the applied voltage. However these terms describe a situation in which the full system voltage comes in direct contact with the ground or ‘return’ side o f the circuit thus bypassing the load.

Figure 13.1 - Frayed Insulation Causing a Direct Short (or Bolted Short Circuit) This establishes a path for current flow that contains only the negligible resistance present in the current-carrying conductors and is an unintentional path o f current flow. In certain situations, a direct short can interrupt a part o f the vessel’s power supply. Whenever a short circuit occurs in an unprotected circuit, the current will continue to flow until the circuit is damaged, or until the power is removed manually. The value o f the peak short-circuit current o f the first cycle is the highest and it is referred to as “the peak letthrough current” (Ip). Simultaneously the system voltage falls to a value close to zero. This is because o f the fact that according to Ohm ’s law, if the resistance tends to fall to zero, the current will tend to rise to infinity; due to this effect, the potential difference in the affected system will tend to fall to zero. There can be ‘bolted’ short-circuits when two or more live conductors are brought directly into contact with each other; ‘arcing’ short-circuits will produce arcs or sparks. The maximum destructive energy let-through (I2t) is a measure o f the energy associated with this current. It is capable o f producing enough heat to melt conductors. For instance, an arcing fault in a main switchboard can dissipate several megawatts o f energy and the arc temperature will reach several thousands o f degrees Celsius. The result o f a short-circuit, if not interrupted, is the thermal damage o f all components carrying the short circuit, and mechanical stresses resulting from the forces imposed on the system by the interaction between the high currents in different phases. 490

Marine Electrical Technology

Fault Protection Devices The minimum distances for bare conductive parts in switchgear and control gear assemblies (according to the requirements by some classification societies) are mentioned in Table 2.3 below. However creepage distances cannot be accurately specified due to the nature o f the environment at sea and insulation also used. They are not expected to be less that the clearance distances mentioned below or less than 16mm per 100V (rated voltage), whichever is greater. Minimum clearance Rated Voltage (V)

between phases and earth (mm)

Minimum clearance between phases (mm) ;

Earthed Neutral

Insulated Neutral



Modular design with small probability o f total loss o f propulsion power.

>

Sharply reduced number o f moving mechanical parts.

>

Proven technology based on decades o f operating experience.

>

Redundant drives with one propeller are also possible.

>

Designs are also possible for maximum redundancy requirements.

17.13.1.3

E n viron m en tal C om patibility

Diesel-electric propulsion systems protect the environment because the pollutant emission o f diesel engines is reduced by operating the engine at the optimal speed and load ranges. 17.13.1.4

O perating C onvenience

Diesel electric propulsion systems are very convenient for the user, because o f the following: > Excellent dynamic response from zero to maximum propeller speed. >

Short reversing times.

>

Availability o f maximum torque across the entire speed range at the propeller.

>

Quiet operation.

>

Minimum mechanical vibration.

17.13.1.5

F lex ib ility

Their modular construction makes diesel-electric drives especially flexible in ship designs through: >

Flexible arrangement o f components in the ship.

>

Simplified mechanical requirement for the propeller shaft.

>

Reduced space requirements in the shaft system.

>

Design and engineering o f the propeller is independent o f the drive.

>

Flexibility in the choice o f diesel engine speed. Marine Electrical Technology

653

Chapter 17 1 7 .1 3 .1 .6

U s e o f H a r m o n ic F ilt e r s

These are used at the source o f the harmonics as induction motors heat-up i f the supply voltage is distorted. Negative sequence harmonics, which rotate in the direction opposite to the direction o f rotation o f the motor reduce the motor torque and cause heating resulting in a burn-out (Refer article 6.14). 1 7 .1 3 .2

S I M A R D r iv e C y c lo - T h e D r iv e w ith th e C y c lo c o n v e r te r

A cycloconverter feeds a three-phase synchronous or asynchronous motor. Such drives are used wherever outstanding synchronous running qualities, low torque pulsation, and high dynamic response at low speeds are required. The cycloconverter consists of three-part converters, each o f which feeds one phase o f the motor, as shown in Figure 17.14. The part-converters “cutout” suitable curve segments from the line voltage and re-arrange them so as to produce a voltage o f near sinusoidal form. T h re e -p h a s e B ridge fo r P o sitive C u rren t Flow

( ( |£ |

rr

rW--£n

■ -K F — H3-

-RHRJrW T h re e -p h a s e B ridge for N eg a tive C u rren t Flow P ro p e lle r f D rive M oto r l

^

M

) )

S ynchronous or A synchronous M otor

Figure 17.14 - The Propeller Drive with a Cycloconverter (SIMAR Drive Cyclo) 654

Marine Electrical Technology

Electrical Propulsion Systems This method gives an output frequency o f up to 40% o f the power system frequency. In order to obtain higher line-to-line output voltages, the part-converters are controlled to give approximately trapezoidal output voltages. The converter is then better utilized and its line-side power factor is improved. The lineto-line output voltage remains sinusoidal and is increased by a factor o f 1.15. The motor current flows through at least two part-converters. In the case o f faults, the current controls o f both part-converters counteract possible short circuits. Together with the TRANSVEKTOR® closed-loop control, SIMAR Drive Cyclo provides dynamic response and a torque waveform superior to the d.c. motor. The slight torque waveform results in an extremely low-noise drive. And as far as output performance is concerned, practically any size required can be implemented. Three ferries for Washington State Ferries “Tacoma”,” Wenatchee” and “Puyallup” are each equipped with four SIMAR Drive Cyclos for 4.475MW. The cycloconverters are as shown in Figure 17.14 or as 12 pulse models. In the lower output range, the converters are provided with air-cooling and in the medium and upper output range, with direct water-cooling. 17.13.3

SIMAR Drive Synchro - The Drive with the Electronic Commutator

SIMAR Drive Synchro is most closely related to the familiar d.c. drive. It comprises a rectifier, current source link, inverter and synchronous motor. The inverter acts as an electronic commutator and replaces the mechanical commutator o f the d.c. machine. Aided by the TRANSVEKTOR® control, SIMAR Drive Synchro possesses the characteristics o f a d.c. drive, which distinguishes it essentially from the synchronous motor in operation. SIMAR Drive Synchro can neither pull out o f step nor does it tend toward inconvenient oscillation. The rectifier is simple in design and requires only normal power thyristors. As shown in Figure 17.15, series connection o f the thyristors enables virtually unlimited output power.

Marine Electrical Technology

655

Chapter 17

Line-side Converter

Link Reactor

He Synchros). Once again this leads to a large reduction o f the phase effects on the power system compared to a 12-pulse design. 17.13.4 SIMAR Drive PWM - The Drive with IGRTs (Insulated Gate Bipolar Transistors) Progress in the field o f power semiconductors has made it possible to use switch-able components in converters for asynchronous and synchronous motors that function without a commutator (Refer F igure 17.16). Since asynchronous and synchronous motors do not supply any reactive power, the motor-side converter m ust itself enforce switching o f the current from one bridge arm to the next. IGBTs make this possible in a simple manner: A low-voltage design o f the SIMAR Drive PW M is available for voltages up to 690 V and a medium voltage design for voltage up to 6000V. In a standard design both have a 12-pulse diode-rectifier input bridge. Optional for both systems is the Active Front End (AFE) which reduces the phase effect on the system to such a degree that it is even possible to m eet naval requirements (STANAG 1008) without taking special measures. Both the low-voltage and the medium voltage converters use IGBTs as power semiconductors. In low-voltage technology IGBT power semiconductors are standard, worldwide. Siemens / Eupec have continued to develop this technology to increase blocking capabilities and to obtain higher current carrying capacities. Unlike IGBTs, conventional GTO thyristors and IGCTs cannot be fully controlled via the gate. With IGBTs, each time the device is tumed-on/tumed-off, the current and voltage transient can be fully controlled.

Marine Electrical Technology

657

Chapter 17

Figure 17.16 - Schematic Diagram of a SIMAR Drive PWM 658

Marine Electrical Technology

Electrical Propulsion Systems And this is where HV-IGBTs really stand apart: >

SIMAR Drive PWM converters for medium voltage are immune to short circuits at the output as they limit the current, contrary to GTO and IGCT drive converters

>

Far fewer components are required for gating which is different for GTO and IGCT technology This means that IGBTs enhance reliability

>

IGBTs require a significantly lower gating power than GTOs or IGCTs

>

No snubber circuitry is required

>

HV-IGBTs, SIMAR Drive PW M converters are simple, compact and also extremely reliable The low voltage converters are available in air / direct freshwater-cooled versions. The

medium voltage converters are water-cooled equipped with a direct dionized water cooling system. 17.13.4.1 The Basics of PWM This type o f drive is known as a pulse-width modulated drive. It is cheap as it does not involve magnetic components. The output voltage waveform is synthesised from constant-amplitude, variable-width pulses at high frequencies in order to simulate a sinusoidal output; these frequencies can range from 500 Hz to 10 kHz. The main advantages o f this drive are smooth torque, low output harmonic currents and no cogging. The disadvantages however are that since every switching causes an energy loss in the output devices and their suppressors, the total losses at high speed go up considerably over six steps (six switches per cycle) i f the same devices are used. In order to overcome this, faster transistors and IGBTs are used. Some PWM converters do not have voltage regulators; they deliver a preset fraction o f the output voltage at any given frequency. I f the input fluctuates so does the output. The highfrequency switching may also cause acoustic noises in the motor. All said and done, the PWM drive is becoming the most widely used method o f control for motors below 100 horsepower. However there are GTO PWM units also available in the range o f 1 to 3000 horsepower at 460 V a.c. Marine Electrical Technology

659

Chapter 17 17.13.4.2

1.

P recautions w h ile H an dlin g 1GB Ts

Since IGBT gates are insulated from any other conducting region, care should be taken to prevent static build up, which could possibly damage gate oxides. IGBT modules are generally shipped from the factory with conductive foam contacting the gate and emitter control terminals.

2. Never touch the gate terminals during assembly and keep the conducting foam or copper grounding straps in place until permanent connections are made to the gate and emitter control terminals. 3.

Always ground parts touching gate terminals during installation

4.

In general, standard ESD precautions applicable to MOSFETs should be followed namely using a grounded work station with grounded floors and grounded wrist straps when handling such sensitive devices.

5. Use a 100 ohm resistor in series with the gate when performing curve tracer tests. 6. Never install devices with power connected to the system. 7. Use soldering irons with grounded tips when soldering to gate terminals. 17.13.4.3

IG B T T esting

Most IGBT devices are 100% tested before shipping and guaranteed to meet the published parametric data. Re-testing by the customer is not recommended because o f the potential o f damaging the device. I f it is necessary to assess the electrical characteristics o f the IGBT the following tests can be performed: 1. Always use static-safe i.e., electrostatic discharge-safe handling procedures and replace the conductive gate-emitter foam after testing. 2. Never apply collector to emitter voltages greater than the IGBT V ce’s rating and never apply gate to emitter voltages greater than the IGBT V qe ’s rating. 3. When using a curve tracer, ramp the voltage up and back down for each test. 4. Never apply a voltage greater than 20V or the prescribed voltage to the collectoremitter junction with the gate terminal open. 5. Avoid thermal shock. Never put a cold device on a pre-heated hotplate; the temperature should not increase at more than 10°C per minute. 660

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S

Test P rocedure (w ith a D ig ita l M ulti-m eter)

Equipment Requirement - DMM with diode check mode and battery voltage less than 20V. (Typical units using 9V battery are acceptable).

V C ollector-E m itter Junction Test: 1. W ith the module out o f circuit, remove the conductive foam and short the gate to emitter. 2. W ith DMM in the “Diode-check” mode, the collector to emitter should give a normal diode reading with the positive lead on the emitter and negative lead on the collector. 3. The DMM should read open or infinite with the positive lead on the collector and negative lead on the emitter. Damaged IGBTs may test as shorted in both positive and negative directions, open in both directions, or resistive in both directions. S

G ate O xide Test:

W ith the DMM in resistance mode the resistance from gate to collector and gate to emitter should read infinite on a good device. A damaged device may be shorted or have resistive leakage from gate to collector and/or emitter. 17.13.5 17.13.5.1

M odu lar C onverters W ater-cooled C onverters

In the medium and upper power range, SIMAR DRIVE uses compact, water-cooled converters, designed to make them short-circuit proof, i.e., they m ust be capable o f carrying the maximum short-circuit current until a higher-level protective device, in this case a circuitbreaker operates. This method o f rating provides large thermal reserves for normal operation. 17.13.5.2

A ir-co o led C onverters

In the lower power range, SIMAR Drive uses air-cooled converters. Dissipated heat is drawn off by integrated cabinet fans or diverted into cooling ducts. 17.13.6

S S P P ropulsor

The Consortium SSP, a consortium o f Schottel GmbH and Co. KG and Siemens AG, Marine Engineering has developed a new podded azimuth diesel-electric propulsion system for power outputs in the range o f 5 to 30 MW per unit (Refer Figures 17.17(a), (b) and (c)); Figure 17.17(d) is a sectional view o f a mermaid pod. Marine Electrical Technology

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Figure 17.17(a) - Side View of the SSP P ropulsor W ith optimum hydrodynamic design and the new permanently-excited propulsion motor, the SSP Propulsor is the first diesel-electric drive system which proves significantly more efficient than a conventional diesel direct drive system or azimuth thruster. These benefits, combined with the proven excellent manoeuvrability o f an azimuth drive, explain the new system’s attraction to cost-conscious ship owners. The new podded diesel-electric azimuth drive is especially suitable for all kinds o f vessels requiring high electric power demands aboard and high manoeuvrability. It is also suitable for vessels w ith frequent changes o f power output, such as cruise ships large ferries and passenger vessels, medium-sized cargo vessels (feeder container vessels and chemical tankers for instance), ice-going vessels, offshore vessels and structures o f all kinds plus navy vessels.

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Image Courtesy: ABB F igure 17.17(c) - Podded P ropulsion Systems Marine Electrical Technology

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Chapter 17 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Propeller Radial Bearing Shaft Earthing Device Brake and Locking Device Drainage Compartments Exciter Thrust Bearing internal Seal Electric Motor Rotor Propeller Shaft External Seal A ir Flow Seal Seating Slewing Bearing Steering Motors Slip-ring Unit A ir C ooler Cubicle

Figure 17.17(d) - Sectional View o f the M erm aid P od P ropulsor As already proven by tank tests and full-scale tests, the SSP Propulsor guarantees energy saving o f more than 10% over conventional diesel-direct systems or conventional azimuth thrusters. The following are the benefits o f the SSP Propulsor: 17.13.6.1

B en efits o f S S P

S

Efficiency increased by more than 10 %

S

No external cooling necessary

S

Elimination o f rudder, shaft line, bossing, aft tunnel thrusters

S

Suitable for a wide variety o f stem hull designs

S

No cooling systems or cooling air ducts and fans, which saves space and simplifies installation

S

Flexible design options for stem and engine room

S

Increase in cargo space

^

The modular design principle allows installation o f the propulsion module ju st before the vessel is launched

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Mounting and dismounting o f the propulsion module is possible while the vessel is afloat

V Optimum maneuverability without additional stem thrusters, especially at low vessel speed S

Minimized crash stop-time without loss o f vessel control

S

Increased safety and easier handling o f the vessel

S

High on-board comfort on account o f extremely low noise and vibration levels

v' Low service and maintenance costs due to the minimized number o f rotating parte v'' Reduced exhaust gas emission at rated vessel speed as a result o f power consumption and optimum maneuverability 1 7 .13.7

F u tu ristic Trends o s Q u oted in th e N a va l A rch itect J o u rn al - S eptem ber 2004

Quote New-generation LNG carriers have provided a further boost for electric drives as owners seek alternatives to the traditional steam turbines o f such tonnage. The first mainstream ship with a dual-fuel diesel-electric power plant, but traditional shaft lines, will enter service late this year. The 74,000m3 Gaz de France Energy, with her dual-fuel Wartsila-driven alternators and Alstom electric m otor is nearing completion at Chantiers de l'Atlantique; two similar but much larger vessels, also for Gaz de France, are on order at the same yard, and it seems quite likely that such plant will become more common-place in the currently buoyant LNG market. Future LNG carrier contracts feature an electrical option in their ship designs - although this will have to compete with mechanical proposals. While most marine people think o f LNG tankers as being large vessels, there are still openings at the opposite end o f the scale, where last year the 1100m3 mini LNG tanker Pioneer Knutsen was completed by the Bijlsma Shipyard in The Netherlands. Although twin electric motors are used to drive the propellers here, the layout is a mechanical Z-drive from the Schottel stable. One o f the principal benefits to naval architects o f any electric-power concept is the huge flexibility that is offered in positioning machinery, coupled with quite substantial savings in installed power, plus the possibility o f releasing extra space for cargo, or cabins on a passenger ship. Marine Electrical Technology

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Chapter 17 The Finnish consultancy Deltamarin believes that as much as 6.5% extra cargo space can be realized for a typical product or chemical tanker by adopting a vertical-position concept. At the same time, such ideas have given impetus to the two competing manufacturers o f gas turbines, GE and Rolls-Royce, since the light weight and high power density o f gas turbines make their installation high up in the hull, or even the superstructure, a distinct possibility - as one or two cruise liner companies have already realised. For example, Queen Mary 2 has her two 25,000kW GE/Brush turbo-alternators mounted in the base o f her funnel. Still to come are the highly interesting prospects for superconducting electric motors Japanese work w as first reported in this journal in November 1991 (page E509). Principal benefits include smaller size and volume and reduced weight, plus greater efficiency, especially at part loads. In the USA, the American Superconductor Corp is currently working on development o f a 5MW high temperature motor, in association with Alstom, for the US Navy; this will be followed by a 36.5MW version. The company anticipates that the weight o f its larger motor could be 69 tonnes, compared with more than 200 tonnes for an advanced induction design, and notes that such a design would also be suitable for large cruise liners and cargo ships.

The 36.5MW Superconducting Motor Unquote

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E lectrica l B ooster D rives

With regard to its functions the booster drive can be considered as an additional yet power-suitable and speed-independent “further cylinder o f the main diesel motor”. This “additional cylinder” converts the electrical energy generated in the ship’s network into propulsion power like a “step-less automatic gearbox”, which applies directly to the propeller. In this way the diesel-electric booster drive can support the diesel motor especially in critical operational conditions, e.g., when starting, in the acceleration phase (middle speed range) as well as with maximum output power. The possibility to provide the booster drive as an emergency and / or take-home drive also exists, and can be utilized for manoeuvring in the port or for a very low speed operation. In a technical sense, the diesel-electric booster drive does not differ significantly from a standard diesel-electric propulsion system. The system can be described as depicted in Figures 17.18 (a) to (d).

Figure 17.18(a) - Shaft Motor Installed in the Shaft Line

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m

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F ig u re 17.180b) - Shaft M otor Installed a t th e N on-drive E n d of a M ain Engine

F igure 17.18(c) - High-speed S haft M otor C onnected via a G earbox a t the N on-drive E nd o f a M ain Engine

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Figure 17.18(d) - High-speed Shaft Motor Connected via a Gearbox Integrated in the Shaft Line 17.13.8,1

A dvan tages o f B ooster D rives

>

Due to the disadvantageous torque characteristics o f the diesel motor, that can only deliver a part o f its basic torque in the lower to middle speed range, long acceleration times result with large two stroke engines, before the ship has reached the required end speed.

>

The diesel motors in this operational condition are hereby often loaded right up to the technically justifiable thermal limit. The efficiency o f the diesel motor in this operational area is likewise unconvincing.

>

The new booster drives have shown that they can continually compensate the “torque weakness” o f the diesel motor. This is explained through the capability o f the electrical booster drive, to deliver its rated torque in the whole speed range from about zero r.p.m. up to its rated speed.

>

Diesel motors and booster drives can thus dispose o f a higher torque for propulsion together, than the diesel motor could on its own. Through a lower thermal load o f the diesel motor a long-term lower abrasion will follow. In summary it is evident that the ship clearly reaches its end speed earlier under improved, economical conditions. Marine Electrical Technology

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Chapter 17 >

With excellent efficiency in the complete performance area, higher power density per unit volume and outstanding control dynamic; the diesel electrical booster drive is an intelligent technical supplement for the diesel motor.

>

The standardization o f the booster drive in the power range from 2 MW to 15 MW has led to a large flexibility o f the system with short deliveiy times.

117.14 Thruster Propulsion Systems This article is an extractfrom a publication titled "Controllable Pitch Main Propellers " by Kamewa. After the article was written Kamewa has been a part of Rolls-Royce. The different products in the new product range are named Kamewa Ulstein (CPP and Tunnel Thrusters), Kamewa (FPP and Waterjets) and Ulstein Aquamaster (Azimuth Thrusters). The Rotatable or Azimuthing Thruster has been used for some decades for propulsion of small vessels o f low power like river barges, ferries etc. In the latest decade quite powerful thrusters have been used in the offshore industry for propulsion and positioning o f special vessels like semi-subs, drill ships, etc. Typically these thrusters are rated at 2-3 MW/unit for continuous operation and built to satisfy the requirements o f various classification societies and authorities. Available bevel gear technology allows thrusters o f about 6000 kW to be built. To introduce thruster propulsion in a bulk/container vessel design is more o f a conceptual novelty than a technical one. In a Rotatable Thruster the following functions are generally included: y

A propeller system including a Controllable Pitch Propeller (Fixed Pitch optional)

S

A power train including necessary bearings, shaft seals and shaft speed reduction gear(s)

S

A step-less thrust magnitude / direction control through the CP-system

S

A rudder /steering gear function through the azimuth control.

^

A transverse thrust control by the option o f selecting a transverse setting (90°) o f the thrust direction.

y

An optimized astern thrust by the option o f turning the unit 180° (depending on the stem design)

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A n integral remote control for required horizontal ship movements. Combined control o f propeller pitch and engine RPM and / or constant RPM operation are available.

S

Automatic engine load / overload control is an added standard feature whose function is based on true engine load sensing and pitch correction. Interface for an autopilot is easily arranged.

17.14.1

H ydrodynam ic Perform ance

A new propulsion system should have approximately the same efficiency as the conventional one. In the case o f thruster propulsion the position o f the unit in the stem is normally further aft when compared with a conventional propeller. In general this allows for the adoption o f a relatively larger propeller diameter. Furthermore, the introduction o f a High Skevj propeller blade design allows for smaller tip clearance and larger propeller diameter. The above increase in diameter will, in combination with an optimized shaft speed, increase the efficiency. Two alternatives with regard to the installation concept are available: 1) Controllable Pitch (CP-type) thruster in combination with skeg 2) Traction thruster (CP - or Fixed Pitch - type) 17.14.2

M achinery Arrangem ents

The thruster propulsion offers a great flexibility regarding the propulsion machinery: 1)

The propeller unit includes one built-in bevel gear. A n upper bevel gear may be added. The gear ratio o f these gears can be selected to give a total speed reduction from the engine to the propeller ranging from 3:1 to 18:1 (depending on the torque situation); because o f this it is possible to select an optimum propeller shaft speed in combination with any practical r.p.m. o f the power plant (diesel, electric motor etc.)

2)

By choosing suitable length o f the couplings / intermediate shafts between the engineupper gearbox propeller unit, it is possible to locate the power plant at the most suitable position, both vertically and longitudinally.

3)

The upper gear can be arranged for single, twin and triple input. When a CP-type thruster is installed, one can make full use o f constant speed driven auxiliaries connected to the engine or gearbox power take-off shaft(s). Marine Electrical Technology

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Chapter 17 4)

Diesel engines are natural for most ships. If, however, the vessel has a high demand on electric power capacity (dredgers, crane vessels, maintenance / service vessels, flotels (floating hotels), etc., diesel-electric drives may be an attractive solution.

5)

The simplest inexpensive thruster drive is arranged through simple non-synchronous a.c. motors with relatively high shaft speed in combination with a CP-type thruster. If space allows, the motors can be direct-coupled to the thruster input shaft. The thruster propulsion system is m ost suitable for remote control and remote monitoring

(i.e., unmanned engine room operation) as the number o f control areas are minimised when compared with the conventional ship’s propulsion / manoeuvring system. 17.14.3

S hipbuilding Aspects

The most striking advantages with the propulsion thruster installation regarding hull design, production and hull disposition are: 1. The engine room length can be remarkably shortened. For a given overall ship’s length this means that the length o f the payload section can be increased and in turn increased bale capacity, normally in the order o f 5 -10%. In m ost cases also the dead-weight capacity increase is o f the same magnitude when compared with a ship o f conventional design (Refer Figures 17.1,17.2 and 17.3). 2. A simple stem, for instance the so-called barge stem, can easily be adopted. This means not only reduced production costs but also the possibility o f adopting a broader stem, displacement gain and improved stability. 3.

The number o f individual seatings and bedplates is diminished with a reduced number o f main systems. The installation work for the whole propulsion / manoeuvring system is reduced for the same reason and so is the time for testing and tuning.

4.

The thruster unit with an upper gearbox can be mounted in a “container” (circular or rectangular) which is flanged to a corresponding well in the stem. By this arrangement the thruster can be installed at a late stage o f the building program, after the hull has been launched. In summary, the thruster-equipped vessel needs less engineering and less qualified

shipbuilding techniques than the conventional ship. Production o f such vessels can therefore take place at yards with simpler facilities.

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Electrical Propulsion Systems If builders o f main engines and thrusters arrange for package delivery including engineering (possibly also containing some major direct-driven auxiliaries) this would mean an additional simplification for the building yard. 17.14.4

T hruster M achinery and P ropeller Concepts

Basically two options for the machinery concept are available: diesel-mechanical or diesel-electric drive. Several factors and conditions influence the choice o f a concept. Optimal overall operational economy is o f course the design goal for any system. The dieselmechanical machinery is a natural and cost efficient system for many applications. However, depending mainly on the operation load profile and the demand for auxiliary electric power, the diesel-electric drive may also be an attractive solution. As discussed above, installation aspects and flexibility with regard to hull disposition may also be advantages o f the electric machinery. The penalty o f the diesel-electric system is the higher cost and higher transmission losses. A multi-diesel generator-set power plant with its power management system provides for optimal operating conditions for the engines. This allows for operation with a higher efficiency close to the design conditions for diesels and generators. This in turn is advantageous with regard to wear and maintenance. One further advantage, o f growing importance, is that the exhaust emissions can be minimized. The rotatable thruster can be o f the FPP type or the CPP type. Depending on the type o f machinery, the main alternatives for the combination o f machinery and propeller-type are as shown in Figure 17.19. 17.14.5

D iesel - M echanical D rives

For a diesel-mechanical drive, CPP and FPP are possible. In the case o f a fixed pitch propeller, power control is accomplished by varying the engine shaft speed. Thrust reversal is obtained by engine reversal or by turning the thruster 180°. I f the thruster operates behind a skeg, turning is not possible and thus engine reversal is necessary. In the case o f CPP, thruster power and thrust control is achieved by pitch control. This facilitates rapid and smooth thrust reversing without engine reversal or turning o f the thruster. This means improved manoeuvrability and reduced stopping / acceleration time and distance. A CPP also allows for constant shaft speed operation and the use o f constant-speed-driven auxiliaries connected to the engine or gearbox powers take-off. I f reversal o f the propeller rotation is avoided, installation o f high skew blades is possible, which will provide for low-pressure pulses and hull vibrations. Marine Electrical Technology

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Figure 17.19 -Thruster Machinery Concepts 674

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D iesel-E lectric D rives

The simplest and least expensive system is arranged through constant speed nonsynchronous a.c. motors with a relatively high shaft speed in combination with CPP-type thrusters. Depending on the operation load profile, two-speed motors could also be an interesting alternative. If operating for long periods at low power this may give the best economy. For the FPP thruster, variable speed motors are required for power control. Different types o f electric systems and concepts for shaft speed control are available. However, growing interest is being shown for a system based on frequency conversion and a.c. motors. The main advantage o f the variable speed system is improved fuel economy at fractional loads. However, the investment cost o f the electric equipment is high compared with the extra cost o f the CP propeller. The rather complex electronic equipment requiring expert knowledge and further training o f personnel should also be considered. Hydraulic motors from Hagglunds are used for the steering o f luxury cruisers. The diesel or turbine-electric drive system has an approximately 15 to 25,000 HP electrical motor built into the actual propeller housing. The steering works in such a way that the entire propeller housing can be turned right round, i.e. through 360 degrees, driven by hydraulic motors from Hagglunds. This means that the conventional rudder has been completely replaced. To summarise, each specific ship requires an extensive evaluation o f the above discussed and other conditions in order to arrive at the optimal machinery-propeller concept. 17.14 .7

R elia b ility S ervice an d M aintenance

As mentioned above, the rotatable thruster concept minimises the number o f systems and facilitates integration o f functions. This makes the thruster propulsion machinery most suitable for remote control and remote monitoring (i.e., unmanned engine room operation). It also seems logical to assume that the reduced number o f components and separate systems will improve and augment system safety. I f more than one thruster is installed, system redundancy is obtained as independent operation o f the thrusters is normally provided for. Integration o f functions and a reduced number o f systems will also simplify and reduce the amount and cost for service and maintenance work. Marine Electrical Technology

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Chapter 17 The work to be carried out on board by the crew o f the vessel is reduced which makes reduced manning possible. In case o f complicated service procedures, specialists from suppliers or other land-based personnel could be utilised. W hen including containerisation o f the thrusters, and in the case o f lightweight power units, the maintenance work can be further rationalised. In that case the lifting ashore o f engines and thruster units can be arranged in order to carry out major maintenance work. In the case o f multi-thruster/engine installation it m ay be economical to invest in spare units for quick exchange. Lifting can be facilitated by on board or land-based lifting gears. I f containerisation o f the thruster is adapted, it is possible to inspect / maintain the outboard parts without dry-docking the ship. A need for dry-docking will only appear when the ship’s bottom needs reconditioning. Since the need for dry-docking is considerably reduced, the vessel could safely operate for prolonged periods in areas with a minimum of yard / docking facilities. 17.1 4 .8

Thrusters f o r B o o ster P ropu lsion o f E x istin g Ships

Existing ships may for several reasons need additional propulsion power and / or improved manoeuvrability. In the first case exchange o f the existing engine is one solution which, however, will often require a new gearbox (where applicable), shaft line with bearings and a propeller. An alternative solution is the installation o f one or two thrusters with propellers specially laid out for propulsion. As the original installation normally includes sufficient means for steering in the free running condition, the additional thruster can be arranged for a fixed mounting, i.e. without steering gear. Figure 17.20 shows the principal arrangement o f additional thrusters for existing single and twin-screw vessels respectively. In both cases either a thruster mounted behind a skeg or a thruster o f the traction type with a guide-vane-shaped stay are solutions.

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Figure 17.20 - Existing Single Screw with Additional Booster Thrusters Note: There could also be two existing screws with one additional booster thruster Figure 17.21 shows the required propeller thrust versus ship speed and the delivered thrust versus ship speed o f existing propeller equipment and additional booster thrusters. From the diagram the speed gain obtained from additional booster thrusters can be determined. O f course the speed increase greatly depends on the resistance curve o f the ship in question. A typical 20% power increase could give a 12-14% thrust increase and a 5-10% increase o f the ship's speed. The choice o f the prime mover for the booster thruster will depend on the overall situation o f the main / engine auxiliary power for the vessel in question. The most straightforward way o f powering is the installation o f a separate high-speed diesel. With suitable clutch arrangements the same diesel can be utilised for driving an alternator in case electric power is in demand.

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Thrust

Figure 17.21 - Speed Gain from Additional Thrusters A diesel-electric drive by a simple a.c. motor is another possibility provided sufficient electric power is available. I f there is a demand for improved manoeuvrability during docking / undocking the thruster should include a steering gear enabling the direction control o f the thrusters in at least a transverse direction. Examples o f installation o f rotatable thruster are Danish single-screw ro-ro (roil-on-roll-off) ships in which two rotatable thrusters have been installed. The reason for this conversion was the demand for higher speed and improved manoeuvrability. 17.14.9 17.14.9.1

R otatable Thrusters M ain C on trol System Type TPC

The Control System Type TPC is a system assembled from modules, which can be combined in many ways, in order to obtain a flexible control system for an actual number o f installed thrusters, type o f prime movers, number o f control stations and an interface with other systems, etc. With the aid o f the lever on the control head, both the thrust and direction o f thrust (rotation) can be controlled by one hand. One control head can be used for a group o f thrusters for parallel action. 678

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□ o □

□ o □

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Chapter 17 Thrust control means that the load o f the prime mover is sensed and that the load (approximately equal to thrust) is controlled by pitch changes, load being proportional to the lever position. The control head is connected to the two controllers for thrust and rotation. The thrust controller thus compares the ordered thrust with actual load and pitch. The rotation controller compares the rotation order signal with the feedback rotation transmitter signal. Both controllers act on the proportional hydraulic valves for pitch and rotation respectively. The terminal unit forms the central part for all connections. The complete system is supplied with 24V d.c. via a power supply unit (Refer Figure 17.22). 17.14.9.2

Indication of Thrust /Pitch and Back-up Control

For back-up control and indication o f thrust (or pitch) and rotation, an independent system with separate valves and transmitters is used. The system is designed so that back-up control o f pitch and rotation is possible without any connections to the main control system. The back-up and indication unit is intended to be placed alongside with the control head but can also be used as a control station in the machinery control room (MCR). It contains indicators for thrust / pitch and rotation, control mode switch, back-up command switches, indicating signal lamp for control, indicating mode and alarm. Indication units without controls are available for control stations where back-up control is not required. The back-up system includes a connection box for the thruster room. This box contains all switch-over relays, power supply and valve interfaces for both main and back-up control systems (Refer Figure 17.23).

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Figure 17.23 - Indication o f T h ru st / Pitch and Backup C ontrol Marine Electrical Technology

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17.14.10.1

Additional Subsystems Polar Joystick Control System

The Polar Joystick System is the solution, when control o f several rotatable thrusters with one lever is required. With the joystick lever - similar to the lever for a single thruster - a number o f thrusters can be manoeuvred with one hand. The joystick lever is located in conjunction with a Heading Control unit, which is connected to the gyro compass and by which an automatic heading control can be achieved both in positioning and transit control modes. 17.14.10.2

Manoeuvre Responsibility System

This is a common system for one or more rotatable thrusters and provides selection between control stations and also to be used if the Polar Joystick System or a Dynamic Positioning Control System is connected to the thruster controls. 17.14.103

Thrust Reduction Unit

This is an optional unit which monitors the generator plant (up to 8 generators) to prevent overload and a possible blackout. The generator load sensing transmitters consist o f current transformers. The output from the thrust reduction unit is connected to all installed thrust controllers and proportionally reduces the ordered thrust command signals. 17.14.11

Features o f the Electronic Control Systemfor Rotatable Thrusters

★

Thruster and rotation can be controlled by one lever and the lever position gives clear information o f the thrust magnitude and direction.

★

The built-in delay and load control functions permit fast handling o f the command lever without risk o f over loading the prime mover.

★

Independent back-up control system for both pitch and rotation allows full manoeuvrability in case the main system is out o f order.

★

All modules o f the system have front space dimensions o f only 144 x 144 mm.

★

Thus each control station can be tailor-made to meet a wide variety o f requirements.

★

Each module has a cable connector and is therefore simple to install and replace.

★

The system is designed to accept a Polar Joystick System, an Automatic Anchor Assist System and can easily be adopted for connection o f a Dynamic Positioning System.

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Description of the Main Control System

17.14.12.1

Transnutter Alarm Circuit

In order to obtain reliable supervision o f the input-output signals connected to the control amplifiers, all important signals are o f the usual proportional current type - (4 to 20 mA). Only two conductors are used for each transmitter. The transmitters are therefore equipped with an electronic circuit, which converts the measured value into a proportional current consumption. The controllers are provided with an alarm detector for each transmitter signal. This detector senses whether the transmitter signal is within correct levels and in case o f failure, activates an alarm to the common sensing alarm circuit, which also checks that all supply voltages are correct. Each alarm channel is equipped with light indicators on the alarm circuit board. 17.14.12.2

Thrust Controller

The command signal from the control head is converted into a proportional +10V d.c. signal after it has passed the alarm detector. This command signal then proceeds via a delay circuit with an adjustable time from 0 to 100 seconds. For an electric motor drive, the delay function is normally not used. After this possible delay the command signal is connected to a multiplier, which operates only when a load reduction unit is installed and produces a decreasing signal. The command signal is then compared with the response signal in the control error comparator. The output signal from the comparator is now used for activation o f the proportional value, controlling the pitch hub servomotor. In order to avoid increasing the pitch above an allowed level, the control error signal is processed in a pitch-limiting circuit. The valve command signal is converted into a proportional value o f current and when it has passed the alarm detector, it is connected to the valve interface in the thruster room. As mentioned earlier, thrust control means that the load shall be proportional to the lever position. Therefore the response signal mainly consists o f the load value.

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W hen an electric motor drives the thruster, a current transformer is used as the load transmitter and when a diesel engine drives the thruster, the load-controlling circuits are more complicated. After passing the alarm circuits, the pitch and load signals are converted into proportional d.c. signals. In the load calculator block, the load signal is multiplied by the pitch-size signal, which is saturated at 25% pitch. Above this limit the load signal dominates as response. As the load signal does not change polarity for ahead-astem pitch, the response signal must be given correct polarity by sensing the pitch direction. The output from the load calculator block representing thrust is then connected to the comparator. As thrust is shown in the indication unit, the response signal is also connected to the thrust-pitch indication instrum ent By a switchover relay, pitch alone can be used as a response indication signal. Thus a pitch control mode is possible, i f desired. However, in this mode the load o f the prime mover is not controlled. After die comparator a detector block is connected for sensing the control error. An error signal above a determined level gives a signal to the thrust deviation signal lamp in the indication unit (during manoeuvring, this lamp is therefore illuminated briefly when pitch changes). 17.14.12.3

Rotation Controller

The rotation controller utilizes a system with sine-cosine voltages and is used for representing the command and response signals. For utilizing only two conductors for the sine-cosine transmitter signals, a multiplexing system is used, where the current pulse’s magnitude and polarity represent the two signals. W hen turning the lever head, the rotation transmitter in the lever unit feeds the command signals, which are first sensed by the alarm circuits and then go via the multiplexer de-multiplexer blocks that are connected to the rotation error calculator inputs. The error calculator then compares the sine-cosine signals with the response signals, which are received from the rotation feedback transmitter in a similar way to the command one. The calculation o f the control error is made in such a way that the thruster always rotates in a direction to achieve the ordered position in the shortest possible time. 684

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The error signal is now used for control o f the proportional valves, which regulate the oil flow to the two hydraulic turning servomotors. As supervision o f the valve command signal is also desirable, converter and alarm circuits are used in this signal path. The interface circuits for the control valve are normally placed in the thruster room. In order to obtain good control stability, the hydraulic valves are equipped with an inner loop feedback transmitter connected to the valve interface circuit. A signal to the rotation control deviation signal lamp in the back-up indicating unit is available in the circuit, sensing the control error in a similar manner to that in the thrust controller. 17.14.13 17.14.13.1

Indication of Thrust/Pitch - Rotation and Back-up Control Indication of Thrust/Pitch

The indicator is a moving coil instrument concentrically placed in the rotation indicator. Either thrust or pitch can be indicated and two signal lamps above the indicator depict which one is being shown. 17.14.13.2

Thrust Indication Mode

As the signal to the instrument is taken from the thrust response calculator in the main control system, the thrust indication mode can only be used when the main control system works in this mode. 17.14.13.3

Pitch Indication Mode

Pitch indicating is used when the main control system is working in the pitch mode. When changeover to back-up control is made, a separate pitch transmitter integral with the back-up system is connected, which also indicates pitch. 17.14.13.4 Indication ofRotation The pointer for the rotation indicator is driven by a servo system with an electric servomotor and a sine-cosine feedback transmitter. The input signal to the servo amplifier is taken from the separate rotation transmitter for the back-up control system (or alternatively from the main control sine-cosine response signals).

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17.14.13.5 Back-up Control o f Pitch-Rotation The back-up indicating unit is equipped with a three position switch. They are: 1. “Control system” - The ordinary thrust-rotation control system is connected. 2. “Stand by” - The ordinary control system is connected but in case o f a failure in this system, the back-up control system is automatically connected. 3. “Back-up” - The back-up system is connected. The toggle switch and the rotation by turning the knob above this switch can manoeuvre the pitch. Signal lamps indicate which control mode is connected. Both the pitch and the rotation change as long as the corresponding switch is manipulated. In the back-up control mode the indicators are necessary to check the actual position (i.e. it is not a follow-up system). When the back-up control system is used, the ordinary proportional control valves are disconnected; instead an on-off solenoid valve is connected in each hydraulic system when the changeover valves are activated. Each solenoid valve is controlled from its corresponding switch via the valve interface in the comiection box in the thruster room. This box also contains the necessary power supply unit for the back-up control system. 17.14.14

Tunnel Thruster

The stem o f a vessel is controlled by means o f a propeller and rudder, but the bow is left to itself. In order to overcome this, a tunnel thruster (also known as a lateral, bow or side thruster) may be installed in a transverse tunnel in the bow (Refer Figures 17.24(a) and (b)). The need for such a system has already been explained in article 17.1.3 (A Manoeuvring System). Thus a tunnel-type side thruster can be defined as a system in which propellers are installed inside a tunnel that passes laterally through the hull o f the ship both fore and aft. By taking water in from one side and discharging it out at the other side, the ship can be turned or moved sideways. Such thrusters are used on a wide variety o f ships including freighters, cargo barges, passenger liners, ferries, fishing boats, and work barges. The thruster may have fixed blades o f a helical shape but with equal pressure and suction sides, with no section camber (arch i cxirve). I f it is o f the controllable pitch type, the blades are made “plane” with zero design pitch. As the blades have to operate equally well in either direction, they must be symmetrical. 686

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F igure 17.24(a) - T unnel (Bow) T hrusters

Image Courtesy: Aqua Manoeuvra Systems, Dubai - mvw.ams-thrusters.com Figure 17.24(b) - Sectional Views of the Prototype o f a M odern Tunnel T h ru ster Using a Synchronous M otor w ith a P erm anent M agnet R otor Marine Electrical Technology

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17,14.15

Side Thruster on a Liquefied Natural Gas Vessel

The side thruster, another name for die tunnel thruster mentioned in die previous article, is also a transverse propelling device with its propeller mounted in the lateral through-tunnel in the hull such that the water je t generated by this propeller gives a lateral thrust to the hull. This facilitates the departure o f the ship from and its coming alongside the pier (Refer Figure 17.24(c). It also helps to improve the ship’s manoeuvrability when it is running at a low speed or in a narrow waterway. From experience it has been found to be effective when the ship is propelling at speeds less than 4 knots; this effect is negligible at speeds o f about 7 knots. This could be due to the forward shift o f the pivot point (of the vessel) as speed increases and also due to the increased flow rate o f water over the opening on both sides o f the tunnel. The side thruster can be a controllable pitch thruster having incorporated in it a propeller pitch controlling mechanism, so planned that the propeller pitch can be remotely controlled from die control stand on the bridge. This device is composed o f the actuating section comprising a drive motor. Some features o f the flexible coupling, thruster and the propeller pitch control device are as follows: (1) Adoption o f 4-bladed skewed controllable pitch propeller which is effective for reducing vibration. (2) Since the propeller pitch is controllable, this allows the use o f a constant speed unidirectional motor. (3) The possibility o f controlling propeller pitch also enables continuous and quick change o f the thrust in either port or starboard directions. (5) Easy operation for all operation controls from that for starting the motor to that for regulation o f the propeller pitch are collectively arranged in the control stand on the bridge. / 7.14.16

Other Thruster Applications

There is a wide spectrum o f vessels equipped with thrusters o f various types. The L-thruster is the most common and is used on almost every type o f displacement merchant vessel, the largest being a 265,000-dwt tanker equipped with a 1620-kW (2172 hp) unit in the bow and 1100-kW (1475 hp) unit in the stem. The highest L-thruster total power installed to date on a single vessel is on each o f the two drill ships, Discoverer 534 and Discoverer Seven Seas, w ith 13,400 kW (18,000 hp approximately). While the use o f L-thrusters on conventional vessels is quite well known, the application o f L-thrusters as well as Ro-thrusters on vessels operating on offshore oil fields is still quite new. 688

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Figure 17.24(c)- Side Thruster on a Liquefied Natural Gas Carrier Marine Electrical Technology

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Chapter 17 117.15

In teg rated Steering / Propulsion Systems

Such systems manoeuvre a vessel solely by changes to propulsion settings and do not use a rudder. Two examples are cycloidal propellers (a Voith system) and the Z-drive / Z-peller. Such systems provide a full 360° propulsion thrust output, which is especially advantageous on dredges, ferries, and towboats. 17.15.1

VoithSchneider Propulsion System

A Voith-Sehneider Propulsion Unit is a vertically mounted paddlewheel with blades that are controlled to feather at different angles during each rotation o f the u n it This is also commonly called the W ater Tractor (Refer Figure 17.25).

Figure 17.25 - V oith-Sehneider P ropulsion Unit 17.15.2

The Active Rudder

The active rudder is constructed with a small propeller built into the trailing edge. The propeller axis turns with the rudder, and the propeller itself can be reversed to produce a flow direction onto the rudder. An active rudder is used for accurate manoeuvring when the vessel is stopped or nearly stopped and also for propelling the vessel at very slow speeds. An active rudder propeller may be driven either by a reversible electric motor mounted in the rudder or by a mechanical shaft passing through the rudder stock (Refer Figure 17.26).

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Figure 17.26 - The Active Rudder 17.15.3

Dynamic Positioning Systems

Dynamic positioning (DP) is a rapidly maturing technology, having been bom o f necessity as a result o f the increasing demands o f the rapidly expanding oil and gas exploration industry in the 1960s and early 1970s. Even now, when there exists over 1,000 DP-capable vessels, the majority o f them are operationally related to the exploration or exploitation o f oil and gas reserves. The demands o f the offshore oil and gas industry have brought about a whole new set of requirements. Further to this, the more recent moves into deeper waters and harshenvironment locations, together with the requirement to consider more environmentalfriendly methods, have brought about the great development in the area o f Dynamic Positioning techniques and technology. The first vessel to fulfill the accepted definition o f DP was the “Eureka”, o f 1961, designed and engineered by Howard Shatto. This vessel was fitted with an analogue control system o f a very basic type, interfaced with a taut wire reference. Equipped with steerable thrusters fore and aft in addition to her main propulsion, this vessel was o f about 450 tons displacement and had a length o f 130 feet. Marine Electrical Technology

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Chapter 17 By the late 1970s, DP had become a well established technique. In 1980 the number o f DP capable vessels totalled about 65, while by 1985 the number had increased to about 150. And in 2002 it stood at over 1,000 and is still expanding. It is interesting to note the diversity o f vessel types and functions using DP, and the way that, during the past twenty years, this has encompassed many functions unrelated to the offshore oil and gas industries. A list o f some activities executed by DP vessels would include the following: ^

exploration drilling (core sampling)

& production drilling & div6r support & pipe and cable laying (rigid and flexible pipe) hydrographic survey ^

platform supply

w floating production (with or without storage) yfr heavy lift cargo transport oceanographical research manoeuvring conventional vessels 17.15.3.1

Basic Principle

Dynamic Positioning can be described as an integration o f a number o f shipboard systems to obtain the ability o f accurate manoeuvrability (Refer Figure 17.27). DP can be defined as: “A system which automatically controls a vessel’s position and heading exclusively by means of active thrust” The above definition includes not only remaining at a fixed location, but also precision manoeuvring, tracking and other specialist positioning abilities. The prime function o f a DP system is to allow a vessel to maintain position and heading. A variety o f further sub-functions may be available, such as track-follow, or weathervane modes, but the control o f position and heading is fundamental.

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Figure 17.27 - Elements of a Dynamic Positioning System Dynamic positioning is concerned with the automatic control o f surge, sway and yaw. Surge and sway, o f course, comprise the position o f the vessel, while yaw is defined by the vessel heading. Figure 1.2 depicts the degrees o f freedom o f a vessel at sea. Both o f these are controlled about desired or “set point” values that are fed into the system by the operator, i.e. position set point, and heading set point. Position and heading m ust be measured in order to obtain the error from the required value. Marine Electrical Technology

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Chapter 17 Position is measured by one or more o f a range o f position references, while heading information is provided from one or m ore gyrocompasses. The difference between the set point and the feedback is the error or offset, and the DP system operates to minimise these errors. The vessel must be able to control position and heading within acceptable limits in the face o f a variety o f external forces. I f these forces are measured directly, the control computers can apply immediate compensation. A good example o f this is compensation for wind forces, where a continuous measurement is available from wind sensors. Other examples include plough cable tension in a vessel laying a cable, and fire monitor forces in a vessel engaged in firefighting. In these cases, forces are generated which, if unknown, would disturb station keeping. Sensors connected to the cable tensioners, and the monitors allow direct feedback o f these “external” forces to the DP control system and allow compensation to be ordered from the thruster. In addition to maintaining station and heading, DP may be used to achieve automatic change o f position or heading, or both. The DP operator (DPO) may choose a new position using the control console’s facilities. The DPO may also choose the speed at which he wants the vessel to move. Similarly, the operator may input a new heading. The vessel will rotate to the new heading at the selected rate-of-tum, while maintaining station. Automatic changes o f position and heading are simultaneously possible. Some DP vessels, such as dredgers, pipe laying barges and cable laying vessels have a need to follow a pre-determined track. Others need to be able to “weather-vane” about a specified spot. This is the mode used by shuttle tankers loading from an offshore loading terminal. Other vessels follow a moving target, such as a submersible vehicle (ROV), or a seabed vehicle. In these cases the vessel's position reference is the vehicle rather than a designated fixed location. Every vessel is subjected to forces from wind, waves and tidal movements as well as forces generated from the propulsion system and other external elements (fire monitors, pipe- lay tension, etc). The response to these forces is vessel movement, resulting in changes o f position and heading. These are measured by the position reference systems and gyro compasses. The DP control system calculates the offsets between the measured values o f position and heading, and the required (or set point) values, and calculates the forces that the thrusters m ust generate in order to reduce the errors to zero. 694

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Electrical Propulsion Systems In addition the DP control system calculates the wind force acting upon the vessel, and the thrust required to counteract it based on the model o f the vessel held in the computer. Modelling and filtering enable a ‘dead reckoning’ or ‘D R’ mode (often called ‘mem ory’) to operate i f all position references are lost. The vessel will continue to maintain position automatically, although the position-keeping will deteriorate with the increasing length o f time since the last position data received. In practical terms, this means that the DPO does not need to immediately select “manual” control upon the loss o f all position reference. The difference between the thrust calculated from the model and the wind speed and direction is the force taken as the current. The current force or ‘sea force’ is therefore a summation o f all the unknown forces and errors in the DP model and displayed in the model as the speed and direction o f the current. 17.15.3.2

User Interface

The Vessel Control System holds the vessel in position by utilizing position reference sensors (such as GPS / DGPS receivers, fan beam laser radar or acoustic positioning). Combined with gyro heading, wind angle, wind speed, roll and pitch sensors, this information is processed by a computerized mathematical model o f the vessel which allocates proportional commands to thrusters, rudders and propellers, as required, to hold position. Sensor signals are continually monitored and adaptively processed to optimize accuracy. Apart from automatically holding position, there are additional operating modes which allow automatic holding in one axis with manual control in the other axis, changing position, joystick control in which thrusters are computer controlled to manually hold position, autopilot mode, autopilot with track following, interface to ECDIS (electronic chart plotting), manual steering control, manual thruster control, ROV follow, pipe laying, cable laying, dredging, etc. 17.15.3.3 17.15.3.3.1

Reference Systems Ultra- or Super-Short Baseline Acoustic System

The principle o f position measurement involves communication at hydro acoustic frequencies between a hull-mounted transducer and one or more seabed-located transponders. The ultra-short or super-short baseline (SSBL) principle means that the measurement of the solid angle at the transducer is over a very short baseline (i.e. the transducer head) (Refer Figure 17.28). Marine Electrical Technology

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Chapter 17 A n interrogating pulse is transmitted from the transducer. This pulse is received by the transponder on the seabed, which is triggered to reply. The transmitted reply is received at the transducer.

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Acoustic Ranging

| Transponder

Figure 17.28 - SSBL Principles The transmit/receive time delay is proportional to the slant and range. So range and direction are determined. The angles and range define the position o f the ship relative to that o f the transponder. The measured angles must be compensated for values o f roll and pitch. The vessel must deploy at least one battery-powered transponder. They can be deployed by a down line from the vessel, by an ROV or simply dropped overboard. The performance o f an acoustic system is often limited by acoustic conditions in the water. Noise from vessel thrusters and other sources, aeration and turbulence will all be detrimental to efficient acoustic positioning. Thus the limits o f the system are ill-defined. In addition, layering can cause errors, especially when the horizontal displacement from the vessel is large. 696

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Electrical Propulsion Systems Acoustic systems are supplied by a number o f manufacturers, notably Kongsberg Simrad, Sonardyne and Nautronix. All use frequencies in the 20-30 kHz band. Some transponders are compatible with more than one supplier’s equipment. 17.15.3.3.2

Taut W ire P o sitio n R eferen ce

A taut wire is a useful position reference, particularly when the vessel may spend long periods in a static location and the water depth is limited. The most common type consists o f a crane assembly on deck, usually mounted at the side o f the vessel and a depressor weight on a wire lowered by a constant-tension winch. A t the end o f the crane, boom angle sensors detect the angle o f the wire. The weight is lowered to the seabed and the winch is switched to constant tension, or ‘mooring’ mode. From then on, the winch operates to maintain a constant tension on the wire and hence to detect die movements o f the vessel. The length o f wire deployed, together with the angle o f the wire, defines the position o f the sensor head with reference to the depressor weight once the vertical distance from the sheave o f the crane boom to the seabed is known. This is measured on deployment (Refer Figure 17.29). Gimbal Head Sensors

Depressor Weight

Figure 17.29 - Taut Wire Principle These angles are corrected at the taut wire or by the DP control system for vessel inclinations (roll and pitch angles and motion). Vertical taut wire systems have limitations on wire angle because o f the increasing risk o f dragging the weight as angles increase. A typical maximum wire angle is 20°, at which point the DP system will initiate a warning. Marine Electrical Technology

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Chapter 17 Some vessels also have horizontal or surface taut wires that can be used when close to a fixed structure or vessel from which a position must be maintained. The principle o f operation is the same, but a secure fixing point is required rather than a weight. 17.15.3.3.3

L on g B aselin e System

In deepwater locations, where the accuracy o f the other types are questionable, the long baseline (LBL) becomes m ore appropriate. LBL systems are in extensive use in drilling operations in deep water areas (> 1,000m) (Refer Figure 17.30).

Figure 17JO - The LBL System The long baseline system uses an array o f three or more transponders laid on the seabed in the vicinity o f the worksite. Typically the array will form a pentagon (5 transponders) on the seabed, with the drill ship at the centre above. One transducer upon the vessel interrogates the transponder array, but instead o f measuring range and angular information, ranges only are measured, because the baseline distances have already been calibrated (distances between transponders). Position reference is obtained from range-range geometry from the transponder locations. 698

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Electrical Propulsion Systems Calibration is done by allowing each transponder to interrogate all the others in the array, in turn. If, at the same time, the vessel has a DGPS or other geographically-referenced system, then the transponder array m ay also be geographically calibrated. Accuracy is o f the order o f a few metres, but the update rate can be slow in deep water because the speed o f sound in sea water is about 1,500 m/sec.

17.16

Water Jet Propulsion

In water jet propulsion systems, the propeller is exchanged for a pump in a tube inside the vessel. The water is drawn in through an opening in the hull bottom, led through a guide wheel and discharged through a nozzle at the transom. The discharge nozzle is pivoted so that the je t can be diverted at an angle o f + 30° for steering, and no rudder is needed. Reversing is obtained by means o f a reversing bucket or flaps directing the je t forward - downward. Water je t propulsion is suitable mainly for small, fast craft. There is no rudder, no external shaft or shaft brackets causing resistance or being susceptible to damage in the event o f grounding, etc. The impeller vanes are fixed, but unlike fixed pitch propellers, the water je t can supply full power and thrust right from zero speed. The disadvantages are friction losses due to the wetted area in the tube, the weight o f the water carried in the tube, and low efficiency when applied to slow vessels. The Kamewa water je t product range includes: 17,16.1 K am ew a S -series

A major part o f the worlds high speed ferries are equipped with Kamewa S-series water jets. More than 1400 units have been delivered including single units rated up to 22 000 KW and complete installations o f 70 800 KW. Projects with single units rated at 50.000 KW and complete installations o f 250 000 KW are under progress. All Kamewa S-series water jets flow from the same basic design (Refer Figure 17.31).

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Figure 17.31 - The Kamewa W ater Jet - S Series 17.16.2

K am ew a A -series

Kamewa A-series represent an aluminium water je t available with mixed-flow pumps, an innovation that has improved the efficiency with 5-10% compared to competitive aluminium jets. The Kamewa A-series jets are available in sizes ranging from 40 to 56, with power ranging from 500 to 2800 KW/unit and have rapidly proven to be the propulsion solution for all types o f high speed vessels. A m ajority o f the world’s fast lightweight ferries are equipped with Kamewa water jets, which are propulsion devices using water-pump technology instead o f propellers to provide thrust. The successful application o f water jets to large vessels is a relatively new but highly successful phenomenon. Projects are in progress to develop single units rated at 50MW and complete installations o f 250MW. The new A-series o f aluminium water jets that has recently been introduced spans mid-range ratings o f 500kW to 3MW.

Figure 17.32 - The Kamewa Water Jet - FF Series 700

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Electrical Propulsion Systems 17.16.3

The SC H O TTE L P um p-Jet

The following text is a brief extract from the technical specification Rev. 4 of the SCHOTTEL Pump-Jet Type SPJ 82 RD. 17.16.3.1 P rin cip le o f O peration

The SCHOTTEL Pump-Jet is a propulsion system made by the Schottel Company o f Germany, a world leader in the manufacture o f marine propulsion equipment.

Figure 17.33 - The SCHOTTEL Pump-Jet The SCHOTTEL Pump-Jet as shown above in Figure 17.33 is a water-jet propulsion system similar to a rotary pump and totally integrated into the ship’s hull. Utilizing the principle o f a centrifuge pump, water is drawn in from beneath the boat, pressurized into a pressure casing where it is accelerated and expelled through three symmetrically arranged nozzles. The casing can be rotated about its vertical axis by 360° endlessly left and right, and then discharged as a water je t in any desired direction to create propulsive force. Thus, full thrust can be generated in each direction controlled by the steering gear. Marine Electrical Technology

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Chapter 17 The hydraulic steering motor is fitted to the steering gear. Besides providing excellent manoeuvrability, maximum propulsive force can be obtained even in very shallow water. As no ballast time is required, this system is widely used as a thrusting system on freighters, cargo barges and passenger liners, and as the main propulsion system on fishing boats and work barges. The power from the horizontally mounted motor is transmitted to the horizontal input shaft o f the Pump-Jet and via spiral bevel gears to the impeller. The spiral bevel gears o f the SPJ gearbox are made o f high quality case-hardened steel. The shafts are made o f high grade steel and run in roller bearings. The shaft seals are made from seawater resistant material. Radial sealing rings are running on coated surfaces. A mounting flange is used for the installation o f the Pump Jet, into the well. The Pump Jet has to be installed from underneath, into the vessel. 17.16.3.2

S elf-clean in g o f P um p-Jet

The SPJ is self cleaning by changing the rotation sense o f the impeller and maximum speed o f the propulsion motor should not be more than 50% o f advance r.p.m. Flushing mode for selfcleaning should not be longer than 1-1.5 minutes. 17.16.3.3

S teerin g

The thrust can be directed to any desired direction through the SCHOTTEL-Pump-Jet, around its vertical axis. The SCHOTTEL units in Figures 17.34 and 17.35 are equipped with the following steering system: SCHOTTEL steering system SST 602, Copilot 2000 proportional; a way-dependent electro-hydraulic steering system. The steering pump is electrically driven and must be connected to electric power supply 440 V / 60 Hz. The ingress protection standard o f the electric motor m ust be at least IP 54. Power consumption is about 5 kW. The desired position o f the SCHOTTEL-Pump-Jet is pre­ selected by means o f the control wheel o f the co-pilot control unit. This activates the hydraulic motor transmitting the steering torque to the SCHOTTEL-Pump-Jet via an electronic control unit. The proportional steering system works in such a way that with a small angle o f steering, the corresponding steering speed is low and with a larger angle o f steering, the steering speed is higher. In case o f failure o f the electronic steering system, the steering is automatically switched over to a time-dependent emergency steering system. This failure is indicated by a visual and audible alarm. 702

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Electrical Propulsion Systems The SCHOTTEL Pump-Jet position is indicated via an electric feed-back system by a SPJ thrust direction indicator.

(b) Speed Astern

(d) Turning to Starboard

Figure 17.34 - The Double Stern Unit

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(a) Speed Ahead (c) Turning to Port (e) Traverse (b) Speed Astern (d) Turning to Starboard

Figure 17.35 - The Double Bow Unit 117-17

Relevant Rules

17.17.1

R eleva n t S O L A S R egulations

Part C - Machinery Installations - Regulation 31 - Machinery Controls Part E - Additional Requirements - Regulation 49 - Control o f Propulsion Machinery from the Navigation Bridge 17.17.2

1)

704

Sum m ary o f R egulations

M ain and auxiliary machinery essential for the propulsion, control and safety o f the ship shall be provided with effective means for its operation and control. All control systems essential for the propulsion, control and safety o f the ship shall be independent or designed such that the failure o f one system does not degrade the performance o f another system. Marine Electrical Technology

Electrical Propulsion Systems 2)

Where remote control o f propulsion machinery from the navigation bridge is provided, the following shall apply: a) The speed, direction o f thrust and, i f applicable, the pitch o f the propeller shall be fully controllable from the navigation bridge under all sailing conditions, including manoeuvring; b) The control shall be performed by a single control device for each independent propeller, with automatic performance o f all associated services, including, where necessary, means o f preventing overload o f the propulsion machinery. Where multiple propellers are designed to operate simultaneously, they may be controlled by one control device; c) The main propulsion machinery shall be provided with an emergency stopping device on the navigation bridge which shall be independent o f the navigation bridge control system; d) Propulsion machinery orders from the navigation bridge shall be indicated in the main machinery control room and at the manoeuvring platform; e) Remote control o f the propulsion machinery shall be possible only from one location at a time; at such locations interconnected control positions are permitted. At each location there shall be an indicator showing which location is in control of the propulsion machinery. The transfer o f control between the navigation bridge and the machinery spaces shall be possible only in the main machinery space or the main machinery control room. This system shall include means to prevent the propelling thrust from altering significantly when transferring control from one location to another; f)

It shall be possible to control the propulsion machinery locally, even in the case o f failure in any part o f the remote control system; it shall also be possible to control the auxiliary machinery, essential for the propulsion and safety o f the ship, at or near the machinery console

g) The design o f the remote control system shall be such that in case o f its failure an alarm will be given. Unless the Administration considers it impracticable the preset speed and direction o f thrust o f the propellers shall be maintained until local control is in operation;

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Chapter 17 h) Indicators shall be fitted on the navigation bridge, the main machinery control room and at the manoeuvring platform, for propeller speed and direction o f rotation in the case o f fixed pitch propellers and propeller speed and pitch position in the case o f controllable pitch propellers i)

An alarm shall be provided on the navigation bridge and in the machinery space to indicate low starting air pressure which shall be set at a level to perm it further main engine starting operations. I f the remote control system o f the propulsion machinery is designed for automatic starting, the number o f automatic consecutive attempts which fail to produce a start shall be limited in order to safeguard sufficient starting air pressure for starting locally.

j)

Automation systems shall be designed in a manner which ensures that threshold warning o f impending or imminent slowdown or shutdown o f the propulsion system is given to the officer in-charge o f the navigational watch in time to assess navigational circumstances in an emergency. In particular, the systems shall control, monitor, report, alert and take safety action to slow down or stop propulsion while providing the officer in-charge o f the navigational watch an opportunity to manually intervene, except for those cases where manual intervention will result in total failure o f the engine and/or propulsion equipment within a short time, for example in the case o f overspeed.

3)

W here the main propulsion and associated machinery, including sources o f main electrical supply, are provided with various degrees o f automatic or remote control and are under continuous manual supervision from a control room the arrangements and controls shall be so designed, equipped and installed that the machinery operation will be as safe and effective as if it were under direct supervision for this purpose. Particular consideration shall be given to protect such spaces against fire and flooding.

4)

In general, automatic starting, operational and control systems shall include provisions for manually overriding the automatic controls. Failure o f any part o f such systems shall not prevent the use o f the manual over-ride.

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1)

A Cycloconverter is a ________ .

2)

The number o f thyristors required for Three phase to Three phase 3-pulse type cyclo converter a re ________ .

3)

Write short notes on APS and THS.

4)

How do modem electrical propulsion systems help to optimize cargo space? Explain this innovation with simple sketches.

5)

W hat is a synchronous motor? Where is it used?

6)

With the help o f suitable diagrams, justify the application o f synchronous motors in m odem propulsion systems.

7)

State the synchronous condenser operation for power factor correction.

8)

How do thyristors help to control speed in an electrical propulsion network?

9)

What is the significance o f a cycloconverter in the speed control circuit o f a propulsion motor? Justify its application with the help o f suitable waveforms and basic circuits.

10)

W ith a simple sketch, explain the application o f a Tunnel Thruster.

11)

With a neat diagram explain the Turbo-Electric Propulsion System.

12)

State the safety precautions associated with the operation o f a high voltage electric propulsion system.

13)

With a neat diagram explain how induction motors are used in a multi-thruster special purpose vessel?

14)

With a neat diagram, explain the method o f using a DC propulsion motor powered by an alternator.

15)

With a neat diagram, explain the synchronous-motor based Propulsion System.

16)

List the precautions when replacing MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) and IGBTs (Insulated Gate Bipolar Transistor) in electronic circuits.

17)

With the help o f a chart, explain the basic operating principles o f various Thruster Systems. Marine Electrical Technology

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Chapter 17 18)

Write a short note on the advantages o f Booster Drives.

19)

List the advantages and disadvantages o f electrical propulsion systems.

20)

How are asynchronous propulsion motors coupled with controllable pitch propellers more capable o f speed variations?

21)

Briefly explain the advanced diesel-electric propulsion concept.

22)

Why are harmonic filters needed?

23)

What is PW M ? Where is this used?

24)

Write short notes on the following: (a) Voith-Schneider propulsion (b) Active Rudder (c) DPS (d) Water Jets

25)

What is the significance o f an Ultra- or Super-Short Baseline Acoustic System?

26)

What do you understand by taut-wire position reference?

27)

What is a significance o f a long baseline system? W here is it used?

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Chapter Steering and Stabiliser Systems

18.1.1

S teerin g G ear

It is the machinery, rudder actuators, steering gear power units, if any, and ancillary equipment and the means o f applying torque to the rudder stock (e.g. tiller or quadrant) necessary for effecting movement o f the rudder for the purpose o f steering the ship under normal service conditions. Elementary steering gear first o f all comprises a steering wheel (or maybe a control knob) on the bridge, generally operated by the helmsman. The helm order or desired angle o f the rudder is then transmitted from here to the steering control unit. This results in the operation o f the rudder to which it is linked. A negative feedback signal o f this ordered (or desired) angle is transmitted automatically through ‘hunting gear’ to the steering gear’s control unit. This (the negative feedback) gradually nullifies the control signal to the steering gear and causes the rudder to stop when the desired angle has been achieved. In the case o f an electro-hydraulic system it is possible for the control unit to receive negative feedback signals through rotary transformers, potentiometers, etc. Now “helm angle” is the position o f the steering wheel relative to the midship position. The steering control dials are normally graduated in such a manner that the rudder moves in tandem with i t For example, if the rudder is designed to move ±35°, then the control wheel will also have a fixed dial and pointer arrangement graduated from 0° to +35°.

Marine Electrical Technology

Chapter 18 In some steering gear systems, be it a wheel, knob or handle, the steering control element does not maintain a one-to-one relationship with the rudder. In such an instance, an independent rudder indicator is deemed necessary and will be in the form o f the type mentioned in article 18.8. Instead o f just a fixed dial and pointer, a two-element synchro chain (i.e., a pair o f transmitting and receiving synchros) may be used to indicate to the operator the position that is desired o f the rudder. Article 18.9 explains the theory behind synchros. The indicator is also called a helm indicator. Classification rules now specify that an independent rudder angle indicator be fitted when the rudder is power operated. 18.1.2

Steering Gear Power Unit

1. In the case o f electro hydraulic steering gear, an electric motor and its associated electrical equipment and connected pump. 2.

In the case o f other hydraulic steering gear, a driving engine and connected pump.

3.

In the case o f electric steering gear, an electric motor and its associated electrical equipment.

18.1.3

Auxiliary Steering Gear

It is the equipment other than a part o f the main steering gear necessary to steer the ship in the event o f failure o f the main steering gear but not including the tiller, quadrant or components serving the same purpose. 18.1.4 Steering Gear Control A Steering gear control system is the equipment by which orders are transmitted from the navigation bridge to the steering gear power units and locally from the steering gear space. These m ay be any acceptable arrangement like manual operation o f hydraulic valves, electrical or electro-hydraulic systems - with the help o f an operating handle, wheel or joystick. Steering gear control systems comprise o f transmitters, receivers, electro-hydraulic converters, hydraulic control pumps and their associated motors, motor controllers, piping and cables. For the purpose o f the Rules, steering wheels, steering levers, and rudder angle feedback linkages are not considered to be part o f the control system. A steering console that is installed on the bridge or wheelhouse o f a ship is depicted in Figure 18.1. The control system may comprise o f the Auto-pilot / Follow-up type that has a console as shown in Figure 18.1 and/or the ‘non-follow-up’ type as depicted in Figure 18.2. Non-follow­ up control units are also depicted in Figures 18.14 and 18.15. 710

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Steering and Stabiliser Systems

Figure 18.1 - AutoNav’s Steering Console In the non-follow-up mode, as long as a wheel / lever is held , or pushbutton is pressed, to energise the steering gear so that the rudder moves in one direction - either to port or starboard, the steering gear functions. It de-energises automatically at its mechanical limit normally 30° to 35°. The rudder may also be moved a few degrees at a time, either to port or starboard, by deflecting the control (off 0°) for brief periods. The helmsman’s keen sense o f judgement is vital in order to ensure effective control o f the rudder. Many a time, novices steer the vessel in a zigzag manner thereby wasting time and precious resources on board. The second type o f controller causes the rudder to automatically align amidships the moment the helmsman releases the controller. Refer article 18.3.2.2 titled Dual Non-Followup (Dual NFU) later in this chapter. The third variant is the full follow-up system. It is also explained further on in this chapter. The modem version senses any existing error between the helm (the position o f the rudder controller) and the rudder’s true position with the help o f a comparator. The error which is in essence an algebraic stun o f the desired and true angles o f the rudder is amplified and fed to the rudder control unit. This moves the rudder to the desired angle either port or starboard. As already mentioned, the rudder slops only when the negative feedback signal cancels out the desired angle signalled by the helm. The rudder is held in position so long as the difference is equal to zero. The rudder will move once again when a difference arises b y moving the helm or due to the drifting o f the rudder on account o f hydrodynamic forces.

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Chapter 18 18.1.5

N on - fo llo w Up S teerin g (o r Tim e D epen den t S teerin g)

The following explanation supports the Non-follow-up Control Diagram depicted in Figure 18.2. As briefly mentioned earlier, we will understand that while using the ‘Non­ follow-up’ (NFU) system, the steering gear will function as long as the controller is held in an actuating position i.e., either to Port or Starboard, and will only stop when it moves back to an ‘O ff or the central position or until the steering gear has reached its mechanical limit. The control from the bridge is by means o f a NFU lever or sometimes a wheel that is spring loaded. Since the rudder movement depends on the duration that the control is held offcentre, this variant o f a steering control system is sometimes called ‘time dependent steering’.

Figure 18.2 - Non-follow up Control Diagram of a Rudder The NFU lever operates a switch that energises either a port or starboard solenoid, depending upon the direction o f movement required. These solenoids in turn operate a pilot valve that brings about the operation o f the main control valve. 712

Marine Electrical Technology

Steering and Stabiliser Systems As seen in Figure 18.2, the solenoid-operated pilot valve is a two-way-three-position one. It is designed to divert hydraulic pressure through direct or cross-connected ports. A fixed delivery pump serves to deliver hydraulic pressure to the steering gear. Depending upon the application o f pressure to a particular side or the ram, the desired direction o f rudder movement is thus achieved. When the NFU lever is released, springs return it to the central position. This causes the control valve to return to its neutral position; the main control valve in turn also returns to neutral thus bypassing the pump delivery. The steering gear stops for two reasons - first o f all because there is no hydraulic pressure and secondly because, as we will see in Figure 18.2, the hydraulic fluid is trapped on either side o f the ram due to the blind ports in the main control valve. The rudder indicator serves as a negative feed back device. It is capable o f only serving as a visual feedback device. Thus the onus is on the helms-man to control the movement o f the rudder. He is as an important link in the control chain and serves as a virtual hunting gear! 18.1.6 Remote Control Systems In short they can relate to mechanical, hydraulic, electrical, and electronic subsystems. Ancients systems even resorted to using shafts, wire rope, sprockets and chains, push-pull flexible control cables, and their combinations to transfer motion proportional to that o f the helm. This was connected from remote steering stations to the local control station o f the steering gear but in most cases proved troublesome. These mechanical means are simple, reliable and still used in smaller vessels. As will be mentioned later, the use o f an electro-hydraulic unit can be used instead. The fundamental type o f hydraulic control system is the hydraulic telemotor that has been in use for ages. It consists o f a telemotor unit located in the wheelhouse and an aft unit in the steering gear flat. Pipelines are used to connect the unite and cylinder. The pipelines o f the cylinder are attached to the steering gear local control unit. The hydraulic pressure causes the steering gear to move the rudder in the direction chosen by the helmsman. Similar systems can be found in Figures 18.3,18.5 and 18.6. 18.1.7 Electro-hydraulic Control Turning a wheel causes a potentiometer in a balanced bridge network, to unbalance the electrical circuit The error signal activates an intermediate powered servo that supplies the local control unit and moves a follow-up potentiometer linked to the rudder. This balances the circuit once again and cuts-off the servo, when the helm and relative servo angular positions neutralise each other.

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Chapter 18 The powered servo is in most cases a rotary electro-hydraulic unit that can be likened to a miniature steering gear. It is designed for application with any steering gear variant, and serves as one link in a chain o f servomechanisms from the steering wheel to the rudder. Several vessels achieve local (manual) control o f the steering gear with the help o f a mechanical differential control. This allows the helmsman to rotate the steering wheel at a desired rate. The helm signal is transmitted to the mechanical differential through the rotary electro-hydraulic unit and then to the steering gear pump’s control. The steering gear itself follows, moving the rudder at its pre-determined rate. A mechanical follow-up linkage feeds the rudder position to the mechanical differential. The reduction o f the pump stroke is with the help o f differential control, when it is within 5° o f the ordered angle. It is fully off stroke when the ordered angle is reached. Electro-hydraulic control has many variants. A twin-ram system is depicted in Figure 18.3. Hydraulic pressure can also be made available with the help o f a dedicated hydraulic system’s accumulators. This has many advantages, a few o f which are: to Compatibility with complex control systems to Less running hours for pumps to Increased reliability Easy and quick changeover to a standby hydraulic system to Local control from the unit itself in case o f an emergency This method is much simpler and can be one o f the m any modes o f operation in a complex auto pilot or electronically controlled steering system. The next sub-heading briefly explains an electronically controlled system where-in, though the use o f electronics is resorted to, hydraulic pressure is required to move the rudder.

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Auto Pilot i

.... A..... Full Follow-Up Controller

Auto Pilot

i 0

> © ◄-------- Full Follow-Up

i

----------

\

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► *

*

1

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Chapter 18 18.1.7.1

The F ou r R am Type o f S teerin g G ear

It consists o f four hydraulic cylinders supplied with oil by two electrically driven pumps. Rams operate the rudder tiller through a crosshead and Rapson Slide mechanism. The pumps are o f the variable displacement axial piston type. Each pump is located inside its own oil storage tank, from which it takes suction and is driven by an electric motor, mounted outside the tank, through a flexible coupling. The steering gear is capable o f operating as two totally independent and isolated steering systems. The second pump unit can be connected at any time by starting the motor. No. 1 pump has a hydraulic system which connects it with No.3 and No.4 hydraulic cylinders whilst No.2 pump is connected with No. 1 and No.2 cylinders. The steering gear is provided with an automatic isolation system which is actuated should there be a pump failure or oil loss from the working system; the automatic isolation system isolates the defective hydraulic system and makes the other system sound so that it can remain fully operational. Both hydraulic systems are interconnected by means o f electrically operated isolating valves that, in normal operation, allow both systems together to produce the torque necessary for moving the rudder. In the event o f failure that causes a loss o f hydraulic fluid from one o f the systems, the float switches in the expansion tank are actuated. This gives a signal to the isolation system, which automatically divides the steering gear into two individual systems. The defective system is isolated, whilst the intact system remains fully operational. This reduces the rudder torque to 50% o f the system’s rated torque and so the ship’s maximum speed should be reduced to less than half o f its maximum speed. The steering gear is remotely controlled by the auto pilot control or by hand steering from the wheelhouse. Emergency control is carried out by the operation o f the pushbuttons on the solenoid valves on the auto pilot units. All orders from the bridge to the steering compartment are transmitted electrically. Steering gear feedback transmitters supply the actual position signal for the systems. The rudder angle is limited to 35° port or starboard. The variable flow pumps are operated by a control lever, which activates the tilting lever o f the pump cylinder, which causes oil to be discharged to the hydraulic cylinders. W hen the tiller reaches the set angle, the tilting lever is restored to the neutral position, which causes the pump to cease discharging. N o.l pump unit is supplied with electrical power from the emergency switchboard and No.2 pump unit from die main switchboard. Under normal circumstances, all four cylinders will be in use, with one pump unit running and the second pump unit ready to start automatically. 716

Marine Electrical Technology

Steering and Stabiliser Systems 18.1.7.2

P rocedu re to P u t th e S teerin g G ear in to O peration

1. The system valves are assumed to be set for normal operation. 2. Check the level and condition o f the oil in the tanks and refill with the correct grade as required. 3.

Check that the control lever is correctly set for operation from the bridge and not locally from the steering flat.

4. Ensure the rudder is in the mid position. 5.

Start the selected electro-hydraulic pump unit.

6.

Carry out pre-departure tests.

7. Check for any abnormal noises. 8. Check for any leakages and rectify if necessary. 9. Check the operating pressures. 18.1 .8

A u tom atic Iso la tio n S ystem

This steering gear is so arranged that in the event o f a loss o f hydraulic fluid from one system, the loss can be detected and the defective system automatically isolated within 45 seconds. This allows the other actuating system to remain fully operational with 50% torque available. 18.1.8.1

C onstruction

This system consists o f the following equipment: (a) Two isolating valves (b) Two oil tank level switches with low and low-low level positions; one for each system tank (c) A n oil tank divided into two chambers for level switches and system test valves (d) An electrical control panel for automatic isolation system (e) A n alarm panel for the automatic isolation system

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Chapter 18 1 8 .1 .8 .2

O peration

I f failure o f one o f the systems occurs, the ship’s speed should be reduced, as only 50% o f the torque for the steering gear operation is available. 18.1.8.3

F ailu re S equence w ith O n e P um p R unning

I f any loss o f oil occurs with say, N o.l pump running and No.2 pump stopped, the following sequence will take place: 1. The oil level in N o.l oil tank goes down to the “Low” level; audible and visual alarms are activated on the navigating bridge and in the machinery space. 2. At the same time the N o.l automatic isolating valve, is energised and the hydraulic system associated with No.2 pump is isolated. 3. I f the oil loss is in the hydraulic system associated with No.2 power system, the steering process is continued by N o.l power system and with the No.2 system isolated, there will be no further oil loss. 4.

I f the oil loss from the system is associated with N o.l power system, the tank oil level will continue to fall and when it reaches the Low-Low position. N o.l automatic isolating valve will be de-activated and N o.l pump is automatically stopped.

5.

System No.2 automatic isolating valve is activated and No.2 pump is automatically started. The hydraulic system associated with N o.l pump is isolated and so no further oil loss will occur. Steering is now being carried out by No.2 pump and its two related cylinders (N o.l and No.2).

6. I f the oil loss occurs in No.2 tank, steering is continued to be carried out by No. 1 pump and its two related cylinders (No.3 and No.4) with 50% torque. 18.1.8.4

F ailure S equ en ce w ith B oth P um ps R u n n in g

I f oil the level in No. 1 tank goes down first: 1. Oil level in N o.l tank goes down to the Low position and the audible and visual alarms are activated on the navigating bridge and in the engine room. 2. No. 1 automatic isolating valve is energised and the hydraulic system associated with No.2 pump is isolated.

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Marine Electrical Technology

Steering and Stabiliser Systems 3. I f the oil loss is associated with No.2 pump system, the oil level in No.2 tank will fall to the Low-Low position and No.2 pump will be automatically stopped. No further oil loss will take place and steering will continue at 50% torque with N o.l system working alone. 4. I f the oil loss is associated with N o.l pump system the oil level in N o.l tank will fall to the Low-Low level and N o.l automatic isolating valve will be de-energised thus isolating No. 1 system. No. 1 pump is stopped and No.2 automatic isolating valve IV-2 energised. No.2 pump and its associated cylinders No. 1 and No.2 provide 50% o f the normal rudder torque. IS. 1.8.5

S ystem Testing

The oil tank float chamber can be isolated and drained to test the system’s automatic isolating operation. This should be carried out as part o f the pre-departure checks. 18.1.9

E lectron ic S teerin g C ontrol

This method may use a microprocessor-based circuit to receive the helm order and the rudder position feedback and compare them. The AutoNav Autopilot Model A -1500, to be explained later in this chapter is one such example. In other cases an operational amplifier could also be used instead. Cumbersome mechanical linkages and differential controls are replaced by quick-response electronic servo control valves on the hydraulic pump, which receive the order from the microprocessor and stroke the pump in the direction and the degree requested. The variant o f this is a system where the electronic signals from the controller and the feedback device are compared, amplified by the power amplifier whose output controls solenoids within the electro-hydraulic u n it The electro-hydraulic unit serves as an interface between the computing circuit and the hydraulically-operated rams. It directs the hydraulic pressure to the cylinders (Refer Figure 18.4). The follow-up element, which is either a potentiometer or a rotary transformer, is moved in direct proportion to the motion o f the rudder-stock or simpler said, the ram itself. It provides the negative feedback signal to the control circuit to de-stroke the pump and stop the rudder at the ordered angle or, in the other case, to nullify the output o f the operational amplifier which in turn forces the output o f the power amplifier to zero. This brings the solenoid valve to the neutral position. The blind-ports are then aligned with the hydraulic lines leading to the rams; this action results in holding the rudder in the desired position by trapping the hydraulic fluid within the cylinders. Marine Electrical Technology

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Chapter 18 A n error in the feedback system caused by a new helm or autopilot order (in the case o f operating in the automatic mode as shown in Figure 18.5) or by motion o f the rudder due to external dynamic forces reactivates the control system; other signals that influence the control o f the rudder are: $

The ship’s speed;

$

The turning radius (may be set manually also);

& The set course; 0

The rate o f change o f course; The present position o f the rudder itself.

Figure 18.4 - Electronic Steering Control - Manual Mode

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Figure 18.5 - Electronic Steering Control - Auto-pilot Mode 18.1.10

In dicators f o r M on itorin g th e O perating C onditions o f th e S teerin g G ear

The indicators for monitoring the operating conditions o f the steering gear, provided in the wheel house and ECR are designed to comply with SOLAS Regulations 29 and 30. They are as follows: a) Phase failure - in case o f single-phasing o f the pum p’s motor, an alarm is activated b) M otor overload - especially when the winding is overheated (the motor’s control circuit is to have short circuit protection) c) Isolation (auto shut-off) valve operated e.g.» in case o f excessive flow rates d) Hydraulic Oil tank level low e) High oil temperature Note: Some steering systems have an air-cooled system that ensures the system will not be activated until thefan is started. 18.1.11 P rocedure f o r C hange-O ver fro m N orm al to E m ergency M ode o f O perations 18.1.11.1

1.

R equirem ents

Changing over from automatic to manual steering and vice versa shall be possible at any rudder position and be effected by one, or at the most two manual controls, within a time lag o f 3 seconds. Marine Electrical Technology

721

Chapter 18 2.

Changing over from automatic to manual, steering shall be possible under any conditions, including any failure in the automatic control system.

3.

When changing over from manual to automatic steering, the automatic pilot shall be capable o f bringing the vessel to the preset course.

4. Change-over controls shall be located close to each other in the immediate vicinity of the main steering position. 5. Adequate indication shall be provided to show which method o f steering is in operation at a particular moment. 18.1.11.2

Basic Actions

a) Establish communication between the navigation bridge and the steering flat b) Changeover to the manual mode (not auto pilot); c)

Set the wheel to the midship position;

d) Switch o ff the telemotor i.e. disconnect the remote control circuit. e)

Steer from the steering flat by operating the manipulators or similar arrangements; this will be as effective as the NFU mode except that it is done locally and the operator may have to resort to monitoring the rudder angle with the help o f the mechanical pointer on the rudder stock itself i f the helm indicator too is not operational.

118.2

Anschiitz A uto Steering

The variety o f devices required for navigation and monitoring in the bridge area necessitates a functional design meeting work-sequence-oriented and ergonomic demands. For more than 75 years, Anschutz has been accumulating experience in this field. In 1969, Anschiitz introduced modular equipment technology in the area o f steering control. Today approximately 10,000 ships use Anschiitz steering control all over the world. A typical system is depicted in Figure 18.6. Figure 18.7 is an example o f a control system on the bridge.

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Marine Electrical Technology

Steering and Stabiliser Systems

24V Speed, Gyro, 24V d.c., 110/220 V d.c.

Status Alarm

Feeding Main Pump 1

Feeding Main Pump 2

Figure 18.6 - AuschUtz A uto Steering

Figure 18.7 - Steering C ontrol System - NautoSteer (Thisfigure includes other systems too)

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Chapter 18 18.2.1

S a lie n t F eatures o f N au toS teer

& Safety, reliability, redundancy. & Integral component o f the Integrated Navigation and Bridge System, expandable. & Proper design o f co-operating functional groups. & Versatile installation possibilities. & Increased reliability by self-explanatory designation o f important sequences. & Reliable legibility o f instruments. & Night design with individual or central dimming. & Neutral and nori-reflective colours. & Service-friendly design. & In compliance with all National and International Classification Rules, especially the IMO resolution A.325 IX and the SOLAS resolution M SC.l (XLV), Chapter II, Part C, Regulation 29. 18.2.2 18.2.2.1

System Types D u a l F ollow -U p (D u al FU)

The required rudder angle is selected on the mechanical rudder position indicator at the follow-up hand wheel or tiller (Refer Figure 18.8). One amplifier operates the servomechanism o f the steering gear and the rudder is moved until it reaches the required angle (1 amplifier per pump or valve according to IMO or SOLAS). The feedback unit transmits the actual rudder position (Refer Figure 18.12).

Figure 18.8 - Rudder Angie Indicator and Tiller

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Marine Electrical Technology

Steering and Stabiliser Systems 18.2.2.2

D u a l N on-F ollow -U p (D u al N F U )

To command a rudder* electrical movement contacts are made by moving the NFU hand wheel or the NFU tiller. The rudder position is changed as long as the contact is held. The steering gear is controlled according to IMO or SOLAS (1 contact set per pump or valve). During the steering process, the actual rudder angle should be checked on the rudder position indicator. 18.2.2.3

F ollow -U p/D ual N on -F ollow -U p

Depending on the type o f steering selector at the steering mode selector switch, the steering gear is controlled by the follow-up or the non-follow-up control system. Each o f the two steering systems is able to control both pumps o f the steering gear. Due to the redundant (dual) design o f the non-follow-up controls, this system is the main steering control in this configuration according to IMO or SOLAS. 18.2.3

S ystem S tru ctu re

18.2.3.1 C on trol C om ponents F ollow -up (F U ) con trols - C ontact steerin g is b y a fo llo w -u p am plifier.

Figure 18.10 -F U Tiller

figure 18,9 - FU Hand-wheel Unit

Figure 18.11 - Follow-up Amplifier

Figure 18.12 - Feedback Unit

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Chapter 18 & Actuator

Figure 18.13 - Solenoid Valve with On/Off Function #

Non-Follow-up (NFU) Controls Direct contact steering elements:

Figure 18.14 - NFU Hand-wheel Unit

Figure 18.15 - NFU Tiller

® Actuator Proportional valve or torque motors

Figure 18.16 - Proportional Steering By Analogue Amplifier/Main Pump ® Selection of Remote Steering Stands In principle, all basic steering control systems (dual FU, dual NFU, and dual FU/NFU) can be extended by remote steering stands. The steering stands, e.g. bridge wings, are selected by a steering mode selector switch and by the electronic ‘Take-over System’. The steering mode selector switch has an additional position *Remote’ by which all remote steering stands can be activated (Refer Figure 18.17).

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Figure 18.17 - Steering Mode Selector Switch #

R u dder P osition Indicator

The scope o f supply o f the Raytheon Marine steering control system includes the feedback unit, which is usually installed on the rudderstock. In addition to the limit switches, this feedback unit includes various potentiometers. One o f these potentiometers is used as a transmitter for the actual electrical rudder position indicators (Refer Figure 18.18). Hence, no additional feedback unit and no additional mechanical connections are required at the rudder stock.

Figure 18.18 - Rudder Position Indicators #

U n iversal S ig n a l D evice

Up to 15 alarm and status indications o f the steering gear system can be free configured as the Nautoalarm. The steering mode selector switch can also be supplied with illuminated status information on steering control modes (Refer Figure 18.19).

Figure 18.19 - Universal Signal Device #

O ver-ride C ontrol

FU and NFU tillers can be extended by an over-ride function if desired by the customer. ‘Over-ride’ means an immediate disconnection o f the automatic mode, such as autopilot or track control and the activation o f manual control (Refer Figure 18.20). Marine Electrical Technology

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Chapter 18 The override signal unit indicates this mode visually and audibly. It enables a return to the automatic mode on completion o f the manual steering manoeuvre via a push-button switch.

Figure 18.20 - Override Control 0

M o n ito rin g System

The steering failure alarm device offers online monitoring from the rudder order element o f the FU steering control to the rudder blade as well as synchronisation monitoring o f mechanically independent double steering gears (Refer Figure 18.21).

Figure 18.21 - Monitoring System #

Emergency Controls

Some classification societies require a dual emergency control in the steering gear room. A separate change-over switch - ‘Bridge / Steering Gear’ in the steering gear room as well as a dual FU tiller meet this task in connection with a steering repeater compass (Refer Figure 18.22). The changeover switch can be locked against unauthorised use. It electrically isolates the steering control in the steering gear room from all other steering controls on the bridge. This ensures galvanically separated operation. I f the mechanical rudder position indicator cannot be seen on the stock, the Raytheon Marine electric rudder position indicator can be introduced as an additional feature.

Figure 18.22 - Emergency Controls 728

Marine Electrical Technology

Steering and Stabiliser Systems The following article is an extractfrom www.sperry-marine.com and a related website (Speiry Marine, with worldwide headquarters in Charlottesville, Va., is part of Northrop Grumman's Electronic Systems sector). These have been insertedfor the sheer simplicity, yet rich content that is capable ofletting the readerpractically visualise whatever is explained!

118.3

Sperry Marine Steering Gear

This steering system is provided to control the rudder in response to helm commands from the bridge. The system consists o f the following subsystems: Steering commands are given to the dual-control gyro pilot steering stand located on the ship’s bridge. In the steering engine room the commands are received by two linear hydraulic power units and compensated hydraulic pumps and transmitted to two Heleshaw radialpiston pumps. The radial piston pumps direct pressurized hydraulic oil to four hydraulic rams which moves the rudder. Precise control o f the rudder position is accomplished by means o f a differential gear train and follow-up mechanism. An emergency hand pump is supplied for use in the event o f failure o f the normal hydraulic system and also for filling and draining the system, and all hydraulic components o f the system are coupled together with high and low pressure piping systems. Each o f the above mentioned components will be discussed in detail as follows: 18.3.1

Steering Design Specifications Max. Rudder Torque-Ahead at 35° Rudder Angle

3,048,000 in-lbs

Max. Rudder Torque-Astern at 35° Rudder Angle

4,370,000 in- lbs

Max. Pressure-Ahead at 35° Rudder Angle

735 psi

Max. Pressure-Astem at 35° Rudder Angle

1055 psi

Relief Valve Setting

1300 psi

Rudder Angle Hard Over (H.O.) to Hard Over

70°

Time - H.O. to H.O. (One Power Unit Operating)

2° per second

Time - H.O. to H.O. (Both Power Units Operating)

4° per second

No. of Turns for Trick wheel (70° H.O. to H.O.)

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9.1

729

Chapter 18 18.3.2

D u a l C ontrol Gyro P ilo t Steering Stand

The Sperry dual control gyro pilot steering stand provides three types o f rudder control: automatic control using the gyrocompass input to maintain the selected heading, hand steering with follow-up, and hand steering without follow-up. The rudder control selector switch on the steering stand is used to change from one mode o f rudder control to the other. Regardless o f wliich mode o f rudder control is utilized, an electric signal is sent to one o f the two independent electro-hydraulic steering controls located in the steering engine room. The heart o f each automatic steering system (port or starboard) is a potentiometer bridge. Each bridge contains two potentiometers connected in a balanced Wheatstone bridge arrangement. One potentiometer o f each bridge is called the control potentiometer. It is located in the steering stand, and is positioned by both the steering wheel and the gyro compass which acts on it through a mechanical differential gear train. The other potentiometer o f each Wheatstone bridge is called the follow-up or repeat-back potentiometer. It is located in the linear hydraulic power units and controlled by the rudder positioning equipment. W hen the control potentiometer is turned by either the steering wheel or by the gyro­ compass, a d.c. signal called the course error signal is sent to a solenoid-operated directional vaive located in each linear hydraulic power unit. The polarity and magnitude o f this course error signal indicates the direction and amount o f corrective rudder action required. When the linear hydraulic power unit transmits this rudder order to the radial piston pumps the follow-up or repeat-back potentiometer generates a d.c. signal opposite in polarity to the control signal. W hen the magnitude o f this opposite signal increases to equal the value o f the course error signal, the effective signal level to the hydraulic power unit reduces to zero, and rudder action ceases. Thus, full follow-up control is provided. Double cabling connects the steering stand in the wheel house with the hydraulic power units located in the steering engine room. Indicating lights on the steering stand show which system is operating and whether the other system has power available. 18.3.3

L in e a r H yd ra u lic Power U n it

The linear hydraulic power unit consists o f a double ended hydraulic control cylinder, manifold-mounted directional and bypass valves, parallel rack, outside limit switches, inside limit bypass relay and repeat-back potentiometer. 730

Marine Electrical Technology

Steering and Stabiliser Systems The power unit receives electrical signals from the dual control gyro pilot steering stand. In response to these signals, the piston rod is positioned by means o f hydraulic fluid delivered under pressure by the Vickers hydraulic pump units. The position rod, in turn, is directly connected through a differential gear train to the floating ring o f the radial piston Hele Shaw pump. The amount o f travel o f the piston is made proportional to the order o f the dual-control gyro pilot steering control. Also, limits are provided to prevent over-travel o f the piston. 18.3.4

Piston Operation

The controlling element o f the linear hydraulic power unit is the directional valve which is a solenoid-controlled, pilot-operated, four-way valve. A control signal from the steering stand energizes one o f the solenoids in the valve. The solenoid pushes the pilot spool offcentre, thus porting pilot fluid to offset the main spool valve. This connects one side o f the cylinder to the input pressure and the other side to the return line, causing the piston rod and hence the floating ring o f the Heleshaw pump to move. The direction o f flow, and thus the direction o f the control cylinder movement, will depend upon which solenoid is energized by the steering control. A parallel rack, which activates the repeat-back potentiometer and limit switches is attached to and moves with the piston. When the piston rod reaches the ordered position, die electrical follow-up signal balances the control signal thereby de-energizing the directional valve. A bypass valve in the power unit opens when the automatic or hand-electric controls are not in use, allowing oil to flow freely from one end o f the power unit cylinder to the other. The ship’s steering mechanism can then be operated by separate means with the hydraulic power unit still connected. When the system is energized, hydraulic pressure closes the valve to permit operation. The bypass valve is a hydraulically pressure-operated, spring-offset four­ way valve requiring at least 50 psi o f pressure for its operation. Although the bypass valve is a four-way type, its use in this system is limited to either the open or closed position. This is accomplished by blocking one set o f ports. When the system is not in operation, or in the event it should become inoperative, the bypass valve allows oil to flow from one side o f the control cylinder to the other so that the piston rod may be moved by an alternate means o f steering such as a trick wheel or telemotor. When the pump is turned on to start the system in operation, there is an immediate pressure build-up in the system, due to the check valve. This pressure closes the bypass valve thus allowing the control cylinder to respond to the operation o f the directional valve. Marine Electrical Technology

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Chapter 18 18.3.5

L im it Switches, Relay, and F ollow -up Potentiom eter

The hydraulic power unit contains two pairs o f limit switches, designated “inside limit switches” and “outside limit switches”. The inside limit switches restrict electrical operation normally to ten degrees o f rudder motion in either direction in order to optimize performance when steering automatically. Thus, when an error signal tends to drive the cylinder beyond moderate rudder angles, an inside limit switch opens the circuit to the energized solenoid o f the directional valve. The outside limit switches are set to open the solenoid circuit at the hard over m dder positions. Also, these switches are always set to prevent the piston from hitting its mechanical stops. In the hand-electric mode o f steering, a relay in the power unit, controlled from the steering stand, closes the circuits across the inside limit switches and allows movement o f the rudder up to the angle determined by the outside limit switches. In the normal mode o f operation both pairs o f limit switches are closed. A control signal is applied to one or the other solenoid o f the directional valve depending on the direction o f the rudder order. The valve operates to port in order to move the piston and rod. This also moves the attached rack. The rack drives a pinion which couples through a gear train to the limit switch cam shaft. The gears are chosen at the factory in accordance with the travel distance o f the piston rod, so that the cam shaft rotates 270° when the piston rod moves from one position to the other. The cams are set on the shaft during installation for the specific limits required by the particular vessel. 18.3.6

Inside L im it Switches

W hen the piston rod has moved sufficiently to produce a rudder angle o f about 10° either side o f amidships, a cam ''pens the limit switch in series with the energized solenoid valve and the steering mechanism is held in this position until control current is applied to the other solenoid. If less than 10° o f rudder were called for, an inside limit switch would not operate. 18.3.7

Outside L im it Switches

In the hand-electric mode o f operation, a cam operated switch in the steering stand energizes the inside limit bypass relay in the power unit when a rudder order o f approximately 8° is applied by the helmsman. A few degrees short o f maximum travel, a cam opens the normally closed snap-action outside limit switch thereby de-energizing the directional valve solenoid and holding the steering mechanism in position until the helmsman orders a return o f the rudder toward amidships. Thus the outside limit switches determine hard-over rudder angles and prevent the power unit from operating to its mechanical limits o f travel. 732

Marine Electrical Technology

Steering and Stabiliser Systems The camshaft also drives the rotating wiper o f a 5000-ohm wire-wound oil-filled potentiometer. This potentiometer is accurately positioned so that when the piston rod is at its mid position, the wiper o f the potentiometer is at “m id resistance”. In this way the potentiometer provides an electrical signal proportional to the power unit position for connection into the follow-up circuit o f an automatic or hand-electric steering control. In other words, this repeat-back potentiometer generates a follow-up signal which is sent to the steering stand. The directional valve solenoid is de-energized when the follow-up signal cancels the control signal. Both ends o f the pov/er unit piston rod carry a clevis, one o f which is connected mechanically through the differential gear train to the Hele Shaw rotaiy pump crosshead. The power unit is capable o f transmitting a force o f about 6,800 pounds, either as a push or a pull.

118.4 1H.4.1

Gyroscopes D e fin itio n

A gyroscope is any device consisting o f a rapidly spinning wheel set in a framework that permits it to tilt freely in any direction i.e., to rotate about any axis. The momentum o f such a wheel causes it to retain its attitude when the framework is tilted; from this characteristic a number o f valuable applications are derived. Gyroscopes are used in such instruments as compasses and automatic pilots onboard ships and aircraft, in anti-roll equipment on large ships, inertial guidance systems and many other systems where stabilisation is a mandatory requirement. The m arine gyrocompass is a three-frame gyroscope with its spin axis horizontal. In order to achieve the north-seeking and actual location (or meridian settling) properties o f a gyroscope, use is made o f the tilting effect o f the spin axis when it is not pointing to the true north. As soon as tilt develops, a pendulum type device introduces torques that precesses the spin axis towards the meridian, causing it to describe a spiral with an ever-decreasing radius. When stabilised, the spin axis is maintained in the meridian plane by a precession equal but opposite to the drift at the particular latitude. When there is no tilting effect the marine gyrocompass will lose its directional properties and become useless. This is the case at the poles and also when a vehicle moves due west with a speed equal to the surface speed o f the Earth. Because the latter condition can easily exist in an aircraft in the middle and upper latitudes, it cannot be used for air navigation. Vertical three-frame gyroscopes with penrecorder attachments are often used to analyse rolling and pitching movements o f ships. Marine Electrical Technology

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Chapter 18 18.4.2

The Three-Frame Gyroscope

I f the base o f a three-frame gyroscope is held in the hand with the rotor spinning and turned about any o f the three axes, the rotor axle will continue to point in the original direction in space. This property is known as gyroscope inertia. If the speed o f the wheel decreases, the gyroscope inertia gradually disappears, the rotor axle begins to wobble and ultimately takes up any convenient position. Rotors with a high speed and concentration o f mass towards the rim o f the wheel display the strongest gyroscopic inertia (Refer Figure 18.23).

Figure 18.23 - The Three-framed Gyroscope It is apparent that gyroscopic inertia depends on the angular velocity and the momentum o f inertia o f the rotor, or on its angular momentum. The rotor (wheel) is subject to the laws o f rotational motion and inertia in that a freely rotating, well-balanced body, whose mass is equally distributed along its circumference, will maintain a fixed direction in space, tends to preserve its angular momentum, or spinning action, unless acted upon by some external force. The consequence o f gyroscopic inertia is that to the observer on Earth, the spin axis o f a gyroscope makes an apparent movement over a period o f time, although this apparent motion merely reflects the revolution o f the Earth about its axis. There is one exception to this, that when the spin axis points towards the polar star, there is no movement o f the spin axis with respect to the observer’s surroundings, as the axis is parallel to the Earth’s axis and points toward the Celestial poles. As the direction o f the Earth’s rotation is counter clockwise when seen from above the North Pole, the relative direction o f this end will change through Northeast, East, Southeast, South, etc. 73 4

Marine Electrical Technology

Steering and Stabiliser Systems This clockwise movement will continue until, at the end o f one period o f rotation o f the earth (23 hours 56 minutes), the rotor and spin axis revert to their original position with respect to the observer on the Earth’s surface. While this is taking place, the top end is apparently tilting upward. The change in azimuth (direction) o f the spin axis is often referred to as drifting. Sometimes tilting and drifting are collectively called apparent wander. If, while the rotor o f a three-frame gyroscope is spinning, a slight vertical downward or upward pressure is applied to the horizontal gimbal ring at the top, the rotor axle will move at right angles in a horizontal plane. But no movement will take place in the vertical plane. Similarly i f a sideways pressure is applied at the same point the rotor axle will tilt upward or downward. This second property is called precession. A precession or angular velocity in the horizontal plane is caused by the application o f a couple, i.e. parallel forces equal and opposite, in the vertical plane perpendicular to that o f the rotor wheel. Precession is the tendency o f the rotor’s axis to move at right angles to any perpendicular force that is applied to it. The unrestrained or free three-frame gyroscope has little practical use because its spin axis is subject to tilting and drifting owing to the rotation o f the Earth. In the controlled state it is widely used. The term control o f a gyroscope implies that the spin axis, by small continuous or intermittent application o f torque (twisting force), is made to precess so that it oscillates around a mark fixed in relation to co-ordinates on the Earth rather than in relation to space. 18.4.3

C ontrolled Gyroscopes

Controlled gyroscopes fall into three categories: •

The north-seeking gyroscope is used in marine applications. In the settling (or normal) position the spin axis is kept horizontal and in the plane o f a meridian.

•

The directional gyroscope is used in aircraft and is sometimes called a self-levelling free gyroscope corrected for drift. With its spin axis horizontal it has directional properties but does not automatically seek the meridian.

•

The gyrovertical has its spin axis vertical and is used to detect and measure angles o f roll and pitch.

These types o f three-frame gyroscopes are called displacement g)>roscopes because they can measure angular displacements between the framework in which they are mounted and a fixed direction - the rotor axis. Marine Electrical Technology

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Chapter 18 The fo llo w in g educative a n d in te re s tin g a rtic le is an un -e d ite d version p u b lis h e d by P a u l W agner, E xecutive C hairm an, A utoN av M a rin e Systems Inc. I t is inserted w ith h is k in d consent a n d can be fo u n d as p a rt o f the w ebsite www.autonav.com; the fig u re s how ever, have been d ig ita lly enhanced / redraw n f o r c la rity .

j18.5

Compass Considerations for Steering and Autopilots

The autopilot compass is perhaps the most critical component in an autopilot system. No matter how good the other components may be, no autopilot can steer better than die heading information provided by its compass. Most modem autopilots use an electronic Flux Gate compass, or equivalent, which directly senses the earth’s magnetic field. These direct sensing compasses are frequently claimed to be far superior to “old fashioned” fluid compasses. In fact, electronic compasses have been in use for over 70 years and their limitations are well known to compass experts. They are in common use today mainly because they are less expensive to manufacture than the conventional fluid-filled compass with its floated card, magnets, pivots, jew els and sealing system. The flux gate consists o f field sensor, usually an inductor, mounted to a gimbaled platform which is intended to sense the horizontal component o f the earth’s magnetic field. The earth’s field has two components: the horizontal field, which gives directional information, and the vertical field, which provides no useful heading information (see figure 1). If the sensor should move from its intended horizontal position due to roll, pitch or slamming in a seaway, the sensor will pick up some o f the vertical field, mixing it with the horizontal field and causing an error in actual course. The same problem would occur in a conventional fluid compass except that the pivot and jew el offers a second line o f defense in decoupling the sensor (card and magnets) from vessel motion. It is a seldom recognized fact that this extra isolation from vessel motion, coupled with fluid damping, results in a conventional fluid compass having much greater stability than any electronic compass under most conditions.

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Marine Electrical Technology

Steering and Stabiliser Systems

Figure 1 - Simplified section through the Earth’s magnetic field. B andR signify blue and redpoles. The lines marked V and HH show the vertical and horizontal directions in various latitudes. At mid latitudes in the U.S., one degree o f sensor tilt off horizontal will have an apparent two-degree shift in indicated heading, even though the vessel is still on course. In higher latitudes where the horizontal field decreases in strength and the vertical field increases, one degree o f tilt can cause over 10 degrees compass error. The Great Lakes and Eastern parts o f Alaska are particularly bad areas in this regard (see figure 2). A few electronic compass manufacturers fill their compass sensor with a heavy oil to dampen the gimbal action and minimize these vertical field errors. Others resort to electronic damping, which either increases the compass dead band (lowers its sensitivity) or averages the heading (delays the availability o f current heading information). Some designs use more sophisticated signal processing, but the end result is roughly the same. Any delay in autopilot response to heading changes, especially in quartering seas, results in excessive yaw and the need for excessive rudder corrections. Some electronic compass manufacturers recognize this deficiency by offering a rate gyro which provides a more current short-term heading reference than their sluggish and over damped electronic compass is capable of. Marine Electrical Technology

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00 03

—X

X li o

Chapter 18

X 66°40' \

t

= 0.474

H = 0.22

Veo0 XT

Z « 0.436\ (a)

=

0.44

Z = 0.38

\

(b)

F igure 2 - H orizontal an d vertical com ponents o f the E a rth ’s m agnetic field, H, Z, and T are respectively the horizontal component, the vertical component, and the total force, all expressed in oersted units. Thefield is shown for (a) London; (b) Northern Spain, and (c) the Sahara Desert. Another electronic compass manufacturer has a “turn” button on the compass display. They recommend that this button be activated when a change o f course is made. This button simply changes the compass damping to minimum and is a tacit admission that the normal amount o f damping, which is required to provide a steady display, causes such a delay in heading indication that the helmsman would overshoot a course change. Clearly, any autopilot using this heading information would have great difficulty steering in quartering seas where immediate correction o f course changes is essential. To verify the severity o f compass errors induced by electronic compasses, a simple test can be made using a well-known brand o f hand-bearing compass which uses an ungimbaled flux gate sensor in a flat hand-held digital readout configuration. The user m ust maintain this sensor perfectly horizontal to avoid errors induced by sensing the earth’s vertical field. I am not sure how this is to be achieved on a heeled and roiling deck! To measure the tilt errors, hold the hand-bearing compass down flat on the edge o f a seat with the vessel at the dock, i.e., no vessel motion to confuse the measurement. Take a reading, and then, without rotating the compass to a different heading, tip it a few degrees up or down and note the change in indicated heading. I f this compass were controlling your autopilot, you may appreciate the resulting sloppy steering. 738

Marine Electrical Technology

Steering and Stabiliser Systems

Figure 3 - A Schematic Diagram of the Compass Over 50 years ago, during World War II, direct sensing electronic compasses using flux gates were used on aircraft and even on vessels, due to the ability to have the compass sensor remotely mounted away from strong magnetic interference and to provide multiple repeaters (see figure 3). However, due to high acceleration such as turning and banking in an aircraft and rolling and pitching on vessels, it was determined that simple pendulous support o f the sensor resulted in unacceptable course instability. To resolve this problem on aircraft, a vertical gyro was integrated into the gimbaied sensor so the sensor always remained horizontal. On ships, a directional gyro was used for heading reference, but since these would slowly drift away from North, they were slaved to a flux gate sensor. Marine Electrical Technology

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Chapter 18 This sensor would be unstable for the reasons previously stated, but when averaged over about one minute would provide a reasonably stable reference for the drifting gyro and keep the heading smooth and reasonably accurate. The cost to produce these systems with the older technology precluded their use on small commercial vessels or yachts. A more sophisticated and more modem approach to a flux gate controlling the directional gyro is now produced by a leading compass manufacturer, who uses multi-axis gyros to provide enhanced stability in the face o f heavy acceleration from roll and pitch, etc. in addition, this newer design provides heading output to the NM EA 0183 standard, making it usable with a wide range o f equipment, including autopilots, which require better compass stability than that available from simple, non-fluid damped flux gate sensors. While considerably more expensive than simple flux gate compasses, it provides an economical alternative to conventional north-seeking marine gyrocompasses. None o f the above flux gate or equivalent direct sensing electronic compasses eliminate compass errors due to vertical heeling error. This phenomenon is largely unheard o f and is almost never mentioned by compass manufacturers who claim to have automatic compass compensation for magnetic deviation. As discussed earlier, the earth’s magnetic field consists o f vertical and horizontal components. O n a vessel having ferrous (e.g., steel) construction, the steel in the hull distorts the horizontal component and causes errors in the reading of compass course. With conventional liquid card type compasses, these errors were compensated for by placing magnets in the horizontal plane around the compass, to provide equal but opposite fields to those Caused by the steel in the ship. All electronic compasses only correct for this horizontal field error. On non-ferrous vessels, this is generally acceptable; however, on vessels with large amounts o f ferrous metal on board, the earth’s vertical field induces a changing horizontal field in the magnetic deviation, as the vessel rolls or heels; hence the name, vertical heeling error. As a vessel roils, the compass error varies, causing oscillation in the indicated course, even though the vessel may still be on the same heading. Not only does this make hand steering difficult, but it also causes autopilots to wander and cause unnecessary steering corrections. On sailing vessels, which may remain heeled for 30 degrees or more for some time, large fixed errors occur, and variable errors due to rolling are superimposed on this. Under these conditions, an uncompensated vertical heeling error can have serious consequences. 740

Marine Electrical Technology

Steering and Stahitiser Systems The flux gate stabilized gyrocompass may give acceptable performance where the roll period o f the vessel is well below the averaging time o f the unstable flux gate North reference, but if the vessel has a long rolling period, this can begin to degrade the North reference stability. O n a heeled sailboat, within minutes o f heeling, large errors can develop, since the roll filtering is no longer effective. The only way to correct this is to use the tried and true technique o f installing a permanent magnet directly above or below the sensor and through its center, while adjusting the distance and polarity for minimum heeling error. Professional compass adjusters use a “vertical force instrument” which measures the vertical field errors and allows precise compensation. Vessel owners may perform an approximate compensation by adjusting the magnetic in a similar manner by monitoring for maximum compass stability (minimum autopilot activity) while rolling in a seaway. On commercial vessels or any vessel going offshore and where vertical heeling error is considered to be a possibility, the services o f a qualified compass adjuster should be contemplated. There are some compass adjusters whose experience m ay be limited and do not correct for or even understand what vertical heeling error is, and i f they express any hesitation about making this correction or don’t seem to understand what you are talking about, find another compass adjuster! Incidentally, this vertical heeling error correction is only valid for the magnetic latitude at which the compensation was made. I f the vessel is expected to go on long ocean voyages where the latitude will change by more than approximately 5 degrees, an additional correction made with “Flinders Bars” should be carried out. Just as horizontal compensation requires quadrantal spheres (soft iron balls) to be placed around the compass, vertical compensation also requires a vertical soft iron corrector to be mounted and operated in conjunction with the vertical heeling error magnet to maintain compensation over wide latitude changes. The ultimate in compass stability is achieved with a true North seeking gyrocompass. While very expensive, the stability is unmatched. Traditional technology uses a spinning inertia wheel which possesses high directional stability despite vessel roll or pitch. Through mechanical or electronic means this gyro wheel is controlled to point to the earth’s geographic North Pole and transmitting devices send this information to various repeater stations. The more modem designs provide this to NMEA 0183 format. Unfortunately, the cost o f the gyrocompass precludes its use on smaller yachts and workboats. In summary, a simple air-suspended flux gate or equivalent electronic direct sensing compass is only suitable for calm seas and/or low latitudes, and a fluid-filled sensor is acceptable for heavier seas and higher latitudes. Marine Electrical Technology

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Chapter 18 A n improvement in performance, especially on steel vessels may be achieved with a flux gate-aided directional or rate gyro, and for the m ost demanding applications, a North-seeking gyro compass is the preferred choice. W ith the above increasing performance, increasing costs may be expected. While there are many factors that could be discussed concerning the design and construction o f autopilots, it may be seen that the compass and its stability are o f prime importance. No autopilot can steer better than its heading reference.

118.6

The AutoNav Flux Gate Compass (as installed on ships)

AutoNav’s unique fluid-damped flux gate compass offers unmatched stability and instantaneous course correction. The compass, due to its direct effect on course accuracy, is the most important part o f any autopilot. A n unstable compass will cause unnecessary rudder movements allowing the vessel to fall o ff course. The AutoNav Flux Gate compass achieves steady course holding accuracy through the use o f a fluid damped gimbal system which is designed to absorb vessel heel angles up to an exceptional 45 degrees! Conventional flux gate compasses lack this stability forcing manufacturers to use excessive electronic damping which slows autopilot response to course errors resulting in the user turning the pilot o ff in heavy weather...just when it is needed the most.

Figure 18.24 - Fluid-filled Flux Gate Compass

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Marine Electrical Technology

Steering and Stabiliser Systems AutoNav’s digital signal processing and rate o f course change algorithms enable responsive performance especially in quartering seas. The AutoNav compass can be used in the heaviest o f seas and in high magnetic latitudes; in all the places where normal flux gate compasses fail to perform. AutoNav’s state o f the art automatic compass deviation technology can automatically correct compass deviation errors, so compass readout and steering performance are perfectly accurate on all headings. The flux gate compass unit is waterproof to allow greater installation options which helps in finding a suitable mounting location away from magnetic interference caused by electronics, steel machinery, tanks, and motors. 18. 6.1

E asy-M ou nting S en sor

On large vessels, and frequently on steel vessels, it is preferred to use the vessels magnetic compass for the autopilot The C l 200 sensor can be mounted to the vessel’s compass and will provide very stable heading references for the autopilot. 18.6.2

G yro In terface f o r A ccu racy a n d S ta b ility

On large vessels, a gyro compass is often used to obtain the utmost in heading accuracy and stability. A Gyro interface to connect NMEA 0183 heading data, and stepper or synchro heading data, for transmission to the autopilot can be provided.

118.7

Rudder Position Indicator

As mentioned at the beginning o f this chapter, rudder position indicators are meant to continuously transmit the actual position o f the rudder to control consoles on the bridge, the engine control room and possibly the engine room and steering flat too. 18.7.1

The P recision P oten tiom eter a n d S tep p er M otor Type (T ype A 070)

This equipment is available in different sizes and styles according to the various requirements on a ship and can be installed in all types o f vessels. The system in mention consists o f a power supply unit, a transmitter mechanically coupled to the rudder shaft by a lever-drive or flange coupling, and one or more receivers (indication instruments). 18.7.2

O perating P rin ciple

The changing o f the rudder position is registered by a precision potentiometer in conductive plastic, installed in a watertight aluminium casing. The output current loop (4 - 20 mA) is driven by precision operational amplifiers. Thus, the influence o f wire resistance and voltage fluctuation is eliminated. The receiver electronically controls a high-resolution stepper motor, which enables the pointer to settle within 0.5° o f the rudder position. Marine Electrical Technology

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Chapter 18

Figure 18.25 - Rudder Position Indicators 18.7.3

T echnical D ata

Supply Voltage

110 V or 220 V A C

Nominal Voltage

24 V DC ± 2 0 %

Current consumption

Transmitter 0,1 A ; Receiver 0,5 A

Degree o f enclosure Transmitter and watertight indicators

IP 65

Desk-mounting indicators

IP 23

Temperature range

-25 t o +70 °C

Relative Humidity

95% Max

Vibration strength

2 to13 c/s; amplitude = 1 mm; 13 to 100 c/s; acceleration = 0,7 g

Transmission accuracy

0.5°

Standard scale version

Indication range 2 x 45° Rudder angle Primary colour: white, Inscription: red (PORT), green (ST’BD)

Illumination 3-side indicator

30V 5W, Socket Ba 15d

Other indicators

28V 40 mA Bi-pin socket Pin spacing 3.17 mm

Electrical connection

Terminals 2.5 mm2

Ships connection

Recommended 1.5 mm2 Cu

Approval

Different Classification Societies

74 4

Marine Electrical Technology

Steering and Stabiliser Systems 18.7 .4

☆

O u tstandin g F eatures

High indication accuracy and repeatability

'k Independent o f voltage and frequency fluctuations k

Automatic control o f voltage supply

ik

Additional 4 - 20 ma current loop available

ik

Connection possibility is up to 20 indicators

ik

Suitable for tropics

ik

Easy installation

☆

N o maintenance required

☆

Low reflection glass

☆

Adjustable scale illumination (red light if required)

118.8

Synchros for Rudder Angle and Cou rse Indication

Synchros are electromagnetic transducers, commonly used to convert the angular position o f a (rotating) shaft into an electrical signal; these devices are AC position indicating motors, consisting essentially o f two basic components, a stator and a rotor, whether transmitter or receiver. Figure 18.26 shows a simple layout. The laminated iron core consists o f three windings connected in star, 120° apart, H-shaped, with both rotors connected to the same supply. Although the name “Synchro” is universally used in the instrumentation field, trade names such as Seisyns, Microsyns and Autosyns are used for these instruments. There are two types o f synchro systems namely: (i) control or error detecting type and (ii) torque transmission type. Torque transmission types o f systems are used only to drive very light loads, such as pointers. Ironically, torque transmission systems have very little output torque. W hen large torques and high accuracies are needed, control type synchros are used. In this topic only the latter will be discussed as this is the most common application onboard a ship. Initially winding the rotor (primary) windings o f both the synchro transmitter and receiver are aligned with the stator winding S2 for maximum (flux) coupling as shown in Figure 18.26. Marine Electrical Technology

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Chapter 18 Transmitter (Tx)

Receiver (Rx)

Figure 18.26 - Two-element Synchro Chain (Initial Position of the Synchro Transmitter and Receiver) The coupling between the stator windings S}and S3 and the rotor are cosine functions and are proportional to Cos60°; since Cos60 = V2, for an applied voltage V, the resultant values o f em f in S2 = V and equal to V/2 in the case o f the other two windings Si and S3. As long as the two rotors remain in this position, there is no torque generated as no current flows between the windings, the reason being that their voltage vectors are exactly opposite to each other. The moment the rotor o f the transmitter, which for example is linked to the rudder stock, is rotated as shown in Figure 18.27, resultants are generated due to a generation o f EMF which is a result o f the imbalance. Assuming that the rotor is turned by 30°, the stator windings’ voltages o f the transmitter will be changed to

in the stator

windings Si and S2 and 0 in S3 respectively. The resultant torque causes the rotor o f the receiver to rotate until it aligns with the stator, ideally taking up the same position, i.e., 30°. 746

Marine Electrical Technology

Steering and Stabiliser Systems Transmitter (Tx)

Receiver (Rx)

Figure 18.27 - Torque Transmitter 18.9

Steering Gear Testing and Drills

These are carried out in accordance with SOLAS Regulation 26 Quote 1

W ithin 12 hours before departure, the ship’s steering gear shall be checked and tested by the ship’s crew. The test procedure shall include, where applicable, the operation o f the following: .1

the main steering gear;

.2

the auxiliary steering gear;

.3

the remote steering gear control systems;

.4

the steering positions located on the navigation bridge;

.5

the emergency power supply;

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Chapter 18

2

3.1

.6

the rudder angle indicators in relation to the actual position o f the rudder;

.7

the remote steering gear control system power failure alarms;

.8

the steering gear power unit failure alarms; and

.9

automatic isolating arrangements and other automatic equipment.

The checks and tests shall include: .1

the full movement o f the rudder according to the required capabilities o f the steering gear;

.2

a visual inspection o f the steering gear and its connecting linkage; and

.3

the operation o f the means o f communication between the navigation bridge and steering gear compartment.

Simple operating instructions with the block diagram showing the change over procedures for remote steering gear control systems and steering gear power units shall be permanently displayed on the navigation bridge and in the steering compartment.

3.2

All ships’ officers concerned with the operation and / or maintenance o f steering gear shall be familiar with the operation o f the steering systems fitted on the ship and with the procedures for changing from one system to another.

3

In addition to the routine checks and tests prescribed in paragraphs 1 and 2, emergency steering drills shall take place at least once every' three months in order to practise emergency steering procedures. These drills shall include direct control within the steering gear compartment, the communications procedure with the navigation bridge and, where applicable, the operation o f alternative power supplies.

4

The Administration may waive the requirements to carry out the checks and tests prescribed in paragraphs 1 and 2 for ships which regularly engage on voyages o f short duration. Such ships shall carry out these checks and tests at least once every week.

5

The date upon which the checks and tests prescribed in paragraphs 1 and 2 are carried out and the date and details o f emergency steering drills carried out under paragraph 4 shall be recorded.

Unquote

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Steering and Stabiliser Systems j18.10

Roll Stabiliser for Ships

Fixed type o f fin stabilisers (fixed, extended fins) had been initially installed on defence ships and patrol boats for the purpose o f safe taking off and landing o f a helicopter. In recent years a retractable type fin stabiliser has been adopted on car ferries and passenger ships from the viewpoint o f improvement o f habitability and prevention o f cargo collapse. However the roll reduction performance o f the fin stabiliser on an actual ship is affected by external sea conditions, hull parameters including hull details and electrical / mechanical parameters o f the fin stabiliser system itself. The system is installed around the midship area o f the vessel; a pair o f fins (one on either side) tilts in the reverse direction mutually, with the help o f hydraulic pressure, based on the electric signals through the control unit from the roll motion sensor (Refer Figure 18.28). Gyroscopes sense the vertical angular displacement and the roll velocity and provide proper control for the fms.The lift is generated by tilting the fins and the velocity o f sea water flowing into the fins acting as a righting couple, thus resulting in reduction o f ro ll.

Hull Roll Sensor

Control Unit

X

Pressure Switch

Figure 18.28 - A Conceptual Diagram of a Stabiliser Marine Electrical Technology

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Chapter 18 This is based on an article by Sperry Marine, Charlottesville, Virginia, USA and is published with due permission. Log on to www.sperry-marine.comfor more information. These are installed on vessels to enhance comfort and alleviate damage to cargo. Automatic control o f the fin movement is achieved with the help o f gyroscopes Sperry Marine, a leader in ship stabilisation, introduces the next generation o f ship stabilisers that feature a new digital control system, the “Lift Control” design and upgraded machinery units. The latest advances in technology are applied to a proven and robust design resulting in a system that truly delivers ship comfort and safety even in the roughest seas. (Refer Figure 18.29)

Figure 18.29 - The Stabiliser and Ships on which the Gyrofin Stabiliser is fitted 18.10.1

H ow L ift C ontrol Works

-

“L ift Control,” a key feature o f Sperry Marine’s innovative and patented fin stabilisation system, is made possible by mounting displacement transducers within the fin shaft. The transducers produce an electrical signal proportional to the lift force generated by the angle of attack o f the fin to the direction o f the local water stream. This lift signal is compared with the instantaneous value o f lift required for roll stabilisation. The difference is used to drive each fin until it achieves the desired lift, thereby automatically compensating for variations on the local water stream direction. The angle o f the fin will change as required until the desired lift is achieved even though direction o f the local water stream is continuously changing. 75 0

Marine Electrical Technology

Steering and Stabiliser Systems “Lift Control” prevents the fin from being driven at times into the cavitation zone and at other times from producing a shortfall in lift. The lift forces required for stabilisation are more faithfully produced, giving improved stabilisation efficiency. 18.10.2

L ift C ontrol Advantages

Stabiliser systems without “Lift Control” cannot maintain die required peak lifts because o f the fluctuating conditions within the cavitation region. Sperry Marine’s “Lift Control” produces maximum fin efficiency not available from any other stabiliser system. 18.10.3

Key Benefits and H ighlights

s

Improved safety and vessel performance

v'

Increased passenger comfort

•/ Enhanced cargo protection Lower fuel costs s

Reduced cargo lashing

S

Worldwide Sperry Marine Service

s

Proven design to ensure maximum performance and reliability

■ / Innovative “Lift Control” feature that enhances stabilisation efficiency and machinery service life v'

Combined with lift control the fin unit, utilising a tail flap, improves the lift to drag ratio thereby, minimising drag and saving fuel

■ / New Digital Control System with serial connectivity and simple to use touch screen controls v' Upgraded machinery elements to enhance performance and supportability 18.10.4

New D ig ita l C ontrol System

★

New Bridge Control Unit offers large colour LCD displays, versatile, easy-touse touch screen controls for operator interface

★

Serial data communication between the units enhances system performance and reduces cabling requirements

★

Electronics allow operating up to 4 fin stabilisers simultaneously

★

Improved system reliability by the use o f a new Roll Motion Sensor with no moving parts, Proximity Sensors in place o f mechanical switches Marine Electrical Technology

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Chapter 18 ★

Redundant operator interfaces at both die Bridge Control and Main Control Units - I f a failure o f the Bridge Control Unit occurs, the fin stabiliser can be maintained by the Main Control Unit.

★

System diagnostic capabilities with intelligent alarm messaging

★

A unique “Quick Test” feature that performs a comprehensive test o f the entire system from the Bridge Control Panel (Refer Figure 18.30).

★

Each fin unit has an auxiliary pump and motor set which can stow the fin in the event o f an emergency, via the ship’s emergency electrical supply in compliance with the SOLAS requirements for passenger vessels

★

Emergency stowing o f the fins is controlled from the standard control stations or it can be initiated automatically

Figure 1830 - A Typical Gyrofin Installation

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Marine Electrical Technology

Steering and Stabiliser Systems 118.11

Relevant Rules

18.11.1

Relevant SOLAS Regulations

Chapter II—1 Part A - General - Regulation 3 - Definitions relating to parts C, D and E Chapter I I - 1 Part C - Machinery Installations - Regulation 29 - Steering Gear Chapter I I - 1 Part C - Machinery Installations - Regulation 30 - Additional requirements for electric and electro hydraulic steering gear Chapter V - Regulation 26 - Steering Gear: Testing and Drills 18.11.2

Sum m ary o f Regulations

1)

The main steering and rudder stock shall be o f adequate strength and capable o f putting the rudder over from 35° on one side to 35° on the other side with the ship at its deepest seagoing draught and running ahead at maximum ahead service speed and, under the same conditions, from 35° on either side to 30° on the other side in not more than 28 seconds.

2)

The auxiliary steering gear shall be o f adequate strength and capable o f steering the ship at navigable speed and o f being brought speedily into action in an emergency; it must be capable o f putting the rudder over from 15° on one side to 15° on the other side in not more than 60 seconds with the ship at its deepest seagoing draught and running ahead at one half o f the maximum ahead service speed or 7 knots, whichever is the greater.

3)

Main and auxiliary steering gear power units shall be arranged to restart automatically when power is restored after a power failure and capable o f being brought into operation from a position on the navigation bridge. In the event o f a power failure to any one o f the steering gear power units, an audible and visual alarm shall be given on the navigation bridge.

4)

Steering gear control shall be provided for the following: a) The main steering gear, both on the navigation bridge and in the steering gear compartment; b) The auxiliary steering gear, in the steering gear compartment and, i f pow eroperated, it shall also be operable from the navigation bridge and shall be independent o f the control system for the main steering gear.

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5) Any main and auxiliary steering gear control system operable from the navigation bridge shall comply with the following: a) I f it is electric, it shall be served by its own separate circuit supplied from a steering gear power circuit from a point within the steering gear compartment, or directly from switchboard busbars supplying that steering gear power circuit at a point on the switchboard adjacent to the supply to the steering gear power circuit; b) means shall be provided in the steering gear compartment for disconnecting any control system operable from the navigation bridge from the steering gear it serves; c) the system shall be capable o f being brought into operation from a position on the navigation bridge; d) in the event o f a failure o f electrical power supply to the control system, an audible and visual alarm shall be given on the navigation bridge; and e) short circuit protection only shall be provided for steering gear control supply circuits. 6)

The electrical power circuits and the steering gear control systems with their associated components, cables and pipes required shall be separated as far as is practicable throughout their length.

7)

The angular position o f the rudder, independent o f the steering gear control system shall be recognizable in the steering gear compartment and if the main steering gear is power-operated, be indicated on the navigation bridge.

8)

With reference to 33 CFR Ch. I (7-1-05 Edition), a telephone or other means of communication for relaying headings to the emergency steering station. Also, each vessel o f 500 gross tons and over and constructed on or after June 9, 1995 must be provided with arrangements for supplying visual compass readings to the emergency steering station.

9)

A low-level alarm for each hydraulic fluid reservoir must give the earliest practicable indication o f hydraulic fluid leakage. Audible and visual alarms shall be given on the navigation bridge and in the machinery space where they can be readily observed.

10)

Every tanker, chemical tanker or gas carrier o f 10,000 gross tonnage and upwards shall comply with the following: a) Two independent steering gear control systems shall be provided each o f which can be operated from the navigation bridge. This does not require duplication o f the steering wheel or steering lever;

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Marine Electrical Technology

Steering and Stabiiiser Systems b) I f the steering gear control system in operation fails, the second system shall be capable o f being brought into immediate operation from the navigation bridge; and c) Each steering gear control system, if electric, shall be served by its own separate circuit supplied from the steering gear power circuit or directly from switchboard busbars supplying that steering gear power circuit at a point on the switchboard adjacent to the supply to the steering gear power circuit. 11)

Means for indicating that the motors o f electric and electrohydraulic steering gear are running shall be installed on the navigation bridge and at a suitable main machinery control position.

12)

Each electric or electrohydraulic steering gear comprising one or more power units shall be served by at least two exclusive circuits fed directly from the main switchboard; however, one o f the circuits may be supplied through the emergency switchboard.

13)

An auxiliary electric or electrohydraulic steering gear associated with a main electric or electrohydraulic steering gear may be connected to one o f the circuits supplying this main steering gear. The circuits supplying an electric or electrohydraulic steering gear shall have adequate rating for supplying all motors which can be simultaneously connected to them and may be required to operate simultaneously.

14)

Short circuit protection and an overload alarm shall be provided for such circuits and motors. Protection against excess current, including starting current, i f provided shall be for not less than twice the full load current o f the motor or circuit so protected, and shall be arranged to permit the passage o f the appropriate starting currents. Where a three-phase supply is used an alarm shall be provided that will indicate failure o f any one o f the supply phases. The alarms required in this paragraph shall be both audible and visual and shall be situated in a conspicuous position in the main machinery space or control room from which the main machinery is normally controlled and as may be required by Regulation 51.

15)

W ith reference to 33 CFR Ch. I (7-1-05 Edition), simple operating instructions with a block diagram, showing the change-over procedures for remote steering gear control systems and steering gear power units, permanently displayed on the navigating bridge and in the steering gear compartment.

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Chapter 18

1)

The three important regulations that govern the operation o f steering systems a re _____ .

2)

Failure o f the steering motor due to single-phasing operates an alarm T / F.

3)

State the conditions under which a steering gear motor would activate an alarm.

4)

What is special about steering gear overload safety?

5)

List the indicators provided for monitoring the operating condition o f the steering gear m otor and their locations.

6)

What are the main components that comprise the steering system? Briefly explain the function o f each.

7)

W ith a suitable diagram, explain the Anschutz Steering Control system.

8)

With a suitable diagram, explain the Electro hydraulic Steering Gear Control system.

9)

W ith the help o f a chart, explain the basic operating principles o f various Thruster Systems.

10) 11)

Draw block diagrams and explain the non-follow-up method o f steering control system. Draw block diagrams and explain the Twin-ram Electro-hydraulic Steering Control System.

12)

What is the significance o f negative feedback in a steering control system?

13)

W hat do you know about Electronic Control for a Steering System?

14)

What are the basic features o f the AutoNav steering system?

15)

Differentiate between dual follow-up and dual non follow-up.

16)

With a suitable diagram explain the basic principle o f a 3-frame gyroscope.

17)

Briefly explain the operation o f a flux gate compass.

18)

What is the principle behind a synchro element? Where is it used?

19)

W hy is a stabiliser needed for a ship? How does lift control work?

20)

Explain the methods o f carrying out operational checks o f the steering gear in normal and emergency modes o f operation as per classification society regulations.

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Marine Electrical Technology

D

e

C

h

c

k

a

p M

a

t e

r

c

h

i n

e

r y

The basic dimensions o f an anchor windlass depend on the anchor weight and chain size. The size o f the vessel, the nature o f the service, and the desired anchor handling and stowage arrangements also contribute to the choice o f an anchor windlass. It is also usual to involve capstans for warping. Combination windlass mooring winches / warping head systems have often been supplied for large ships as will be explained in article 19.1.2. 19.1.1

The H o rizo n ta l Windlass

One o f the two fundamental configurations o f anchor windlasses is the horizontal windlass that is in fact a specialised winch, powered by a hydraulic or electric motor or in rare cases, by a steam engine. The motor is then connected to a gear train that drives one or more chain sprockets, called wildcats, through sliding-block locking heads or comparable jaw clutches. Figure 19.1 depicts a pictorial diagram o f a windlass, Figure 19.2 depicts a typical electric windlass and Figure 19.3 is a schematic diagram o f a horizontal electro hydraulic type o f windlass. The specification for cargo vessels often require the combination o f a horizontal mooring winch with a clutched drum driving a chain wildcat through an auxiliary reduction gear and jaw clutch or sliding pinion. The chain-lifting unit consists o f a rigid framework holding an axle for the support o f the integral gearwheel wildcat brake rim and the pinion shaft with bearings.

Marine Electrical Technology

Chapter 19

Figure 19.1 - Pictorial Diagram of a Windlass From tiie maintenance point o f view, although enclosed gears running in an oil-bath are preferable on deck, an open gear protected by a guard is generally accepted on a chain-lifting unit. The large gear teeth are not corroded easily. The open gear facilitates the transfer o f torque directly to the wildcat from the gear rim that allows the design o f a simple and rigid chain-lifting unit o f moderate weight. A n automatic lubricator (greaser) for the gearwheel eases operation and maintenance.

Figure 19.2 - A Typical Electric Windlass

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Marine Electrical Technology

Deck Machinery

Hydraulic Motor

Limit Switch & Control Handle

Figure 19.3 - Schem atic D iagram o f a H orizontal E lectro-hydraulic W indlass

Marine Electrical Technology

759

Chapter 19

Figure 19.4 - Layout of Mooring Winches and Windlasses on a VLCC One or both pinion shaft ends are fitted with a coupling for connection to mooring winches. In the case o f combination units, each wildcat and mooring winch rope drum is provided with a band brake as shown in Figures 19.6 and 19.8. The wildcat brake restrains the chain when the anchor is let go under a controlled drop to veer the chain as desired, and to hold the chain while the chain stoppers are being attached. Usually the winch drum or intermediate shafting has one or more warping heads keyed to them. The gear train driving the warping head usually allows a line pull in the order o f one quarter o f the available wildcat chain pull, at four times the normal chain speed. A selfcontained horizontal type o f windlass is the cheapest to install. But it requires more maintenance than the vertical type because the windlass machinery is completely exposed to the harsh weather conditions and to spray and waves that wash over the bow during stormy conditions. In the preliminary stages o f a vessel’s construction, it is advisable to develop the anchor­ handling arrangement to the extent that the whole arrangement is confirmed to be satisfactory. In the case o f ships with large bulbous bows, the anchors must be located further aft or closer to the rail so that either anchor will not hit the bulb when it is dropped. This proved difficult when the first ships were retrofitted with bulbous bows and there are cases when, while letting-go, the anchor hit the bulbous bow! In the present scenario, two separate windlasses are generally installed, each set at an angle to the ship’s centerline in order to obtain proper alignment with the hawse-pipes, and facilitating an “anchor-on-deck” arrangement (Refer Figure 19.4 above). A well-rounded fairing plate guides die chain and anchor. The arrangement is integrated with the deck structure and is incorporated with means o f securing the anchor at sea and sustaining the load o f the chain when anchored.

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Marine Electrical Technology

Deck Machinery

19.1.2 19.1.2.1

T ypical Deck M o o rin g E lectro-hydraulic System on a VLCC F orw ard System

Triple control stations are situated on the port and starboard sides o f the forecastle; they operate the two anchor windlass / winches and also the forward mooring winch. Double control stations are situated on the port and starboard sides o f the forward main deck; they operate die two forward main deck mooring winches (Refer Figure 19.4). 19.1.2.2

A ft System

Double control stations situated to port and starboard o f the aft main deck operate the two aft main deck mooring winches. Triple control stations situated to port and starboard o f the after end o f the poop deck operate the after three mooring winches on the poop deck (Refer Figure 19.4). 19.1.2.3

Suggested Procedure fo r the O peration o f the H yd ra u lic Power U nits

a) Check the level o f the tank. If it is low, then transfer oil from the reserve tank. b) Ensure that the filters are clean and that their shut off valves are secured in the normal position. c) Check that the shut off valves are secured in the normal position. d) Check that the changeover cocks are secured in the normal position. e) Close the isolators for each pump and start the supply fan. f)

A t the auxiliary function panel for the power unit, set the cooling fan and circulation pump selection to “Auto”. The fan and circulation pump will cut in when the oil temperature has reached 45°C and cut out at 40°C.

g) Start the required pumps one by one in the “standby low pressure” mode. h) After starting, check for any leakage and ensure the sound o f the pump is normal. i)

When the mooring operation is ready to commence, switch the start control to either working pressure or standby high pressure.

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Chapter 19 19.1.2.4 Pressure Selection Definition 1. Standby low pressure: For starting the pumps and system warm-up 2. Standby high pressure: For anchor payout operation on auto-mooring 3.

Working pressure: For mooring operation or anchor heaving operation

19.1.2.5 Stopping the Units a)

Set each pump to standby low.

b)

Stop each pump one by one.

c)

Stop the supply fan.

19.1.2.6

Controls

A local control valve is mounted on each hydraulic motor and is activated by a three position lever which, on release, is spring-centred to the neutral stop position. The other two positions are ‘heave’ and ‘lower’. The speed is variable, according to the amount the lever is deflected towards the heave or lower positions, within die range o f the hydraulic unit. On the side o f the local operating valve is a range valve. This valve is a two-position manual lever “Auto Speed Selection” and “Low Speed”. For anchor handling duty, the speed setting should remain in the “Low Speed” position.

Lamp Switch Push Button

Figure 19.5 - Remote Control Panel on a VLCC 762

Marine Electrical Technology

Deck Machinery

19,1.3 The V ertical Windlass

It mainly consists o f a wildcat (with the chain wrapped 180° around it). The wildcat is mounted on a vertical shaft, which is carried in a rugged set o f main bearings in a casting or welded structure. The structure is in turn bolted or welded to the deck (Refer Figures 19.6 and 19.7). It thus adds to deck strengthening. The strengthening o f the deck and supporting structure on board the ship is by way o f this assembly which is usually made adequate to sustain all anticipated loads due to the chain pull. It is independent o f the main shaft extension from the deck beneath it. The wildcat is also brought as close as possible to the deck so as to minimize bending moments due to the chain pull. The chain is usually wrapped approximately 180° around the wildcat, and then it enters a deck-pipe leading to the chain locker. The shafts from vertical wildcats and associated capstans are normally extended to the deck below where they are coupled to the main and intermediate shafts o f the transmission gearing. Some vessels have been using vertical windlasses with the gearbox and clutch being supported under the deck on which the wildcat is mounted. This alleviates the problem o f relative deck deflection and simplifies the installation and alignment o f the windlass, as the wildcat, transmission and band brake are supported from a common structure. Vertical windlasses usually incorporate completely enclosed gears. If necessary the shaft couplings allow limited relative vertical motions between the decks. The weather deck below protects the brake drum and locking head (clutch) from harsh weather conditions. This also facilitates the location o f the wildcat to be as close as possible to the weather deck. A capstan head may be keyed to the main shaft, above the wildcat to enable handling o f warping lines. However unless there is a speed change in the gearing or hydraulic transmission, the light-line hauling rate may be unsatisfactorily slow. An alternative arrangement is a capstan that is located adjacent to the wildcat and driven from the windlass gear train by a separate shaft whose r.p.m. is about four times as fast as the wildcat. The capstan head may be keyed to the drive train and allowed to rotate or remain idle when the windlass is used for anchor hauling duties.

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Chapter 19 A vertical windlass is advantageous wherein it permits flexibility when setting up o f anchor handling and mooring arrangements. It can range from the simplest single unit to as many independent systems as there are anchors installed.

Figure 19.6 - Schematic Diagram of a Vertical Electro-hydraulic Windlass

Figure 19.7 - Pictorial Diagrams of Figure 19.6 764

Marine Electrical Technology

Deck Machinery

Some systems incorporate a power unit driving a single variable stroke pump. The hydraulic system comprises selector valves thus permitting the single pump to drive either of two hydraulic motors serving two wildcat or capstan systems. Two power plants are installed and electrical, mechanical, and hydraulic cross connections or their combinations, depending on the basic characteristics o f die system, are included so that both anchors may be recovered despite a failure in any one system. This enhances reliability o f the entire system. A good anchor windlass brake must stop the anchor chain within about two seconds after the brake is activated. Due to the short time available for the brake to absorb most o f the kinetic energy possessed by the anchor and chain, the brake lining’s surface temperature usually rises. Elevated temperatures can bring about degradation o f the brake lining’s frictional characteristics such that the brake can be no longer capable o f arresting the anchor chain while running. I f the situation becomes uncontrollable, the situation worsens; i.e., as the anchor will continue running after the brake is activated, the quantity o f energy that the brake must absorb also increases and more heat is generated. This justifies the necessity for the brake to immediately arrest the anchor and chain after it is activated. Usually anchor windlass brakes are o f the lined-band type. The effectiveness o f these brakes is felt when the periphery o f the drum is wrapped almost completely. Auxiliary power-assist mechanisms for using the brake have also been used extensively on very large windlasses.

Figure 19.8 - A Band Brake 19.1.4

In d ica tio n and C ontrol

A windlass chain counter is an integral part o f an electronic or mechanical display o f the number o f feet or fathoms o f chain that have been paid out. It is also linked to a digital indicator in the wheelhouse. This enables the duty officer to remotely monitor the amount o f chain in use without sending another person to the windlass itself to check the marking on the chain. Marine Electrical Technology

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Chapter 19 The indicator also helps in calculating the amount o f chain to be paid out especially in known depths. Remote control o f the windlass brake enhances safety o f both the operator and the ship. I f the brake mechanism is not spring-loaded, the first step in remote operation is to set the brake. The wildcat clutch is then disengaged and any devices used to secure the chain at sea such as the devil’s claw or chain stopper are to be removed from the chain. The operator must then remain in an area where one can see the chain both outboard and on deck. Chain length monitors and speed indicators help the operator in controlling the anchor drop. A hydraulic cylinder, supplied from a dedicated accumulator, mechanically overrides the brake screw mechanism. A pilot or solenoid valve directly operates the activating valve for this cylinder. Inbuilt speed governing limits the rate o f fall o f the anchor. With speed control and solenoid valve operation, a remote control system for operation from the wheelhouse can be incorporated. The hand-wheel for the hydraulic stroking device is arranged in a horizontal plane. A n auxiliary hand-wheel for the stroking device must be located to assist in servicing or warming-up the unit. The stroke-control mechanism should be provided with a spring detente for the neutral position o f the stroking spindle. Limit switches are provided to prevent the pump from starting unless the pump and servo controls are in the neutral position. This ensures that the anchor does not start moving when the pump is started. Electrical master switches with detentes help die operator to identify the speed position selected. Hand wheels for controlling hydraulic windlasses should be equipped with a speed indicator marked "V4 , “l/2*, "3/ reduces it to grey spongy lead (Pb). The water formed by this action dilutes the electrolyte thereby causing the density to fall and the cell discharges. Measurement o f the density change with a hydrometer will indicate the state (of charge or discharge) o f the cell. At the negative plates o f the cell, sulphate ions (SO 4 ") combine with the pure lead o f the negative plates to form a layer o f white lead sulphate (PbSC>4).

Marine Electrical Technology

819

The lead sulphate layer increases during discharge and finally covers the active material o f the plate so that further reaction is stifled. Some sulphate also forms on the positive plates, but this is not a direct part o f the discharge reaction. Note: A digital load tester may be a good choice if you need to test sealed batteries 21.8.5 21.8.5.1

Additional Indicators Open Circuit Voltage Test 1) Can find gross faults 2) Can indicate the approximate state o f charge 3) Does not measure capacity 4) Does not measure internal resistance

21.8.5.2

Discharge Test 1) Accurately measures battery capacity 2) Can estimate remaining time 3) Heavy discharge can indicate weakness

The results o f your testing should be as follows: 1) Hydrometer readings should not vary more than 0.05 between cells. 2) Digital Voltmeters should indicate the voltages as required in the manuals. 3) The sealed AGM and Gel-Cell battery voltage (full charged) will be slightly higher in the 12.8 to 12.9 ranges. I f the voltage readings are in the 10.5 volts range on a charged battery, this indicates a shorted or defective cell. 4) For a maintenance free wet cell, the only ways to test it are with the help o f a voltmeter and a load test.

21.9

Polarization of the Cell

The chemical action that occurs in the cell while the current is flowing causes hydrogen bubbles to form on the surface o f the anode. This action is called “polarization”. Some hydrogen bubbles rise to the surface o f the electrolyte and escape into the air. Some remain on the surface o f the anode. If enough bubbles remain around the anode, the bubbles form a barrier that increases internal resistance. W hen the internal resistance o f the cell increases the output current decreases and thus the voltage o f the cell also decreases. 820

Marine Electrical Technology

Batteries and Battery Charging A cell that is heavily polarized has no useful output. There are several methods to prevent polarization or to depolarize the cell: 1. A vent on the cell allows the hydrogen escape into the air. A disadvantage o f this method is that hydrogen is not available to reform into the electrolyte during recharging. This problem is solved by adding water to the electrolyte (also known as topping-up), such as in an automobile battery. 2. This method uses a material rich in oxygen, such as manganese dioxide, to supply free oxygen to combine with the hydrogen and form water. 3. Here, a material such as calcium, to absorb the hydrogen is used. The calcium releases hydrogen during the charging process. All three methods remove enough hydrogen so that die cell is practically free from polarisation.

21.10

Local Action

When the external circuit is removed, the current ceases to flow, and theoretically, all chemical action within the cell stops. However, commercial zinc contains many impurities, such as iron, carbon, lead, and arsenic. These impurities form m any small electrical cells within the zinc electrode in which current flows between the zinc and its impurities. Thus, the chemical action continues even though the cell itself is not connected to a load. Removing and controlling impurities in the cell greatly increases the life o f the battery.

|21.11

N ickel-Cadmium Storage Batteries

This type o f battery possesses good qualities such as a high-rate o f discharge, low temperature capability, flat voltage and excellent cycle life. The active materials o f the positive and negative plates in each cell o f a charged nickel-cadmium (NICAD) battery are nickel hydrate and cadmium, respectively (Refer Figure 21.12). The chemicals are retained in the supporting structure o f perforated metal plates and the design is such as to permit maximum contact between active compounds and the electrolyte. The strong alkaline electrolyte is a solution o f potassium hydroxide in distilled water (with an addition o f lithium). The ions produced in the formation o f the potassium hydroxide solution (K+ and OH-) act as current carriers and participate in an ion transfer. 21.11.1 D isch arge A ction.

During discharge the chemical action at the positive plates (hydrated oxide o f nickel) results in hydroxyl ions (OH-) being introduced into the electrolyte. As this progress, the nickel hydrate is changed to nickel hydroxide. Simultaneously, hydroxyl ions (OH- ) from the electrolyte form cadmium hydroxide with the cadmium o f the negative plates. Marine Electrical Technology

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Chapter 21 Effectively, the hydroxyl ions (O tT ) move from one set o f plates to the other, leaving the electrolyte unchanged. There is no significant change in density through the discharge / charge cycle and the state o f charge cannot be found by using a hydrometer.

Charged - N i(0)0H Discharged - NiO(OH)2

Figure 21.12 - A Nickel Cadmium Cell 21.11.2

E lectrolyte

Potassium hydroxide solution is aqueous and strongly alkaline and the physical and chemical properties o f potassium hydroxide closely resemble those o f caustic soda (sodium hydroxide). The electrolyte also contains Lithium Bromide that reduces local action. It is corrosive, so care is essential when topping-up batteries or while handling the electrolyte. In the event o f skin or eye-contact, the rem edy is to wash the affected area with plenty of clean water for at least 15 minutes in order to dilute and finally remove the solution quickly. Speed o f response is vital in order to prevent any damage due to bums; and water, which is the best flushing agent, must be readily available. Neutralising compounds (usually weak acids) cannot always be located easily, although they should be available in battery compartments. The density o f the electrolyte in a Ni-Cd cell is about 1210 and this does not change with charge and discharge as in lead-acid cells. However, over a period o f time the strength o f the solution will gradually drop and renewal is necessary at about a density o f 1170.

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Marine Electrical Technology

Batteries and Battery Charging 21.11.3

C ontainers

The electrolyte slowly attacks glass and various other materials. Containers are therefore o f welded sheet steel, which is then nickel-plated, or moulded in high-impact polystyrene. Steel casings are preferred when batteries are subject to shock and vibration. Hardwood crates are used to keep the cells separate from each other and from the support beneath. Separation is necessary because the positive plate assembly is connected to the steel casing. 21.1 1 .4

P lates

The active materials for nickel-cadmium cells are improved by the addition o f other substances. Positive plates carry a paste initially made up o f nickel hydroxide with a small percentage o f other hydroxides to improve its capacity, and 20% graphite for better conductivity. The material is brought to the charged state by passing a current through it, which changes the nickel hydroxide to hydrated nickel oxide, NiO(OH). An addition o f 25% iron plus small quantities o f nickel and graphite improve the performance o f cadmium in the negative plates. Active materials may be held in the pockets o f sintered plates. The formers are made up o f nickel-plated mild steel strips, shaped to form an enclosing pocket. The pockets are interlocked at their crimped edges and held in a frame. The electrolyte reaches the active materials through perforations in the pockets. Sintered plates are produced by heating the powdered nickel to 900°C, which has been mixed with a gas-forming powder and pressed into a grid or perforated plate. This process forms a plate, which is 75% porous. Active materials are introduced into these voids. 2 1 .1 1 .5

S ea led N ickel-C adm ium B atteries

This type o f battery also possesses good qualities such as a high-rate o f discharge, low temperature capabilities and excellent cycle life. Gassing occurs as a conventional battery approaches its folly charged state; it increases during any overcharge due to electrolysis o f water in the electrolyte, by the current that is supplied but no longer being used in charging. The gas is released through the vent to prevent pressure build-up and this loss, together with loss from evaporation, makes topping up necessary. While on charge, the active material o f the plates is being transformed, but when the transformation is complete and no further convertible material remains, the electrical charging energy starts to break down the electrolyte._________________________________________________ _____________ _________ Marine Electrical Technology

823

Chapter 21 Oxygen is evolved at the positive plates and hydrogen at the negative. These are designed to be maintenance-free and, although developed from and having a similar chemical reaction to the open type, will not lose water through gassing or evaporation. The seal stops loss by evaporation and gassing is inhibited by modification o f the plates. Sealed cells are made with surplus cadmium hydroxide in the negative plate so that it is only partially charged when the positive plate is fully charged. Oxygen is produced by the charging current at the positive plates, the formula being 4 0 H “ + 2H20 + 4 e ' + 0 2 but no hydrogen is generated at the negative plate because some active material remains available for conversion. Further, the oxygen from the positive side is reduced with water at the negative plate ( 0 2 + 4e~ + 2H20 40H~), hence replacing the hydroxyl ions used in the previous action. The process leaves the electrolyte quantity unaffected. The hydroxyl ions, acting as current carriers within the cell, travel to the positive electrode (Refer Figure 21.13).

2H20 + 4e* + 0 2

-* ■

0 2 + 4e + 2H20

Figure 21.13 - A Sealed Nickel-Cadmium Cell 824

Marine Electrical Technology

Batteries and Battery Charging Sealed batteries will accept an overcharge at a limited rate, indefinitely without any pressure rise. Charging equipment is therefore matched for continuous charging at low current rates, or fast charging is used with an automatic cut-out to prevent an excessive rise o f pressure and temperature. Rise o f pressure, temperature and voltage all occur as batteries reach the overcharge zone, but the last two are mostly used as signals to terminate the “fullcharge” process. Recent developments have ensured longer life-spans in nickel-cadmium batteries which is one reason why they are regarded as economical for emergency purposes despite initial costs. W hen compared with other types, nickel-cadmium cells give an extremely good high-rate discharge performance. Thus, a smaller capacity battery is adequate for high-performance applications such as engine starting. The ripple content to which batteries may be subjected in high-technology applications does not affect the nickel-cadmium cell and maintenance o f these cells is infrequent. 21.12

Silver-Zinc Cell

The silver-zinc cell is used extensively to power emergency equipment. However, it is relatively expensive and can be charged and discharged fewer times than other types o f cells. W hen compared to lead-acid or nickel-cadmium cells, these disadvantages are outweighed by the light weight, small size, and good electrical capacity o f the silver-zinc cell. The silver-zinc cell uses the same electrolyte as the nickel-cadmium cell (potassium hydroxide and water), but the anode and cathode differ. The anode is made o f silver oxide, and the cathode is made o f zinc. This type o f cell has the highest specific energy, very good high-rate capability, low cycle life and high cost. 21.13

Silver-C adm ium Cell

The silver-cadmium cell is a recent development for use in storage batteries. The silvercadmium cell combines some o f the virtues o f nickel-cadmium and silver-zinc cells. It has more than twice the shelf-life o f the silver-zinc cell and can be recharged many more times. The disadvantages o f the silver-cadmium cell are high cost and low voltage production. The electrolyte o f the silver-cadmium cell is potassium hydroxide and water as in the nickel-cadmium and silver-zinc cells. The anode is silver oxide as in the silver-zinc cell, and the cathode is cadmium hydroxide as in the N1CAD cell. This type o f cell has high specific energy, good charge retention, moderate cycle life and high cost. Marine Electrical Technology

825

Chapter 21 j21.14

Lithium Ion Battery for 406 MHz EPIRBs

T he latest digital EPIRB is automatically deployed by a hydrostatic release and activated if the vessel sinks. It transmits on 406 MHz - 5 watts +2dB (COSPAS Space System for Search o f Distress Vessels - a Russian acronym and SARSAT i.e., Search and Rescue Satellite-Aided Tracking) with your registered, digitally-coded distress signal, and 121.5 MHz - 50mW +3dB (SAR homing frequency allowing aircraft and rescue craft to quickly find the vessel in distress). The AM signal frequency (406 MHz) has been designated internationally for use only for distress. A new type o f 406 M Hz EPIRB has an integral GPS navigation receiver with an accuracy o f about 100m (110 yards) GPS position accuracy, an optimum value allowed by COSPASSARSAT. This EPIRB will send an accurate location as well as identification information to rescue authorities immediately upon activation through both geostationary (GEOSAR) and polar orbiting satellites. The EPIRB o f this nature uses a special type o f Lithium Ion Class 1 battery for colder temperature operation. Once activated, it is possible o f operating a minimum o f 48 hrs at -40°C (i.e., -40°F). Some manufacturers claim about 11 years’ storage life for the battery. It is also designed for long-term low-power consumption operation. B ut the batteries m ust be replaced after five years or by the date indicated on the EPIRB label using the model specified by the manufacturer. It should be replaced by a dealer approved by the manufacturer. If the replacement battery is not o f the proper type, the EPIRB will not operate for the duration specified in a distress. Lithium-ion batteries can be formed into a wide variety o f shapes and sizes so as to efficiently fill available space in the devices they power. Li-ion batteries are lighter than other equivalent secondary batteries - often m uch lighter. The energy is stored in these batteries through the movement o f lithium ions. Lithium is the third lightest element, giving a substantial saving in weight as compared to batteries using much heavier metals. However, the bulk o f the electrodes effectively serve as a “housing” for the ions and add weight, and in addition, “dead weight” from the electrolyte, current collectors, casing, electronics and conductivity additives reduce the charge per unit mass to little more than that o f other rechargeable batteries.

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Marine Electrical Technology

Batteries and Battery Charging The advantage o f using Li-ion chemistry is the high open-circuit voltage that can be obtained in comparison to aqueous batteries (such as lead acid, nickel metal hydride and nickel cadmium). Li-ion batteries have a low self-discharge rate o f approximately 5% per month, compared with over 30% per month in nickel metal hydride batteries and 10% per month in nickel cadmium batteries. Lithium-ion batteries have a nominal open-circuit voltage o f 3.6 V and a typical charging voltage o f 4.2 V. The charging procedure is done at a constant voltage with current limiting circuitry. This means charging with constant current until a voltage o f 4.2 V is reached by the cell and continuing it with a constant voltage applied until the current drops close to zero. Typically the charge is terminated at 7% o f the initial charging current. In the past, lithium-ion batteries could not be fast-charged and typically needed at least two hours to fully charge. Current generation cells can be fully charged in 45 minutes or less; some reach 90% in as little as 10 minutes.

[21.15 Battery Charging Battery charging equipment uses a transformer-rectifier arrangement with reverse-current protection, to supply the required d.c. voltage to the cells. Reverse current protection prevents a failed battery charger component from discharging the battery. Rules require that the normal charging facilities are to be such that a completely discharged battery can be recharged to 80% capacity in not more than 10 hours. The magnitude o f voltage depends on the battery type and the method o f charging required. Check the manufacturer’s instructions for details o f the charging voltage needed. Do not allow electrolyte temperatures to exceed 45°C during charging. At 50°C, the charging must be terminated. N ote: Adding the active ingredient to the electrolyte of a discharged battery does not

recharge the battery. Adding the active ingredient only increases the specific gravity of the electrolyte. It does not convert the plates back to active material, and so does not bring the battery back to a charged condition. A charging current must be parsed through the battery to recharge it.

WARNING! A m ixture o f hydrogen a n d a ir can be dangerou sly explosive. N o sm oking, electric sparks, open fla m e s o r fla m m a b le m a teria l sh o u ld b e p erm itted in th e vicin ity o r in th e b a ttery com partm ent.

Marine Electrical Technology

827

Chapter 21 The following types o f charges may be given to a storage battery, depending on the condition o f the battery: y

Initial charge

y

Normal charge

y

Equalizing charge

y

Floating charge

y

Fast charge

21.15.1

In itia l C harge

When a new battery is shipped in its dry state, the plates are in a discharged condition. After the electrolyte has been added, it is necessary to charge the battery. This is done by giving the battery a long low-rate initial charge. The charge is given according to the manufacturer’s instructions, which are shipped with each battery. 21.15.2

N orm al C harge

A normal charge is a routine charge given according to the nameplate data during the ordinary cycle o f operation in order to restore the battery to its charged condition. 21.15.3

E qu alizin g C harge

An equalizing charge is a special extended normal charge that is given periodically to batteries as part o f a maintenance routine. It ensures that all sulphate ions are driven from the plates and that all the cells are restored to a maximum value o f relative density. The equalizing charge is continued until the density o f all cells, corrected for temperature, shows no change for a four-hour period. 21.1 5 .4

F loatin g Charge.

In a floating charge, the charging rate is determined by the battery voltage rather than by a definite current value. The floating charge is used to keep a battery at “frill charge” while the battery is idle or on light duty. It is sometimes referred to as a “trickle charge” and is done with low current - this is explained in article 21.15.5 o f this chapter. In larger installations, the bus bar voltage is maintained at a value that is at least 5 volts higher than the battery voltage. This prevents motoring o f the source (most often a generator); however this condition can also be prevented by simply providing reverse current protection within the circuit; such a provision normally trips the generator breaker in the event o f a reverse current condition.

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Marine Electrical Technology

Batteries and Battery Charging 21.15.5

F ast Charge o r Q uick Charge

A fast charge is used when a battery must be recharged in the shortest possible time. The charge starts at a much higher rate than is normally used for charging. It should be used only in an emergency, as this type o f charge may harm the battery. 21.15.6

C harging Rate

Normally, the charging rate o f storage batteries is given on the battery nameplate. If the available charging equipment does not have the desired charging rates, use the nearest available rates. However, the rate should never be so high that violent gassing occurs.

|21.16

Charging of Lead-Acid Batteries

To charge lead-acid batteries, the cell is disconnected from the load and connected to a d.c. charging supply o f the correct voltage. The positive terminal o f the charging supply is connected to the positive terminal of the cell, and likewise, the negative terminal o f the charging supply, to the negative terminal o f the cell. Flow o f current from the charging sources reverses the discharge action o f the cell. Lead sulphate on the plates is thus broken down. The accumulated sulphate goes back into the electrolyte as sulphate ions (SO4"), leaving the plates as pure lead. Water in the electrolyte breaks down, returning hydrogen ions (H*) to the solution, and allows the oxygen to recombine with lead o f the positive plate to form lead peroxide (PbOj). A fully charged cell will be capable o f producing about 1.95 volts on load and the relative density o f the electrolyte will be at a maximum value (say 1280-1300). The general crossover (maximum) voltage o f a new cell can be as high as 2.42V. 21.16.1

C harging Systems

In various installations, batteries are kept ‘floating on the line’ and are so connected that they are being charged when load demands are light and automatically discharged during peak periods, when load demands are heavy or when the usual power supply fails or is disconnected. In some other installations, the battery is connected to the feeder circuit as and when desired, allowed to discharge to a certain point when they are removed and re-charged for further requirements. For batteries other than the ‘floating’ and ‘system-governed’ type, the following two general methods (though there are some variations o f these) are employed.

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Chapter 21 21.16.2

C onstant-current System

This is usually used for nickel-cadmium and nickel-metal hydride cells or batteries; it is not widely used for batteries o f the lead-acid type. In this method, the charging current is kept constant by varying the supply voltage to overcome the increased back EMF o f cells, the process being much longer as compared to the constant voltage method. This method may be adopted to overcome sulphation in batteries that have been discharged beyond limitations or those that have been undercharged. If a charging booster is used, the current supplied by it can be kept constant by adjusting its excitation. It charges on a d.c. supply; varying the rheostat connected in the circuit, controls the current. The value o f charging current should be so chosen that there would not be excessive gassing during the final stages o f charging and, also, the cell temperature does not exceed 45°C. This method takes a comparatively longer time. A s mentioned earlier, charging must be interrupted if the temperature rises to 50°C. It may be resumed after a spell o f cooling and agitation o f the electrolyte, when the temperature falls to about 30°C. R h e o s ta t

Figure 21.14 - Constant C urrent System 21.16.3

C onstant-voltage System

A constant voltage charger is basically a DC power supply which in its simplest form may consist o f a step down transformer from the mains with a rectifier to provide the DC voltage to charge the battery. In this method, the voltage is kept constant but it results in very high values o f charging current in the beginning when the back EMF o f the cells is low and a low current when their back EMF increases on being charged. 830

Marine Electrical Technology

Batteries and Battery Charging

AW R

H| Ijl

Eb Note: With this method, the time of charging is almost reduced to half It increases the capacity by approximately 20% but reduces the efficiency by approximately 10%. F igure 21.15 - C onstant Voltage System 21.16.4

Calculations

When a secondary cell or a battery o f such cells is being charged, then the EMF o f the cell acts in opposition to the applied voltage. I f V is the supply voltage which sends a charging current o f 1 against the back EMF Eb (battery voltage), R being the total circuit resistance including internal resistance o f the battery, then the input power is VI but the power spent is E f. This power Ehl is converted into the chemical energy which is stored in the cell. By varying R, the charging current can be kept constant throughout.The charging current can be thus found from the following equation: 1= V- Eh R 21.16.5

Trickle Charging

When a storage battery is kept entirely as an emergency reserve, it is very essential that it should be found fully charged and ready for use when an emergency arises. Due to leakage and other open-circuit losses, the battery deteriorates even when it is idle or in an open-circuit condition. Hence, to keep it ‘fresh’, die battery is kept on a trickle charge. The rate o f trickle charge is low and is just sufficient to balance the open-circuit losses. For example, a standby battery for station bus bars capable o f giving 2000 A for 1 hour or 400 A h at the 10-hours’ rate will have a normal charging rate o f 555A.

Marine Electrical Technology

831

Chapter 21 A continuous ‘trickle’ charge o f 1A or so will keep the cells fully charged (without any gassing) and in perfect condition. When an emergency arises, the battery gets discharged; it is re-charged at its normal charging rate and then is reverted to a (continuous) trickle charge. 21.16.6 Indication s o f a F u lly - Charged C ell

The indications o f a folly charged cell are: (i) Gassing (it) Voltage (in) Density (iv) Colour of theplates (i)

Gassing

When the cell is fully charged, it liberates hydrogen at the cathode and oxygen at the anode; this process is known as ‘gassing’. Gassing at both plates indicates that the current is no longer doing any useful work and hence should be stopped. Moreover when the cell is folly charged, the electrolyte assumes a milky appearance. Towards the end o f a charging process and during an overcharge, the current flowing into the cell causes a breakdown or electrolysis o f water in the electrolyte, shown by bubbles at the surface. Both hydrogen and oxygen are evolved and released through ceil vent-caps into the battery compartment. There is an explosion risk i f hydrogen is allowed to accumulate (the flammable range is 4% to 76% o f hydrogen in air because at the upper limit, there is still a possibility o f 18 to 21% o f oxygen being present in the atmosphere to support the combustion, the other gases being negligible). Ideally the explosive value is 3% in an enclosed space and 4% in an open space. Thus regulations require good ventilation o f the battery compartment in order to rem ove this hazardous gas; precautions m ust also be taken against the use o f naked lights and prevention o f the occurrence o f sparks in an enclosed battery compartment. (ii)

Voltage

The voltage ceases to rise when the cell becomes folly charged. The value o f die voltage o f a folly - charged cell is a variable quantity being affected by the rate o f charging, the temperature and density o f the electrolyte etc. The approximate value o f the EM F is 2.1V to 2.42V. However Table 21.2 clarifies voltages under various conditions that a lead-acid is normally subjected to.

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Marine Electrical Technology

Batteries and Battery Charging

i Number j Nominal | of Cells | Voltage

Fully-Charged Float Voltage

Fully-Discharged Float Voltage

1 6

2 12

2.15 12.9

1.9 11.4

12

24

25.8

22.8

Discharge Voltage at Ah/20

2.0-1.7 12-10.2 24-20.4

Charge Voltage at Ah/5

2.1 - 2.30 j 12.6 - 13.8 | 25.2 - 27.6 |

Table 21.2 - Voltages under Various Conditions of a Lead-acid Cell Note: The state of charge of a sealed battery is to be estimated based on its open-circuit voltage (measured with a digital voltmeter thatpossesses 0.5% accuracy or higher. a)

12.6V -* 100% charged

b)

12.4V -* 75% charged

c)

12.2V -* 50% chaiged

d) 12.0V -* 25% charged e) < 11.9 -* 0% charged. (iii)

Density ofthe Electrolyte

A third indication o f the state o f charge o f a battery is given by the density o f the electrolyte (it must be remembered that the density o f pure water is taken to be 1000 and here, the values are relative to pure water). W e have seen from the chemical equation, that during discharging, the density o f electrolyte decreases due to the production o f water, whereas it increases during charging due to the absorption o f water. The value o f density when the cell is fully charged ranges from 1225 - 1285 (1300 maximum) and 1075 to 1130 when discharged up to 1.8 V (1.73V minimum). Density can be measured with a suitable hydrometer that consists o f a float, a chamber for the electrolyte and a squeeze bulb (also known as a siphon). However, electrolyte temperature affects the indication o f the correct density; it also depends upon the life o f the cell. Newer cells would obviously achieve higher levels o f relative density and older cells would achieve lower levels o f relative density. Thus for every 1.5°C that the electrolyte temperature is above the reference value, add 0.001 to the reading because it is noticed that the density and temperature relationships are inversely proportional. Likewise, for every 1.5°C that the electrolyte temperature is below the reference value, subtract 0.001 to the reading. Table 21.3 will serve as a guideline in this regard.

Marine Electrical Technology

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Chapter 21 Temperature

Correction

-5°C

-0.020

0°C

-0.016

+5°C

-0.012

+10°C

-0.008

+15°C

-0.004

+20°C

0

+25°C

+0.004

+30°C

+0.008

+35°C

+0.012

+40°C

+0.016

+45°C

+0.020

Table 21.3 - Correction for Density Readings based on Electrolyte Temperature Table 21.4 provides a general guideline to help identify the state o f charge with respect to the electrolyte density o f an average lead-acid battery: Density - Temperate Climates

Density - Tropical Climates

State o f Charge

1260-1280

1225

Fully Charged

1225-1250

1195

75% Charged

1200-1220

1165

50% Charged

1170-1190

1135

25% Charged

1110-1130

1075

Discharged

Table 21.4 - State of Charge versus Density in Temperate and Tropical Zones (iv)

C olour

The colour o f plates, when a cell is fully charged, is a deep chocolate brown for positive plates and a clear slate-grey for negative plates. 21.16.7

Topping Up

Batteries suffer water loss due to both gassing and evaporation, with a consequent drop in liquid level. Note: There is no loss ofsulphuric acidfrom the electrolyte (unless through spillage). Regular checks are to be made to ensure that the liquid level is above the top o f the plates and distilled water is added as necessary. Exposure o f die cell plates to the atmosphere will rapidly reduce the life o f the battery. Overfilling will cause the electrolyte to bubble out o f the vent. 834

Marine Electrical Technology

Batteries and Battery Charging 21.17

Charging of Nickel Cadmium Batteries

A direct current supply for the purpose o f charging is obtained from the a.c. mains, through the transformer and rectifier in the battery charger. The positive terminal o f the charging supply is connected to the positive terminal o f the cell, and likewise, the negative terminal o f the power supply to the negative terminal o f the cell. Flow o f current from the charging source reverses the discharge action. The reactions are complicated but can be summarised by the simplified equation in Table 21.5. Charged

Discharged

2 NiO(OH) + Cd

2Ni(0H)2 H20

Hydrated Cadmium Oxide of Nickel

+ Cd(OH)2

Nickel Hydroxide + Cadmium Hydroxide

Table 21.5 - Chemical Reactions in an Alkaline Cell The state o f charge o f an alkaline battery cannot be determined from its density value. The electrolyte density does not change during charge / discharge cycles but gradually falls during the lifetime o f the battery. The only indication o f a fully charged alkaline cell is when its voltage remains at a steady value o f about 1.6V - 1.8V. New alkaline cells have a density o f around 1190. When this falls to about 1145 (which may take 5-10 years depending on the duty cycle) the electrolyte must be completely renewed or the battery replaced. Discharge o f alkaline cells should be discontinued when the cell voltage has fallen to about 1.1V. 21,17.1 Gassing - N icke l Cadmium Types

An alkaline cell gasses throughout the charging period. The gases evolved during charging are oxygen (at the positive plates) and hydrogen (at the negative plates). The rate of production o f gas increases during periods o f overcharge. W hen hydrogen in the air reaches a proportion o f about 4% and up to 74% as mentioned earlier, it constitutes an explosive mixture. Good ventilation o f battery compartments is therefore necessary to remove the gas. Equipment likely to cause sparking or arcing must not be located or introduced into battery spaces. Vent-caps are non-return valves, as shown diagrammatically (Refer Figure 21.6) so that the gas is released, but contact by the electrolyte with the atmosphere is prevented. The electrolyte readily absorbs carbon dioxide from the atmosphere and deterioration results because o f the formation o f potassium carbonate. For this reason, cell vent-caps must be kept closed. Marine Electrical Technology

835

21.17.2 Topping-up Gassing is a consequence o f the breakdown o f water in the electrolyte. This, together with a certain amount o f evaporation means that topping up with distilled water will be necessary from time to time. High consumption o f distilled water would suggest overcharging. 21.18

T h erm al R unaw ay

Thermal runaway is a condition in which the current for a fully charged nickel-cadmium battery rises out o f all proportion to the impressed voltage. It is caused by heat from oxygen recombination and an inherent property o f most rechargeable batteries that causes their voltage to drop as they get hot. The battery can become dangerously hot, gas excessively, and eventually spew electrolyte. During overcharge, the oxygen generated in the cells can pass through or around the separator and recombine on the negative plates. The oxygen recombination generates heat, causing the battery temperature to rise and the battery voltage to drop, so that it draws a higher charge current. I f allowed to continue, the cadmium plate may ignite and bum. 21.18.1 Proceduresfor Detecting and Handling Thermal Runaway Check for the following conditions. 0

I f flames are present, use a CO 2 fire extinguisher

0

I f no flames are present, but smoke / fumes are noticed, or electrolyte is spilling out from the battery or vent tubes, spray the battery with low-velocity water fog.

W ARNING! S

Never spray COzfrom a portable fire extinguisher into a battery compartment for cooling or to displace explosive gases. The static electricity generated by the discharge could explode the gases trapped in the battery compartment especially if it is inadequately ventilated.

21.19

M ethods o f C harging

21.19.1 Charging with Supplyfrom a DC Source The circuit tor charging from a d.c. source o f supply includes a resistance connected in series, to reduce the current flow from the higher mains voltage. A simple, older type of charging circuit is shown in Figure 21.16.

836

Marine Electrical Technology

Batteries and Battery Charging Feedback (reverse current) from the battery that is being charged is prevented during a failure o f the main supply by the relay (which is de-energised) and springs that are so arranged as to automatically disconnect the battery. The contacts are spring operated; gravity opening is not acceptable to marine installations due to the many degrees o f freedom that a ship possesses. This is ju st a basic circuit. Today there are many more solid-state battery charging circuits that use electronic switching circuits. These circuits automatically cut-in the supply when the battery voltage falls below the desired level and likewise cut-out the supply when the battery is fully charged, thereby alleviating the difficulty encountered due to the harsh prevailing conditions at sea.

I

$

D C Supply

Spring

Figure 21.16 - Battery Charging with DC Supply (Battery Charged in the Figure) 21.19.2

C harging w ith Supply fro m an A C Source

The a.c. voltage from the main source o f supply is reduced by a transformer to a suitable value and then rectified to give a direct current for charging. The supply current may be taken from the 230-volt section and stepped down to say 30 volts for charging 24-volt batteries (the charging voltage is at least 5 volts greater than the battery voltage). Various transformer / rectifier circuits are available and any o f these could be used i.e. a single diode (half-wave rectification), two or four diodes (full-wave rectification), or a three-phase-sixdiode circuit Smoothing is not essential for battery charging but would be incorporated for power supplies to low-pressure d.c. systems with standby batteries, and for systems with batteries on float i.e., batteries that are constantly on the bars and the discharge occurs only if a failure o f the main system occurs. Marine Electrical Technology

837

Chapter 21 As seen in Figure 21.17, there is a transformer and bridge o f four diodes with a resistance to limit current. Many manufacturers may build the resistance into the transformer secondary. Voltage is dropped in the transformer and then applied to the bridge rectifier, which is connected to the batteries. The state o f charge that is held by a lead-acid battery' is best indicated by a check on the electrolyte density, using a hydrometer. A fully charged lead-acid cell has a density o f about 1270 to 1285 which falls to about 1100 when fully discharged. The cell voltage also falls during discharge and its value can also be used as an indication o f the state o f charge. Step-down

Figure 21.17 - Battery Charging from AC Supply Generally, alkaline cells are more robust, mechanically and electrically, than lead-acid cells. Nickel cadmium cells will hold their charge for long periods without recharging and so are ideal for standby duties. They also thrive on a ‘float charge’ to provide a reliable emergency supply when the main power fails. Emergency power or temporary emergency power can be provided by automatic connection o f a battery at the loss o f main power. A simple arrangement is shown in Figure 21.18 for lead-acid batteries.

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Marine Electrical Technology

Batteries and Battery Charging

Rectified AC or DC Emergency Load

------ 1

< ---------- >

F igure 21.18 - Em ergency B attery C ircuit This type o f secondary cell loses charge gradually over a period o f time. The rate o f loss is kept to a minimum by maintaining the cells in a clean and dry state, but it is necessary to make up the loss o f charge; the system shown has a trickle charge circuit incorporated within. Under normal circumstances the batteries are on standby with load switches (L) open and charging switches (C) closed. The relay’s solenoid holds the contacts in position against the pressure o f the spring. Loss o f main power has the effect o f de-energising the relay and the contacts are changed-over by spring pressure moving the operating rod. The batteries are disconnected from the mains as switch C opens, and connected to the emergency load by closing o f L. Loss o f charge is made up when the batteries are on standby, through the trickle charge, which is adjusted to supply a continuous, constant current. This is set so that it only compensates for losses, which are not the result o f an external load. The current value (50 to 100 mA per 100 ampere-hours o f battery capacity) is obtained by cross-checking with a trial value that the battery is neither losing charge (hydrometer test) nor being overcharged (gassing). When batteries have been discharged on load, the trickle current, set only to make up leakage, is insufficient to recharge them. Switching over to the quick charge regime restores a full charge. Afterwards batteries are put back on trickle charge. Marine Electrical Technology

839

Chapter 21 j21.20

Single-Rate an d Tw o-Rate B attery C hargers

Battery systems for generator sets are very important for m any reasons. Their reliability is o f much importance when critical loads are a factor. While the battery and charger work together to ensure uninterrupted power, the battery is ultimately responsible for reliable engine starts. If the battery fails the generator m ight not start resulting in the failure o f power supply to essential equipment and facilities. It is the battery charger’s duty to maintain batteries in their folly charged condition at all times in order to assure positive engine starts under routine or emergency conditions. We m ust remember that regulations require that the battery m ust be capable o f catering to at least three consecutive starts o f an emergency generator. The battery charger is designed to provide the batteries with a charging current anytime the charge level falls below acceptable limits. 21.20.1

Some Simple Steps to Select the Correct Charger

Step 1 Determine what type o f battery or batteries are being charged i.e., wet-cell (flooded), maintenance-free, AGM (absorbed glass mat), gel cell or VRLA (valve regulated lead acid). In m ost cases one charger will work for all except for the gel cell. However, some gel cell chargers will wofk well with the other battery types too. Step 2 Determine the capacity o f the battery; that means how many Ampere-hours (AH) the battery stores. Sometimes it is required to rate the charger for a quick recharge, therefore requiring more amperes from the charger. A lower-rated charger will mean increased charging time. The most important thing is to make sure there is enough power to do thejob required in the allocated time. Step 3 Know the desired outcome. Sometimes it is required to keep a battery charged all the time. In such a case, a simple, low-current charger will work fine. In other cases, a fast and powerful charger will be required to quickly restore a battery set. For example the batteries onboard for the generator sets could have two modes name ‘Trickle Charge” and “Boost Charge” In this case, the charger is capable o f providing a minimum o f two charging rates one very low and the other, higher. There are many designs o f battery chargers, but most modem ones are single-rate, two-rate or smart chargers.

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Batteries and Battery Charging 21.20.2 Single-Rate Charger Also known as a ‘trickle charger’, this unit will produce its maximum current for only a very short time, and then it begins to fall as the batteries’ state o f charge increases. The only way to maintain a higher current output into the battery would be to increase the charger’s output voltage. This, however, would cause serious overcharging problems as the battery becomes fully charged. Thus, the voltage chosen for any single-rate charger is a compromise between fast charging and optimum batteiy maintenance voltage. The most common problem with the single-rate charger is the loss o f electrolyte in batteries caused by a ‘boiling’ effect as they are overcharged. This leads to extensive wastage o f man-hours while replacing or servicing batteries. It should also be noted that due to their nature, most single-rate chargers sold as ‘5-ampere’ types never achieve its designed current output and therefore may offer the performance o f a 2-ampere charger! 21.20.3

Two-Rate Charger

Also known as ‘float chargers’, the two-rate charger automatically operates at a ‘boost’ charging voltage that allows the battery to draw the charger’s maximum output until it is almost fully charged; when the battery reaches a high rate o f charge, the charger shifts to the optimum float voltage condition so as to minimise the battery’s electrolyte consumption. Since the charger monitors the battery 100% o f the time, the optimum charge is provided without adjustments. The correct two-rate battery charger will offer fester charging performance and will reduce the requirement for maintenance. No compromise is made. The benefits o f the tworate battery' charger ensure the availability o f a much more reliable battery system. However, attention must be given to the various designs out in the marketplace. The minimum requirements should include: (a)

Temperature compensation which ensures correct charging in most conditions,

(b)

Output voltage regulation to maintain rated output regardless o f input voltage and frequency variations,

(c)

Current limiting,

(d)

Overload protection.

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Chapter 21 21.20.4

S m art C harger

This charger is designed to charge lead-acid type batteries based on computer-generated algorithms. Simply put, the charger collects information from the battery and adjusts the charge current and voltage based on this information. This allows the batteries to be charged quickly, correctly, and completely when using a smart charger (Refer Figure 21.19). 21.20.5

C alculation o f a B attery C h arger’s C apacity

As an example, if a cranking battery were about 100 ampere-hours, it would take a 20A charger approximately 6 hours to recharge it if the battery were completely dead. Another example, a marine deep cycle battery may be rated at 100 ampere-hours, so it would take a 10-ampere charger about 12 hours to recharge a dead battery to near a full charge i.e., from a completely dead condition. As a rule o f thumb take the ampere-hour rating o f the battery and divide by the charger rating (amps) and then add about 10% for the extra time to totally top off the battery; this determines the total charge time. The following formula can also be used to determine die required two-rate-charger ampere rating to recharge a battery used in an engine-starting application. This formula assumes that there is little or no continuous current drain on the charger and it is useful only in calculating the ampere rating o f two-rate chargers. It is also assumed that the charger will replenish only the ampere-hours withdrawn by the engine-cranking event. The voltage o f the battery system is immaterial to this calculation. Determine the current that the starter draws for the entire starting cycle. For example, assume die following: •

The starter (motor) draws 900 amperes o f current (worst case scenario).

•

The maximum cranking time per start attempt is 15 seconds, which equals 0.00416 o f an hour

•

The maximum nu^N er o f start attempts will be 5

Ampere-hours (AH) drawn by the starterfor this example is 900x 0.0042 x 5 — 18.72 AH. Decide how quickly you wish the battery to be recharged. Assume, for example, that you wish to recharge this battery in 5 hours. In order to find the charger’s ampere rating use 1.4 for lead acid and 1.8 for nickel-cadmium when asked for inefficiency constant: Total AH drawn by the starter multiplied by the recharge inefficiency constant / desired recharge hours = 18.9 x 1.4 = 5.29 ampere charger. 5

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Batteries and Battery Charging (21.21

Automatic Thyristor-controlled Battery Charger

3

21.21.1 Specifications Continuous

Rating Input

440V, 60Hz, 3-phase

Output (automatic)

V Float Voltage - 26 to 27 volts S Equal Voltage - 29 to 30 volts S Current range - 0.5 to 60 amperes

Output {manual)

•

Voltage range - 0 to 34 volts

•

Current range - 0.5 to 60 amperes

Commutation method

All-wave (3-phase)

Insulation Resistance

> 5MO (using a 500V megger immediately after load-testing)

Dielectric Strength

1500V AC for one minute

Alarms available

V 440V AC Power Failure v' Charging Failure (DC voltage failure or rectifier failure) V Earth Leakage V Over Current (60 amperes)

S Under Voltage (22 volts) The specifications o f the battery associated with the above charger are as follows: Battery

Lead Acid - Maintenance-free Type

Nominal Capacity

200AH

Rating

10 hours’ discharging

Float Charge

26 to 27V

Equalising Charge

29 to 30V

Quick Charge

50 amperes

Table 21.6 - Specifications of an Automatic Battery Charger 21.21.2

Control Modes

The selector switch permits the selection o f the following modes: 21.21.2.1

Auto Mode

In this mode, the voltage is automatically maintained at 26 to 27 volts and can be fed not only to the battery but also to the distribution board o f the battery charger.

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21.21.2.2 Equalising Mode In this mode, the voltage is 29 to 30 volts based on the battery condition and can be noted with the help o f the volt meter. 21.21.2.2 Manual Mode The voltage can be changed from 0 to 34 volts 21.21.3

Operating Principle

An isolating transformer is used to isolate the mains supply from the DC circuit. It also changes the mains voltage to a suitable level which is the rectified by the SCR bridge to obtain the DC output. The conducting duration o f each half period o f the A C waveform through the SCRs determines the DC output level. A filtering network consisting o f inductors and capacitors are used to smooth out the DC output to obtain low output ripple voltage. The control circuit monitors the output voltage, varies the conducting duration o f the SCRs to keep the output voltage constant irrespective o f mains supply voltage fluctuation and load current variation. When the load current exceeds the preset limit, a current limiting circuit reduces the conducting duration o f the SCRs to reduce output voltage hence the load current.

Figure 21.19 - Functional D iagram o f a S m art C h arg er

Marine Electrical Technology

Batteries and Battery Charging

D E S C R IP T IO N

SYMBOL H L

AH AL Af El Dl

HI LI

CONTENTS

DROP OCR DROP YD OVERVOLTAGE UNDERVOtTAGE POWER FAIL l/P TIMER EARTH iIP TIMER OVERCURRENT 1IP OVERVOLTAGE VP UNDERVOLTAGE l/P

11 iS

FLOAT EQ

FLOAT ALARM SETTING MODE

SOURCE

EQ

CHARGING MODE

POWER EARTH OVER UNDER FAIL LEAK CURRENT VOLT

OVER VOLT

RESET ALARM MODE

Figure 21.20 - A VLCC’s Battery Charger’s Monitoring Panel The slow-start circuit raises the output voltage gradually from zero to its preset level while starting. This minimizes the effect o f the inrush current surge on the mains supply; it also protects the rectifier from excessive current surge i f the output is short-circuited while starting. This feature is advantageous when it is charging up a fully discharged “flattened” battery which almost resembles an output short-circuit. Surge suppressors are incorporated to protect the rectifier against transient mains voltage. Any transient across the output is also suppressed.

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Chapter 21

Battery Charger Panel

Charger Monitoring Panel

Main Circuit Breaker

Emergency Feed C ircuit Breaker

24V Distribution Board

Earth insulation Meter Earth Lamps

Earth Test Push Button

■

Lamp Test Push Button

Figure 21.21 - A Modern Battery Charging System on a VLCC

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Batteries and Battery Charging

Figure 2 1 . 2 2 - Block Diagram of the Battery Charger

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From ESB 440 V Feeder Battery Charger 125A

Z }

> 24V - 200 AH (iO H rs) Lead-Acid Battery

63A 30A

Engine Control Console

3

Chart Console

10A 10A 5A 10A 10A 5A 10A 10A !0A IOA

Cargo Control Console -

Main Switchboard (Transient Light in ECR)

-

Fire Alarm Relay Box

- Automatic Telephone Exchange r ---------- --------------------------j Emergency Switchboard Transient Light) Elevator Control Panel Public Address System Miscellaneous Spare Spare

24V Distribution Board

F igure 21.23 - A Basic B attery C h arg er a n d D istribution Board Figure 21.23 above depicts a basic distribution scheme o f a 24V system. However, the following are generally supplied with power from the 24V DC Distribution Board on a VLCC: 848

Marine Electrical Technology

Batteries and Battery Charging Accommodation Temporary Light

•

Hospital Call System

Automatic Fog Bell and Gong

•

Lighting Control Panel

Automatic Telephone Exchange

•

Loran C Receiver

Bridge Alarm Console

•

Magnetic Compass

Bridge Main Console

•

Navigation Light Panel

Cargo Control Console

•

Navtex Receiver

Echo Sounder Depth Indicator

•

No. 1 GPS Receiver

Echo Sounder M ain Cabinet

•

No.2 GPS Receiver

Electric Clock

•

N o.l Gyrocompass

Elevator Control Panel

•

No.2 Gyrocompass

Emergency Generator Room Temporary Light

•

Public Address System

Engine Control Console

•

Satcom Power Supply

Engine Room 24V Distribution

•

Wheelhouse Group Control Panel

|21.22

Battery Installations and Safety Measures

21.22.1

Com m on C auses o f B attery a n d B attery C h arger F ailure

First o f all it is necessary to find out why the battery is dead! This m ay be attributed to the following: 21.22.1.1

D evice S w itches , L igh ts or O ther E lectrica l D evices L e ft On

Never leave any system (load) that is connected to a battery (source) unattended or if need be, the same should be monitored at regular intervals to avoid discharging the battery to dangerously low levels. 21.22.1.2

S h o rt E n gin e R u nning P eriods

The engine’s starting battery cannot be charged without running the engine long enough; however this may be overcome by having an external battery charger that cuts o ff when the engine starts. Engine starting batteries are n o rn rL y installed in the same space where the engine is installed and are located close to the engine. 21.22.1.3

K ey O ff-loads

Loads that are still drawing current from the batteries (even with the switch in the off position) many electronic circuits in the panel (being fed by the same battery) could often cause this problem. Marine Electrical Technology

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21.22.1.4

Parasitic Drain

M inor short-circuits in the wiring o f one or more pieces o f equipment connected to the battery. These are generally not large loads or they would trip a fuse or breaker. They cause batteries to gradually discharge over time. Electrolyte on the cell-top also leads to this... 21.22.1.5

Deficient Charging

W hen the system cannot fully charge the battery during a normal cycle o f operation, there is a decline in capacity (shorter run time) and reduced battery life. 21.22.1.5.1

Typical Causes

•

Engine alternator voltage and / or amperage is too low,

•

Engine run time / Charger ‘'On time” not long enough to recharge batteries.

«

High accessory loads (lights, bells, alarm systems, etc.)

21.22.1.5.2

Solutions

•

Shut o ff accessories (if any) when possible or leave the engine running

•

Periodically use an external charger to folly charge the batteries; i f there is a boost­ charging facility, which is generally the case, switch to “Boost-charge”

21.22.1.6

Mixing Different Types ofBatteries Together

W hen batteries o f different types, for which different electrolytes are used, are installed in the same room, they are to be segregated and effectively identified. Connecting different types o f batteries together will lead to shorter battery life and possible overcharge or undercharge problems with individual batteries. Premature failure L bound to happen; the solution to this is to only connect together, batteries o f identical make and model. Never mix different battery types. 21.22.1.7

Leaving Batteries in a Discharged Condition

At a low state o f charge, without some type o f energy input, as little as 24 hours in hot weather and several days in cooler weather, leads to sulphation on the plates. This reduces battery capacity and leads to premature battery failure. Irreversible damage can occur in a very short period. Sulphation can also be caused due to the following: 1) Deep discharging an engine starting battery; remember these batteries can’t stand deep discharge. 850

Marine Electrical Technology

Batteries and Battery Charging 2) Undercharging o f a battery e.g., to charge a battery to 90% o f its capacity will allow sulphation o f the battery using the 10% o f the battery’s chemicals not reactivated by the incomplete charging cycle. 3) Low electrolyte level - battery plates exposed to the air will immediately sulphate. 4) Incorrect charging levels and settings. Most cheap battery chargers can do more harm than good. 5) Cold weather is also hard on the battery. The chemistry does not make the same amount o f energy as a warm battery. A deeply discharged battery can also freeze solid in sub zero weather as the relative density will be very close to that o f fresh water! 6) Heat o f temperatures greater than 100° F, increases internal discharge. As temperatures increase so does internal discharge. 7) A new fully charged battery left sitting 24 hours a day at 110° F for 30 days would most likely not start an engine. Note: For every 10°C rise in temperature beyond the nominal value, the self discharge rate doubles! 21.22.1.8

Positive Grid Corrosion and Flaking

This can be caused due to: 1) Over voltage, over charging, wrong orfaulty charger 2) High temperatures 21.22.1.9 Loss ofElectrolyte This can be caused due to: 1) Over voltage, over charging, wrong orfaulty charger 2) Decompression ofcell, faulty valve, mishandling 3) High temperatures especially during charging 21.22.1.10

Cell Poisoning

This can be caused due to impurities in the electrolyte accumulating over a period o f time.

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Chapter 21 21.22.2

F ailure o f S o ld ered J oin ts C arryin g H igh C urrents as in B a ttery C hargers

The following article is an extract from a paper by Aditya Kumar, Zhong Chen, C. C. Wong, S. G. Mhaisalkar, Vaidhyanathan Kripesh, and Published by Materials Research Society, USA (www.mrs.org) Quote E lectric C urrent In d u ced B rittle F ailure o f E u tectic L ea d a n d L ead-free S o ld er J o in ts with E lectroless N i-P M etallization

The mechanical properties o f thermally-aged and electric current-stressed eutectic lead (Sn-37Pb) and lead-free (Sn-3.5Ag) solder joints with electroless Ni-P metallization were investigated using tensile testing. Multi-layered test samples, electroless Ni-P / solder / electroless Ni-P, having two electroless Ni-P/solder interfaces were prepared. Tensile testing results showed that for both types o f solder, high density electric current causes the brittle failure o f solder joint. The eutectic lead solder joint was found to be more prone to current induced brittle failure compared to the lead-free solder joint. In the eutectic lead solder joint, brittle failure always occurred at the cathode side electroless Ni-P/Sn-37Pb interface (where electrons flowed from Ni-P to solder), whereas no such polarity effect was observed in the case o f lead-free solder joint. Unquote 21.22.3

S a fety M easures W hen W orking W ith B atteries

We m ust think about safety when we are working around and with batteries. Just remember you are working with corrosive acid, explosive gases and sometimes, 100’s o f amperes o f electrical current. The explosion risk in battery compartments is reduced by: 1) Ensuring good ventilation so that the hydrogen cannot accumulate; and 2) Taking precautions to ensure that there is no source o f ignition. Ventilation outlets are arranged at the top o f any battery compartment where the lighterthan-air hydrogen (16 times lighter than oxygen) tends to accumulate. If the vent is other than direct to the atmosphere, an exhaust fan is required, and in any case would be used for large installations. It is obvious that the fan is in the air-stream from the compartment and so the blades must be o f a material that will not cause sparks from physical contact or due to an electrostatic discharge.

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Batteries and Battery Charging The fan motor is to be o f a certified safe type and installed outside o f the ventilation passage with seals to prevent entry o f gas to its casing. The exhaust fan m ust be independent o f other ventilation systems and capable o f completely changing the air in the battery room in not more than two minutes. Where the ventilation capacity is based on low-hydrogen emission type batteries a warning notice to this effect is to be displayed in a visible place in the battery room. All outlet vent ducts are made o f a corrosion-resistant material or protected by a suitable type o f paint. Ventilation inlets should be below battery level. With these and all the openings, consideration should be given to weather-proofing. The use o f naked lights and smoking are prohibited in battery rooms and notices are required to this effect. All safety notices should be backed up by verbal warnings because the presence o f dangerous gas is not obvious. Gas risk is highest during charging or if ventilation is reduced. W hen working on batteries there is always the risk o f short circuit faults occurring by accidentally dropping metal tools across tenninals (metal jugs are not to be used as distilled water containers for this reason). Cables must be adequately rated and connections m ust also be made well. Emergency switchboards are not placed in the battery space because o f the risk o f arcing. This precaution is extended to also include any non-safe electrical equipment, battery testers, switches, fuses, and cables other than those for the battery connections. Externally fitted lights and cables are recommended, with illumination o f the space through glass ports in the sides or deck head. Alternately, flameproof light fittings (Ex d equipment) are permitted. Ideal temperature conditions are in the range o f 15°C to 25°C. Battery life is shortened by temperature rises above 50°C and low temperatures reduce capacity. In addition to the safety measures already mentioned, observe the following safety precautions when working with batteries: Do’s El Handle all types o f batteries with care. 0

Wear protective clothing, such as a rubber apron and rubber gloves when working with batteries. Electrolyte will destroy everyday clothing such as overalls / boiler suits.

0

Wear chemical splash-proof safety glasses when maintaining batteries,

0

Take care to prevent spillage o f electrolyte especially when topping up cells.

0

Batteries with a liquid electrolyte should always be transported in the upright position to avoid spillage o f electrolyte. Marine Electrical Technology

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Chapter 21 0

Use carrying straps when transporting batteries.

0

Remove all jew elry - Even watches are dangerous!

0

Carryout regular inspection and maintenance especially in hot weather as gassing is accelerated.

0

Recharge batteries immediately after discharge.

0

W hen electrolyte is being prepared, the concentrated sulphuric acid must be added slowly or trickled into the distilled water.

WARNING/ I f d istilled w ater is added to th e acid, th e h ea t g en era ted m ay cau se evolution o f steam , sp a tterin g a cid a ll over th e p la c e / The consequences o f a c id burns a re w ell know n to a ll D o n ’ts

0

Don’t short the terminals o f a battery (also avoid wearing watches and jewellery).

0

Don’t exchange battery tools (including hydrometers) between lead-acid, batteries and nickel-cadmium batteries.

0

Don’t install alkaline and lead-acid batteries in the same compartment - even tools must not be common.

0

Don’t add new electrolyte (acid) to batteries that are in use.

0

Don’t use unregulated high output battery chargers to charge batteries.

0

Don’t keep the battery in a discharged state. It is not advisable to delay the recharging of batteries.

0

Don’t disconnect battery cables while the engine is running (your battery acts as a filter).

0

Don’t add ordinary water as it may contain minerals that will contaminate the electrolyte.

0

Don’t discharge a battery any deeper than you possibly have to.

0

Don’t let a battery get hot to the touch and boil violently when charging.

21.23

First Aid Treatment for Contact due to Spillage

We already know that an alkaline cell has an electrolyte o f potassium hydroxide while a lead-acid cell uses sulphuric acid. Both are diluted with distilled water. The first aid treatment if you were splashed with either electrolyte in both cases would be to rapidly wash the affected parts e.g., the eyes and skin with lots o f fresh water.

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Marine Electrical Technology

Batteries and Battery Charging The electrolyte o f alkaline cells causes skin burns, which should be treated with boracic powder, and the eyes must be washed out with a solution o f boracic powder - (one teaspoonful to 500ml o f water). Sulphuric acid splashes can be washed with a saline solution two teaspoonfuls o f household salt to 500ml o f water. For both types o f battery, first aid equipment should be available in the battery compartment.

121.24 Reclaiming, Recycling and Re-using Lead Acid Batteries Modem batteries, with a lifespan between 3 to 5 years under normal circumstances, are over 99% recyclable and virtually all batteries are completely recycled. Less than 20% o f the lead used to make lead-acid batteries today is virgin lead. How does the recycling process work? Battery users return their used “junk” lead-acid batteries to the point o f sale - retail store, service station, etc., or to a highly regulated and monitored drop-off site. Fleets o f trucks owned and/or operated by lead-acid battery manufacturers or secondary smelters transport “junk” batteries to secondary lead smelters. There the lead plates o f a battery are melted and refined and the plastic is separated and sent to a “reprocessor”. Purified lead is delivered to battery manufacturers and other lead-use industries. The acid is collected and either recycled or neutralized.

i21.25 21.25.1

Relevant Rules R eleva n t S O L A S R egu lation s (C hapter ll~ l)

Part D - Electrical Installations: Regulation 42 - Emergency source o f electrical power in passenger ships - Sub-paragraphs 2.3; 2.6; 3.2; 4; 4.1.2; 5.3 and 6. Regulation 42-1 - Supplementary emergency lighting for ro-ro passenger ships - Subparagraph 1.1. Regulation 43 - Emergency source o f electrical power in cargo ships - Sub-paragraphs 2.4.4; 2.6.2; 3.2; 4; 5.3 and 6 Regulation 44 -- Starting arrangements for emergency generating sets - Sub-paragraphs 2; 2.1 and 3.

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Chapter 21 21.25.2

Sum m ary o f SOLAS Regulations

1)

The batteries must be capable o f being independently charged, i.e,, there must be dedicated charging systems with a re-charge capacity o f at least 6 hours, using the Constant Voltage (quick-charge method).

2)

The electrical fittings in the Battery Room (compartment) must be Flame-proof.

3)

Alkaline and Lead-acid Batteries must not be kept in close proximity to each other.

4)

Battery Rooms m ust be fan-ventilated for 20 KWh ratings and above. However it is always advisable to have a well-ventilated battery room that is also hazard free.

5)

Batteries must always be fixed to their bases. Placing them on a wooden grating makes good sense.

6)

Spillage o f electrolyte must be avoided.

7)

The batteries m ust be able to provide a “full output” for at least 30 minutes especially where they serve as transitional sources o f power during black out situations.

21.25.3

Relevant A B S Rules

Extractfrom ABS Rulesfor Building and Classing Steel Vessels - 2012 Part 4 Vessel Systems and Machinery - Chapter 8 Electrical Systems Section 3 Electrical Equipment Quote

5.9

Battery Systems and Uninterruptible Power Systems (UPS) (2008)

In addition to the applicable requirements in 4-8-3/5.3, equipment for essential, emergency and transitional sources o f power are to comply with the following. Such equipment would include die battery charger unit, uninterruptible power system (UPS) unit, and the distribution boards associated with the charging or discharging o f the battery system or uninterruptible power system (UPS). 5.9.1 Definitions (2008) Uninterruptible Power System (UPS) - A combination o f converters, switches and energy storage means, for example batteries, constituting a power system for maintaining continuity o f load power in case o f input power failure. Off-line UPS unit - A UPS unit where under normal operation the output load is powered from the bypass line (raw mains) and only transferred to the inverter if the bypass supply fails or goes outside preset limits. This transition will invariably result in a brief (typically 2 to 10 ms) break in the load supply.

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Marine Electrical Technology

Batteries and Battery Charging Line interactive UPS unit - An off-line UPS unit where the bypass line switch to stored energy power when the input power goes outside the preset voltage and frequency limits. On-line UPS unit - A UPS unit where under normal operation the output load is powered from the inverter, and will therefore continue to operate without break in the event o f the supply input failing or going outside preset limits. DC UPS unit- A UPS unit where the output is in DC (direct current). 5.9.2 Battery Charging Rate (2008) Except when a different charging rate is necessary and is specified for a particular application, the charging facilities are to be such that the completely discharged battery can be recharged to 80% capacity in not more than 10 hours. See also 4-8-3/5.9.6(c). 5.9.3 Reversal o f Charging Current (2008) An acceptable means, such as reverse current protection, for preventing a failed component in the battery charger unit or uninterruptible power system (UPS) unit from discharging the battery, is to be fitted. 5.9.5 Location (2008) 5.9.5(a) Location. The UPS unit is to be suitably located for use in an emergency. The UPS unit is to be located as near as practical to the equipment being supplied, provided the arrangements comply with all other Rules, such as 4-8-4/5,4-8-4/7 and 4-8-4/9 for location of electrical equipment. 5.9.5(b) Ventilation. UPS units utilizing valve regulated sealed batteries may be located in compartments with normal electrical equipment, provided the ventilation arrangements are in accordance with the requirements o f 4-8-4/5.3 and 4-8-4/5.5. Since valve regulated sealed batteries are considered low-hydrogen-emission batteries, calculations are to be submitted in accordance with 4-8-4/5.5 to establish the gas emission performance o f the valve regulated batteries compared to the standard lead acid batteries. Arrangements are to be provided to allow any possible gas emission to be led to the weather, unless the gas emission performance o f the valve regulated batteries does not exceed that o f standard lead acid batteries connected to a charging device o f 0.2 kW. Unquote

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Chapter 21

1)

When choosing a battery for a particular application, major consideration should be given to the battery’s ________ . a)

ampere-hour capacity (b) terminal polarity

c) stability under charge (d) ambient temperature rise 2)

In a temperate climate, the specific gravity o f a fully discharged lead-acid battery i s __.

3)

In a tropical climate, the specific gravity o f a fully discharged lead-acid battery i s ___ .

4)

Leaving a battery discharged will lead t o ________ .

5)

Electrolyte will destroy everyday clothing such as overalls / boiler suits. (T / F).

6)

Batteries with a liquid electrolyte should always be transported in the upright position to av o id ________ .

7)

In a lead acid battery, the number o f negative plates i s _________the number o f positive plates.

8)

The state o f the charge in a lead acid battery is best indicated by th e _______ .

9)

Separators are provided in the lead acid battery t o _______ .

10)

The active elements on the positive and negative plates o f a fully charged lead acid battery a re ________ .

11)

The colour o f a fully charged positive plate o f a lead acid cell i s ________ .

12)

The capacity o f a storage battery is measured in ________ .

13)

If acid is added to distilled water, the heat generated may cause evolution o f steam, spattering acid all ov^r the place! (T / F).

14)

Since lead-acid batteries and nickel-cadmium batteries supply emergency systems, battery tools (including hydrometers) must be common for them (T / F).

15)

I f externally fitted lighting is not used, __________ are permitted in battery compartments

16)

Ideal temperature conditions in battery compartments are in the range o f.________ .

17)

Battery life is shortened by temperature rises a b o v e _________ and low temperatures reduce capacity.

18)

In Battery compartments, externally fitted lights and cables are not recommended, especially with illumination o f the space through glass ports in the sides or deck head. (T /F ).

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Batteries and Battery Charging 19)

Gas risk is highest during battery______ or if ventilation is reduced.

20)

Only metal jugs are to be used as distilled water containers for batteries as they are rugged. (T / F).

21)

Emergency switchboards are not placed in the battery space because o f the risk o f ___ .

22)

In a battery compartment, ensure good ventilation so that the ______ cannot accumulate.

23)

W hen batteries are utilised / maintained, take care to ensure th a t________ .

24)

The electrolyte in a lead-acid storage battery consists o f distilled water a n d ________ . a)

calcium chloride (b) sulphuric acid (c) muriatic acid (d) none o f the above

25)

A

lead-acid battery may become hotter than normal during a charge i f it has a ________ cell.

26)

When choosing a battery for a particular application, major consideration should be given to the battery’s _________ .

27)

The electrolyte used in a nickel-cadmium battery is distilled water a n d ________ . (a)

28)

29)

dilute sulphuric acid (b) potassium hydroxide (c) lead sulphate (d) zinc oxide

With respect to an emergency standby lead acid battery: a)

Describe the safety aspects considered in the design and construction.

b)

State, with reasons, two causes o f short circuit.

c)

State, with reasons, five causes o f fall off in rated capacity.

Describe the changes whi ch take place within a lead acid battery during discharge and when charging is taking place. Explain: a)

How the rate o f charge will affect gassing,

b)

The risks associated with gassing and safeguards in battery compartments,

c)

The reasons for distilled water top up,

d)

The remedy for spillage o f electrolyte on the skin.

30)

Batteries used for diesel engine starting should be o f th e __________ type.

31)

When lead-acid batteries are charging, they always give off hydrogen gas that is harmless (T / F).

32)

Battery charging rooms should be well ventilated because the charging process produces___________. Marine Electrical Technology

859

Chapter 21 33)

Regarding battery charging rooms, (intermittently / continuously / never).

34)

When a battery is continuously exposed to low temperatures, the best procedure to keep it from freezing is t o ________ .

35)

The freezing point o f the electrolyte in a fully charged lead-acid battery will be

36)

W hat is an acceptable lining for battery trays containing alkaline batteries?

37)

W hen charging lead-acid batteries, the charging rate should be reduced as the battery nears its frill charge to ensure th a t__________ .

38)

The electrolyte used in a nickel-cadmium battery is distilled water an d ____________.

39)

The capacity o f a storage battery is measured in __________ .

40)

A lead-acid battery is considered frilly charged when th e __________ .

41)

When the electrolyte level o f a lead-acid storage battery has decreased due to normal evaporation, the level should be restored by adding__________ .

42)

The standard procedure for maintaining the charge in an emergency diesel starting battery is t o __________ charge the battery.

43)

W hen a lead-acid battery begins gassing freely while receiving a normal charge, the charging current should b e __________ .

44)

When a nickel-cadmium battery begins gassing while connected to the battery charging circuit, you should_________ .

45)

W hen charging lead-acid batteries, you should reduce the charging rate as the battery nears its full charge capacity to ensure th a t__________ .

46)

From the standpoint o f safety, you should never allow salt water to enter a lead-acid storage battery or come in contact with sulphuric acid because____________.

47)

A lead-acid battery can deliver 10 amperes continuously for 10 hours with an amperehour rating o f _________ .

48)

A 24 volt lead-acid battery is constructed o f _________ cells.

49)

If violent gassing occurs when a lead-acid storage battery is first placed on charge, it is due to_________ .

50)

The electrolyte in a lead-acid storage battery consists o f distilled water and

860

ventilation should be _______

Marine Electrical Technology

provided

Batteries and Battery Charging 51)

The relative density o f the electrolyte solution in a lead acid battery i s _________ when charged.

52)

When checking the specific gravity o f the battery electrolyte with a hydrometer, you should be aware o f the following: __________ .

53)

List types o f batteries for marine applications.

54)

State the specific gravity o f lead-acid electrolyte in fully charged and fully discharged conditions.

55)

What is an appropriate method to check the condition o f a lead-acid battery?

56)

What is the range o f voltages o f a lead acid cell?

57)

What is battery capacity rated as?

58)

Explain the capacity test on lead acid batteries.

59)

Why must the two types o f cells be kept separate?

60)

What does the value o f the electrolyte density indicate in lead acid batteries?

61)

What arrangement is used in battery charging equipment?

62)

During charging which gas is given off?

63)

What is the maximum allowable temperature o f the electrotype during charging?

64)

Discuss the precautions needed for a maintenance free battery (Alkaline battery).

65)

What is the only indication o f a fully charged alkaline cell?

66)

Identify the type o f battery used in an EPIRB (Emergency Position Indicating Radio Beacon).

67)

What are the chemical changes that occur during the Charging and Discharging processes o f a Lead-acid cell? Explain with suitable equations,

68)

Are there any related effects while charging and discharging?

69)

With suitable diagrams and graphs, explain the Constant Voltage and Constant Current Charging Methods.

70)

What are the chemical reactions that occur while charging and discharging? Are there any related effects?

71)

How is charging from AC mains done? Explain the same with a neat diagram.

72)

How is charging from DC supply done? Explain the same with a neat diagram.

73)

What are the indications o f a fully charged cell? Marine Electrical Technology

861

Chapter 21 74)

W hat is the voltage o f a folly-charged lead-acid cell?

75)

How is the voltage o f a cell achieved?

76)

W hat do you know about voltage regulators in battery circuits? Explain the Auto regulator with a neat diagram.

77)

With a neat diagram, explain the construction o f a basic Nickel-cadmium Battery.

78)

With a neat diagram, explain the construction o f a Lead-acid battery.

79)

With a neat diagram, explain the construction o f a sealed Nickel-cadmium Battery.

80)

With a neat diagram, explain the role o f standby emergency batteries and the method of charging them.

81)

With suitable graphs explain the electrical characteristics o f a Lead-acid Cell.

82)

Write a short note on First-aid when spillage o f electrolyte occurs.

83)

Write a short note on Trickle charging.

84)

List the detrimental effects on batteries, if left in discharged condition over long periods

85)

Describe the causes o f buckling o f battery plates.

86)

Sketch an emergency standby battery charging-discharging circuit o f the four pole switch type which allows one section o f the battery to be on charge while the other section is available for discharge. Include in your sketch the arrangement for quick charge and trickle charge.

87)

What is polarization and local action?

88)

Differentiate between silver cadmium and nickel cadmium cells.

89)

List the hazards and precautionary measures associated when working with batteries and in the battery room.

90)

State / Demonstrate the precautions needed for a maintenance free battery.

91)

Nam e the material suitable for lining the battery holding enclosure.

862

Marine Electrical Technology

Chapter 22 - A * Lighting Systems At the end of this chapter you should be able to: j★

Id e n tify v a rio u s in c a n d e s c e n t la m p s a n d e x p la in th e ir m e th o d s o f fu n c tio n in g

|★

Id e n tify v a rio u s d is c h a rg e la m p s a n d e x p la in a s s o c ia te d c irc u its

|★

E x p la in th e n a v ig a tio n a n d s ig n a l lig h tin g s y s te m

1★

J u s tify th e n e e d fo r e m e r g e n c y lig h tin g a n d e x p la in a b a s ic s y s te m

★

C o m p ly w ith re le v a n t r e g u la tio n s g o v e rn in g lig h tin g s y s te m s

22.1

T he Basics

Lighting o f the ship’s deck areas, engine room and accommodation to meet specified levels o f illumination is provided by various light fittings designed to work safely in their particular locations. They also m eet the safety and comfort levels o f illumination required throughout a ship. The power ratings o f the lamps used will vary from a few watts for alarm indicator lamps to a few kW for deck floodlights and searchlights (e.g. a Suez Canal Projector Light). The amount o f light falling on a particular area can be measured with an ‘illuminance m eter’ or m ore commonly known as a ‘lux meter’. This light meter is calibrated in units called lux (lx), where one lux is the illumination o f one lumen per square metre (lm/m2) and a lumen is the unit o f luminous flux. The minimum illumination standards required in a ship are given in Table 22.1. The luminous efficiency o f a light-fitting is defined as the ratio o f lumens / watt; it is also known as Efficacy, whereas a Candela is the unit o f luminous intensity, an expression o f the illuminating power o f a light source in a given direction. This efficiency deteriorates in time mainly because o f a lamp ageing and the lumens emitted gradually reduce while the watts (power) input remains constant. Dirt on the lamp reflector and the lamp itself will also reduce its luminous efficiency. Group replacement o f lamps is often considered by shipping companies to be more economical and convenient than individual replacement following a lamp’s failure. Cleaning o f die fittings can also be carried out during the replacement o f iamps hence maintaining a high luminous efficiency. Marine Electrical Technology

Chapter 22

A v e r a g e illu m in a tio n (lu x )

P o s itio n C a b in s

200 250 150

i Mirrors I Desks General areas B a th r o o m / W a t e r C lo s e t

200 50

Mirror General areas W h e e lh o u s e

Chart table / General areas Radio Console General areas

200 250 50

| O ffic e s

250 100

Desks General areas !

G a lle y

\ Tables

Pantry / General areas

250 100

M a c h in e r y S p a c e s

150 100

Operating areas Passageways W o rksh o p s

500 100

Operating plane General areas D eck

50 25

Operating areas General areas M is c e lla n e o u s A r e a s

Lifeboat areas Laundry Refrigeration spaces Storerooms / Passageways Mess rooms / Recreation spaces Hospital (Sickbay)

25 100 50 50 200 100

Note: The lighting on the navigation bridge and chart table must be dimmable in nature; this facilitates unhindered vision when visibility reduces at night or inpoor weather conditions.

Table 22.1 - Typical Illumination Levels Onboard a Ship 864

Marine Electrical Technology

Lighting Systems |22.2

Incandescent Lamps

The most common lamp used for general lighting is the incandescent lamp, which is known as the simple filament type or a GLS (general lighting service) lamp. A current is passed through the fine wire tungsten filament, which raises its temperature to around 3000°C when it becomes incandescent and glows. The glass bulb is filled with an inert gas such as nitrogen or argon, which helps to reduce filament evaporation. This ensures an operating life expectancy o f about 1000 hours. One variation o f the basic lamp design has a special coiled-coil filament, which increases the life expectancy o f low-power lamps (up to 150 watts), which are referred to as *double­ life' lamps. (Refer Figure 22.1). Specially reinforced construction lamps (rough service) have a tough filament for use in areas where shocks and vibration are expected - this type is useful with portable (hand) lamps. Other variations include - clear glass bulb, inside frosted glass bulb (pearl) to reduce glare, tubular construction, internal reflector lamps, decorative lamps and heating lamps (Refer Table 22.2). Coiled - coil

Single -c o ll [

Power (W)

Light {lm}

Power (W)

Light (lm)

15

150

-

-

25

200

-

-

40

325

40

390

60

575

60

665

-

75

100

1160

100

1260

150

1960

150

2075

i

|

885

I o ; o ^

\

i___

2720

300

4300

500

7700

750

12400

-

1000

17300

-

-

j

Table 22.2 - Typical Lamp Power Ratings and Average Light Outputs (At 240V Supply) For lighting in areas where high vibration is imminent, single coil filament lamps are preferred, as they are more robust than the double wound type. (Refer Figure 22.1) Marine Electrical Technology

865

Chapter 22 The luminous efficiency o f 100W single - coil and coiled-coil lamps are as follows: 11.6

lumens per watt and 12.6 lum ens per watt respectively; and

Efficiency = Out out (lumens) Input (watts) A n increasingly popular variation o f the incandescent lamp is the Tungsten-Halogen type. This construction has a gas-filled quartz tube or bulb, which also includes a halogen vapour such as iodine or bromine. When the filament is heated, evaporated tungsten particles combine w ith the halogen vapour to form a tungsten-halide.

Specially Moulded Neck {to prevent loosening) Arc-quenching Foam Filling Fused Connector (for electrical safety)

Support Wires (to permit usage in all positions) Robust Tungsten Filament High Internal Gas Pressure (pre-washed and pressurised to increase filament life)

Figure 22.1 - Ordinary Filament Lamp (General Lighting Service Lamp) A t the high filament temperature, the tungsten vapour re-forms onto the filament. This regenerative process continues repeatedly creating a self-cleaning action on the inner surface o f the glass tube or bulb. In an ordinary (GLS) lamp, the tungsten evaporation from the filament causes an internal blackening o f the glass bulb which is overcome in the tungstenhalogen lamp. Two basic lamp forms for the tungsten-halogen design are the ‘linear doubleended’ lamp and the ‘single - ended’ lamp. Some tungsten - halogen details are mentioned in Tables 22.3 and 22.4.

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Marine Electrical Technology

Lighting Systems Power (W)

Light Output (Im)

300

5000

500

9500

750

15000

1000

21000

1500

33000

2000

44000

Table 2 2 3 - Tungsten - Halogen lamps - Power vs. Luminosity - Double-ended Refer Figure 22.2 for a Linear - Double-ended Tungsten - Halogen lamp that is rated at 240V and has a life expectancy o f 2000 hours. Voltage (V)

Power(W)

Life (h)

Light Output (Im)

6

10

100

210

6

20

100

420

6

20

2000

350

12

100

2000

2150

12

50

2000

900

12

20

250

450

24

250

2000

5750

240

300

2000

5000

240

300

2000

8500

Table 22.4 - Tungsten - Halogen lamps - Power vs. Luminosity - Single-ended Linear tungsten-halogen lamps must be used horizontally, otherwise the halogen vapour will concentrate at its lower end, which results in rapid blackening o f the tube and a reduced operating life. Both are particularly useful for display, floodlighting and spotlighting. Tungsten-halogen lamps m ust be carefully handled while being fitted. I f the outside surface o f the quartz tube or bulb is touched with dirty or greasy hands premature failure can occur due to fine surface cracks in the glass.

Marine Electrical Technology

867

Chapter 22 Handle the tube by its ends only, or use a paper sleeve over the lamp during fitting. If accidentally handled, the lamp glass may be cleaned with a spirit solvent, carbon tetrachloride or trichloroethylene. (Refer Figure 22.2)

1 ■. i

1

MH

‘ ----------------------------- - ' V " -------7-----------

Figure 22.2 - L in ear Tungsten-H alogen L am p - Double-ended 122,3

D ischarge lam ps

The light output from a discharge lamp is generated by the flow o f current in an electric arc between two electrodes through a gas and metal vapour inside a sealed glass bulb or tube. Mercury (as used in a fluorescent tube) and sodium are the most common metal vapours employed in discharge lamps. Low and high-pressure types o f mercury and sodium lamps are available. A suitable voltage applied between the electrodes o f a discharge lamp causes an arc discharge through the gas. This ionisation o f the gas either creates visible light directly or by secondary emission* from a phosphor coating on the inside wall o f the lamp glass. * Ionisation may also be caused by thermionic emission or by both - high voltage and thermionic emission. The discharge lamp’s current m ust be carefully controlled to maintain the desired light output and some form o f current-limiting ‘ballast’ is required. This ballast is often an ironcored inductor coil (choke) but special transformers and electronic regulator ballast circuits are also used. The ballast must ‘match’ the lamp (e.g. a 20W fluorescent tube must have a matching 20W ballast unit) in order to ensure correct lamp operation for high luminous efficiency and long life. s22.4

H ot C athode Low P ressure M ercu ry Fluorescent Lam ps

The most obvious example o f this type is the popular fluorescent tube. It is classified as a MCF lamp (M - mercury, C - low pressure, F - fluorescent coating) (Refer Figure 22.3). The inside wall o f the glass tube is coated with a fluorescent phosphor, which emits white or coloured light. The coating is meant to absorb ultraviolet rays and convert them into visible light. Variations o f fluorescent material create different hues o f white light. The most commonly known are given in Table 22.5.

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Marine Electrical Technology

Lighting Systems Fluorescent tubes are available in lengths from 150mm to 2400m m with power ratings from 4 Watts to 125 watts. The tube ends are usually fitted with bi-pin lamp caps or miniature bi-pin caps for the smaller tubes. Typical luminous efficiency for a fluorescent tube is approximately 70 lumens per watt with an average operating life o f 5000 hours. It is capable o f giving a glare-free, cool daylight effect.

Material

Colour

Calcium Tungstate

Blue

Magnesium Tungstate

Blue-white

Zin c Beryllium Silicate

Yellow-white

Cadmium Borate

Pink

Zinc Silicate

Green

|

Table 22.5 - Common Variants of Fluorescent Tubes To ‘strike’ a fluorescent tube, its gas filling (usually argon or krypton) m ust be ionised by a voltage between its cathodes that is slightly higher than that required to maintain normal discharge. These electrodes may be coated with an oxide (e.g., barium oxide) to improve thermionic emission. I f a suitable voltage is applied between the two electrodes a discharge strikes between them and the mains voltage is then sufficient to maintain the discharge. This occurs in low pressure so that the lamp will function at a comparatively low temperature and so will not affect the fluorescent coating. The electrons from the electrode collide with the mercury atoms.________________________________________________ _ Marine Electrical Technology

869

Chapter 22 This dislodges an electron from the atom making the mercury atom a positively charged ion. As the dislodged electron returns to the influence o f the ion (i.e. the electron changes from one energy level to another) a certain amount o f electro-magnetic radiation (i.e. a photon) is given o ff in the form o f ultra-violet lig h t These rays activate the fluorescent coating and the luminous surface provides a glare-free efficient light. Two common methods are used to start the tube glowing: (a) The Switch-start C ircuit (b) The Transformer Quick-start Circuit. A typical switch-start circuit is found in Figure 22.4. The starting action is initiated by a ‘glow’ type starter switch connected between the opposite ends o f the tube. It contains two bi­ metallic electrodes in a small glass tube filled with Helium that prevents oxidation o f the electrodes. When the supply voltage is applied to the circuit (ideally 220V), the entire voltage appears across the starter switch. A glow discharge occurs between the starter’s contacts, which quickly heat up, bend and touch each other. This allows current to flow through the lamp cathodes which will cause the tube ends to glow before the tube actually strikes. The tube strikes when the starter switch re­ opens as it cools down during its closed (non-glow) period. When the starter switch opens, it interrupts an inductive (choke) circuit which produces a surge voltage (approximately 700 to 1000V, which can be about 3 to 4 times the applied voltage) across the tube which then ‘strikes’. The tube glows brightly now and the reduced arc voltage across it is not sufficient to re-start the glow discharge in the starter; hence its contacts remain open. The voltage falls to about 120 V due to the effect o f the supply frequency (normally 50 Hz) that causes a rise in impedance o f the choke (as the inductive reactance XL = 2IT/L). I f a d.c. source is used then a ballast resistor (which may be an incandescent light) must be used. Thermionic emission is now maintained by ionic bombardment. In fact, in the normal operating condition the starter switch can be unplugged from its bayonet cap base and the lamp will function normally. The tube will not, o f course, re-strike after switching off without the help o f the starter. Two or three ‘strikes’ may be required to get the lamp working normally as the starter contacts may open before the cathodes are sufficiently heated.

870

Marine Electrical Technology

Lighting Systems Main Switch

figure 22.4 - Typical Glow-starter Switch Circuit Such ‘cold-striking’ reduces the lam p’s life by eroding the cathode material, which causes irregular lamp flashing. Severe blackening at the tube ends is a sure sign that its useful life is finished. Each time die starter’s thermal contact closes, there are high-current surges through the choke coil, which increases its temperature. Excessive lamp flickering must, therefore, be swiftly corrected by replacement o f the lamp or starter. More often than not it is the starter that is defective when such symptoms are observed. Only old or ‘weak’ lamps flicker due to their inability to bring about ionisation within. Most choke coils are ‘potted’ in a thermosetting polyester compound within a steel case. While an earth fault is unlikely to occur within a choke, an open circuit is possible and can be simply checked with an ohmmeter. No repair o f a choke is feasible so it must be completely replaced with an identically rated unit. Similarly, glow starters should only be replaced with an equivalent, which matches die wattage o f the tube that it is to be used with. The commonly available glow starters have a dual rating i.e. 20 / 40 watts. Electronic starter switches are now available to eliminate flicker while switching on. These circuits are also called starter-less circuits or referred to as rapid start or instant start, where a drop in potential between the electrode and an earth strip is sufficient to ionise the gas adjacent to the electrode and this ionisation then spreads across the whole tube. A n example o f a transformer quick-start circuit is shown in Figure 22.5.

Marine Electrical Technology

871

Chapter 22

Figure 22.5 -Transformer Quick-start Circuit The lamp discharge begins as soon as the cathodes reach their operating temperature. A capacitive effect between the cathodes and the earthed metalwork o f the fitting ionises the gas and the tube ‘strikes’ very quickly. Most tubes have a conducting path through the phosphor coating or, alternatively, a special metal earth strip running between the end-caps assists the starting process. The transformer ballast gives an immediate start but some difficulty can occur with low ambient temperatures, low supply voltage and poor earthing. Many other variations o f the quick-start circuit using transformers and resonant effects are used. Capacitors are used with a discharge tube for: (i) Power factor correction (PFC) (ii) Radio interference suppression (RIS) The PFC capacitor is used to raise the power factor o f the power supply to around 0.9 lagging and is connected in parallel with the power supply. Without this capacitor, the power factor may be as low as 0.2 lagging due to the choke’s inductance. For a 125 W tube, a PFC value o f about 7.2 pF is typical. The ionisation process o f the discharge causes radio interference from discharge tubes. This is suppressed by a capacitor fitted across the tube’s ends. In glow-switch circuits, the RIS capacitor is actually fitted within the starter. RIS capacitor values are approximately 0.0005 pF. I f the RIS capacitor in a glow-switch fails due to a short-circuit condition, the tube would not strike but would glow at its ends while the choke may overheat and eventually fail. A similar result would occur if the bi-metallic strips o f the starter are welded together. 22.4.1

872

Advantages s

Greater efficacy, about 5 times the lumens per watt o f tungsten filament

*

About 5 times the life o f incandescent lamps (5000 Hrs approximately) Marine Eiectrical Technology

Lighting Systems 22.4.2 Disadvantages x

Higher initial cost Power loss in a d.e plant due to the ballast resistor Stroboscopic effect; two may be placed 90° out o f phase

122.5

High Pressure Mercury Fluorescent Lamps

A typical high-pressure lamp and its circuit is shown in Figure 22.6. This type o f lamp is coded as MBF (M = Mercury, B = high pressure, F = fluorescent coating). An additional suffix to lamp codes may be /U or /V meaning that the lamp is designed for fitting in a Universal or Vertical position respectively, e.g. MBF/U.

Figure 22.6 - High Pressure Mercury Fluorescent Lamp Circuit The high pressure mercury lamp comes in sizes ranging from 50 to 1000 W and is fitted with Edison Screw (ES) or Goliath Edison Screw (GES) lamp caps. Its luminous efficiency is in the range o f 40-60 lm/W with an average lifespan o f around 7500 hours. The lamp takes several minutes to achieve maximum brightness. It will not immediately re-strike when rapidly switched-off and then switched-on, because the vapour pressure (o f several atmospheres) prevents this from happening. Re-striking will occur only when the discharge tube has sufficiently cooled down; ionisation o f the gas occurs between a secondary electrode and one o f the main electrodes fitted close to it, which warms up the tube and an arc now strikes between the main electrodes. Mercury vapour, which was condensed on the tube wall, now vaporises and the main arc passes through it. The secondary electrode ceases to function as the lamp pressure builds up. Marine Electrical Technology

873

Chapter 22 22.6

Low Pressure Sodium Vapour Lamps

A low-pressure sodium lamp is coded as SOX (SO stands for ‘sodium vapour’, X stands for ‘standard single-ended lamp o f integral construction’). A typical lamp shape and its circuit are shown in Figure 22.7.

Figure 22.7 - Low Pressure Sodium Vapour Lamp Circuit The lamp has a U-shaped arc tube containing metallic sodium and an inert gas such as neon. Common power ratings o f the lamp are 35W, 55W, 90W, 135W, and 180W with luminous efficiencies in the range o f 120-175 lm/W and with an average operating life span o f 6000 hours. The low-pressure sodium lamp needs a high voltage (480-650V) provided by a special transformer ballast circuit. When first ignited, the SOX lamp gives a red glow as the discharge is initially through the neon gas. As the lamp warms up, the sodium begins to evaporate to take over the discharge from the neon while the lamp colour changes from red to yellow. The time taken to reach maximum intensity is approximately 6-15 minutes.

|22.7

High Pressure So^um Vapour Lamps

A typical lamp and its circuit is shown in Figure 22.8. The basic lamp type is coded as SON (SO = Sodium vapour, N = high pressure), but two other variants are labelled as SON-T (a tubular clear glass type) and SON-TD (a tubular double-ended clear quartz type). The illumination by the SON lamp is wide-spread and delivers a golden-white light. Lamp starting is achieved with the help o f a high-voltage pulse from an igniter circuit, which ceases to function once the main arc has been struck. A t least 5 minutes are required for the lamp to achieve its maximum intensity and will usually re-strike within 1 minute o f extinction from a hot condition. SON lamp power ratings range between 70 and 1000W with corresponding luminous efficiencies being between 80 and 120 lm/W.

874

Marine Electrical Technology

Lighting Systems

Figure 22.8 - High Pressure Sodium Vapour Lamp Circuit 22.8

Disposal of Lamps Containing Mercury

The average four-foot fluorescent lamp contains about 8 milligrams, or about 100 times less mercury than is contained in a typical 700-milligram fever thermometer. A typical compact fluorescent lamp (CFL) m ay contain even less mercury; lamp breakage would appear to cause virtually no risk o f harm. However, the legal requirements for disposal are quite different. Increasingly, people have become familiar with the environmental and human health impacts associated with mercury and its compounds; however, many are not aware that mercury is an essential component o f m ost energy-efficient lamps. Although fluorescent and high intensity discharge bulbs and tubes contain mercury, they are more energy efficient than incandescent or halogen bulbs. By requiring less energy, these bulbs reduce the amount o f pollution from energy production, which includes the emission o f mercury (e.g., from coal combustion). Any mercury-containing bulb, generally with “Hg” encircled, regardless o f the amount o f mercury, should not be discarded in the trash. Bulbs containing mercury should be handled as hazardous waste, stored carefully prior to disposal in order to avoid breakage, and properly disposed. If the mercury concentration o f a defective lamp exceeds the regulatory Toxic Characteristic Leaching Procedure (TCLP) lim it o f 0.2 mg/L, the lamp fails the toxicity test and must be managed as universal or hazardous waste. Green tip or low-mercury fluorescent lighting contains less mercury, but still should not be placed in the trash; in addition, these lamps may contain small quantities o f lead that can be potentially harmful to human health and the environment. Labeled mercury-containing products are banned from landfills. Bulbs should be disposed o f through a licensed hazardous waste transporter. Marine Electrical Technology

875

Chapter 22 22.8.1

Id e n tify in g B ulbs th a t C ontain M ercury

The following types o f bulbs contain mercury: •

Fluorescent, compact fluorescent, black lights.

•

High intensity discharge bulbs (HID). These bulbs are commonly used in security, outdoor and warehouse lighting. HID lighting is becoming popular for use in commercial areas and dockyards. The following are HID bulbs:

•

o

Mercury vapour

o

Metal halide

o

High pressure sodium vapour

HID lighting is also used in vehicle headlamps. HID headlamps can be identified by their characteristic bluish-white tint when lit. Some halogen bulb manufacturers are now applying a blue coating to their bulbs which makes them look like HID bulbs when lit, however, halogen bulbs do not contain mereuiy.

22.8.2

B u lb Storage and H a n d lin g

•

Store bulbs in an area and in a way that will prevent them from cracking or breaking, such as in boxes the bulbs came in or in boxes supplied by a bulb recycler.

•

Do not break or crush bulbs because mercury may be released.

•

I f a bulb is accidentally broken, follow the recommended clean-up procedure applicable to any mercury-containing lamp: 1. I f a lamp breaks, first o f all try to ventilate the area so as to disperse any vapour that may escape, and leave the space for at least 15 minutes. 2. Then carefully scoop up the fragments with a stiff paper (do not use your hands) and wipe the area with a disposable paper towel to remove all glass fragments and place these in a container. The container must be closed, structurally sound, compatible with lamps, and lacking any evidence o f spillage. 3. Do not use a vacuum cleaner as this disperses the mercury over a wider area. All fragments should be placed in a sealed plastic bag and properly disposed of.

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Marine Electrical Technology

Lighting Systems 122.9 5___

Lamp Caps

Lamp end-caps are o f various types, but the most common are bayonet and screw fittings. The old names, e.g., Goliath Edison Screw (GES) and Bayonet Caps (BC) are now re­ designated to indicate the cap type and its dimensions. A selection o f old and new designations is shown in Table 22.6. Some lamp cap shapes are shown in Figure 22.9. The dimension code indicates the type o f cap first by mi alphabet. The first two figures indicate the nominal outer diameter o f the cap barrel or screw thread in millimetres. The next two figures indicate the overall length and the last two the diameter o f the flange. Old Designation

Description

Current Designation

GES

Goliath Edison Screw

E40/45

ES

Edison Screw

E27/27

SES

Small Edison Screw

E14/23 x 15

MES

Miniature Edison Screw

E10/13

LES

Lilliput Edison Screw

E5/9

BC

Bayonet Cap

B22/25 x 26 (may be 2 or 3 pin)

SBC se e

Small Bayonet Cap

B 15/24 x 17 (double or single contact)

MCC

Miniature Centre Contact

BA9s/14

Table 22.6 - Lamp Caps

SES

Figure 22.9 - Lamp Caps’ Shapes

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Chapter 22 |22.10 Effects of Voltage on the Performance of Lamps A ll lamps are designed to produce their rated luminous output at their rated voltage. An over-voltage on an incandescent lamp produces a brighter and whiter light because the filament temperature is increased. Its operating life is, however, drastically reduced. A mere 5% increase over its rated voltage will reduce the lamp life by 50%. Conversely, reduction in the voltage will increase the operating life o f a GLS lamp but it produces a duller, reddish light. Lamps rated at 240V are often used in a ship’s lighting system operating at 220V. This ‘under-running’ should more than double the lamp life. Similar effects on light output and operating life apply to discharge lamps but if the supply voltage is drastically reduced (below 50%), the arc discharge ceases and they will not re-strike until the voltage is raised to nearly its normal value. Fluorescent tubes will begin to flicker noticeably as the voltage is reduced below their rated value. The normal sinusoidal a.c. voltage waveform causes discharge lamps to extinguish at the end o f every h alf cycle, i.e. every 10ms at 50Hz or every 8.33ms at 60Hz. Although this rapid light fluctuation is not detectable by the human eye, it can cause a stroboscopic effect whereby rotating machinery in the vicinity o f discharge lamps m ay appear to be stationary or rotating slowly, which could be dangerous to operators. There are a few methods to alleviate a stroboscopic problem. A few o f them are:

{22.11

878

Use a mixture o f incandescent and discharge lighting in the same area.

(ii)

Use twin discharge lamp fittings with each lamp wired as a ‘lead-lag’ circuit, e.g. the lamp currents are phase-displaced so that they go through zero at different times; hence the net light output is never fully extinguished.

(iii)

Where a 3-phase supply is available, connect adjacent discharge lamps to different phases (e.g. red, yellow and blue) so that the light in a given area is never extinguished.

Navigation and Signal Lights

22.11.1

(a)

(i)

M andatory Requirements

The navigation lights must be connected directly or through a transformer to the main or emergency switchboard i.e., no switches are to be in between the source and the dedicated distribution board. The distribution board must be easily accessible to the officer o f the watch. Marine Electrical Technology

Lighting Systems (b)

The masthead, side and stem lights shall be connected separately to the above distribution board which is reserved for this purpose.

(c)

Each light shall be controlled and protected in each insulated pole by a switch and fuses or by a circuit breaker mounted on the above distribution board (in case o f failure o f the ship’s mains, the double pole switch may be changed over to an emergency source of supply).

(d)

The bulb shall provide a uniform intensity output as mentioned in Table 22.7 below hence a cage winding is used.

(e)

Each light shall be provided by an automatic indicator which gives an audible and / or visual warning in the event o f an extinction o f the light. I f an audio device alone is used, it shall be connected to a separate source o f supply - e.g., a battery. I f a visual signal is used, which is in series with the light, means must be provided to prevent the extinction o f the light owing to the failure o f the visual signal.

(f)

The visual device must be so connected that its failure does not extinguish the navigation light circuit.

(g)

Provision shall be made on the bridge for such navigational lights to be transferred to an alternative circuit. Lamp

Intensity (Candelas)

Corresponding Visible Range (Nautical Miles)

Masthead Lamp

94

6

Side Lamp

12

3

Stem Lamp

12

3

Low Lamp

12

3

Other Navigation Lights

12

3

4.3

2

Ship at Anchorage less than 50m long

Table 22.7 - Intensity Requirements of Lamps The International Maritime Organisation (JMO) and National Authorities prescribe the number, position and power rating o f navigation lights aboard ships. By far the m ost common arrangement is to have five specially designed navigation ‘running’ lights referred to as Fore mast, Main mast (or Aft mast), Port, Starboard and Stem. Two anchor lights fitted forward and aft, may also be controlled from the Navigation Light Panel. Marine Electrical Technology

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Chapter 22 The sidelights are red for port and green for starboard while the other lights are white. Special incandescent filament lamps are used each with a power rating o f 65W, but 60W and 40W ratings are permitted in some cases. The following formula will help to calculate the minimum permissible intensity o f navigation lights: Luminous Intensity I = 3.43 x 106 x T x D2 x K 'd; Where T = 2 x 10 7 lux; D is the visible range in nautical miles; K is the atmospheric transmissivity constant at a value o f 0.8 corresponding to an approximate meteorological visibility o f about 13 nautical miles. Due to the essential safety requirement for navigation lights, it is becoming a common practice to have two fittings at each position, or two lamps and lamp-holders within a special dual fitting. Each light is separately supplied, switched, fused and monitored from a Navigation Light Indicator Panel in the wheelhouse. The electric power is provided usually at 220V a.c. with a ‘main’ supply fed from the section o f the main switchboard that is meant for the essential services. (Refer Figures 3.9 and 4.2). An ‘alternative’ or ‘stand by’ power supply is fed from the emergency switchboard. A changeover switch on the Navigation Light Panel is used to select the main or standby power supply. The Navigation Light Indicator Panel has indicator lamps and an audible alarm to warn o f any lamp or lamp-circuit failure. Each lampcircuit has an alarm relay which monitors the lamp’s current The relay may be electromagnetic or electronic. A basic double navigation light’s schematic circuit with alternative power supplies is shown in Figure 22.10. A similar circuit is shown in Figure 22.11. Obviously the light fittings are in exposed positions, so during maintenance checks, one should concentrate on water-tightness and on the condition o f the supply cable.

Buzzer

Figure 22.10 - A Basic Double Navigation Light Schematic Circuit 880

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Lighting Systems Audio Alarm (Buzzer) Double Pole Supply Chat

m

N/C Contact Indicator Lamp

Main Supply

Fuses Emergency Supply

Navigation Light

Figure 22.11 - Alternative Navigation Light Circuit (Circuit Energised) Note: Dimmers are not permutedto be incorporated with navigation lights ’ circuits 22.11.2

Operation

Referring to Figure 22.11, when the double pole switch is closed the navigation light is illuminated. Current in the relay circuit causes the relay coil to energise, which pulls the NC (normally closed) contact open so that the audio alarm (buzzer) circuit is now open. Only a low voltage lamp is needed for the indicating lamp. This ensures a small voltage drop across that part o f the circuit. Keeping regulatory requirements in mind, if the indicating lamp fails, the circuit is completed through the back-up resistor, so the navigation light does not fail. In the event o f a short circuit, the dedicated fuses will rupture and thus isolate the defective lamp. So, if the navigation light fails, or if a fuse blows, the current in the circuit ceases and the relay is de-energised. The NC contact springs back to activate the buzzer circuit. In case o f failure o f the ship’s mains, the double pole switch may be changed over to emergency supply. A similar circuit is depicted in Figure 22.12. The recommended caged filament lamp is also shown. Caged filaments last longer as they can withstand shock and vibration. Now various signal lights with colours o f red, green, white, and blue are arranged on the signal mast. These lights are switched-on to provide a particular combination in order to signal states relating to various international and national regulations. Pilotage requirements, health, dangerous cargo condition, etc., are signalled with these lights. White Morse code flashing lights may also be fitted on the signal mast.

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Chapter 22

Figure 22.12 - A Navigation L ight C ircuit (C ircuit De-energised) The NUC (Not Under Command) state is signalled using up to two all-round red lights (depending on the length o f the vessel), vertically mounted and at least 2 metres apart. Such important lights are fed from the 24V d.c. emergency supply system but some ships may also have an additional NUC light-pair fed from the 220V a.c. emergency power supply. Lights and signals are to be exhibited appropriately to other ships so as to indicate whether the vessel is under way, at anchor, not under command or even aground. The required lights are to be shown from sunset to sunrise, and also during bad visibility. Note: ^ A ship is ‘Under Way’ when she has no ropes running ashore or to the buoy, nor is she at anchor or aground. A ship is ‘Not Under Command’ generally when either thepropeller or the rudder is not functioningsatisfactorily.

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Lighting Systems 22.12

Signals for a Pow er-driven Ship U nder W ay (A t Night)

22.12.1

M asthead L ig h t

(If the vessel is less than 50 metres in length, then only one light would suffice). These lights are carried one behind the other on the two masts when the vessel is greater than 50 metres in length. The forward light is lower than the after light. Their visibility is at least 6 miles and they show a white light over an arc o f 225°, namely from right ahead to 22.5° abaft the beam on either side o f the vessel. 22.12.2

Side Lights

A green light on the starboard side and a red light on the port side; these are generally carried abreast o f the bridge and are provided with special screens so that each light is visible over an arc o f 112.5° to show light on its own side from right ahead to 22.5° abaft the beam on its respective side. Screens are painted matt black. Visibility o f these lights is at least 3 miles. 22.12.3

One Stem L ig h t

It is fixed right astern and shows a white light over an arc o f 13 5° (67.5° from right aft on each side o f the vessel); visibility is to be at least 6 miles. A towing light shall be a yellow light above the white stem light but with the same characteristics 22.12.4

Ship a t Anchorage

A ship o f 50 metres or more in length will have one white light in the fore part of the vessel and a second white light at a lower level near the stem. Both lights are to be visible all around the horizon (360°) for a distance o f at least 3 miles. If the ship is less than 50 metres long, only one all round white light is required. It may be o f reduced visibility as well, but o f not less than 2 miles. 22.12.5

S hip N ot Under Command

If not making way through water, the ship shall show only two red lights one over the other at least 1.5 to 2 metres apart, visible all around the horizon for a distance o f at least 3 miles. These lights are either hoisted on a flag halyard on the bridge or shown on the Christmas Tree if the ship has one. I f the ship is making way through the water, she shall also show the two sidelights and the stem light. 22.12.6

Ship A ground

Every ship when aground shall show the appropriate anchor lights and the two red lights for vessels not under command.

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|22.13

Emergency Lighting

Depending on the ship’s classification, e.g. ferry, RORO, gas carrier, etc., and tonnage, the SOLAS Convention prescribes minimum requirements for emergency lighting throughout the vessel. The arrangement o f the emergency electric lighting system is to be such that a fire or other casualty in spaces containing the emergency source o f electrical power, associated transforming equipment, i f any, the emergency switchboard and the emergency lighting switchboard will not render the main electric lighting systems required inoperative. Emergency iight fittings are marked, often with red paint, etc, to indicate their function. The emergency lighting is continually powered from the ship’s emergency switchboard at 220V a.c.; Chapter 4 elaborates this aspect. A few emergency lights may be supplied from the ship’s 24V d.c. battery, e.g., at the communication post in the wheelhouse, and in the machinery space, including the steering fiat (Refer Figures 3.9 and 22.13). M odem emergency lights are generally installed within the normal lighting fixtures and some shipping companies now fit special battery-supported light-fittings along main escape routes in the engine room and accommodation, and at the lifeboat station on the deck. These lights can also be found to be an integral part o f the general light-fitting e.g. a fluorescent tube can be designed to house an emergency light that is supplied from an independent emergency source o f supply as mentioned in the regulations. Generally, such emergency lights in the accommodation are arranged to produce light immediately upon mains failure; emergency lights at the boat station are switched on when required. Inside the fitting, a maintenance-free battery, usually the nickel-cadmium type is continuously trickle charged from the normal mains supply via a transformer/rectifier circuit. The battery is then available to supply the lamp via a d.c. to a.c. inverter when the main power is absent. Usually the battery will only function for a few hours. Such battery-supported light fittings can be simply tested by switching o ff the normal mains power supply or, in some cases, by a test switch on the actual fitting. Periodic inspection and testing o f all emergency lights is an essential requirement on all ships. This is done once a week or as would be deemed necessary. To conclude, it is important to note that installations on board vary not only between different classes o f vessels, but also may be customised. The relevant circuits and layout diagrams must be referred to in order to obtain adequate information o f your ship’s lighting system, as it is vital to everyone around you!

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Lighting Systems

Figure 22.13 - Basic Emergency Lighting Circuit 22.14

Relevant Rules

22.14.1

Relevant SOLAS R egulations ( Chapter I I - l )

P a rt D - E lectrical Installations

Regulation 41 - Main Source o f Electrical Power and Lighting Systems Regulation 45 - Precautions against shock, fire and other hazards o f electrical origin 22.14.2

1)

Sum m ary o f SOLAS Regulations

A main electric lighting system which provides illumination throughout those parts o f the ship normally accessible to and used by passengers or crew shall be supplied from the main source o f electrical power.

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Chapter 22 2)

The arrangement o f the main electric lighting system shall be such that a fire or other casualty in spaces containing the emergency source o f electrical power, associated transforming equipment, if any, the emergency switchboard and the emergency lighting switchboard will not render the main electric lighting system inoperative.

3)

Lighting fittings shall be so arranged as to prevent temperature rises which could damage the cables and wiring, and to prevent surrounding material from becoming excessively hot.

4)

All lighting and power circuits terminating in a bunker or cargo space shall be provided with a multiple-pole switch outside the space for disconnecting such circuits.

Extractfrom Convention on the International Regulationsfor Preventing Collisions at Sea Part C - Lights and Shapes - Rule 22 - Visibility ofLights Quote The lights prescribed in these Rules shall have an intensity as specified in Section 8 o f Annex I to these Regulations so as to be visible at the following minimum ranges: (a)

In vessels o f 50 metres or more in length: - a masthead light, 6 miles, - a sidelight, 3 miles; - a stemlight, 3 miles; - a towing light, 3 miles; - a white, red, green or yellow all-round light, 3 miles.

(b)

In vessels o f 12 metres or more in length but less than 50 meters in length: - a masthead light, 5 miles; except that where the length o f the vessel is less than 20 metres, 3 miles; - a sidelight, 2 miles; - a stemlight, 2 miles; - a towing light, 2 miles; - a white, red, green or yellow all-round light, 2 miles.

Unquote

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Marine Electrical Technology

Lighting Systems

1)

An alarm relay in a navigational light circuit is connected in parallel with the navigation lights. True / False

2)

In the event o f failure o f a navigation light, the alarm relay g e ts ________ .

3)

The occurrence o f a short circuit in navigational lights is protected b y _______ .

4)

List the various Navigation Lights required on board a ship. W hat do know about their associated circuitry?

5)

With a neat diagram explain a basic lighting system on board a ship.

6)

Explain the operation o f a Glow-starter Switch Circuit for a Fluorescent Tube.

7)

Explain the operation o f a High-pressure Mercury Fluorescent Lamp circuit

8)

Explain the operation o f a High-pressure Sodium Vapour Lamp circuit.

9)

Explain the operation o f a Low-pressure Sodium Vapour Lamp circuit.

10)

Explain the operation o f a Transformer Quick-start Circuit for a Fluorescent Tube.

11)

How does a tube-light work?

12)

What is the function o f a choke?

13)

W hat is the function o f a starter in a tube-light circuit? Explain with a suitable diagram.

14)

State the precautionary measures for the storage, use and disposal o f high pressure mercury vapour lamps used on the deck.

15)

List the various Navigation Lights required on board a ship. What do know about their associated circuitry?

16)

What is the indication available on the navigation lights panel in the event o f failure of one o f the navigation lights and the provision to allow continued indication o f the light?

17)

W hat is the protection provided in the navigational lights panel in the occurrence o f a short circuit?

18)

Describe the characteristics o f an incandescent lamp as in the chart room table light and the procedure for dimming.

19)

With a self-explanatory diagram explain a basic Emergency Lighting system on board a ship.

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Chapter 22 20)

How are emergency lighting and essential services provided for on a large crude carrier? Describe, briefly, the plant involved and make a diagrammatic sketch o f the electrical circuits supplied and their interconnection with the main switchboard, justifying the installation.

21)

What is luminous efficiency and how is it affected?

22)

How does voltage affect lamp performance?

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Marine Electrical Technology

Chapter 23 * * * , Alarm Indication Systems At the end of this chapter you should be able to: |★

S ta te th e re q u ir e m e n ts o f a fire a la r m a n d d e te c tio n s y s te m

|★

E x p la in th e o p e ra tin g p rin c ip le s o f v a rio u s d e te c to rs in a fire a la r m s y s te m

11 ★

U n d e rs ta n d th e im p o rta n c e o f a c r a n k c a s e oil m is t d e te c to r

*

B rie fly e x p la in th e o p e ra tio n o f a m o d e m s y s te m fo r m is c e lla n e o u s a la r m s

★

C o m p ly w ith re g u la tio n s g o v e rn in g a la r m s a n d a la r m in d ic a tio n s y s te m s

I

saisKgsgsja ? . •

123.1

: .■■

F ire A larm s an d Detection

We know that the cargo deck, cofferdams, paint stores and machinery spaces are classified as hazardous areas. Since fire detection is o f paramount importance onboard a ship or for that matter everywhere, the explanation o f the various sensors that lead to its detection have taken priority over other alarm indication systems. As a fire grows it passes through four stages o f development namely: 1. Incipient (developing) 2.

Smouldering

3. Open Flame 4. High Heat 23.1.1 Requirements o f a Basic Fire Alarm System S

Must be able to sense a fire

S

Must have some means to indicate the occurrence o f a fire to persons concerned

S

M ust always be in a state o f operational readiness

23.L 2

Requirements of a Typical Fire Detection System

S

Quick Response

S

Accurate Detection

S

False Alarm Immunity

S

Fault Tolerance

S Networked panels Marine Electrical Technology

Chapter 23 S

Enhanced Survivability

S

Expandability

^

Audio Integration Interface with other safety systems

S

Adequate coverage o f the hazardous area

23.1.3 Initiating Devices Initiating devices can automatically transmit an alarm signal when a condition indicative o f fire to which they respond occurs. Apart from manual arrangements they can be: '*ar Smoke Detectors (light obscuration, light scatter or ionization types) ^

Heat Detectors (they operate at a fixed value or with the rate o f rise in temperature) Flame Detectors (they react to radiation emanating from flames) Sprinkler-line flow-switches

23.1.4 Indicating Devices These devices generate a signal that indicates / notifies that a condition indicative o f fire has occurred. The primary purpose o f such a device is to notify occupants that an emergency condition has arisen. The most common indicating devices are: ® Homs ® Bells ® Strobe Lights ® Sirens ® Chimes ® Audio Speakers 23.1.5

890

Control Panel

^

Provides the central annunciation o f alarms or faulty conditions

^

Generates displays and indicates the nature o f an abnormal condition

^

It is also a means o f transmitting an alarm to a remote location Marine Electrical Technology

Alarm Indication Systems 23.1.6 Power Supply M Most systems supply 24V directly to the panels. If the main power supply to the control panel is the line voltage, internal transformer-rectifier blocks are used to stepdown and convert the line voltage to a low voltage (typically 24V d.c.). M In the event o f interruption o f the main power supply, these systems have auxiliary power sources (battery back-up) to keep them on-line (Refer Figure 3.9). 23.1.7

Virtues of an Intelligent Fire Alarm System

#

User-friendly operator interface

#

Accurate and reliable detection o f fire

#

Incredibly fast response time

^

Networking o f panels

#

Integrated user prompts

#

Integrated audio system

^

Process-adapted, flexible automation

#

Real-time data capturing and routing

^

Uninterrupted operations and enhanced survivability

An automatic fire alarm system comprises o f components for automatically detecting a fire, initiating an alarm and other actions as arranged; the system may also include manual call points. A system with at least these basic capabilities is to be installed in the cargo, accommodation and service spaces o f certain classes o f passenger ships and in the cargo spaces o f ships carrying explosives. Tankers too are no exception to this rule. Such systems are also fitted in the machinery spaces o f cargo ships specially designed for unmanned operation and on the vehicle decks o f certain roll-on-roll-off ships depending on the conditions o f carriage o f the vehicles contained therein. Fire detectors operating on various different principles are currently available and the types presently found in service onboard ships will be included in the following list: 1) Heat detectors which operate at a predetermined temperature; 2) Heat detectors which operate when the rate o f temperature rise o f the surrounding air reaches a set limit; Marine Electrical Technology

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Chapter 23 3)

Smoke detectors which operate when smoke obscures a light beam falling on a photoelectric cell;

4)

Smoke detectors which operate when a beam o f light is scattered by smoke and caused to fall on a photoelectric cell;

5) Combustion products detectors which operate when an electric current flowing through an ionised atmosphere is changed; 6) Flame detectors which react to radiation emanating from flames; 7) The sprinkler system also incorporates an automatic fire alarm and detection system but this can be described in the section dealing with fixed fire extinguishing systems and hence is beyond the scope o f this chapter. 8) Other types o f fire detectors are available such as line detectors, laser beam detectors, etc.

|23.2

Heat Detectors

23.2.1

Fixed Temperature Type

The means o f operation is extremely simple usually being either a bi-metallic strip or a soldered joint. In the first type, the bi-metallic strip is used to make or break an electric circuit at a pre-set temperature, which in some cases is 78°C. When it is arranged to make a circuit, the contacts are usually encapsulated in glass to prevent the contacts from being affected by the atmosphere, since any corrosion may prevent the passage o f current when the contacts are required to complete and activate the circuit. Heat detectors incorporating bi-metallic strips are especially useful in places such as boiler rooms where rapid variations o f temperature are likely to be encountered and preclude the use o f the rate-of-rise type o f detector. 23.2.2 Rate ofRise Type This type o f detector works on the principle that provided the rate o f increase in the temperature o f the surrounding air is above a given minimum value, the detector will operate between given time limits, the latter depending on the rate o f increase o f temperature. It is likely to operate when the temperature exceeds 54°C fo r a rate o f rise o f less th an one m in u te an d generally operate before the tem perature exceeds 78°C. B oth fixed temperature and rate-of-rise detectors are required to have response times in accordance with the graph in Figure 23.1. Detectors o f this type may be o f Grade I, Grade II or Grade III. D ie response times o f heat sensitive detectors depend on various factors one o f which is the height that the detector is situatedabovefloor level. 892

Marine Eiectrica? ’’"echnology

Alarm Indication Systems As the magnitude o f the fire to which a detector will respond increases considerably with the height o f the deck head on which it is mounted, the Grade I detector is m ore suitable for higher deck heads. In unattended machinery spaces too, any heat-sensitive detector o f this specification should be o f a Grade I type, on account o f its faster response time. It is o f interest to note that the standard also requires all detectors made in compliance with it to function at a given temperature, according to its grade, when the rate o f increase in temperature is very low. Thus, in effect, this type o f device now incorporates the fixed temperature characteristic o f the previously described detector. 1,2,3- Upper Limits 4 - Lower Limit

Response Time (minutes)--------------►

Figure 23.1 - Upper and Lower Limits of Response Time Referring to Figure 23.1, for any given rate o f rise o f temperature, it m ust operate between the left hand curve (4) and one o f the other three depending upon its grade. Marine Electrical Technology

893

Chapter 23 It is obvious that if the response time lies to the right o f the curves 1 ,2 or 3 the detector is not sensitive enough. All marine engineers are familiar with the attitude of mind thatfalse alarms engender and it isfor thisparticular reason, i.e. to avoid over-sensitivity, that curve 4 has been introduced and interpreted as the shortest response timefor a given rate ofrise. Two typical ways o f affecting the required result are illustrated. In Figure 23.2 a pneumatic type is shown in which an otherwise sealed chamber is fitted with a bleed-off orifice.

F igure 23.2 - R ate o f Rise-type F ire D etector Means for permitting expansion o f the chamber due to increase in temperature are provided so that when a predetermined limit o f movement is reached, an alarm is sounded. Thus, under normal changes o f day-night temperatures, the bleed-off hole will be able to exhale and inhale air such that the alarm condition is never reached. Under the action o f a rapid rise in temperature when a fire occurs, the air expands faster than it can exhale through the bleed-off orifice, the resulting expansion ultimately activating the alarm.

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Marine Electrical Technology

Alarm Indication Systems Figure 23.3 shows a thermal type o f detector that works on the bi-metallic strip principle. It consists o f two such strips; one insulated from rapid changes o f temperature and the other although enclosed, being exposed to such changes. Contacts at the ends o f the strips form a part o f an electric circuit. Thus, on a slow rise o f temperature - say due to normal climatic conditions, the heat input to both strips is similar and hence the contacts remain apart. On a rapid increase in temperature, the unprotected strip responds more quickly than the insulated strip with the result that the contacts meet and the alarm is activated.

Layout Of Components

Operating Principle

Figure 23.3 - The Rate of Rise Detector (Bi-metallic Strip Type) 23.3

Combustion Detector

Most o f the detectors o f this type use two ionised chambers in series, as shown in Figure 23.4. One o f the chambers is open to the surrounding atmosphere while the other is enclosed. The atmosphere in both chambers is ionised by a radioactive source with elements such as americium* and radium being used. * Pronounced as “am-er-iss-ium”, an artificially made radioactive metallic element (symbol Am), that emits gamma radiation (The Oxford Paperback Dictionary -1988 Edition). Marine Electrical Technology

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Chapter 23

Figure 23.4 - Ionization Chamber of the Combustion Defector The ionisation o f the atmospheres in the two chambers under normal conditions permits a minute current to flow, caused by the positive and negative ions created by the radiation and moving in opposite directions (Refer Figure 23.5). The supply voltage across the two chambers is therefore divided. On the products of combustion (which may even be too small to be seen by the naked eye) entering the outer chamber, the tendency is for them to collect ions by collision.

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Marine Electrical Technology

Alarm Indication Systems

Radioactive Source

Galvanometer Figure 23.5 - Principle of Operation of the Ionization Chamber The ‘aerosols’, as they are known, are much larger than the ions; the latter are virtually stopped by the collision. A reduction in the ion flow obviously means a reduction in the voltage across the chamber and hence a change in the voltage at the common terminal. The latter, as stated, is connected to the input o f a transistor amplifier, the output o f which activates an alarm relay (only one transistor is shown in order to simplify the concept). The alarm will continue to operate until the electrical power is switched off. The sensitivity o f this type o f detector can be varied by altering the levels o f radiation in the chambers or by modifying the transistor amplifier circuit. Such a detector is most suitable for use in machinery spaces and can be adjusted to a high level o f sensitivity especially when unmanned operation is required and where prompt detection o f fire is imperative. However, this very feature is to some extent a disadvantage as false alarms can occur when the sensitivity is too high; too many false alarms produce a loss o f confidence in the equipment. It is worth noting here that this type o f detector does not depend on the combustion products being visible; it is the number o f particles that is important. Thus, a large number o f particles although they may be invisible will cause the indicator to activate the alarm while a smaller number o f particles, which may be in the form o f smoke, may not necessarily do so. From a more practical point o f view, the presence o f steam in the outer chamber will activate this type o f detector and it is important to site them well clear o f any steam leak-off source, e.g., from turbine glands. Note: Modem techniques a\>oid the use of radioactive elements in detectors as disposal of defective / old ones poses manypractical difficulties.

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Chapter 23 123.4

Detectors Reacting to Radiation Emanating from Flames

Detectors o f this type can be either the infrared or ultra-violet light type. They are intended to respond to radiated heat and light emitted from flames during combustion, and to avoid false alarms being given by natural or artificial light. The detectors thus ignore fixed light sources and rapidly flickering illumination predominantly produced by lighting. The detectors discriminate between flames and other light sources by responding only to low frequency flickering produced by flames (typically 1 to 15Hz). This is achieved with the help o f appropriate light filters. Heat radiating from hot machinery will therefore not affect this type o f detector. The circuitry o f the system is also arranged such that the detector will not activate the alarm on immediately sensing radiation, e.g. the striking o f a match to light a cigarette, but only i f the flame persists for a pre-determined time. The flame flicker techniques have the advantage o f still allowing the detection o f flames through a thin layer o f oil, water vapour, ice or dust. (Refer Figures 23.6(a) (b) and (c)). One obvious drawback o f such a detector is that i f smoke happens to screen the detector from the fire before the detector has an opportunity o f sensing it, its operation is unlikely. Another disadvantage is the possibility o f the detector reacting to light being received from a vibrating source. Careful positioning during installation, o f course, can overcome this. W ith the presently mentioned drawbacks in mind, these radiation type detectors are seldom, if ever, used independently. They always operate in tandem with types previously mentioned.

Figure 23.6(a) - Infra-red Flame Detector 898

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Alarm Indication Systems

Figure 23.6(b) - Ultra-violet Flame Detector J Figure 23.6(c) - Flame Detector Housing It is considered that a fire detector system for use in machinery and for boiler spaces should ideally consist o f smoke or ionisation-type detectors in the main areas. These must be backed-up by one or two infra-red detectors, so situated as to monitor the protected space as much as possible and one or more thermal detectors o f the rate-of-rise-type for use in spaces such as boiler rooms. Hence flame detectors are used in the engine room especially above the main engine, boilers, purifier spaces and above the waste oil incinerator. It goes without saying that such intricate equipment is worthless if it is not regularly serviced and tested. One is advised to be conversant with the practicalities o f the system in his ship, i f not with the intricate electronics involved.

123.5

The Fire Alarm Control Panel

The main fire control panel, which is fitted in wheelhouse, is capable o f the following: 1. It monitors the condition o f all fire detectors, that are divided into zones and loops i.e., it identifies the defective detectors, open or short circuits and earth faults in detector loops. 2. It indicates the location o f the fire detectors and call points that are activated so as to quickly reach the scene o f fire or take any necessary action.

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Chapter 23 3. The panel is also linked to manual call points in the engine room, accommodation and other locations. Manual call points are essentially switches connected in the fire detector loops. 4. W ith Reference to ABS Rules for Building and Classing Steel Vessels 2012 - Part 4 Vessel Systems and Machinery, Chapter 7 Fire Safety Systems, Section 3 Fire­ extinguishing Systems and Equipment, paragraph 11.3.1 - Locations Requiring Manually Operated Call Points (2012), Manually operatedcall points are to be installed throughout the accommodation spaces, service spaces and control stations. One manually operated call point is to be located at each exit. Manually operated call points are to be readily accessible in the corridors ofeach decksuch that nopart ofthe corridor is more than 20 m (66ft)from a manually operated call point. The statement “Manually operated call points are to be installed throughout the accommodation spaces, service spaces and control stations” above, does not require the fitting o f a manually operated call point in an individual space within the accommodation spaces, service spaces and control stations. However, a manually operated call point is to be located at each exit (inside or outside) to the open deck from the corridor such that no part of the corridor is more than 20 m (66 ft) from a manually operated call point. Service spaces and control stations which have only one access, leading directly to the open deck, are to have a manually operated call point not more than 20 m (66 ft) (measured along the access route using the deck, stairs and/or corridors) from the exit. A manually operated call point is not required to be installed for spaces having little or no fire risk, such as voids and carbon dioxide rooms, nor at each exit from the navigation bridge, in cases where the control panel is located in the navigation bridge.” 5.

They are operated by breaking the front glass and regularly tested with the help o f a customised key. The main purpose o f the manual call station is to raise an alarm in case o f emergency like fire or flooding.

6. It activates the ships general alarm/fire alarm in case fire detector is activated any where in engine room, accommodation or other locations after a preset delay time. The smoke, heat and flame detectors are tested with appropriate test kits provided by the manufacturer.

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ALARM MUTE ALARMS IN QUEUE

EXTERNAL CONTROL ACTIVATED • SECTION / DETECTOR NOT RESET

■CS3

ALARM RESET

Figure 23.7 - A Modern Fire Alarm Panel

Figure 23.8 - A Modern Fire Detection System Here the brief explanation pertains to a product tanker. This arrangement is similar in other types o f vessels too with o f course minor alterations to suit the requirements on board. Marine Electrical Technology

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Chapter 23 The main supply to the panel is 220 V a.c. fed from both the main and emergency switchboards. 24V d.c. is also fed from the battery charger and 24V d.c distribution board. Manually operated call points in the control stations, accommodation and service spaces are installed about 20 metres from each other and generally at each exit. As mentioned earlier, the detectors and manual call points are connected in a loop. This helps to quickly identify the scene o f fire and likewise, faults in the circuit (Refer Figure 23.9). £5

Loop A shown in Figure 23.9 will also include locations such as the passageways on other decks (from the D deck to the A deck, upper deck), the galley (which has heat detectors and manual call points), emergency generator room, cargo control room, foam and fire control room and dry provision stores.

Figure 23.9 - Loop A of the Fire Alarm System 902

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Alarm Indication Systems 23

Loop B will include the engine room area starting from the Engine Control Room, passageways, casing, entrance, workshop and store. Thermal detectors will be installed in the ECR and workshop while smoke detectors will be installed elsewhere (including these two locations). The Navigation Bridge is informed before inhibiting the fire sensors in workshop. The alarms can generally be disabled only from the bridge. Fire detectors are inhibited (especially during hot work) by means o f a tim er that keep the fire detector isolated for the specified time. After the specified time, the workshop fire detector will get connected to the fire control panel once again.

23

Loop C includes the engine room 2nd and 3rd decks and the engine room floor (smoke detectors will be installed in these locations).

23

Loop D will take care o f the shaft generator room and comprise o f smoke detectors and manual call points.

23

Loop E comprises o f the air conditioning room, the gas sampling locker and paint / bosun stores which have smoke detectors and manual call points too.

23

Loop F is shown in Figure 23.10 and includes the incinerator, auxiliary boiler, inert gas system, main engine, generator engines and the purifier system. This loop incorporates flame detectors too and they will be installed in the vicinity o f the engines (generally above them). Smoke detectors are also installed in the engine room but are carefully located so as to prevent nuisance alarms.

Note: • •

The span ofcoverageper detector is generally not more than 12 metres (horizontally). Every zone has the means to isolate its associated detectors by cutting off the power supply. This may be necessary when hot work is carriedoutfor example in the workshop.

•

A “Switched Off" zone is indicated by a “Zone Off” indicator; this is also known as “Zone Isolation ”. Press the samepush button again to switch the zone “On

the “Zone

OFF" indicator goes off. •

A test button isprovided to check the operation of the alarms.

•

The continuity of the wiring in the detector is continuously monitored such that interruption in the circuit (due to a break in wiring) would activate an audio-visual Marine Electrical Technology

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Chapter 23 warning. Most systems have different sounds to help one distinguish between a genuine alarm and afault condition. •

The detection offaulty detector heads is also incorporated in most modem systems. In the absence of such a facility, the only way to prove that a detector is working is by simulating the condition e.g. with a hot air blower or smoke, provided it is safe to do so.

Purifier ® q

Release push button for local fire fighting system

Flame detector

Figure 23.10 - Loop F o f th e F ire A larm System 904

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Alarm Indication Systems 123.6

F ire Fighting System s

There are many methods o f extinguishing a fire and this includes the popular and most commonly used fire main system. However, research has found that high pressure water mist is more effective for suppressing fires. The high pressure, high fog or functional mist system has a few advantages wherein it requires less than half the flow that is needed for a low pressure system, smaller pipe diameters are effective and there are lower friction losses. In oil tankers, (fluoro-protein) low expansion foam generators are used. Low expansion foam gives a good “throw” and is resistant to wind drift.

Figure 23.11(a) - Foam Pum p

F igure 23.11(b) - Foam P um p S tarter

Figure 23.12 depicts a basic C 0 2 fixed fire extinguishing system. This is only to give one a brief idea as to how fire may be extinguished by smothering. On resolving to operate the C 0 2 system in the event o f a fire, the door o f the local control cabinet is opened to activate the pilot cylinders. Marine Electrical Technology

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Chapter 23 This action in turn activates an alarm, which is routed through a door switch, whose circuit is depicted in Figure 23.13. In addition to activating the alarms, the related ventilation systems (fans, etc) are shut down and the vents in that area are closed.

Figure 23.12 - A CO 2Fire Extinguishing System including Smoke Detector Installation Note: Portable fire extinguishers are deliberately not mentioned in this article as they are beyond the scope of this chapter. 23.6.1

A larm s f o r E ngine R oom a n d P um p R oom S ystem s

Should any cylinder discharge accidentally, it will pressurise the main line up to the stop valve. This line is monitored by a pressure switch which will activate the C 0 2 alarms. Overpressure o f the main line is prevented by a safety valve, which will vent the gas to atmosphere.

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Alarm Indication Systems The pressure o f the control air in the release cabinets is monitored by a pressure switch. A drop in pressure will activate the Pilot Air Pressure Low alarm in the control room. I f the system power supply fails, the C 0 2 Power Failure alarm will operate in the control room. In a tanker, the following are possible from within the foam generator and fire control room: S

Stopping o f the cargo pump;

v'' Control o f the engine room fire dampers, showing whether they are opened or closed; S

C ontrol o f ventilation fans in locations such as th e engine room , steering gear com partm ent, accom m odation spaces, etc.

S

The rotating lights and air horns will operate in the engine room.

S

Emergency starting and stopping o f the fire pumps

v" Emergency trips for the engine room fuel and lubricating oil systems

Figure 23.13 - C 0 2 Cabinet Door Alarm Note: To test the CO2 alarm, open the cabinet door Marine Electrical Technology

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Chapter 23 23.6. 2

IM O F ire C on trol Signs

SOLAS Chapter 11-2, Regulation 28.2.10 requires all fire equipment location markings to be photoluminescent material or marked by lighting. Resolution A.654 (16) amending Chapter 11-2, Regulation 20 requires the use o f these symbols in the Fire Control Plan and Booklet as an International Standard. Some o f the symbols relevant to this chapter have been depicted below:

, , 0 , ,. Control Station

Fire Alarm „ . Panel

Emergency Telephone Station

Manually Operated Call Point

Push Button / Switch for Fire Alarm

CQS

Fire Alarm Beil

Fire Alarm Horn

Fire Alarm Horn C02

Fire A lann Horn Halon

Fire Alarm Horn Sprinkler

Flame Detector

Smoke Detector

Heat Detector

^ . ■■ Gas Detector

Release Station „„

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CU 2

Alarm Indication Systems Extractfrom ABS Rulesfor Building and Classing Steel Vessels - 2012 Part 4 Vessel Systems andMachinery - Chapter 8 Electrical Systems Section 2 System Design Quote 11.9

Em ergency Shutdow n Systems

11.9.1 Ventilation Systems 11.9.1(a) Machinery spaces (2005). Power ventilation systems serving machinery spaces are to be fitted with means for stopping the ventilation fan motors in the event o f fire. The means for stopping the power ventilation serving machinery spaces is to be entirely separate from the means for stopping the ventilation o f other spaces. See 4-7-2/1.9.5. 11.9.1(b) Cargo spaces. Electrical ventilation systems installed in cargo spaces are to be fitted with remote means o f control so that the ventilation fan motors can be stopped in the event o f a fire in the cargo space. These means are to be outside the cargo spaces and in a location not likely to be cut o ff in the event o f a fire in the cargo spaces. Particular attention is to be directed to specific requirements applicable to the ventilation systems o f cargo spaces o f each vessel type provided in Part 5C. See also 4-7-2/7.3.4(c). 11.9.1(c) Other than machinery and cargo spaces. A control station for all other ventilation systems is to be located on the navigation bridge, in firefighting station, i f fitted, or in an accessible position leading to, but outside of, the space ventilated. 11.9.2 Fuel Oil, Lubricating Oil and Thermal Oil Systems (2005) Fuel oil transfer pumps, fuel oil unit pumps and other similar fuel pumps, lubricating oil service pumps, thermal oil circulating pumps and oil separators (purifiers, but not including oily water separators) are to be fitted with remote means o f stopping. These means are to be located outside the space where these pumps and separators are installed or at the firefighting station, if fitted, so that they may be stopped in the event o f a fire arising in that space. 11.9.3 Forced-draft Fans Forced- or induced-draft fans for boilers, incinerators, thermal oil heaters and similar fired equipment are to be fitted with remote means o f stopping. These means are to be located outside the space in which this equipment is located or at the fire fighting station, i f fitted, so that the fans may be stopped manually in the event o f a fire arising in that space. Marine Electrical Technology

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Chapter 23 11.9.4 Unattended Machinery Spaces For vessels intended to be operated with an unattended propulsion machinery space, the emergency shutdowns o f equipment in 4-8-2/11.9.1 through 4-8-2/11.9.3, associated with the propulsion machinery space, are to be located in the fire-fighting station, as required by 4-94/21.3. Unquote

123.7

Modern Methods of Fire Detection and Suppression

As seen in the preceding articles, all detection and suppression systems are in vogue due to their tried and tested reliability. However the need o f the hour is minimisation o f damage by speeding up the whole process! This has resulted in numerous debates and innovations. These findings prove that the existing technology has poor reliability as the most commonly used detectors are smoke detectors and we all know that there no smoke without a fire! Delays for manual suppression can also be long and most o f them are not designed for pre-emptive action. The first step in this direction is the development o f early warning and fire detection systems. These will help to increase sensitivity and decrease detection time, simultaneously decreasing nuisance alarms. The future microprocessor-based neural network and fire pattern recognition systems will include acoustic detectors, optical detection systems, gas sensors (carbon monoxide, carbon dioxide and oxygen to name a few). These in turn will be interfaced with alarms that are not only audio-visual types but also would provide alpha-numeric information on the control panels. These advances in technology would undoubtedly help to pinpoint the origin o f the fire by the use o f cubic pattern neural networks i.e. pattern vectors in multi-dimensional spaces. 2 3 . 7.1 E a rly F ire W arning an d D etection

The first step in this direction will be the development o f an extensive database o f signatures for real fire and nuisance alarm sources. It is understood that there are more that 135 patterns or signatures which may be used to design and train the neural network. These patterns contain both magnitude and slope information for each sensor which include ionization, photoelectric, carbon monoxide and carbon dioxide types. The prototypes used in the early fire warning and detection system are proving faster than photoelectric and ionization types.

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Alarm Indication Systems 23.7.2

The S u pervisory C on trol System

This system will be integrated with sensor information modules and the control system itself. It will be capable o f providing situation awareness to personnel in-charge o f the whole operation and also control damage with the help o f self-governing pre-emptive actions; o f course this will mean that algorithms will be used to control smart pumps and smart valves in the fire-main system that have embedded pressure sensors and actuators with embedded microprocessor and network transmitters and receivers. Figure 23.14 depicts a conceptual system and it is not long before ships have such systems installed. Sensors (inputs) Automated Systems

Thermocouples EFWDs

Fluid Systems

Gas Detectors

- Fire Main

Optical Density

- Chilled W ater

Pressure Flow Rates Door Closure, etc.

=c>

1...........

--------

1

m

- Fuel, Lubricating Oil High Pressure W ater Mist Ventilation

Figure 23.14 - Overview of the Supervisory Control System 123.8

Crankcase Oil Mist Detector Extractfrom SOLAS Consolidated Edition 2004 - Chapter II-1

Construction - Structure, Subdivision and Stability, Machinery and Electrical Installations Part E - Additional Requirementsfor Periodically UnattendedMachinery Spaces (Part E applies to cargo ships except that regulation 54 refers to passenger ships) Regulation 47 Fire Precautions Quote 2 Internal combustion engines o f 2,250 kW and above or having cylinders o f more than 300mm bore shall be provided with crankcase oil mist detectors or engine bearing temperature monitors or equivalent devices. Unquote Marine Electrical Technology

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Chapter 23 23.8.1

Crankcase Explosions

Explosions can occur when three ingredients are present; air, fuel, and heat. In a marine diesel engine and its crankcase the danger o f explosion can exist The gravity o f explosions is proportional to the physical size o f a given crankcase and its gas volume. But nonetheless explosions are destructive on any scale. Crankcase explosions are often caused by human error. The failure cycle begins with some root problem, such as lubricating oil contamination, engine over-speed, excessive operating hours, etc. The other reasons are poor maintenance, negligent operation and lack o f basic engineering knowledge. A n explosion originates at any moving element e.g., the bearings in the crank case (with an embedded particle), elongation o f the connecting rod bearing bolts, poor meshing o f gear teeth, defective gudgeon pin or due to defective piston rings and liners allowing hot gases to blow past. It is the hot spot (a source o f heat), which generates vapour, develops an explosive vapour-air mixture and finally combines with the source o f heat to result in a violent explosion. An accident o f this nature cannot suddenly take place if the engine is properly maintained and operated according to m akers’ recommendations. The possibility o f an explosion can be further significantly reduced i f operation and maintenance is carried out by competent crew. This is not a hazard which has to be a part of engine operation. Preventive maintenance and performance monitoring are the key words. In large diesel engines such explosions can result in the loss o f human life, which is reason enough to prevent them. At the very least they also have large economic ramifications, resulting from repair and replacement cost, as well as lost commerce. Damages can be limited by explosion relief doors, designed to vent the explosion, stop flames and the continuation o f feeding oxygen to maintain the combustion. A crankcase explosion can also be prevented by monitoring the space’s oil mist density. Once a situation has been detected, immediate action is needed; primarily to prevent an escalation. In order to prevent it, a slow-down o f the engine and subsequent stopping to investigate the source o f oil mist m ust be carried out. Precautionary measures such as preparing fire-fighting equipment, shutting access to the engine room and evacuating persomiel should also be done.

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Alarm Indication Systems The root o f the problem that eventually leads a component to failure may have occurred many months earlier. Therefore, it is sometimes often impossible to detect the problem through inspection or observation or to take the necessary corrective measures.

<

................................................................................................................................................................................................................... - ...............................- ................... ............... >

May take only a few seconds

F igure 23.15 - Stages Leading to a n Explosion Due to Oil M ist Monitoring bearing temperature or other parameters can be effective in detecting component failure i f it is slowly developing. However, most failures remain undetected until the collateral damage occurs and the oil mist is already increasing. It is important to note at this stage that the flash point o f lubricating oil is as high as 200°C. It may only be a matter of seconds before explosive levels o f oil mist are present. Therefore, monitoring systems must have the ability to quickly activate safety features, such as an engine shut-down or engine slow-down in order to halt the rapid propagation o f the oil mist. Efforts to exhaust and direct the explosion i f it should occur, are necessary, but offer no real assurance o f complete safety since the hot exhausted gases can also have disastrous collateral effects. The best solution is to ensure that the crankcase explosion does not occur at all. Therefore, oil mist monitoring has been in the forefront o f technologies for preventing crankcase explosions for years. Oil

mist, sometimes erroneously called oil smoke, consists o f very fine oil droplets

(colloidal oil) suspended in the crankcase atmosphere. The size o f these oil particles is in a range o f approximately 1 to 10 microns. Marine Electrical Technology

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Chapter 23 The larger oil mist particles are generally the result o f mechanical atomisation. Due to their size, they generally quickly drop-out due to gravitational forces. However, they are just as quickly replaced since the process is continuous. Oil

mist that results from the re-condensation o f vapourised oil, for example during a

failure, tends to consist o f particles in the smaller size range. They tend to remain suspended in the atmosphere, and therefore increase in concentration. The hydrocarbon content o f oil mist varies, depending on several factors. However, experimentation has revealed that the lower explosive limit remains about the same no matter what the hydrocarbon content. Research has proven that the lower explosive limit o f oil mist or the minimum concentration which can explode is 49mg o f colloidal oil per litre o f air. 23.8 .2

C onstruction a n d O perating P rin cip le o f a B asic D etector

A n oil mist detector may be fitted to monitor samples o f the air and vapour mixture taken continuously from the crankcase o f a diesel engine. (Refer Figure 23.16). Such a device will detect the presence o f oil mist at concentrations well below the level at which explosions may occur. The detector consists basically o f two parallel tubes o f equal size, each with a photocell at one end whose outputs are directly proportional to the intensity o f light focussed on them. Output signal to comparator

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Alarm Indication Systems Mirrors reflect two identical beams o f light from a common lamp. The beams o f reflected light pass along the tubes onto the cells, which are then electrically balanced. One tube is sealed and contains clean air thus termed as the reference tube. The other, the measuring tube has connections through which samples o f the crankcase vapour are drawn by an electric extractor fan. Sampling points should be fitted to each cylinder crankcase and their connecting capillaries are brought to a rotary selector valve, which is driven by the fan motor. This repeatedly brings about a sequential connection o f the measuring tube to each test point. Sampling connections must be sloped in order to ensure positive drainage o f oil; they must avoid any loops, which could fill with oil. In the event o f oil mist being detected, the intensity o f light falling on the measuring tube’s cell is lower than that o f the reference tube’s cell. This results in an imbalance, which activates an audio-visual alarm, the rotor stops, indicating the concerned point. The detector should be tested daily and its sensitivity checked. Its lenses and mirrors must also be cleaned periodically. In this model, we know by now that the signalling level is set with respect to clean air. 23.8.3

The C om parison-type C rankcase O il M ist D etector

An alternative model draws samples through both tubes. A mixture from all cylinders is passed through the reference tube while comparison is made with samples from each cylinder crankcase and also from the atmosphere. In this manner a general sample from all cylinders is compared with the normal atmosphere and each is individually checked against the average (Refer Figure 23.17(a)). An alternative circuit is shown in Figure 2 3 .17(b). In the event o f a hot spot or explosive condition being detected, the engine must immediately be slowed down and stopped as soon as possible; the engine must then be turned by the turning gear in order to allow its overheated parts to cool down thereby preventing seizure. A third model uses only one tube for measurement. It is compared electronically with a reference signal proportional to a mist-free (or clean-air signal). Crankcase explosion relief valves must be fitted to all but the smallest crankcase. They open automatically at moderate pressures allowing the pressure o f the primary or minor explosion to be dissipated thus preventing a possible rupturing o f the casing. Marine Electrical Technology

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Chapter 23 These valves instantly close when the pressure drops as ingress o f air could lead to a major explosion. They are fitted with wire gauze that serves as a flame-arrestor so as to prevent the emission o f flames. They may have deflectors that aim hot gases in areas where they will do least damage. Personnel should not remain in the vicinity o f relief valves in order to avoid being scorched.

Vent pipes from the crankcase must not be too large and m ust also lead to a safe place, remote from the engine. In m ost cases the vent pipes lead to a dedicated enclosure in the rear o f the funnel; here the gases escape freely to the atmosphere and the condensate (oil) drains back to a dirty-oil / sludge tank. These must be inspected periodically in order to ensure that they are not clogged, otherwise this could lead to a pressure build-up in the crankcase. On no account must the crankcase be opened until the parts have cooled down considerably. This will prevent a major explosion, once again due to the ingress o f air. Any internal crankcase lighting must be flameproof.

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Alarm Indication Systems

A ir Supply

Oil M ist 1

Oil Mist 2

Oil Mist 3 and so on.

Figure 23.17(b) - Alternative Circuit of a Comparison-type Crankcase Oil Mist Detector The following article has been published with the kind permission of Jim Russel, www.iceweb.com.au. Relevant un-editedextracts have been reproduced andFigure 23.16 has been redrawnfor clarity whileprinting. 2 3.8.4

The L in e o f S ig h t O il M ist D etector

The I.R 6003 line o f sight oil m ist and smoke detector was developed by Shell Thornton, and is now marketed by Wormald Fire Systems.

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23.8.4.1

Principle of Operation

As indicated, the I.R. 6003 is a line o f sight device. The transceiver generates a modulated infrared beam from an 820 nm GaAs source. This light beam is projected from the transceiver to a specially designed cone reflector and back again. I f anywhere along the path o f detection, the beam becomes incident to mist or smoke, the IR light is refracted (scattered) and the received signal is greatly diminished (Refer Figure 23.18). M ain Unit

Sm oke or Oil M ist

5 Reflector C on e

F igure 23.18 - Line of Sight Sm oke / Oil M ist Detector The light scattering is a function o f the particle size and shape, refractive index o f the particle and the wavelength o f the incident radiation. Complex calculations and predictions have been made and used to determine how the received signal will appear after passing through a cloud o f smoke or mist. Specific algorithms programmed into the device allow it to distinguish between beam blockages and alarm conditions. More recently, advanced signal processing has been introduced into the unit, allowing the detector to be used in external / exposed environments. By adding neural networks into the device, further analysis on the scattering characteristics differentiates between oil mist / smoke and other non-hazardous particles such as water mist and fog. I f the beam becomes blocked and remains so for an extended period o f time, an alarm is raised. This alarm is distinguishable from a true alarm (mist detection) condition.

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Alarm Indication Systems 23.8.4.2

A pplication s

The IR 6003 comes as either a smoke or a mist detector. As well as this, it is also available in rapid or moderate response times. The rapid response tim e should be used in critical applications where immediate action must be taken. The moderate response unit takes a few seconds longer to report as the signal is integrated to give better accuracy and less spurious alarms. The use o f intelligent electronics enables monitoring o f large areas o f a plant, whilst still being highly sensitive and suitable for use even in “still air” conditions. The 4 possible combinations make it a suitable device for many different applications. The matrix below specifies what model to use for varying operating conditions: 23.8.4.3 D etection M ode

Response Tim e

M ist

Smoke

Model 1

Model 3

I.R. 6003/1

I.R. 6003/3

Model 2

Model 4

I.R. 6003/2

I.R. 6003/4

5 - 6 Seconds

1 0 -1 2 Seconds

23.8.4.4

M o d el IJ L 6003/1

Used for situations where immediate action upon detection is required. Typically this would be areas like turbine enclosures, pump rooms, engine rooms and lube oil pipe-work. 23.8.4.5

A dvan tages over C on ven tion al Techniques

The main advantage o f this device is the large plant coverage achieved with just one unit. This means better coverage, less maintenance, fewer cables, less configuration and testing and ultimately less cost. Traditional oil mist detection is normally performed using pellistor gas detectors which try to detect the small amounts o f gas that are dissolved in the oil. This has proven unreliable due to the insufficient amount o f gas released from the oil and because the oil poisons the gas detector material. Marine Electrical Technology

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Chapter 23 23.8.4.6 Features ★

S elf checks at power up.

★

Local and remote alarms and healthy alignment signals

★

Functions correctly on up to 90% o f signal reduction due to fouled lenses

★

Indication o f dirty optics

★

Wire operation allowing direct replacement o f point types.

★

Watchdog timer to detect electronics fault No circuit adjustments - configuration and alarm sensitivity set by software.

★

The first instrument to meet the standards o f the loss prevention council

★

Allows serial communication to obtain more analogue information or further signal processing.

23.8.4.7 Conclusions The I.R. 6003 appears to be the beginning to a new wave o f oil mist detection and an efficient means o f detecting the presence o f smoke. The unit seems to have all the functionality and finesse o f a first grade detector o f oil mists and smoke. This combined with the favourable laboratory and field test results highlight its use in both onshore and offshore situations. The I.R. 6003 has clear-cut advantages and would be ideally suited for use in personnel and equipment safety designs - especially in ships’ engine rooms. 23.8.5

Immediate Steps to Be Taken In Case O fan Alarmfrom an Oil Mist Detector

1. Check the mist detector for a false alarm. If it is not a false alarm, it is prudent to expect a hot spot. 2. Inform the bridge about the prevailing condition, 3.

Stop die engine and leave the engine room immediately as recommended by major engine builders and many ship managers.

4. However, every attempt must be made to at least start turning the engine with the help o f the ‘Turning Gear’ immediately for at least 30 minutes without endangering the lives o f crew due to a secondary explosion. 5. Alternatively, attempts m ust be made to give several slow starts to the engine until it cools down. Failure to turn engine may result in engine seizure. 920

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Alarm Indication Systems 6. Do not open the crankcase for at least 30-40 minutes in an attempt to allow the engine to cool down and oil mist to condense; this will provide fresh air and thus cause a fire. 7. Make a thorough inspection o f the crankcase when it is deemed safe to do so. 8. Pay particular attention to bearings, hot pistons, gudgeon pins, and bottom-end bolts and any other potential hotspots.

23.9

Dead Man Alarm

A dead man alarm is also known as an engineer’s safety alarm. It was devised to ensure the safety o f engineer’s and crew in the engine room while carrying out inspections or attending to “alarm” situations alone. 23.9.1

G eneral D escription a n d L ocation o f U nits

Alarm

Buzzer

Pre-warning

Mains On

O

□

'

Figure 23.19 - Main Unit in the ECR

Figure 23.20 - On / Off and Accept Unit

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Chapter 23 In the Engine Room

Figure 23.21 - Bridge Unit in the Wheel House 23.9.2

System S pecification s

Power Supply - 24V DC Current Consumption - Less than 1A Maximum Load on relay contacts - 5A 23.9.3 23.9.3.1

M odes o f O peration Standby

The power is on and the Engine Room is manned. The running sequence can only be started by the On / O ff Unit. I f the “Unmanned” input terminals are jumped, the system goes into the “Unmanned” mode when it is switched on. 23.9.3.2

U nm anned

The system is now armed for being started by the engine alarm system. 23.9.3.3

Running

The monitoring sequence is now running. This is indicated by the flashing green “Running” lamp. The operator has to press the “Accept” button by certain intervals, in order to avoid an alarm. The running mode can only be stopped by pressing the Start / Stop button when leaving the Engine Room. If the running mode was started by the Engine Room alarm system, this has to be accepted first.

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Alarm Indication Systems 23.9.3.4

P re-w arnin g

I f the operator has failed to press one o f the Accept buttons within the preset time (maximum 27 minutes), the pre-warning alarm sounds in the Engine Room and the Accept buttons will light up so that the alarm may be reset by pressing any one o f them to start the Running sequence all over again. 23.9.3.5

D ea d M an A larm

I f the operator does not press the accept button within the preset time (maximum 3 minutes) after the pre-warning alarm, the alarm will now be activated on the Bridge Unit. The Alarm can be muted by pressing a red button or mute switch and reset by pressing the Accept button. 2 3 .9 .3 .6

S ta rtin g th e R u nning Sequence

This is achieved by: 1. Turning on the power 2.

Switching from manned to unmanned mode

3. By pressing the Start / Stop button 4. 23.9.4

By activation from the Engine Alarm System, when it is in the unmanned mode S afeP age 3000™ W ireless P aging, Inform ation a n d D ead M an A larm System

The Following article in reproduced with permission from Henning E. Larsen, Marketing Manager Gertsen & Olufsen AS / G & O Technologies AS Savsvinget 4 Denmark-2970 Horsholm (www.gertsen-olufsen.dk). Quote

Introduction The Safepage 3000™ system is a wireless portable two-way communication system providing crew on ships, oil rigs, production platforms and other locations, the possibility to move freely around the vessel or area and always be in contact with the remaining crew via the Master Unit. The system is designed and approved for usage in intrinsically safe areas (EeX) and is, contrary to other communication systems, supplying an unique radio coverage all over the vessel using very few components e.g. easy to install. The system typically consists o f a number o f Portable Units (EeX), a Master Unit (e.g. on the bridge), 1-2 Repeaters (one on deck and one in the engine room) and 1 central Antenna. Marine Electrical Technology

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Chapter 23

The M a ster U nit

It is supposed to be mounted inside the bridge (or in the Cargo Control Room) and consist o f a box comprising the electronic equipment and a separate external antenna. The Master Unit contains a graphic LCD-display with a dimmable back-light for indication o f alarms and system status, a keypad for acknowledge o f alarms, paging, dimmer and system set-up and finally a LED for “Alarm” indication. The antenna is ruggedly designed to operate in all weather conditions and is connected to the Master Unit via a coaxial cable. It has several digital inputs and outputs and two RS232 and one RS485 serial ports. The P ortable H an dset (EeX)

It contains a graphic LCD-display with a back-light for indication o f alarms and system status, a keypad for resetting the Dead M an Alarm timer, Emergency Call and activation of the back-light for the display. Finally, the handset contains three LEDs indicating “Alarm”, “Paged” and “Battery Low”. When not in use, the portable handset is placed in a battery charger unit. The SafePage 3000™ system provides Wireless Paging, Data Exchange/Information & Safety Monitoring o f up to eight engineers when working alone in remote areas (e.g. cargo area and engine room) allowing the engineers to move freely around. There is an emergency button and Dead M an Alarm timer function in the portable unit with which the engineers at regular intervals can report that they are in good health. Otherwise, the officer on the bridge is alerted by a Dead M an Alarm raised on the Master Unit. Unquote

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Alarm Indication Systems 23.10 Miscellaneous Alarm Indication Systems The other alarm systems in engine rooms can vary extensively from simple individual alarm lights activated by the closure o f electrical contacts, to sophisticated data logging and alarm annunciation systems incorporating such facilities as alarm point scanning, memory banks and automated shut-down operations. This latter type o f equipment could be interfaced with computer systems with which many new vessels are being equipped. An alarm system basically consists o f one or m ore actuating devices, or transducers, providing a signal related to the process variable state. It has a monitoring device such that, i f the variable being measured exceeds preset limits, an alarm is activated. This can be both visual (a flashing light) and / or audible (klaxon or bell). Some o f the systems that require alarms to be incorporated are as follows: c

A ir compressors

z

Main propelling machinery including essential auxiliaries

c

Bilge and ballast systems

z

Transverse thrust units

z

Boilers and th e ir ancillary equipment

z

Oil fuel transfer and storage systems

c

Cargo pumping systems fo r tankers

z

Steering gear

z

Cargo and ballast pumps in hazardous areas

z

Thermal fluid heaters

z

Cargo tank, ballast tank and void space instrum entation where specified (e.g. w ater ingress detection)

z

Miscellaneous machinery o r equipment (where control, alarm and safety systems are specified e.g. the elevator)

z

Controllable pitch propellers

z

Valve position indicating systems

z

Electric generating plant

z

W aste-heat boiler

z

Incinerators

z

W ater-jets for propulsion purposes

z

Inert gas generators

z

Refrigeration Plants

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Chapter 23 23.10.1 The Bridge Alarm Console On a tanker it is generally situated directly behind the main bridge console and could contain the following: Signal light control panel

| ©

•

Two navigation light panels

!

•

No.2 VHF main unit

•

Echo sounder main unit

•

•

•

Autom atic telephone ]•

INMARSAT telephone

Alarm panel with buzzer stop and test button

1•

Message indicator fo r Inmarsat Alarm buzzer fo r Satcom CRT fo r the alarm monitoring system

Changeover switch from A t Sea to Harbour

|•

•

Navtex

1•

•

GMDSS alarm unit

•

Fire alarm pushbutton General and emergency alarm

No.3 operator control panel

23.10.2 Group Control Panel The group panel is situated directly behind the main chart table and contains the following: 1> Master fire alarm panel 2.

Gas detection repeater panel for the ballast tanks and pump room

3.

Elevator alarm, buzzer and direct telephone to the elevator

4.

Master clock

51

Fog and gong automatic system

6.

Rudder and course log printer

7.

IG system indicator panel

8.

Deck lighting control panel

9.

Alarm indication for the infirmary, refrigeration rooms and auto telephone trouble

10. Start/stop for the bilge, fire and GS pumps and emergency fire pump

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Alarm Indication Systems 23.16.3 O peration o f a B a sic System

There are many vessels fitted with simple alarm systems comprising a series o f pressureoperated or temperature-operated electrical switches. When the operating temperature or pressure o f machinery rises above or falls below a pre-set limit, the switch closes a circuit to an alarm light and sounds an audible alarm if necessary. A more advanced version incorporates a flashing unit with ‘Test’ and ‘Accept’ push button facilities, so that when an alarm condition exists, a light flashes and a klaxon sounds. The watch-keeper can then press the ‘Accept’ button, which will cancel the audible alarm and cause the flashing light to remain constantly alight. He will then immediately investigate and take the necessary corrective action to eliminate the alarm condition, such that the alarm light will be extinguished when the process variable is returned to within the pre-set limits. In this type o f system, test buttons are provided to enable the lamps to be periodically checked. Variations o f the alarm indication systems are many, ranging from a simple alarm light with a red-coloured lens, which is extinguished when in the safe condition, to a double light system with red and green lights on the alarm annunciator facia. This may incorporate between 20 and 30 alarms per panel, and several panels may be used in an installation. A third type o f alarm panel may be used, having small opalescent screens engraved or marked with the process variable’s identification; the screens are only illuminated when the alarms are activated. Simple, pneumatically operated alarm systems can also be provided for areas that require flameproof equipment and in this application, the indication will be by a coloured flag and an air-operated whistle. f....-........................................ ................................................................................................................................... ]

23.11

Scanning-type System

Alarm annunciators with scanning-type systems provide m ore comprehensive facilities than the simple system previously described and are obviously more costly. The basic details and functions o f a typical scanning alarm system with various facilities available are illustrated in Figure 23.22. In the normal mode o f operation, which is the ‘alarm scan’, each measured point is automatically selected in sequence and presented to the measuring system. If an alarm situation occurs while a measurement is taking place, a visual and audible warning is given. W ith this system, two scanning speeds are available. In the case o f a slow scan, which has a scanning speed o f 1 point every 3 sec, both the point identification and the measured value o f that point are displayed on a panel during the scan such that a hand-written log o f the points being scanned can be taken, if required. Marine Electrical Technology

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Chapter 23 In the fast scan mode, the measured value indication is that o f the associated point num ber indicated which can be selected manually for any o f the measured points. For example, if ail operator suspects a particular measured point, he can manually switch to that position whilst watching the measured value display, the value o f which is updated each time the scanner ‘looks’ at that point. Measurements and alarm comparisons (set levels) are made on all other points as they are scanned and i f any point reaches an alarm condition, this is displayed on the alarm annunciator separately. The fast scan speed on this system is 1 point / second. Command Control

Timing

Central Processor

Converter

ve

Measured Value Display

c

8

to

Alarm Lamps Programming

Alarm Pin Board

Alarm Detection

Scanned Alarm Annunciators

Audio Aiarm

Self Check System External Contacts

Alarm Lamps Contact Alarm Annunciators Audio Aiarm

Figure 23.22 - Overall System Diagram I f it is required that a specific point be measured without waiting for the scanner to arrive at that point, the point is selected on the manual selection matrix and the ‘read’ button is depressed. The equipment will immediately make repeated measurements on that point until the ‘read’ button is released, when it will revert to the alarm scan mode and commence measuring each point sequentially again.

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Alarm Indication Systems This type o f equipment can have information presented to it from various types of transducers, such as platinum resistance thermometers, thermocouples, potentiometric pressure transducers and contacts for continuously monitored alarms. For temperature measurement, the resistance thermometer is the basic measuring element and the outputs of other transducers are scaled accordingly. The resistance thermometers (RTDs) are supplied from a current source, which is controlled by the voltage developed across the transducer is such a way as to provide a linear output signal voltage directly proportional to the temperature. Where thermocouples are used, they are connected to a data transmitting circuit, which produces a current proportional to the thermocouple temperature; this current is suitably scaled and presented to the measuring system. For pressure measurement, the transducers consist o f either a barometric capsule or Bourdon tube, the free end o f which is connected via a mechanical linkage to a wiper, which is free to travel along a resistance element. Thus, if a voltage is applied across the resistance element, a second voltage will be developed between one side o f the element and the wiper; the magnitude o f this voltage being proportional to the applied pressure. The output voltage from the appropriate transducer is selected by the scanner and presented to the measuring system. With contact alarms, the external contacts are connected to the individual alarm annunciators in cases where certain parameters must be continuously monitored for an alarm condition. As these points are not being scanned, no information is available for indication on the measured value display unit. Where contact alarms are being used to monitor liquid levels in tanks, an adjustable delay circuit is placed between the contact and the alarm annunciators so that spurious alarms will not be generated due to the ship’s motion. Referring to Figure 23.17, the basic circuit blocks are clearly shown, each having the following functions: 23.11.1

Scanners

The scanners in this type o f equipment are multi-bank uni-selectors with gold-plated contacts on the signal banks. They perform 3 basic functions for any particular point selected. They are as follows: a) Routing the transducer signal into the analogue to frequency (A/F) converter;

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Chapter 23 b) Selecting the program for the point; the program provides instructions for the type o f measurement, i.e. temperature or pressure, etc. and switching die alarm annunciator and alarm pin board value to that point; c)

Setting up the point identity display in the slow-scan mode or enabling the measured value to be up-dated when the point coincides with that selected on the manual selection matrix, when it is in the fast scan mode.

23.1 1 .2

C onverter

The converter consists o f the control circuitry, which selects the mode o f operation (resistance thermometer or voltage input) and sensitivity, a precision crystal-controlled voltage to frequency converter, an output and an isolation stage. A comparison technique is used when making a measurement and the operation o f the converter is described below for the case o f a resistance thermometer. Initially, a compensated current supply is fed into a reference resistor contained within the converter and the voltage developed is applied to the converter input. The resulting output frequency is fed into the counting chain o f the central processor for a defined period, with the total number o f pulses being stored (reference count). The compensating current is then fed into the resistance thermometer and the voltage developed is applied to the converter input. Again, the output frequency is fed into the counting chain o f the central processor (forward count), but in such a manner that the reference count is subtracted from the forward count Complete d.c. and low frequency isolation between the converter and the central processor is obtained by feeding the output pulses via a transformer. Thus, any electrical noise generated which is external to the equipment, cannot be introduced into the central processor or other logic circuits where it might cause spurious operation. 23.11.3

C en tral P rocessor

The central processor comprises a number o f sub-systems as follows: a) The totaiiser; b) The decoder and measured value memory; c) The alarm comparator. 930

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Alarm Indication Systems The totaliser is a four-decade reversible counter preceded by reversible binary and associated gates having the following method o f operation: The pulses are generated by the converter during the forward and reverse count periods and fed into the reversible counter where they are totalled so that the number o f pulses stored at the end o f measurement is the difference between the total number o f pulses generated in each case. The double count technique ensures maximum accuracy and freedom from drift. The gating times and the operating frequency o f the converter are chosen so that the number o f pulses stored in the totaliser, when decoded, gives the measured value o f the unknown parameter in engineering units. The circuitry is reset to a reference condition determined by the program before the start o f a new measurement. A program also determines the position o f the decimal point. The output from the totaliser is in binary coded decimal (BCD) form and is fed into the decoder and the alarm comparator. 23.11.4

D isp la y R egister, D ecoder a n d L am p D rivers

The BCD information in the totaliser has to be decoded prior to display. In this case the output from the totaliser is transferred to a storage device and then on to the decoder and lamp drivers, which initiate the appropriate lamp in each decade o f the measured value display. In the ‘slow scan’ and ‘read’ modes, the stored information is changed each time a new measurement is made, but in the ‘fast scan’ mode, only the information generated on manually selected points is permitted to enter the storage device. 23.11.5

T im ing C ircuits

The timing circuits consist o f the timing generator and the control logic, with the timing generator being made up o f crystal oscillator and frequency divider boards which generate the precision timing signals required by the control logic. The control logic governs the sequence o f events during the measurement and display cycles, and is responsible for stepping the scanners during the alarm scan mode. 23.11.6

P rogram D istribu tion B oard

The scanner selects the group program required for each specific point via this board and in addition, the particular alarm setting and alarm annunciator are selected. On this board the group instruction is broken down into the individual instructions for the totaliser and converter i.e. ‘decimal point’ - 1O'1, etc. Marine Electrical Technology

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Chapter 23 2 3 .1 1 .7

C om m and C on trol

The ‘read’ and ‘alarm scan’ modes and the ‘measured value updating’ mode are selected in the command control module. 2 3 .11.8

A larm System

The high and low alarm levels are ‘set in’ on the alarm pin board in decimal form, being converted internally into a pre-determined code for use in the measuring circuits. The alarm setting for any particular point is selected and compared with the measured value in the totaliser through the alarm comparison circuits as the measurement progresses. Further logic in the alarm detector establishes whether a normal condition exists i f the measured value exceeds the low alarm setting, or a high alarm condition exists if the measured value exceeds the high alarm setting. I f an alarm condition occurs, an alarm signal is routed to all annunciators and a red background is illuminated on the point identity display. I f no alarm is detected, a ‘clear’ signal is generated. Each alarm point has its own annunciator, and the alarm annunciator will only accept an ‘alarm’ or ‘clear’ signal from the alarm detector when it is selected by the program from the scanners. Thus, although the alarm detector sends out a signal to all annunciators, only that annunciator associated with the alarm level being compared can change its state. I f it is necessary to check for both high and low conditions on a single point, two alarm levels are set up on the pin board. Initially, the low-level setting is compared with the measured value and i f the low level is exceeded, the comparison is switched to the high level setting. In this way, both the high and low limits may be checked during one measuring cycle. Groups o f annunciators may be cut out by means o f inhibit switches on the alarm display panels, a facility which may be required when machinery is shut down. As in the simple type o f alarm system, the alarm lamp operating cycle is as follows:

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S ta te

Lam p

No Alarm

Extinguished

Alarm

Flashing light j

Alarm accepted

Steady light

Accepted alarm reverting to normal

Extinguished

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j

Alarm Indication Systems 23.11.9

Contact Alarms

A number o f alarm systems can be displayed on the equipment, in which case the contacts are brought in through a junction box to alarm annunciators in a visual display unit. Tank level alarms pass through delay circuits, which permit the alarm contact information to be passed to the annunciators only after a discreet period in order to avoid the possibility of displaying spurious alarms when tank levels are disturbed by a ship’s movement. This delay is normally adjustable. 23.11.10 Self-checking System It is normal to incorporate a self-checking system in this type o f equipment, in which case the alarm detection system is checked once in every complete scan. Alarm settings are incorporated on the test point so that, for example, the system would detect a low alarm on the pressure signal test point and a high alarm on the temperature signal test point. Another example is to have a temperature signal test point o f 50°C, which has high and low settings o f 45°C to 55°C and the system requires a clear signal on this point as follows: As the scanner steps through the test points, failure to detect the sequence viz. low alarm, high alarm and clear, will cause the self-check system to give out its own alarm. This also acts as a check on the accuracy o f the measuring system, since if the 50°C test point reads outside the limits o f 45°C to 50°C, an alarm will also be activated. Failure o f the scanner to continue scanning when the monitor is in the alarm scan mode is indicated by an alarm annunciation - all the d.c. power supplies are monitored and failure of any one will initiate an alarm. I f there is a failure in the system, an audible alarm will be activated and the ‘System Failure’ annunciator will flash. If the ‘M ute’ switch is pressed, die audible alarm will cease, the flashing ‘System Failure’ annunciator will glow steadily and the ‘M ute’ annunciator will flash. W hen the fault in the system is eliminated, the ‘System Failure’ annunciator will go off but the ‘M ute’ annunciator will remain on until the ‘M ute’ switch is returned to the ‘o ff position. The self-check system also has test facilities to check itself and it is advisable that the system be tested once a week.

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Chapter 23 23.12 23.12.1

Com m unication Systems Sound Powered and Intrinsically Safe Telephone Systems

The sound powered telephone system is designed for safe and clear communication. The aim o f this system is to provide communication independent o f the ship’s power supplies thereby providing emergency communication. Stations within machinery spaces are fitted with headsets to provide communication within the noisy environment. The headsets are selected by operating the headset/handset toggle switch on the telephone unit. Direct communication by means o f sound power telephones and intrinsically safe systems is provided between the Wheelhouse and the Engine Control Room, Main Engine Emergency Maneuvering Position, Pump Room, Captain’s cabin, C hief Engineer’s cabin, Steering Gear flat and Emergency Generator room, to name a few. 23.12.2

The Engineer*s Call Alarm

The control for the engineers’ call alarm is in the Engine Control Room. The purpose o f this alarm is to call any particular engineer or all o f them to the engine room in case o f an emergency. Whenever any engineer is called by this alarm, a buzzer or bell is activated in the respective cabin. The same can also be operated from common places like the officers’ mess room, duty mess room, officers’ smoke room and gymnasium. ‘A ’ class telephones have the following facilities: 1. Extension to extension calling 2.

Paging facility via the PA system

3.

Priority

4.

Trunk access

‘B ’ class telephones have the following facilities: 1. Extension to extension calling 2.

Paging facility via the PA system

3. Priority ‘C* class telephones have the following facilities: 1. Extension to extension calling 2. Paging facility via the PA system

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Alarm Indication Systems ‘D ’

class telephones have the following facility: 1. Extension to extension calling

C all P ilo t lam p

G e n e ra to r H an d le

R o tary S e le c to r Sw itch H e a d s e t / H an d set S e le c to r Sw itch

Figure 23.23 - A Sound-powered Telephone 123.13

Relevant Rules

23,13.1 Relevant SOLAS Regulations Chapter II-1 - Part C - Machinery Installations - Regulation 38 - Engineers’ Alarm Part E - Additional Requirements for Periodically Unattended Machinery Spaces -

Regulation 47 - Fire Precautions

-

Regulation 51 - Alarm System

-

Regulation 53 - Special Requirements for Machinery, Boiler and Electrical Installations

Chapter II - 2 - Part A - General - Regulation 3 - Definitions Part C - Suppression o f Fire Regulation 7 Detection and Alarm Part D —Escape - Regulation 12 - Notification o f Crew and Passengers Part E - Operational Requirements - Regulation 14 Operational Readiness and Maintenance Marine Electrical Technology

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Chapter 23 Chapter HI - Life-saving Appliances and Arrangements Part B - Requirements for Ships Regulation 6 - Communications 23.13.2 1)

Summary ofRegulations

An alarm system shall be provided to indicate any fault requiring attention and shall: a) Be capable o f sounding an audible alarm in the main machinery control room or at the propulsion machinery control position, and indicate visually each separate alarm function at a suitable position; b) Have a connection to the engineers’ public rooms and to each o f the engineers’ cabins through a selector switch, to ensure connection to at least one o f those cabins. c) Activate an audible and visual alarm on the navigation bridge for any situation which requires action by or attention o f the officer on watch; d) As far as is practicable be designed on the fail-to-safety principle; and e) Activate the engineers’ alarm if an alarm function has not received attention locally within a limited time.

2)

The alarm system shall be continuously powered and shall have an automatic change­ over to a stand-by power supply in case o f loss o f normal power supply.

3)

Failure o f the normal power supply o f the alarm system shall be indicated by an alarm.

4)

The alarm system shall be able to indicate at the same time more than one fault and the acceptance o f any alarm shall not inhibit another alarm.

5)

Alarms shall be maintained until they are accepted and the visual indications o f individual alarms shall remain until the fault has been corrected, when the alarm system shall automatically reset to the normal operating condition.

6)

W ith regard to machinery, boiler and electrical installations, an alarm system shall be provided for all important pressures, temperatures and fluid levels and other essential parameters.

7)

A centralised control in which the following control and indicator functions are centralized shall be arranged with the necessary alarm panels and instrumentation indicating any alarm a) Fixed fire detection and fire alarm systems; b) Automatic sprinkler, fire detection and fire alarm systems; c) Fire door indicator panels;

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Marine Electrical Technology

Alarm Indication Systems d) Fire door closure; e) Watertight door indicator panels; f)

Watertight door closures;

g) Ventilation fans; h) General/fire alarms;

8)

i)

Communication systems including telephones; and

j)

Microphones to public address systems.

For the protection o f machinery spaces, a fixed fire detection and fire alarm system shall be installed in periodically unattended machinery spaces and machinery spaces where: a) The installation o f automatic and remote control systems and equipment has been approved in lieu o f continuous manning o f the space; and b) The main propulsion and associated machinery including the main sources o f electrical power are provided with various degrees o f automatic or remote control and are under continuous manned supervision from a control room.

9)

The fixed fire detection and fire alarm system shall be so designed and the detectors so positioned as to detect rapidly the onset o f fire in any part o f those spaces and under any normal conditions o f operation o f the machinery and variations o f ventilation as required by the possible range o f ambient temperatures.

10)

Except in spaces o f restricted height and where their use is especially appropriate, detection systems using only thermal detectors shall not be permitted. The detection system shall initiate audible and visual alarms distinct in both aspects from the alarms o f any other system not indicating fire, in sufficient places to ensure that the alarms are heard and observed on the navigation bridge and by a responsible engineer officer. When the navigation bridge is unmanned, the alarm shall sound in a place where a responsible member o f the crew is on duty.

11)

For cargo ships, a fixed fire detection and fire alarm system shall be so installed and arranged as to detect the presence o f fire in all accommodation spaces and service spaces and control stations o f cargo ships, providing smoke detection in corridors, Marine Electrical Technology

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Chapter 23 stairways and escape routes within accommodation spaces, except spaces which afford no substantial fire risk such as void spaces, sanitary spaces, etc. 12)

The function o f fixed fire detection and fire alarm systems shall be periodically tested to the satisfaction o f the Administration by means o f equipment producing hot air at the appropriate temperature, or smoke or aerosol particles having the appropriate range o f density or particle size, or other phenomena associated with incipient fires to which the detector is designed to respond.

1)

M ain Power supply to the control panel o f the fire alarm system is normally th e _____ .

2)

The line voltage to the fire alarm panel is stepped down and then rectified to typically volts.

3)

In the event o f interruption o f the main power supply, the fire alarm systems h av e ____ to keep them online.

4)

The dead man alarm is also known a s _______ .

5)

Sprinkler line flow switches are connected with th e ________ .

6)

The means o f direct communication on board ships is with the help o f ________ .

7)

The manual call station switch is connected t o ________ .

8)

Manual call points on board ships are generally located__________.

9)

Fire detectors in engine room workshop are inhibited b y _________ .

10)

I f a fire detector or its cable gets grounded at some point, what is the indication on fire detector panel?

11)

What is the method o f isolating the fire zone loops?

12)

What are the general requirements o f a fire alarm and detection system?

13)

In simple words explain the virtues o f an intelligent fire detection system.

14)

List the various Fire Detectors generally found onboard a ship (indicate their locations too). Explain any two in detail.

15)

What happens when the CO 2 cabinet door is opened? Explain with a neat sketch.

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Alarm Indication Systems 16)

With a simple sketch, explain the Infra-red type o f Flame Detector.

17)

With a simple sketch, explain the Ionization Chamber type o f Detector.

18)

With a simple sketch, explain the Rate-of-rise type o f Fire Detector (air chamber and breather assembly type).

19)

With a simple sketch, explain the Rate-of-rise type o f Fire Detector (bi-metallic strip type).

20)

With a simple sketch, explain the Ultra-violet type o f Flame Detector.

21)

Briefly explain the modem methods adopted for the detection and suppression o f fire.

22)

With a neat diagram explain a supervisory control system for fire detection and suppression.

23)

What are the stages leading to an explosion due to oil mist?

24)

With a neat sketch explain the operation o f a basic crankcase oil mist detector.

25)

What are the special features o f a comparison-type o f crankcase oil mist detector? Explain the operation o f the same with a suitable diagram.

26)

List any 10 systems that require alarms to be incorporated so as to ensure their safe operation.

27)

Describe the controls o f the fire control panel and state the procedure to test the fire installation.

28)

List the types o f fire detection sensors provided in fire control panel and means o f isolation o f fire zone loops.

29)

State how the C 0 2 alarm system is tested.

30)

State the sequence o f “dead mans” alarm and pre-alarm in engine room.

31)

State the locations o f fire detectors in engine room workshop and means o f inhibiting the fire sensors.

32)

Explain the term “manual call station”.

33)

List the critical stations between which direct communications is provided. Marine Electrical Technology

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jAt the end of this chapter you should be able to: l

i★

E x p la in t h e o p e ra tio n o f t h e c o m m o n ty p e s o f c o m b u s tib le g a s in d ic a to rs

★

B rie fly e x p la in c a rb o n d io x id e a n a ly s is

★

E x p la in th e o p e ra tio n o f th e c o m m o n ty p e s o f o x y g e n a n a ly s e rs

24.1.1

Application

It is used to measure the volume o f flammable gas in a gas / air mixture. It gives the result as a percentage o f the lower explosive limit (LEL) or lower flammable lim it (LFL). It is used to ascertain the atmosphere in a cargo tank when planning tank cleaning in a very lean atmosphere or for measuring C/H gas content prior to entry, hot work or other operations by personnel. 24.1.2 Principle of Operation MSA 40 (Marine Safety Appliances) is a commonly used CGI. It works on the Catalytic Combustion Principle. An electric current heats a catalytic filam ent A sample o f the atmosphere to be tested is drawn over it and combustion o f the flammable gas on the filament further raises the temperature. This in turn causes an increase in resistance, which is indicated on the meter as the gas concentration (Refer Figures 24.1 and 24.2). The measuring circuit is a balanced Wheatstone’s bridge and for its operation, three major steps must be taken before samples are drawn into the instrument for analysis: l|

The combustion chamber m ust be swept free o f combustible gases and filled with fresh air.

2. Batteries must be turned on and the proper voltage applied to the bridge. 3. The bridge m ust be balanced to zero deflection on the meter with fresh air in the chamber. Marine Electrical Technology

Chapter 24 The MSA 40 is a dual range CGI, with two scales i.e., 0 - 100% and 0 - 10% o f the LFL (Lower Flammable Limit) o f combustible gas/air mixtures. The meter scale also has a voltage check mark to indicate the correct bridge voltage setting. Selection o f the required range or check facility is by means o f a three-position selector switch.

Figure 24.1 - Combustible Gas Indicator 24.1.3

G uidelines f o r Use:

1. Place the sample intake in fresh air. 2. Turn the selector switch to ‘check’. This places the meter across the active filament where it will indicate the voltage across the filament. 3.

Pull straight up on the switch knob. This operation closes che battery circuit and the meter pointer will be seen to rise on the scale towards the checkpoint.

4.

Flush fresh air into the indicator’s chamber by squeezing the rubber aspirator bulb and allowing it to completely expand. Ten to twelve squeezes will flush the chamber. If the sampling line is more than 5 feet, two additional squeezes will be required for every ten feet o f line.

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Gas Analysers

3 - Voltage Adjustment Knob 4 - Zero Adjustment Knob 5 - Aspirator (Bulb)

Figure 24.2 - Combustible Gas Indicator 5. Adjust the voltage by turning the voltage adjustment knob such that the m eter’s pointer deflects to the ‘check’ position on the scale. This establishes the proper operating voltage to the bridge and heats the platinum filament to about 650°C. 6. Turn the selector switch to 100 % LFL. 7. Adjust the pointer to zero. Turn the zero adjustment knob so that the meter pointer slightly overshoots the zero mark, then turn the knob slowly in the opposite direction until the pointer returns to zero. 8. Turn the selector switch to 10 % LFL; adjust as above. 9.

Turn the selector switch back to 100 % LFL. Marine Electrical Technology

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Chapter 24 10. Place the end o f the sampling line in the area to be tested. 11. Aspirate the sample through the indicator. The aspirator bulb should be squeezed repeatedly until the m eter pointer comes to rest on the scale. 12. After 10 squeezes, if the reading obtained is below 10 % LFL, continue aspirating and take the reading from the top scale. CAUTION! A lw ays ta k e th e f ir s t sam ple with th e selecto r sw itch se t to th e 100 % L F L position .

13. If the meter pointer does not rise on the scale within 15 Squeezes o f the aspirator bulb, the point being sampled may be considered gas free. 24.1 .4

L im itation s o f th e In stru m en t

The instrument should not be used in the following atmospheres: 1) C/H gas + Inert Gas - Since the instrument is based on the burning principle, no burning will occur. 2) C/H gas + Oxy-acetylene - Burning may be too violent to be contained by the flash back arrestors. 3) C/H gas + Oxy-hydrogen - Same as above. 4) Leaded petroleum vapours - A lead oxide deposit on the filament will affect its sensitivity. 24.1 .5

M SA 40 - C alibration P rocedure

1) Set the pointer to zero on the 0-100 % scale. 2) Fit the hose adapter onto the inlet o f the instrument. 3) Assemble the rubber tube and balloon onto the plastic cup. 4) Eliminate the air from the balloon by rolling it up towards the cup. 5) W hile keeping the balloon rolled up, slip the open end o f the rubber tube over the outlet socket on the calibration gas can. 6) Press the nozzle o f the gas can and let the balloon inflate completely. 7) Close the rubber tube near the plastic cup with a pinching clamp and remove it from the can. 944

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8) Slide the rubber tube onto the hose adapter and remove the pinch clamp. 9) Draw the gas sample through the instrument by squeezing the aspirator bulb. 10) Note the scale reading. 11) To check the response, refer to the calibration curve. 12) Read-off methane content in air (shown on the calibration can) on the bottom scale o f the graph and from the response curve, check the reading on the ‘meter reading scale’. Some examples are as follows: a)

Let the value o f calibration gas being used be 2% methane in air.

b)

The response curve indicates that for 2% methane in air, the reading should be between 44% and 62% LFL.

c)

The reading obtained is 50% LFL.

Therefore, the instrument is fit for use. 2 4.1.6

M SA -40 C alibrated on P entane

A full-scale reading o f 100 = the LFL o f pentane (1.4%) or A full-scale reading o f 100 = 1.4 parts o f Pentane in 100 parts mixture, i.e. A full-scale reading o f 100 = 1.4*1000000

100 = 14,000 ppm o f pentane In practice, calibration with pentane will give reliable results for most straight chain hydrocarbons in crude oil and products commonly carried by sea because their LFL is around 1.4%. I f the LFL o f the cargo is different from 1.4% then the correction curves must be used. Thus a full scale reading of 100 = LFL Pentane (1.4%) = 14,000 ppm pentane The main scale is divided into 50 equal parts. Therefore, one division = LFL = 14,000 ppm 50 50 = 2% LFL or 280 ppm The expanded scale, 0 - 10% LFL, is divided into 25 equal parts. Marine Electrical Technology

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Chapter 24 Thus a full scale reading —10% LFL o f pentane - 14.000

ppm

10 = 1400 ppm 1 scale division = 10% LFL - 1400 ppm 25 25 = 0.4 % LFL or 56 ppm A TLV o f about 300 ppm (corresponding to about 2% LFL) is considered normal for mixtures encountered in petroleum liquids transported by sea.

24.2

The Tankscope

A tankscope measures the C/H gas content in a sample o f atmosphere. It has an advantage over the explosimeier wherein it can measure C/H gas content even in an inert atmosphere. This is because it does not depend on combustion o f the gas on the heated filament to change its temperature and consequently the resistance. The construction and working is similar to the explosimeier except that the mere passage o f C/H gas over the heated filament changes its temperature which is indicated on the meter as a percentage o f the volume o f the sample. The basic arrangement within the Tankscope and its related circuit diagram are depicted in Figures 24.3 and 24.4. Type

Combustible Gas Indicator

i Scale

0 - 20% by volume of gas

Calibrating Gas

8% by volume butane in nitrogen/carbon dioxide mixture

Batteries

8 high power leak-resistant cells

Battery Life

Approximately 6 - 8 hours of continuous use

Controls

On / off switch Combined battery-check and a gas percentage range switch Voltage adjustment Zero adjustment Span trimmer

Case

Waterproof aluminium case covered by a PVC outer case

Table 24.1 - Specifications of the Tankscope

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Flam e Arrestors

1 2 3 4 5 6

- On / O ff Switch - “C h eck G a s” Switch - Zero Adjustment Knob - Span Adjustment Knob - Voltage Adjustment Knob - Aspirator (Bulb)

Figure 24.3 - A Tankscope Marine Electrical Technology

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Chapter 24

®

1 - Zero Adjustment 2 - Span Adj. 3 - Check Switch 4 - Compensator Filament 5 - Sensor Filament 6 - Voltage Adjustment 7 - On / Off Switch 8 - 8 Cells (1.5V each)

Figure 24.4 - Circuit Diagram of a Tankscope 24.2.1

'O peration

As seen in Figure 24.4 above, the measuring circuit is a balanced Wheatstone’s bridge. Three major steps must be taken before the samples are drawn into the instrument for analysis: 1. The detector chamber must be swept free o f C/H gases and filled with inert gas or air. 2.

The supply from the batteries m ust be turned ‘O n’, and the proper voltage m ust be applied to the bridge.

3. The bridge must be balanced to zero deflection on the meter with inert gas or fresh air in the filament chamber. 948

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The M SA tankscope is sensitive to tilt, so it is important to keep the instrument in a normal, upright position during operations or there may be a significant error in instrument readings. 2 4.2.2

G uidelines f o r use

1. Attach the sampling line to the inlet and the aspirator bulb to the outlet o f the instrument. Check the air-tight integrity o f the system by pinching a bight on the sampling line and squeezing the aspirator bulb. The bulb should not expand as long as the sampling line is pinched. I f the bulb expands, re-check connections and non-return valves on the aspirator bulb. 2.

Place the instrument in fresh air.

3. Turn the selector switch to ‘Check’. 4.

Switch ‘O n’ the unit by lifting the switch on the top left hand comer o f the instrument.

5. Flush fresh air through the tankscope by squeezing the m bber aspirator bulb and allowing it to expand completely. 8 to 10 squeezes are sufficient to flush the chamber. If the sampling line is used, two additional squeezes will be required for every 3 metres o f line. 6. Adjust the meter pointer to the ‘Check’ position marked on the dial, using the ‘Voltage Adjustment’ knob. 7. Turn the selector switch to ‘G as’. 8. Adjust the m eter pointer to ‘Zero’. 9. Place the end o f the sampling line where die sample is to be taken. Aspirate the sample through the instrument. The aspirator should be operated (squeezed) until the meter pointer comes to rest on the scale. W ith 2 metres o f sampling line and probe, the meter pointer should rise on the scale within fifteen squeezes - otherwise the sample point may be considered gas free. 10. Stop aspirating and note the final reading. The reading should be taken with zero flow through the instrument and with the gas at normal atmospheric pressure. Small deviations from the normal atmospheric pressure in the instrument produce significant differences in the indicated gas concentration. I f a space under elevated or reduced pressure is sampled, it is important to detach the sampling line from the M SA tankscope when aspiration is stopped; this allows the instrument to attain atmospheric pressure before the reading is noted. Marine Electrical Technology

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Chapter 24 11. After each reading flush the sampling line and instrument with fresh air. 12. Recheck ‘Volts’ and ‘Zero’ controls at frequent intervals (steps 5 ,6 and 7). 13. Protect the instrument from weather as much as possible and avoid exposure to very wet conditions. 24.2.3 Troubleshooting

1. If the meterpointer goes below the scale when the selector switch is turned to ‘Gas ’ and cannot be adjustedto 'Zero' with the zero adjustment control: >

The batteries may require replacement.

>

The thermal conductivity filament (detector filament - white housing) may be defective and requires replacement.

2. If the tankscope is sluggish and requires more than the specified number of aspirations for maximum deflection ofthe meterpointer: >

The flashback arrestors may be clogged.

>

There m ay be an obstruction in the aspirator coupling’s flow orifice.

>

The cotton filters may be choked,

> There may be a leak in the flow system. I f service other than that outlined is necessary, send the instrument ashore for repair and maintenance.

jj24.3

Thick Film Technology Gas Analysis

The reaction o f a combustible gas with oxygen on a catalyst is used to give an extremely sensitive measurement o f the concentration o f that gas. Thick Film technology is used in combustion gas analysis applications to allow users to enhance combustion efficiency, saving fuel and reducing emissions (Refer Figure 24.5). This technique relies on the combustion o f carbon monoxide and oxygen over a catalytic surface. A four quadrant track is precision-printed onto a substrate using platinum ink. Each quadrant forms one leg o f a Wheatstone bridge circuit (Refer Figure 24.6). A layer o f protective glaze, having a consistent thickness, is printed over the complete circuit and the catalyst which also has a consistent thickness, across two o f the quadrants. 950

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Platinum track -

Zirconia substrate -

Figure 24.5 - T he Thick Film heat

measuring ce il-------heat-------

F igure 24.6 - T he Sensor The disk is mounted in a measuring cell and heated to 30Q°C at which stage the gas sample enters. Any carbon monoxide in the sample will bum on the catalytic surfaces, causing a heating effect. This alters the current in the circuit to produce an output that is proportional to the carbon monoxide concentration in the sample.

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Chapter 24 24.4

Carbon Dioxide Analysis

24.4.1 Influence of Carbon Dioxide Most fruits continue to live after picking, and breathe even when carried under chilled conditions. In breathing they absorb oxygen and release carbon dioxide into the storage space. The C 0 2 content must be controlled by ventilating the space with outside air for the following reasons: i)

C 0 2 content in excess o f 5% with its associated reduction in oxygen content is dangerous to human life, and levels must be kept below this in case the space has to be entered;

ii) Some port authorities require the C 0 2 content to be below 0.5% before men are allowed to work in the space; iii) Many fruits, particularly apples, suffer from “suffocation” and develop internal browning o f the flesh if they are kept for long in an environment where the C 0 2 content is in excess o f 2%; iv) Some fruits, for example bananas, give off ethylene and this ethylene can initiate ripening o f other bananas. Vigorous ventilation is desirable to keep down ethylene concentration levels. As there is no ready method o f determining ethylene content, C 0 2 content is determined and kept to a fraction o f 1% and ethylene present is thus proportional; v) Some shippers o f citrus fruit request low C 0 2 contents again to control ethylene concentration levels rather than C 0 2 concentration, as they consider this may improve the quality o f the fruit. C 0 2 production is most vigorous on completion o f loading warm fruit, and decreases as temperatures are reduced. The C 0 2 contents should be measured daily after loading until the level has settled down. 24.4.2 Monitoring of CO2 Portable instruments for measuring C 0 2 are electrical-types (based on measuring changes in thermal conductivity o f the gas sample) or chemical-types (based on absorbing the C 0 2 in a cartridge or solution o f caustic soda). They are used with a short length o f rubber sampling tube, which can be lowered down an exhaust ventilator (or coupled to a sampling pipe if one has been built into the cooler room) and a hand aspirator. 95 2

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As portable instruments, the chemical types are more robust, although the electrical types are often used as permanently installed instruments in the engine room, from which a pipe leads to each cargo space. Permanent installations include a suction pump and manifolds for measuring each space in turn. W hatever instrument is used, it is good practice to carry a gas bottle o f known concentration o f C 0 2 for calibration purposes. In the absence o f such a bottle a rough check is to breathe into the instrument; a reading o f about 5 % should be obtained. 24.5

P ortable Oxygen A nalyser - Model: Draeger E -ll

The oxygen analyser is used to evaluate the 0 2 content in the atmosphere. The most important part o f the instrument is the sensor, which can be o f different types in different makes such as 1. Electrolytic cell (Refer Figures 24.7 and 24.8) 2. Paramagnetic sensor 3. Chemical absorption sensor 24.5.1 Operation ofthe Electrolytic Cell Type 1.

Prior to use, check the battery by changing over the ‘Battery Check’ switch to ‘Battery Check’ mode. The meter pointer should lie within the black band marked ‘B a tt\ It is not necessary to switch the instrument ‘O n’ to check the battery. When it is released, the battery check switch being spring-loaded will return to its normal position.

2. Connect the remote head (sensor) to the instrument via the cable. Switch the instrument ‘On’ and allow it to stabilise for 10 minutes. A t the end o f this period, the reading should be 21% + 1% oxygen (Provided the head is in open air). If not, it should be set using the panel potentiometer ‘Set 21% ’ to read 21% oxygen in open air. 3. Just above and below the 21 % mark there is an upper and lower alarm level. These alarms must be tested by turning the ‘Set 21% ’ potentiometer on the front panel above and below the 21% mark until the respective alarms are activated. The instrument must then be returned to read 21% oxygen in air prior to use. 4.

When moving its location, it is recommended that the instrument be kept ‘O n’ owing to the necessary stabilisation time required for the next measurement.

5. Ensure that the alarm switch (on the rear o f the instrument casing) is set to ‘Operate’ when an audible alarm is required, and not to ‘M ute’. Marine Electrical Technology

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Chapter 24

F igure 24.7 - E-11 D raeger Oxygen A nalyser 24.5.2

Technical Specifications o f the E-11 Draeger Oxygen Meter

1. Storage temperature: -3° to 50° C. 2.

M eter display: 0 to 40 % oxygen.

3. Batteries: 2 x TR 132N Mallory Mercury Cells. 4. Maximum warm-up time: 5 minutes. 5. Accuracy: + 5% o f the reading. 6.

Speed o f response: 8 seconds to reach 90% o f the reading.

7. Cell life: 130,000% oxygen hours at 20° C (Guaranteed 6 months o f continuous use in 21% oxygen) 8. Remote head: Standard cable length - 10 meters; other lengths are available on demand. 9. Weight: Total weight o f sensor and instrument is 2.6 kg.

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Cathode (Permeable Membrane)

Figure 24.8 - Draeger Oxygen Analyser E -ll Probe (Electrolytic Cell) 24.5.3 Calibration It is important that the 02 meter be calibrated before use. The E - ll is calibrated by using the aspirator attachment to create a flow system for calibration. 1. Open the front panel by removing the four securing screws. Take care not to break the connections to the alarm isolation switch on the rear panel, which should be set to ‘M ute’. Locate the 4 potentiometers - ‘A ’, ‘B ’s *C \ ‘D ’. 2.

Connect the sensor to the instrument, connect the aspirator attachment to the remote head, switch ‘On’ the device and leave it for 20 to 30 minutes.

3. Permit the nitrogen to flow through and leave it for approximately 10 minutes. Using potentiometer ‘B ’ set the instrument to read 0% oxygen.

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Chapter 24 4. Place the sensor in a suitable concentration within the range o f the instrument and leave it for approximately 1 minute. Fresh air drawn from the atmosphere is ideal and the instrument can be set for 21%. The pointer is set to read 21% using potentiometer ‘A ’. 5. Repeat steps 3 and 4 and perform any adjustments i f necessary. 6.

Replace the front panel and secure it.

24.5.4 Replacement o f the Polarographic Cell The polarographic cell will last for a minimum o f six months in 21% oxygen at atmospheric pressure. W hen the instrument can no longer be adjusted to read 21% oxygen, the cell must be replaced. Replacing the sensor will suffice. As each cell has a slightly different ‘oxygen zero’, it will be necessary to recalibrate the instrument. The vessel can either: ♦

Return the complete instrument to the manufacturer for replacement and calibration.

❖

Return the sensor only for replacement o f the cell i f die vessel has adequate calibration facilities.

24.5.5 Fault Finding 1. Meterpointer deflects over (extreme) positive or negative limits in all ranges o f oxygen : S

Ensure that the batteries are firmly held in the pressure contacts; secure loose batteries or replace them.

^

Check the continuity o f the cable and replace it if necessary.

2. Meter goes to someposition and does not respond to oxygen: S

Check the continuity o f the cable and replace it if necessary.

3. Instrument cannot be set to 21 % in air: S

Inspect the batteries and replace them i f necessary.

S

Inspect the polarographic cell and replace it if necessary.

4. Alarm does not sound:

9 56

S

The alarm mute switch may not be changed over to ‘operate’.

S

Inspect the batteries and replace them if necessary .

Marine Electrical Technology

Gas Analysers 24.5.6 Setting the Alarm Level 1. Using potentiometer ‘A ’ set the pointer to read the required lower alarm level setting of oxygen. Then alter potentiometer ‘C’ so that the alarm ju st sounds. 2. Using potentiometer ‘A ’ set the pointer to read the desired upper level alarm setting. Then alter the potentiometer ‘D ’ until the upper level alarm just sounds. Reading o f the instrument should then be returned to 21% oxygen in air. 24.6

Fixed Oxygen A nalyser - Beckman Oxygen Analyser (Pauling Cell Type)

24.6.1 Principle of Operation The strong magnetic property o f oxygen is virtually unique compared to other gases. Its attraction into a magnetic field (paramagnetism) is the basis for high accuracy oxygen analysis, when fast and reliable measurements are needed. On the other hand, Nitrogen is diamagnetic i.e. it is repelled by a magnet. Other paramagnetic gases are NO, and NO 2. 24.6.2 Construction Two diamagnetic spheres o f glass filled with nitrogen are mounted at the ends o f a bar to form a dumb-bell. This dumb-bell is suspended horizontally from a quartz fibre. It operates in a strong non-uniform magnetic field. The spheres are repelled from the strongest part o f the field and so rotate, twisting the suspension to its zero position when 100% nitrogen is made to flow across the field. The deflection o f the pointer from zero is proportional to the force acting on the two spheres, which in turn is proportional to the oxygen content in the sample. I f the 0 2 content in the field changes, the force acting would change and the dumb-bell will attain a new position proportional to the 0 2 change. The limitation o f this type o f instrument is that the deflection with a change in 0 2 content is not linear and so the calibration o f the scale is also non-linear. Hence although zero setting with nitrogen flow and 21% oxygen setting with airflow can be done, the scale cannot be divided into 21 equal divisions. 24.7

Beckm an Oxygen A nalyser (Munday Cell Type)

24.7.1 Principle of Operation The ‘Zero5 position o f the dumb-bell is sensed by a split photocell. This cell receives light reflected from a mirror, which is fixed on the suspension. The output from the photocell is amplified and fed back to a coil wound on the dumb-bell so that a restoring torque due to feedback current balances the torque due to oxygen in the sample. Marine Electrical Technology

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Chapter 24 The measuring system is thus ‘null balanced’ and has all the inherent advantages o f this type o f system. This electro-magnetic feedback also stiffens the suspension damping it heavily (Refer Figures 24.9(a) and 24.9(b)).

Restoring Force O f Suspension

F igure 24.9(a) - Sensitive Elem ent o f th e M unday Cell Because o f a linear relationship between feed-back current and the susceptibility o f the sample, a proportional voltage can be developed, and various ranges can be obtained. Linearity o f its scale also makes it possible to calibrate the instrument for all ranges by checking it at only two points, i.e., for ‘zero’ using pure nitrogen and 21% using air and dividing the scale into 21 equal divisions unlike the earlier model. The instrument continuously monitors the oxygen percentage in the inert gas at a point after the blowers. There are displays in the cargo control room (CCR), engine control room (ECR) and locally at the instrument. An alarm is incorporated to ring when the O 2 content goes above 8%. The oxygen content can also be recorded continuously. 958

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Figure 24.9(b) - Basic C ircuit o f th e M unday Cell 24.7.2

Construction

This differs from the Pauling cell type wherein a platinum ribbon suspension is used and facilitates a considerable increase in physical strength (Refer Figure 24.10). In addition, an electro-magnetic feedback is used to maintain the dumb-bell at its zero position. In a sample containing oxygen, the dumb-bell tends to deflect. The current required to maintain the dumb-bell at its zero position is measured and shown on the scale. The greater the deflection, the greater will be the current required to restore the dumb-bell to its zero position. Thus, this current represents the magnetic susceptibility o f the gas present in the cell and therefore the 0 2 concentration. The advantages ofthis instrument are: >

Calibration is simple - nitrogen is used for zero calibration and air for span at 21% oxygen.

>

There is virtual independence from variations in the sample gas composition.

>

The scale is linear over the complete range (0 - 100 % oxygen), although the meter itself may only be graduated up to 21% or 40% oxygen.

>

Response time is fast as there is no heating or cooling o f filaments.

>

The analyser is not sensitive to tilt. Marine Electrical Technology

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Chapter 24

Purge Connection

123456789-

Set Temperature Span Adjustment Zero Adjustment Lamp On Feedback On Power On Reset Range (% 0 2) Lamp

Figure 24.10 - An Oxygen Analyser

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2 4 .7 .3

>

S ta rtin g P rocedure

Switch on the supply (the supply is pre-set to 110 or 220V).

N ote: A voltage set higher than this will reduce the life ofthe lamp.

>

Allow at least 2 hours (or as instructed by the manufacturers) for the analyser to warm up before passing the sample through it. This helps the instrument to attain proper sensitivity and prevents condensation o f moisture in the instrument.

>

A green indicator lamp glows when the heater is on. The lamp will go off when: 1. Power supply fails. 2. Temperature exceeds the cut-off point (around 60° C). 3. The thermal fuse fails.

>

Open the sample flow valve to obtain the normal operation flow rate o f 100 cc/min; a total inert gas flow rate between 100 cc/min - 1500 cc/min is acceptable but the instrument should be preferably set at 200 cc/min o f which 100 cc passes through the instrument and the excess 100 cc bypasses to the bubbler unit.

24.7 .4

Shu t-dow n P rocedure

>

Shut o ff the sample gas supply by closing the gas inlet valve.

>

Flush out the sample system with instrument quality dry air for 6 hours or flush with dry nitrogen for 3 hours.

>

After flushing for an appropriate time, switch off the main electric supply and allow the analyser to cool.

>

Maintain flushing until the analyser has cooled internally to within 2° C o f the ambient temperature. This is to avoid condensation o f corrosive moisture o f the gas trapped in the instrument.

24.8

Zirconia Oxygen Analysis

Zirconia is a ceramic that conducts electricity at high temperature by the movement o f oxygen ions. This property is used in oxygen measuring cells for applications such as combustion gas analysis or gas purity measurement This technique gives a robust way of accurately measuring oxygen. Marine Electrical Technology

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Chapter 24 It is used in combustion gas analysis in power stations where aggressive, hot gas mixtures are the norm and other industrial gas applications along with medical and physiological applications where extremely fast response times are paramount. Some ceramics conduct electricity at high temperature through the movement o f charged oxygen ions; zirconia is such a ceramic. This ability can be used to measure oxygen in a gas mixture especially for direct measurement o f hot flue gases; thus the need for sample conditioning equipment in combustion applications is reduced (Refer Figure 24.11). A zirconia disk is mounted between the gas to be measured and a reference gas (usually air), inside a heater. Electrodes are connected to either side o f the disc (Refer Figure 24.12). If there is a difference in oxygen concentration between the two sides o f the disk, a voltage is generated and detected by the electrodes.

Heat Figure 24.11 - The Basic Device

Oxygen Concentration

Figure 24.12 - Construction of the Sensor In use, the zirconia disk is mounted on a flexible diaphragm in a rugged body - making it resistant to both thermal and mechanical shock (Refer Figure 24.13). 962

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Figure 24.13 - The Device in use 24.9 24.9.1

Things to Remember P resen ce o f G as

There may be flammable gas in the tank; it may be at any opening to the tank $

After loading or discharging volatile petroleum

4F After loading non-volatile petroleum into a tank which is not gas-free 24.9 .2

P ressu re

Vapour in tanks may be under pressure 24.9 .3

In S paces D eclared G as-Free , F u rth er G as m ay b e R elea sed ...

#

After loose scale or sludge is disturbed

4F After a heating coil is opened up When a pipeline or valve is opened up When a cargo pump or valve is opened up #• When a cargo vent line is opened up

Marine Electrical Technology

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Chapter 24 2 4.9.4

In O ther S p a ces...

Flammable gas may be in any space into which volatile petroleum may leak such as #■ Pump rooms # ' Cofferdams #

Ballast tanks

0 ' Empty compartments next to tanks used to carry low flash point petroleum N ote: A space declared gas-free is free of gas at the time of the test. The space may not

remain gas-free. Remember thatfurther gas may be released. The absence o f flammable mixtures does not necessarily mean the space is gas-free and safe. R em em ber: toxic gases are not necessarilyflammable.

Before you open a tank, any pressure m ust be released. This has to be done very carefully under controlled conditions. Openings m ust be re-closed as soon as possible. A space may be certified gas-free and be: ^

Safe for men and cold work (includes jobs which can cause sparks or enough heat to ignite any nearby vapour e.g., hammering).

■ S Safe for hot work (work that is so hot that it can actually cause dirty parts o f a tank to give o ff vapour e.g. welding. This vapour can o f course be ignited by the work).

24.10 24.10.1

Relevant Rules R elevan t SO L A S R egulations

Chapter II - 2 - Part B - Prevention o f Fire and Explosion - Regulation 4 - Probability of Ignition and Chapter V I - Part A - General Provisions - Regulation 3 - Oxygen Analysis and Gas Detection Equipment. 24.10.2

Sum m ary o f R egulations

1) Tankers shall be equipped with at least one portable instrument (and means for its calibration) for measuring flammable vapour concentrations, together with a sufficient set o f spares. Suitable means shall be provided for the calibration o f such instruments.

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2) Suitable portable instruments for measuring oxygen and flammable vapour concentrations shall be provided in double hull spaces and double bottom spaces. 3) W here the atmosphere in double hull spaces cannot be reliably measured using flexible gas sampling hoses, such spaces shall be fitted with permanent gas sampling lines that are electrically conductive and unrestrictive. 4) W hen transporting a bulk cargo which is liable to emit a toxic or flammable gas, or cause oxygen depletion in the cargo space, an appropriate instrument for measuring the concentration o f gas or oxygen in the air shall be provided together with detailed instructions for its use.

1.

The Combustible Gas Indicator or Explosimeter displays its result as a percentage o f

2.

The MSA 40 is a dual range CGI, with two scales that rea d _________ a n d __________ .

3.

While using a CGI, it must be remembered that i f the sampling line is more th a n ___ feet.

4.

While using a CGI, it must be remembered t h a t ____ additional squeezes will be required for every ten feet o f line.

5.

A tankscope measures th e _____ gas content in a sample o f atmosphere.

6.

W hy is gas-detection equipment required onboard?

7.

What is an explosimeter?

8.

Give a detailed explanation o f the calibration procedure for a CGI o f the MSA 40 type.

9.

Draw and explain the Munday Cell Oxygen Analyser

10.

Draw and explain the Pauling Cell Oxygen Analyser

11.

The Tankscope is an important device. Explain this with relevant diagrams.

12.

With the help o f a diagram explain the Combustible Gas Indicator.

13.

With suitable sketches, explain gas analysis using thick-film technology.

14.

W hat is the influence o f carbon dioxide on perishable cargo?

15.

How is carbon dioxide analysis done?

_________________________________________

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Chapter 24 16.

Draw and explain the portable oxygen analyser.

17.

Briefly explain the operation o f a fixed oxygen analyser.

18.

W ith suitable sketches, explain the role o f ceramics in oxygen analysis.

19.

What is the correct method o f testing the gas detection equipment normally used on board tankers?

20.

Entering a hazardous space is dangerous; what must be remembered before entering such spaces?

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C M

i s

c

e

l l a

h

a n

p e

t e o

u

r s

2 S

5 y

s

t e

m

s

The list o f miscellaneous marine systems is exhaustive and volumes could be written on the innovative means o f making an engineer’s life easier on board a ship today. For example, although refrigeration and air-conditioning is a subject by itself, it has been briefly covered in this chapter in order to provide a pen-picture o f the few electrical components found there-in. The others briefly covered are cathodic protection, water purity monitoring, water-tight doors, galley and laundry equipment.

125.2

Cathodic Protection

25.2.1

The E lectroch em ical Theory o f C orrosion

The presence o f a solution which can conduct an electric current, an electrolyte, is one o f the first requirements for corrosion. An electrolytic solution is any liquid that contains ions. Remember that ions are electrically charged atoms in a given solution and that even pure water contains both positively and negatively charged ions in equilibrium. Because o f this, solutions o f salts, acids and alkalis are all good electrolytes. In addition to an electrolyte, two electrodes - an anode and a cathode - are required for corrosion to be initiated. The electrodes may consist o f two different types o f metals o r they may be different areas on the same piece ofmetal. In either case, for corrosion to occur, there m ust be a difference in electrical potential between the two electrodes or areas so that electricity will flow between them. In addition to the portion o f the electrical circuit made up o f electrolyte, the circuit must be completed by a conductive path between the two electrodes. Marine Electrical Technology

Chapter 25 I f they are on the same piece o f m etal there is an inherent circuit. I f they are separate pieces o f metal they must be connected in some manner. What takes place at the anode in a corrosion cell when corrosion occurs? Positively charged atoms o f metal detach themselves from the surface and enter into solution as ions, while the corresponding negative charges, in the form o f electrons, are left behind in the metal (oxidation) (Refer Figure 25.1)

F igure 25.1 - D etachm ent o f Positive Charges The detached positive ions bear one or more positive charges. In the corrosion o f iron, each atom releases two electrons and becomes an iron ion carrying two positive charges. The released electrons travel through the metal to the cathode area. What takes place at the cathode? The electrons reaching the surface o f the cathode by passing through the metal circuit meet and neutralize some positively charged hydrogen ions which were present in the electrolyte. In losing their electric charge by gaining electrons the hydrogen ions become neutral atoms (reduction). They then combine to form hydrogen gas. The conversion o f hydrogen ions to hydrogen atoms and then to hydrogen gas results in a decrease in hydrogen ions in the electrolyte (Refer Figure 25.2). This increases the alkalinity o f the electrolyte in the area o f the cathode. The ionic and cationic reactions discussed so far can be written as follows: Anodic Reaction - Fe° -*-Fe++ + 2e Cathodic Reaction - 2e+2H+ -►2H0 -►IH 2

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Miscellaneous Systems

F igure 25.2 - Conversion o f H ydrogen Ions to H ydrogen Atoms The general electrochemical theory o f corrosion states that corrosion proceeds by an anodic or oxidation reaction and a cathodic or reduction reaction. For a metal M, the reactions involving corrosion in an acid medium are given as: Anodic Reaction M —* M+ + e'

(I)

Cathodic Reaction 2H+ + 2e‘ — ► H2

(2)

From these reactions it becomes clear that metal dissolution involves oxidation o f metal into metal ions with a simultaneous release o f electrons. Electrons will flow from the anodic area through the hull to the cathodic area. By hydrolysis negatively charged hydroxyl ions will form. A t the anode electron depletion leads to positively charged Fe (iron) ions. Hydroxyl ions migrate through the water to the anode, here combining with the iron ions to form Fe(OH ) 2 which combines with dissolved oxygen to form Fe(OH ) 3 or rust. In this way the anodic area will corrode. To prevent this it would be necessary to make the entire hull cathodic. Therefore, forcing electrons onto the metal will stop its corrosion. This technique is called “Cathodic Protection”. It is easy to see that several environmental factors can be varied to affect the corrosion rate. If for instance the hydrogen ion concentration is increased (pH reduced) the rate of corrosion is likely to increase since there are more hydrogen ions to receive electrons at the cathode. Conversely if the solution is made more alkaline (by reducing the H ion concentration - pH increased) corrosion can be reduced. Further by reducing the concentration o f dissolved material in the electrolyte the conductivity o f the electrolyte is reduced and the resistance is increased. An increase in resistance impedes the flow o f current and the corrosion rate o f an immersed material can be reduced. Marine Electrical Technology

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Chapter 25 It is important to note that anodes and cathodes can occur randomly on a piece o f metal. This can be illustrated by placing a piece o f steel in a hydrochloric acid solution. Upon immersion o f the piece o f steel in the acid solution, the vigorous formation o f numerous hydrogen bubbles is observed. Hydrogen is evolved seemingly from the entire surface without the indication o f either cathodic or anodic areas. This is, in fact, the case since the anodes and cathodes shift from time to time during corrosion under these conditions (Refer Figure 25.3).

F igure 25.3 - A node a n d C athode O ccurrence on th e Sam e Piece o f Meta! The development o f an anode on a metal surface may result from a variety o f microscopic surface conditions including local impurities in the metal, surface imperfection, orientation o f grains in the metal, localized stresses and variations in environment. The outer surface o f a ship’s hull is subjected to electro-chemical attack by corrosive currents that flow between areas o f the hull, which are at slightly different electric potentials. Dissimilar metals, variations in structural and chemical uniformity in hull plates and welding, differences in paint thickness and quality, water temperature, salinity and aeration combine to cause areas o f the hull to become either anodes (positive) or cathodes (negative). The value o f protection current must be critically controlled to just prevent corrosion, as beyond this value the increase in the rate o f release o f hydroxyl ions will cause sponginess and flaking o f the anti-fouling paint. Reference electrodes can determine the correct value o f protection current. These are either made o f zinc or silver attached to the hull below the waterline, but insulated from it.

970

Marine Electrical Technology

Miscellaneous Systems The principle o f cathodic protection can be well understood with the help o f the E Log I diagram in Figure 25.4.

Figure 25.4 - E Log I Diagram Figure 25.4 above illustrates the nature of polarization and the amount o f current required when steel is subjected to complete protection by this technique. Under freely corroding conditions the metal has a potential (ECOrr) and corrodes at a rate equivalent to the corrosion current ( i c o r r - i ) . I f the potential o f the metal is reduced from Ecorr to Ea (equilibrium potential at the anode) by externally applied current i2then the metal will be protected. Thus Ea is the required potential at which the structure should be polarized for complete cathodic protection. Any amount o f current less than i2 will give partial protection. The voltage measured between the hull and reference electrodes o f an unprotected ship is: a) Zinc - 450 mV negative to hull with seawater as an electrolyte. b) Silver - 600 mV positive to hull. Marine Electrical Technology

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Chapter 25 W hen satisfactorily protected, the protection current will make the hull 200 mV more negative, i.e. a zinc reference electrode will register 250 mV negative with respect to the hull and silver 800 mV positive with respect to the hull. The reference electrode voltage may, therefore, be used to monitor the protection, but more importantly, is used as the signal source to automatically control the value o f protection current (Refer Figures 25.5 and 25.6). Negative «----------------

I D t

Hr Zinc (Ref.)

Positive --------------- ►

I

I 800mV

250mV Steel (Hull)

Sacrificial Anode

o

Silver (Ref.)

Protected Structure - Cathode

F igure 25.5 - Sacrificial Anode System

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>

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Miscellaneous Systems

Figure 25.6 - Sacrificial Anodes 25.2.2

Impressed C urrent Cathodic Protection

Cathodic protection systems fitted on ships consist o f a number of anodes fitted to the hull at selected places below the waterline, and control equipment, which automatically regulates the anode current to the required value (Refer Figures 25.7 and 25.8). Direct current is supplied to the anodes, from the ship’s 440V 60Hz 3-phase distribution system, after transformation and rectification. The amount o f current required by the system depends upon the area beneath the waterline, the quality and state o f the paintwork, the temperature o f the seawater (higher the temperature, greater the electrolysis) and the speed o f the vessel. Since the speed o f the vessel constantly changes, it is most likely to cause changes in the current demanded too. It has been found to be almost twice as high when the vessel is underway (i.e., in motion) as compared to the period when it is alongside i.e., at rest. The control equipment comprises reference electrodes, an amplifier assembly and one or more transformer-rectifier units. There are basically 4 modes o f operation namely: 1. 2.

The Standby Mode - all output currents are zero. The Manual Mode - the output currents are manually adjustable and must be carefully adjusted so as to prevent any permanent damage to the paintwork. As mentioned above, it may be required to double the current when the speed is say between 5 and 10 knots. Further increase in speed generally has little impact on the current demand.

3.

The Automatic Mode - the outputs vary to provide a constant electrode potential. This installation is relatively expensive as compared to the manual system but the end results are better. Here, the potential difference between the hull and the reference electrode is constantly monitored and the output current is controlled. Marine Electrical Technology

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Chapter 25 4.

The Configuration Mode - the controller set points may be adjusted (say one setting when the speed is zero knots and one when the vessel is underway)

Sacrificial Anode

Protected Structure - Cathode

Figure 25.7 - Impressed Current System Supply Current

Figure 25.8 - Impressed Current System Circuit 974

Marine Electrical Technology

Miscellaneous Systems Generally there are two subsystems comprising a Forward Controller Power Unit and an Aft Controller Power U n it One central remote monitoring unit is normally shared between the two units; it is normally located in the engine control room. This unit helps to maintain the daily log o f readings and also caters to housing the alarm circuits in case o f any abnormalities in the system. Some monitoring facilities in the cathodic protection control cabinet / remote monitoring unit may facilitate the measurements o f the following: >

Reference Electrode Voltage (hull potential)

>

Amplifier output voltage

>

Total anode current

>

Individual anode current

Electronic thyristor amplifiers or magnetic amplifiers may control the anode current. The schematic diagrams in Figures 25.9, 25.10,25.11 and 25.12 are examples o f this.

Figure 25.9 - Layout of a Basic Impressed Current Cathodic Protection System The control equipment automatically monitors the magnitude o f anode current required This will vary with the ship’s speed, water temperature and salinity, condition o f paint work, etc. Typical anode current densities range from 10mA/m2 to 40mA/m2 for the protection o f painted surfaces and 100 to 150mA/m2 for bare surfaces. The total controller current for a hull in good condition may be as low as 20A.

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Maximum controller outputs in the automatic mode may be up to about 60GA at 8V. Manual adjustments are also possible. Cathodic protection does not appear to deter molluscular growth on the ship’s hull so a topcoat o f anti-foul (poisonous) paint is still necessary. As seen in Figure 25.8, the anode is insulated from the hull, electrical connection is via cable and ships side gland box. It m ay be made o f lead or Platinised Titanium. W ith the lead anodes, the hydroxyl ions turn the surface o f the lead to a rich brown colour (Pb 0 2 ). A d.c. voltage is applied to ju st overcome the natural galvanic voltage. I f the current is allowed to become too great then the increased hydroxyl release causes sponginess and flaking o f the paint. Referring to Figure 25.10, the cathodic system should make the hull 200mV more cathodic i.e. 200mV negatively charged. The system measures this by checking the hull voltage against an insulated reference anode which has a known value o f galvanic voltage with the hull material. Typically this may be Zinc which is normally at a voltage 450mV more negative than the hull, or silver which is 600 mV more positive than the hull. The Cathodic protection system will tty to make the potential difference between the hull and the zinc reference anode 250 m V (Zinc anode 250mV more negative than the hull), and the silver anode 600mV (Silver anode 800mV more positive than the hull). Measurements should be regularly logged, e.g. location, draught, water temperature, etc. Changes in underwater hull area, speed, water temperature / salinity and paint conditions cause the anode current to vary. The hull potential should, however, remain constant in a properly regulated system. Although the reference electrodes and the monitoring facilities give a reasonable day-to-day check they only measure in the vicinity o f the fitted electrodes. When the ship is moored singly or stopped at sea, voltage reading can be taken between portable silver or zinc test electrode and the ship’s hull. This portable electrode is lowered 6-8 feet below the water surface, as close as possible to the hull at a specified position around the ship. Check the manufacturer’s instructions regarding the storage and setting up o f the portable electrode. Some have to be immersed in a plastic bucket o f seawater for about 4 hours before the hull test.

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Marine Electrical Technology

Miscellaneous Systems With the cathodic protection system switched on and working normally, the voltage measured between the hull and a silver / silver chloride portable electrode should be 750850mV using a high-resistance multimeter (e.g. an AVO meter o f the analog or digital type), the electrode being positive with respect to the h u ll When the ship is dry-docked, ensure that the main anodes and reference electrodes are covered with paper tape to prevent contamination by paint.

Figure 25.10 - Schematic Diagram of an Impressed Current Cathodic Protection System Marine Electrical Technology

977

Chapter 25 In order to ensure that the rudder, propeller and stabiliser fins receive the same degree o f cathodic protection as the hull, it is necessary to electrically bond these to the hull. The rudderstock may be bonded by a wire braid linking the top o f the stock to the deck head directly about it. Carbon brushes in contact with slip rings on the main propulsion shaft effectively bond the shaft to the hull. A periodic inspection o f such earthing is worthwhile as the brushes wear away and may occasionally stick in their brush holders; this also ensures efficient bonding. A second set o f brushes, insulated from the earth, can be used to connect a mV meter that monitors the shaft’s potential. Anode

Reference Cell

Figure 25.11 - Example of System Components (Forward) and Remote Monitor 978

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Miscellaneous Systems

Figure 25.12 - Example of System Components (Aft) 25.2.3 R outine Checks

✓

Record the output current and all voltages, including the reference electrode voltage, on a daily basis.

S

Check and clean the propeller shaft’s slip ring and brushes every week.

S

Inspect the rudder stock earth strap once a month.

■ S Carry out other inspections / maintenance as directed in the ship’s manuals. 25.2.4 Dangers to be avoided

#

Considerations should be given to spark hazards created by introduction o f electric currents into structures situated in hazardous atmosphere. Any secondary structure residing in the same electrolyte may receive and discharge the cathodic protection dc current by acting as an alternative low resistance path. Corrosion will be accelerated on the secondary structure at any point where current is discharged to the electrolyte. This phenomenon is called as “interaction”. Interaction is minimized by careful design o f the cathodic protection system. Therefore where there are chances o f interaction to take place we use SACP systems because they are less prone to interaction. Marine Electrical Technology

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Chapter 25 125.3

Monitoring of Water Purity

25.3.1

The D io n ic Water P u rity M eter

Specific conductivity is measured in mhos / cm3 and is equal to the conductance o f a column o f mercury -1 cm3 in volume. This is a large unit and micro mhos / cm3 is used; when corrected up to 20°C, it is called a Dionic Unit. The electrical conductivity o f water is dependent upon dissolved impurities. Conductivity o f pure water is about 0.5 and fresh water is about 500 dionic units. The meter measures the conductivity o f two columns o f water, in parallel between the (positive) platinum rings and (negative) gunmetal collars o f the sensor (Refer Figure 25.13). The insulating plunger, operated by the bi-metal strip temperature compensator, automatically varies the water flow, to facilitate compensation equivalent to 20°C. Dissolved CO 2 can be produced and should be removed by de-gasifiers. It is important that the measuring electrodes are kept very clean and the electrical connections are tight.

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Marine Electrical Technology

Miscellaneous Systems 25.3.2

The Salinom eter

It is a device which measures the impurity o f water. The conductivity o f the water can be continuously tested by the salinometer. If the device registers an excess o f salinity, it can also dump the product using a solenoid valve and simultaneously activate an alarm. Pure distilled water may be considered a non-conductor o f electricity. The addition of impurities such as salt in solution increases the conductivity o f water, and this can be measured. Since conductivity o f water is, for low concentration, related to impurity content, a conductivity meter can be used to monitor the salinity o f water; Figure 25.14 depicts one such circuit, the explanation o f which is given in the following paragraphs. The incoming a.c. power supply, through the main switch and fuses, is fed to the transformer. A pilot-lamp on the 24-volt secondary winding indicates that the power supply is available. A voltage is applied across the electrode cell and die indicator. The indicator shows salinity by measuring the current (in mA) which at a preset value (set with the help o f the potentiometer) actuates an alarm circuit that incorporates a warning relay. The transformer’s secondary voltage is applied across a series circuit comprising the bridge rectifier, the current limiting resistor R2 and the electrode cell. The current from the rectifier branches out into 2 paths, one through the temperature compensator within the probe, via resistor R2 and the other through the alarm relay’s potentiometer (Pot.), the indicator (a mA meter) and resistor R3. The two circuits join a common return line to the low potential side o f the rectifier. The indicator is protected from overload by a zener diode and resistor R4 (connected in shunt across the indicator itself) and the potentiometer. When the water temperature is at the lower limit o f the compensated range the total resistance o f the compensator (a thermistor) is in the circuit. As the temperature o f the water increases the resistance o f the compensator drops progressively, the electrical path through the compensator now has a lower resistance than the other branch and hence a large portion o f current flows through it. Conversely due to a fall in temperature the resistance o f the compensator rises and a corrected reading is thus obtained over the (compensated) range. The compensator therefore ensures that the circuit is balanced and the total resistance o f the two paths correspond to the water’s conductivity alone. Marine Electrical Technology

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Chapter 25

The alarm setting is adjustable (with the help o f the potentiometer) and the contacts o f the warning relay make so as to complete the electrical circuit for a lamp or to activate an alarm when salinity exceeds the acceptable level. A s mentioned earlier, the salinometer can also control a solenoid valve which dumps unacceptable feed water; this is made possible through the contacts o f the same relay.

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Marine Electrical Technology

Miscellaneous Systems The salinometer’s alarm circuit and solenoid valve reset automatically when the salinity drops to the desired value - based on the setting at the potentiometer due to the changing over o f the relay’s contacts. The alternative to the circuit in Figure 25.14 is to have a digital meter. This is made possible by a comparator circuit where-in a reference signal serves to act as a base. The algebraic output o f the comparator can be amplified, converted and fed adequately to the remote digital readout. M g / litr e

O h m ic v a lu e a t 2 0 ° C

0.5

20632.0

1.0

10319.0

2.0

5162.0

3.0

3443.0

4.0

2583.0

5.0

2068.0

6.0

1724.0

7.0

1478.0

8.0

1294.2

10.0

1036.2

Table 25.1 - Salinity to Ohmic Value

Marine Electrical Technology

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Chapter 25 J25.4

Galley Equipment Extractfrom SOLAS ConsolidatedEdition 2004 Chapter 11-2 - Construction -fire protection, detection, extinction Regulation 10 Suppression offire

Quote 6.4

Deep-fat cooking equipment

Deep-fat cooking equipment shall be fitted with the following: .1

an automatic or manual fire-extinguishing system tested to an international standard acceptable to the Organisation;

.2

a primary and backup thermostat with an alarm to alert the operator in the event o f failure o f either thermostat;

.3

arrangements for automatically shutting o ff the electrical power upon activation o f the fire-extinguishing system;

.4

an alarm for indicating operation o f the fire-extinguishing system in the galley where the equipment is installed; and

.5

controls for manual operation o f the fire-extinguishing system which are clearly labelled for ready use by the crew.

Unquote The electrical power in a galley is largely utilised to produce heat. Ovens, deep trying pans, water boilers and the hotplates on the galley range employ resistive heating elements, which are usually controlled by bi-metallic thermostatic switches. Other miscellaneous electric galley equipment may include oven air-circulating and range-exhaust fans, meat sheers, food mixers and grinders, dishwashers, potato peelers and garbage disposal units. Most o f this equipment will utilise small electric motors together with their necessary control switches, safety interlocks and indicator lamps.

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Marine Electrical Technology

Miscellaneous Systems Due to the large power requirement for food preparation and cooking, the major galley items are supplied from the 3-phase 440V system. Smaller galleys may be supplied from the low-voltage 220V system. The electrical equipment has to work safely in the usual galley atmosphere o f high humidity and high temperature. The galley’s electrical equipment thus operates in such an area where it must be prepared for faults caused by the environmental hazards o f grease, grime, dust and dampness. Heating elements are usually formed from Nichrome wire insulated with magnesium oxide (MgO) powder within an Inconel tube, which forms the outer sheath. Power ratings vary from lkW to about 4kW and some elements are connected so as to give varying levels o f heat. The simplest arrangement is obtained using a 3-heat-level, 4-position switch to control 2 elements within a single hot plate. The switch settings give one a choice o f Off (disconnected), Low (both in series), Medium (one element) and High (both in parallel). In order to obtain the same, the two resistance elements are generally interconnected as shown in Figure 25.15. Better control o f heating elements is obtained by using ‘simmerstat’ switches and electronic switching.

The simmerstat switch-type houses a bi-metallic switch that

‘cycles’ the heating element on and off at a rate pre-determined by the switch setting. Average hot-plate temperature is fixed by the ratio o f time that the element is switched on, to the time it is switched off. Circuit current heats the bi-metallic thermostat, which operates a switch for the same. Oven simmerstat controls have a similar switching action but a temperature sensing capillary tube, located in the oven, deflects a diaphragm or bellows that activates the switch. Electronic switching devices such as transistors, thyristors and triacs may also be used for temperature control o f ovens and hot plates. One m ust be careful not to megger test lowvoltage electronic components during maintenance and faultfinding. Check the manufacturer’s instructions and drawing before locating faults in a control circuit. The most likely fault in a heating element is a simple open-circuit. Earth faults within the element or in the cables supplying it are also probable. Loose wire-connections can cause localised over-heating, with the wire burning away to leave on open-circuit, but the possibility o f short-circuits or earth fault conditions also arise. Connecting wires close to heating elements should be covered with a high-temperature silicone or fibreglass sleeving or with ceramic beads. Marine Electrical Technology

985

Chapter 25 W hen measured cold, the resistance value may be slightly lower than the calculated value. High power ovens and hot-plate ranges are often supplied from the 3-phase, 440V supply. Thermostats control the on-off heating cycle. M icrowave ovens provide rapid defrosting and cooking o f foods. The microwaves are produced by a magnetron operating at around 4000V with a frequency o f approximately 2450MHz. Specialised knowledge is required for the repair o f this type o f oven and internal faultfinding is not recommended without the manufacturer’s guidance. Inspection and maintenance o f galley equipment is most important. The main objective is to keep the electrical parts dry and free o f oil, dust and grease. Pay particular attention to all connection points in high-current heating circuits where loose connections cause overheating and further problems. For operator safety, all related metalwork must be earthed and regular checks o f earthing straps must be given priority. Insulation tests on heating elements, when they are cold, may reveal surprisingly low values (10-100fcQ) even with new elements. This is because the element insulation (magnesium oxide powder) is somewhat hygroscopic (absorbs moisture). The insulation resistance o f a healthy heating element should rise rapidly after being operated for a few minutes. Obviously , i f the insulation resistance o f an element remains low when hot, it is defective and must be replaced.

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Medium

High

Figure 25.15 - Heater Connections for Galley Plates [25.5

Laundry Equipment

Washing machines’ spin driers and tumble dryers utilise heat and mechanical rotation during their laundry processes. The sequence o f events is controlled by timers which are often simple electric timer motors driving cam-operated switches. Alternatively, electronic timers with relay-switching or solid-state switching using thyristors or triacs may be employed. Small washing machines operating on a single-phase supply have motors, which are usually the split-phase type o f the capacitor-start, capacitor-run variety. Larger washing machines operate with 3-phase supply (i.e., with a 3-phase induction motor drive). W hen using such machines, care should be taken to ensure that the right voltage and frequency is available; failure to do so will result in over-heating and consequent damage (Refer Articles 4.3.1 and 4.3.2 for further information). Marine Electrical Technology

987

Chapter 25 Control items in washing machines include water level switches, temperature switches (bi-metallic) and solenoid valves in the inlet and outlet water lines. Lid and door switches interrupt the main power supply if operated after the washing sequences have begun. Spindryers have a safety-door interlock that prevents it from being opened while the drum is still revolving. Tumble dryers often only have one m otor with a double-ended shaft for drum and blower fan drives. Lint and fluff collects on the motor and wiring, which causes no trouble while it remains dry and in small quantities. Periodic removal o f the fluff will help prevent faults arising where dampness may combine with the fluff to cause conductive tracking between live conductors and also to the earth. Small single-phase motors are sometimes protected by a thermal cut-out connected to the stator-end windings. 125.6

W ater-tight Doors

25.6.1

Control o f Doors

]

Where doors are designed for power operation, they will be capable o f being remotely closed from the bridge and are also locally controlled from each side o f the bulkhead. Each power-operated sliding door will be provided with an individual hand-operated mechanism (Refer Figure 25.16). Where designed for power operation, a single failure in the electric or hydraulic power-operated system excluding the hydraulic actuator will not prevent the manual operation o f any door. Where necessary for power operation o f the door, means to start the hydraulic unit, or equivalent arrangement, will be provided at the navigation bridge, and at each remote control position, if provided, and also the local control position. 25.6.2 Monitoring of Doors Displays provided at the control position indicate whether the doors are open or closed. Display and alarm systems are generally self-monitoring such that any failure in the system (e.g. power failure, sensor failure, etc.) will be detected and displayed at the navigation bridge control panel. 25.6.3 Alarm while Closing Power-operated Doors Every power-operated sliding door is provided with an audible alarm which will be activated whenever the door is closed remotely; it will be active for at least five to ten seconds before the door begins to move and will continue until the door is completely closed.

988

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Miscellaneous Systems

25.6.4 Electrical Power Supply The electrical power required for power-operated doors is generally supplied from the emergency switchboard either directly or through a distribution board situated above the bulkhead deck. The associated control and monitoring circuits are supplied from the emergency switchboard also, either directly or through a distribution board situated above the bulkhead deck. The power circuits for power-operated doors are segregated from the power supply to any other systems. The availability o f the power supply is continuously monitored on the load side o f the feeder’s protective device and the loss o f any such power supply activates an audible and visual alarm at the navigation bridge control panel.

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Chapter 25 25.6.5 Protection o f Electric Power, Control and Monitoring Circuits Electric power, control and monitoring circuits are also protected against faults in such a way that a failure in one door circuit will not cause a failure in any other door circuits. Shortcircuits or other faults in alarm or display circuits o f a door will generally not result in a loss o f power operation o f that door. A single electrical failure in the power-operating or control system o f a power-operated door also does not result in accidental opening o f a closed door. This feature is important in areas below the waterline. 25.6.6

Electrical Equipment

As far as is practicable, electrical equipment and components for watertight doors will be situated above the freeboard deck and outside hazardous areas. The enclosures o f electrical components necessarily situated below the freeboard deck provide suitable protection against the ingress o f water. In this context, the following degrees o f protection are to be maintained: ^

IPX7 standard for electrical motors, their associated circuits and control components;.

^

IPX8 standard for door position indicators and associated circuit components; the water pressure testing o f the enclosure is to be based on the pressure that may occur at the location o f the component during flooding for a period o f 36 hours.

3K

IPX6 standard for door movement warning signals.

Doors and hatches fitted with gaskets and dogs are provided with means o f indicating locally and on the bridge, whether they are open or secured closed. For this purpose all dogs are generally monitored individually. When all dogs are linked to a single acting mechanism, then only the monitoring o f a single dog is required. 25.6.7

Displays andAlarms

The alarm system is designed on the fail-safe principle. The display and alarm system will be o f the self-monitoring type and incorporate the following:

990

S

Separate indicator lights are provided on the navigation bridge and on each operating panel to indicate when the doors are closed and that their locking devices are properly positioned.

S

The display panel on the navigation bridge is equipped with a mode selection function “ harbor/sea voyage” , arranged so that an audio-visual alarm is activated if whilst it is in the “sea voyage” mode, the doors are not closed, or any o f the securing devices are not in the correct position. Marine Electrical Technology

Miscellaneous Systems •/ Display o f the open/closed position of every door and every securing and locking device is available at the operating panels. 25.6.8 Indicator Lights Indicator lights are designed so that they cannot be manually turned off. The display panel also incorporates a lamp-test function. 25.6.9 Power Supply The power supply for the display system is independent o f the power supply for operating and closing the doors. 25.6.10 Protection ofSensors Sensors are protected from water, ice formation and mechanical damage. 25.6.11 25.6.11.1

Leakage Monitoring Bow Doors and Inner Doors

For vessels fitted with bow and inner doors, a water leakage detection system with an audible alarm and sometimes television surveillance, provide an indication to the navigation bridge and to the engine control room, o f leakage through the inner door. 25.6.11.2 Side Shell Doors and Stem Doors In the case o f passenger vessels fitted with side shell or stem doors, a water leakage detection system with an audible alarm and television surveillance provides an indication to the navigation bridge and to the engine control room o f leakage through any o f the doors. For cargo vessels fitted with side shell or stem doors, a water leakage detection system with an audible alarm provides an indication to the navigation bridge o f any leakage through the doors. 25.6.12 Drainage A drainage system is arranged in the area between the bow door and ramp and in the area between the ramp and inner door, where fitted. The system is generally equipped with an audible alarm function to the navigation bridge for water level in these areas exceeding half a metre above the car deck level. \

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Chapter 25 25.6.13 Door Surveillance Between the bow door and the inner door a television surveillance system is fitted with a monitor on the navigation bridge and in the engine control room. The system monitors the position o f doors and a sufficient number o f their securing devices. 25.6.14

Features of an Electrical System as Installed Onboard

★

The local indication system consists o f an indication panel and magnetic sensors

★

Magnetic sensors indicate when the equipment (e.g. a door) is closed and all the cleatings are locked.

★

The indication panel is located close to the manoeuvring stand from where the operator also has a good view o f the cargo access equipment being controlled.

★

The bridge indication system comprises a remote indication panel and one control unit

★

The bridge indication panel shows the closed and cleaied status o f all the ship’s watertight items with a green and red light-emitting diode (LED) for each (the green diode indicates that the door, etc, is closed and all cleatings are locked, while the red diode indicates improper securing).

★

A n audible alarm (a buzzer) is activated when any o f the watertight doors, etc are open causing the indications to change from green to red thus immediately drawing attention to this change. This alarm can be reset by operation o f an “alarm accept” button. In order to assist night vision on the bridge, all green diodes can be dimmed; for safety reasons this cannot be done to the red ones. All LEDs can be tested by an indication check push button.

★

The signals indicating closed and cleated status o f watertight items are also continuously transmitted to the ship’s Voyage Data Recorder (VDR), to be used in subsequent investigations in case o f an incident.

★

The control unit is normally located below the bridge indication panel inside the bridge console. This unit controls the bridge indication panel described above.

In addition to its use with watertight items, the indication system can be employed with weather-tight equipment such as bow and stem doors on Ro-Ro vessels, and side shell doors on other ships. 992

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Miscellaneous Systems Extractfrom SOLAS ConsolidatedEdition 2004 Chapter II-1 Construction - Structure, Subdivision andStability, Machinery and Electrical Installations Part B - Subdivision andstability Regulation 15 - Openings in watertight bulkheads inpassenger ships Quote 8.1

The central operating console at the navigating bridge shall have a “master mode” switch with two modes o f control: a “local control” mode which shall allow any door to be locally opened and locally closed after use without automatic closure, and a “doors closed” mode which shall automatically close any door that is open. The “doors closed” mode shall permit doors to be opened locally and shall automatically recluse the doors upon release o f the local control mechanism. The “master mode” switch shall normally be used in an emergency or for testing purposes. Special consideration shall be given to the reliability o f the “master mode” switch.

8.2

The central operating console at the navigation bridge shall be provided with a diagram showing the location o f each door, with visual indicators to show whether each door is opened or closed. A red light shall indicate a door is fully open and a green light shall indicate a door is fully closed. W hen the door is closed remotely, the red light shall indicate the intermediate position by flashing. The indicating circuit shall be independent o f the control circuit for each door.

8.3

It shall not be possible to remotely open any door from the central operating console.

Unquote

25.7

Refrigerating Machinery

25.7.1 The Vapour Compression Refrigeration Cycle Most marine refrigeration plants make use o f the vapour compression refrigeration cycle. As refrigerants are too expensive, it should be ensured that after the refrigerant has done its cooling job, the gas is collected and re-liquefied. This is accomplished by using a compressor to suck gas from the evaporator at low pressure and to deliver it as hot compressed gas to a condenser. The compressor raises the gas temperature above that o f the atmosphere (or seawater) so that either air or water at normal atmosphere temperature can be used as the cooling medium in the condenser (Refer Figure 25.17).

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In order to complete the circuit, the liquid from the condenser passes through a regulator, or expansion valve, which controls the flow o f liquid to the evaporator. The part o f the circuit downstream from the expansion valve to the suction valve o f the compressor is called the low-pressure side o f the system, and that from the compressor delivery valve to the upstream side o f the expansion valve is the high-pressure side. The correct functioning o f the expansion valve is o f paramount importance. Compressors are usually o f the continuous running, fixed-speed-type; the correct functioning o f the expansion valve is necessary in order to maintain the appropriate amounts o f refrigerant in the high and low pressure sides. in order to obtain high efficiency, the amounts o f refrigerant must be correct so that there is enough refrigerant in the condenser for the liquid refrigerant to be sub-cooled and only enough refrigerant in the evaporator to ensure that there is some superheating o f the gas. This correct working o f the cycle is obtained when the total charge o f refrigerant in the system is correct, and the expansion valve is correctly maintaining its distribution between the low and high-pressure sides.

Figure 25.17 - T he R efrigeration Cycle (with a W ater-C ooled C ondenser)

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Miscellaneous Systems 25.7.2 Refrigerants Refrigerants play a vital role in most cooling systems. Amongst refrigerants, R-12 (Dichlorodifluoromethane) with a boiling point o f -30°C, R-22 (Monochlorodifluoromethane) with a boiling point o f -41°C and R-502 (48.8% R-22 and 51.2% R 115) are used. The first two are popular because o f the following characteristics: S Non-explosive ^ Non-combustible S

Non-toxic (however at high temperatures they can decompose causing toxic phosgene gas to be evolved and vapours may turn explosive, so they should not be taken for granted)

■ S Non-irritant S

Odourless, liquid and oil miscible.

S

Available worldwide

25.7.3

Compressor Safety Devices

The compressor is protected by three safety switches; 25.7.3.1

HP or High Pressure Switch

This is fitted to the outlet o f the compressor before the isolating valve. On overpressurisation, dependent on the refrigerant, up to about 24 bars for R22, the switch will trip the compressor and a manual reset is required before restart. 25.7.3.2 LP or Low Pressure Switch When activated (at about 1 bar for R22), it will trip the compressor and require a manual reset before the compressor can be restarted 25.7.3.3

OP switch or Oil Differential Pressure Switch

This compares the measured lubricating oil pressure with the suction (crankcase) pressure. Should the differential pressure fall below a pre-set minimum value, (about 1.2 bars) then the compressor will trip and requires a manual reset to restart. A time delay is built into the circuit to allow sufficient time for the lubricating oil pressure to build-up when starting before preparing the circuit to be capable o f tripping in case o f a drop in lubricating oil pressure below the nominal value. Marine Electrical Technology

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Chapter 25 25.7.4

Compressor ControlDevices

This normally takes the form o f a Low Pressure cut-out pressure-switch with an automatic reset when the pressure rises. The cut-out set point is just above the LP trip point say at about 1.4 bars. An adjustable differential is set to about 1.4 bars to provide a cut-in pressure o f around 2.8 bars. The electrical circuit is so arranged that even when the switch has reset, if no rooms’ solenoid-valves are open the compressor will not start. This is to prevent the compressor cycling due to a leaky solenoid valve. In addition to this, extra LP switches may be fitted which operate between the extremes o f the LP cut in and cut out to operate compressor unloaders. Some modern systems contain a rotary vane compressor with variable speed control (with the help o f frequency variation).

\

■

A Mains On Cold Room

Evaporator

Fan

V

Cool Room

Defrost On

Evaporator Fan

Defrost Switch

Low Oil Pressure 1 - Manual On O-Off 2 - Automatic

Figure 25.18 - A R efrigeration System ’s C ontrol Panel (w ith an A ir-C ooled C ondenser) 996

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Note: Each motor in the circuit will have dedicatedoverload and short circuit protection

Figure 25.19 - A Refrigeration System’s Basic Circuit (with an Air-Cooled Condenser) 25.8 25.8.1

Air-conditioning Systems What Air-conditioning M eans...

Air-conditioning means to modify the temperature and humidity of a room in order to achieve a more comfortable living condition. An air-conditioning system has the capacity to take the air o f a room, treat and deliver it back in a cool and dehumidified state. An airconditioner also normally has the capacity to heat the room when it is equipped with either the reverse cycle system or with an electrical heater. An air-conditioner is also supplied with a room temperature controller (thermostat), an on/off switch and a fan-speed controller with multiple speeds. M arine Electrical Technology

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Chapter 25 If the air-conditioner also has the heating function, then the control panel also gives the choice between cooling and heating. This choice can also be automatically made, in such a way that once you set the desired temperature, the air-conditioner control will automatically choose the functioning mode to reach and maintain the set temperature. In this case the airconditioner becomes an environmental control system. All types o f thermostats are found in air conditioning systems - pneumatic and electrical. By themselves, they are all satisfactory instruments, but the results they achieve are dependent on the correct installation o f their sensing elements. Even the site for a direct acting thermostat to control one single-berth cabin m ust be chosen with care. I f it is masked behind curtains, or too far away from the air inlet, control will be too sluggish. The correct location for a thermostat to control a block o f cabins is more difficult to find. One can pick on a “typical” cabin - but if the occupant opens his porthole he can upset the whole block. Another possibility is to site the thermostat in the alleyway o f the block o f cabins. This position may be affected m ore by an open door or draught in the alleyway than by the temperature o f the cabins. Yet another possibility is to site the thermostat in the re-circulation air trunk, carrying air back from the accommodation to the unit. If the re-circulation grille is close to an outside door, this position too can be affected by outside air temperature when the door is open, rather than by cabin temperature. A large public room, say a lounge, may be impossible to control satisfactorily by one thermostat. If one thermostat is positioned awkwardly, it may sense a temperature higher than the average in the room and cause air to be delivered, which will be too cold for the comfort o f those sitting around the edge o f the room. Similarly, a thermostat sited at the edge o f the room may leave too high a temperature in the central area. The only satisfactory arrangement for such rooms is to have different controls for different parts o f the room. Accurate temperature records for cargo spaces are essential to guide the operating engineer and also as evidence that correct temperatures were maintained iti the event o f any cargo arriving in less than a perfect condition. Remote reading thermometers are usually platinum resistance thermometers, with a resistance o f about 100H at 0°C (known as the PT-100 type), whose bulbs are rubber-covered and vulcanised to the cable to ensure complete water-tightness. The calibration o f this type o f thermometer can also be checked in melting ice.

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Miscellaneous Systems The earliest installations o f this type were connected via a selector switch in the engine room, to an indicator in which manual adjustment o f a Wheatstone bridge circuit was made to ensure zero deflection on a sensitive galvanometer. Great care was taken to prevent accidental circuit resistance, all terminal boxes had joints screwed and soldered and the overall accuracy was +0.1°F. This manual balancing has now been replaced in modem installations with self­ balancing electronic indicators, strip chart recorders, or data loggers. The recorders and data loggers used, have sometimes been general-purpose instruments and not engineered with the accuracy required for refrigerated cargo in mind. Tolerances have increased to + 0.1 °C and even + 0.2°C. Yearly checks on thermometers in melting ice are advisable, and i f the thermometer bulb or probe is not fully watertight, a rubber or plastic covering before testing should protect it. The refrigerated cargoes that demand the most precise control o f temperature are those carried at about 0°C, and great precision is required to be sure that the cargo is not accidentally frozen. Accordingly, testing thermometers in melting ice check the calibration at the point (in the range) where accuracy is most required. Thermistors in place o f resistance thermometers are gaining acceptance as reliable thermometers. The sensor has a resistance o f thousands o f ohms and stray circuit resistances become relatively less important since they are swamped out.

25.8.2

Types o f Air-conditioners

There are three types o f air-conditioners:

a) Independent direct expansion units as self-contained or two-part units which are used to air-condition one or two rooms close to one another The air-conditioner treats the room air and delivers it back directly to the room, through air ducting with sizes from 75 to 175 mm which avoids making complicated and long distribution systems. A n interesting version o f the independent unit is the split model which is built in two parts: a compressor assembly and a separate evaporator/fan assembly which can be installed several metres apart from the compressor, saving cabin space and permitting the air-conditioning installation in boats where there is no space for both components in one piece. The temperature control is made by stopping and running the compressor and also by controlling the fan speed.

b) Central units with direct expansion circuit to several evaporators (fan coils) These are very common units used in land installations where they can be called ‘muitisplit’. In marine applications there are some installations made with this configuration where one (large) compressor supplies several evaporators. M arine Electrical Technology

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Chapter 25 Unfortunately this simplified configuration makes the system rigid and it tends to become unbalanced, particularly when the thermal load is reduced at night and with the large compressor balanced for the high load o f the day, it becomes far in excess o f the reduced night load. This situation can cause an increase in the fan coil noise into the cabins. In addition to this, the piping for the refrigerant connection to each fan coil could become a weak point if it is not correctly designed and installed, as any leak will stop the entire system and the repair could be a real hassle. With this type o f system it is not possible to connect several compressors in parallel on the same circuit; each compressor must have its independent circuit connected to its evaporators. The temperature o f each room is controlled by stopping the fan o f that room or by stopping the refrigerant flow to that fan coil. In either case again the system becomes unbalanced i f not properly designed as the compressor capacity is still the same while the fan coil load is reduced.

c)

Central systems with chilled (or heated) water distribution to several fa n coils each installed in the room to be air-conditioned In this case the central system, which can be made with one or m ore compressors, cools

(or heats) the water o f a closed water circuit which is pumped to each fan coil. This type o f unit has several advantages: The distribution system o f the chilled (or heated) water has the same characteristics as a heating circuit but instead o f a boiler there is one or more chiller compressors and at the place o f the radiator in each room, there are the fan coils. Each fan coil is completely independent o f the central unit, which is set to keep the fresh water circuit temperature at a preset value (normally +12°C in summer mode and +40°C in winter mode); all the fan coils are connected in parallel to the fresh water circuit and the room temperature.

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Miscellaneous Systems 25.8.3

Cooling Mode

The air-conditioner, when used in the cool mode (summer use), is a refrigerating unit which extracts the heat from the room air (directly in the direct expansion systems, and indirectly with an intermediate fluid in case o f ‘chiller’ systems. The heat removed together with the heat generated by the compressor working, must be then dissipated outside the airconditioned space. The marine air-conditioner uses a special marine heat exchanger to dissipate the heat to the sea water, which is circulated by a pump.

25.8.4

H eating Mode

The same air-conditioner which produces cool air in summer can produce heat in winter. In order to produce hot air, the air-conditioner must be equipped either by a ‘reverse cycle valve’ or by an electric resistor. The reverse valve is a special 4-way valve which can “reverse” the refrigerating circuit so that the evaporator becomes a condenser and the condenser becomes an evaporator. In this way the heat is taken from the sea water (which is consequently cooled) and given to the room air which is heated. The heat is sufficient for Mediterranean climate, with mild winter temperature and, more important, sea water temperature above 0°C. The sea water temperature must be carefully considered as the air-conditioner’s efficiency drops dramatically i f the sea water temperature drops below 10°C. I f this happens, the air-conditioner loses efficiency and it can no longer be used. For cold seas it is advisable to install a system equipped with electrical heating, which does not lose its efficiency in cold waters. In the market air-conditioners equipped with electrical heating are also available.

25.8.5 Sea Water Cooling o f the Air-conditioner As the heat is rejected overboard by an air-conditioner when cooling, and the consequent problems in a typical marine installation, all marine air-conditioners are water-cooled, in other words the air-conditioner dissipates the heat into the sea water, using a special marine heat exchanger in which the sea water is circulated by means o f a pump. The pump used to circulate the sea water should be rated for continuous duty and built to marine specifications. It normally uses a marine centrifugal pump which is installed below the water line as the standard centrifugal pump is not self-priming.

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Chapter 25 25.8.6 Safety The marine air-conditioning system has two aspects which must be well considered for safety reasons: a) The system is connected to the mains supply and it is essential that the connections follow the safety rules. b) The air-conditioner unit (or its fan coils) must recirculate the cabin air and possibly a small percentage o f external air. The air intake should never come from a contaminated compartment or even worse, from the engine or generator room. In case o f a problem in the exhaust system o f the engine or generator, the exhaust gas is toxic and i f the airconditioner takes and delivers these gasses, it will be extremely dangerous or even lethal to the people on board.

25.8.7 Autom atic Temperature Controllers The automatic control o f space temperature in general becomes easier as the space becomes larger. This is because the thermal capacity o f the contents provides a “flywheel” effect to overcome temperature variations caused by the functioning o f the controllers. It is sometimes difficult to decide whether the best place for a controller’s sensing element is in the space to be controlled or in the air delivery stream o f the air cooler. For cargo spaces, which may be used for fruit carriage, the air delivery temperature is the correct temperature to control in order to be sure that temperatures never fall low enough to injure fruit. In systems with reversible fans, the sensing element is placed in a by-pass duct (between delivery and suction ducts) so that air delivery temperature is detected whichever way the fan is running. A n electric solenoid valve located before the expansion valve in the liquid line and incorporating a simple on/off thermostat usually controls a direct-expansion cooler. Alternatively, for small installations with only one cooler served by one compressor, the on/off thermostat may control stopping and starting o f the compressor. Small installations, for example, integral refrigeration units on containers, can suffer from “short cycling”, i.e. too frequent stopping and starting o f the compressor, if the control is a simple on/off type and no other controls are fitted.

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Miscellaneous Systems This occurs under light-load conditions, i.e. high container temperature and low ambient conditions and there is little the operator can do to alleviate it, except for possibly restricting the condenser cooling to give the compressor more work to do. The simplest controller used in brine (or chilled water) systems is the direct-acting type, where the temperature-sensing phial is liquid-filled and connected by capillary tubing to the head o f a diaphragm valve in the brine delivery pipe. These controllers are found in air conditioning installations, but not used for cargo storage, as the alteration from one set-point to another cannot be carried out quickly and accurately. A more sophisticated controller is the all-pneumatic controller with a compressed air operated valve in the brine pipe. The controller is mounted outside the battery room and receives a signal from a mercury-filled bulb in the air delivery stream, via a capillary tube and bellows. The controller produces an output signal at varying pressures to maintain the brine valve in a “floating” or modulating, partially-open position. Slight adjustments to the set-point may be required to maintain the same air delivery temperature under ambient conditions as the capillary is affected by ambient temperature. In order to overcome the limitations o f capillary length, pneumatic controllers are available where the temperature-sensing bulb is mounted in the air stream, and integral with it is the pneumatic controller whose output is used to control the brine valve. The pneumatic controller has its set-point adjusted by a pressure signal, which is supplied by a signal line from a central control station in the engine room. This system also provides modulating control. Combined electronic-pneumatic systems are also used; here the sensing element is a resistance thermometer, and the primaty controller is a Wheatstone bridge network arranged so that the output signal operates a solenoid valve (on/off) in a compressed air line. This compressed air line then controls a pneumatically-operated brine valve - but the mode of action is on/off and not modulating. It is essential that all the electric components o f this system are completely watertight, as they are liable to soaking by condensation. Whatever type o f controller is used, it is important that the brine valves are correctly set, and the spring tension is adjusted to facilitate fullyclosed and fully-opened positions at the designed air pressures. I f this is not so, there is a possibility o f valves passing on brine when they are nominally closed. Brine valves are normally mounted within cooler rooms, where they are not subject to undue condensation.

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Chapter 25 However, if they are mounted outside cooler rooms, ice formation on the stems can be troublesome; the valve stem should be coated with low temperature grease. Engineers onboard the ship may be faced with a dilemma when the air delivery temperature as recorded on the recorder or data logger differs from that set on the controller. On such occasions it is not clear as to which is the true temperature. It is recommended that recorders and data loggers should be calibrated regularly and they should be relied upon for accuracy. Where scale adjustment is provided on controllers, this should b e used to calibrate the scale against the recorder or data logger.

1)

W hat do you understand about the electrochemical theory o f corrosion?

2)

With simple sketches explain the Impressed Current Cathodic Protection System

3)

What is the role o f a magnetic amplifier in a cathodic protection system? Explain its basic operating principle.

4)

W hat are the various protections / components in a basic Refrigeration system? Explain.

5)

Briefly explain the operation o f a Dionic Water Purity Meter.

6)

With the help o f a diagram explain the working principle o f a Salinometer.

7)

With the help o f diagrams explain the working principle o f a galley range

S)

What are the main features o f an electrical system for the operation o f water-tight doors? Briefly explain the control system too.

9)

Briefly explain the role o f an air-conditioning system’s temperature controllers.

10)

What is the significance o f pressure switches in a refrigeration plant?

11)

How is the temperature o f a cold room maintained?

12)

What is a self-monitoring alarm circuit?

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Chapter 26 Maintenance and Troubleshooting At the end of this chapter you should be able to: S tate the significance o f different types o f m aintenance List the basic steps in troubleshooting C arry-out basic m aintenance o f specific electrical equipm ent List th e spares, tools and accessories required fo r m aintenance C om ply w ith regulations governing m aintenance

In order to emphasise the importance of maintenance, an extract from the website of LACS - the International Association of Classification Societies (www.iacs.org.uk) is included withpriorpermission of the Permanent Secretariat in London. Quote

IACS A Guide to Managing Maintenance April 2001 Recommendation 74 Introduction One o f the primary responsibilities o f a ship owner and ship management Company is that the ship hull structures, machinery and equipment are maintained and operated in conformity with the applicable rules and regulations and any relevant additional requirements, procedures and standards established by the Company. That responsibility starts from the top Managers o f the Company, who should be committed to direct efforts, resources and investments in order to ensure that their ships are properly maintained and operated by qualified and competent crew. Such a Company’s commitment from the top is the first element to be verified by the ISM Auditors. The objectives o f a responsible Company should be to ensure, as required by the ISM Code, that the procedures for ship maintenance established by the Company are properly implemented ashore and on board. The identification o f factual evidences and possible non­ conformities related to the implementation o f ship maintenance procedures ashore and on board is the second step o f verification against the relevant requirements o f the ISM Code.

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Chapter 26 The Company shall not limit its maintenance and repair interventions to the ones strictly required by Flag and Port State Authorities, classification societies and other interested parties during periodical and renewal-of-certificates surveys. Third-party surveyors and auditors can only verify the compliance o f the ship with respect to the relevant statutory and class requirements, and other applicable standards, at the time and within the scope o f their periodical and renewal surveys, inspections or audits. In accordance with the requirements o f the ISM Code, the Company is the party which is solely responsible for the daily maintenance o f the ship, including the hull structure, machinery and equipment and for the safe and environmentally responsible operation o f the ship. In addition to the safety and environmental considerations, a well-designed and implemented maintenance system should be seen as a sensible investment in a very valuable asset. On the contrary, the management o f shipboard maintenance is too often regarded as an entirely technical matter that somehow has less to do with safety and pollution prevention than do drills and exercises, or familiarization training, for example. It is seen as being the exclusive responsibility o f the technical staff rather than being the concern o f safety managers and designated persons. As a result, shipboard maintenance has tended to be the leastdeveloped and weakest element in many management systems. This increases the risk o f death, injury and damage to property and the environment, and has the potential to cause substantial costs arising from repairs and operational delays. It not only threatens ISM certification, but also increases the risk o f port state control detentions. (O f all the port state control detentions attributed to failures in shipboard safety management systems, more have referred to maintenance than to any other clause o f the ISM Code.) The purpose o f this document is to assist ship-owners, managers and operators in the development and improvement o f maintenance management systems by establishing the principles on which they should be based, and by identifying their fundamental elements. Although it provides useful guidance on what external auditors will be looking for, companies should avoid the temptation to create systems with the sole aim o f keeping the auditor happy. The objective must be to ensure the safe and reliable operation o f the ship and its equipment, and compliance with all the applicable class and flag state regulations. How this is achieved will depend on the size and complexity o f the company and its ships. The system may be entirely electronic, entirely paper-based, or a combination o f the two, and the level of shore-based supervision will vary from one organization to another. All that matters is that the system works, and that it works in a way that best suits the company. I f it does, it can pose no threat to the company’s ISM certification. 1006

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Maintenance and Troubleshooting 1. What the Code Says About Maintenance Paragraph 10.1 o f the ISM Code states, “The Company should establish procedures to ensure that the ship is maintained in conformity with the provisions of the relevant rules and regulations and with any additional requirements which may be established by the Company”. The procedures should be documented, and should ensure that applicable statutory, class, international (e.g. SOLAS, MARPOL) and port state requirements are met, and that compliance is maintained in the intervals between third-party surveys and audits. The maintenance procedures should also include any additional requirements established by the Company. These may arise, for example, from an analysis o f the maintenance histories o f machinery and equipment, from the particular demands o f a ship’s operations, or from a manufacturer’s recommendations. It is important to remember that these requirements apply as much to die maintenance o f the hull, the deck machinery and the life -saving and firefighting equipment as they do to engine room items. Compliance with the requirements o f the ISM Code with respect to the maintenance o f the ship and its equipment involves more than meeting the specific requirements o f clause 10. Several other clauses also apply to this activity, as they do to all others. Examples are: 1.2

Objectives

What are the company’s objectives with respect to safety and pollution prevention? How successful are the maintenance procedures in contributing to the achievement o f those objectives? 4

Designated Person(s)

How effective is the designated person in verifying an efficient flow o f maintenance-related information between the office and the ships, and in securing adequate resources to support shipboard maintenance, (in particular, the prompt provision o f spares and consumables)? 6

Resources andPersonnel

Is the company’s management committed to the provision o f adequate resources to enable prompt and satisfactory maintenance to be carried out? Have inspection and maintenance responsibilities been assigned to adequately qualified and trained members o f staff?

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Chapter 26 9

Reports andAnalysis ofNon-Conformities, Accidents andHazardous Occurrences

Are system non-conformities, accidents and hazardous occurrences being properly reported and investigated? Is appropriate corrective action being implemented? Terms such as ‘defect’, ‘non-conformity’, ‘incident’ and ‘hazardous occurrence’ should be carefully defined to ensure that the appropriate type and grade o f event will be reported. 11

Documentation

Are the publication, amendment and distribution o f maintenance procedures and other essential documents properly controlled? Company verification, review and evaluation

12

Is appropriately analyzed and summarized vessel performance and maintenance information being included in the shipboard and company reviews o f the effectiveness o f the management system? Are these reviews beneficial in terms o f generating improvements in the management o f maintenance? Com pliance w ith relevant rules and regulations

Clause 1.2.3 states, “The safety-management system should ensure: .1

compliance with mandatory rules and regulations; and

.2 that applicable codes, guidelines and standards recommended by the Organization, Administrations, classification societies and marine industry organizations are taken into account. ” Procedures should be in place to control such documents. In other words, the appropriate rules, regulations, codes guidelines and standards must be made available to those departments and people whose activities are governed by them. They should be o f the appropriate edition or revision, and significant changes should be identified and distributed accordingly. The procedures should contain provision for ensuring that obsolete documents do not come into use inadvertently.

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Maintenance and Troubleshooting R eporting and investigation o f technical deficiencies and nonconform ities

Clause 10.2 o f the ISM Code states that the company should ensure that any non­ conformity is reported, with its possible cause, if known, and that appropriate corrective action is taken. (In this context, “non-conformity” should be taken to mean a technical deficiency which is a defect in, or failure in the operation of, a part o f the ship’s structure or its machinery, equipment of fittings. See also clause 9 o f the ISM Code.) Problems reported may be discovered during routine technical inspections or maintenance, following a breakdown or an accident, or at any other time. The fundamental elements o f an effective defect - or non-conformity investigation process are shown in the following diagram. Note that it is not enough simply to take corrective action. The effectiveness o f such action must be verified.

The Corrective Action Process

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26 The Company should also take into account the following when developing and improving maintenance procedures: i)

The maintenance manufacturer;

recommendations

and

specifications

of

the

equipment

ii) The history o f the equipment, including failures, defects and damage, and the corresponding remedial action; lii) The results o f third-party inspections; iv) The age o f the ship; v) identified critical equipment or systems; vi) The consequences o f the failure o f the equipment on the safe operation o f the ship;

2. A Systematic Approach to Maintenance A systematic approach to maintenance will include: i)

The establishment o f maintenance intervals;

ii) The definition o f the methods and frequency o f inspection; iii) The specification o f the type o f inspection and measuring equipment to be used, and the accuracy required o f it; iv) The establishment o f appropriate acceptance criteria (pass/fail); v) The assignment o f responsibility for inspection activities to appropriately qualified personnel; vi) The assignment o f responsibility for maintenance activities to appropriately qualified personnel; vii) The clear definition o f reporting requirements and mechanisms. Maintenance intervals Maintenance intervals should be established based on the following: i)

The manufacturers’ recommendations and specifications;

ii) Predictive maintenance determination techniques (i.e. lube oil analysis, vibration analysis);

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Maintenance and Troubleshooting iii) Practical experience in the operation and maintenance o f the ship and its machinery, including historical trends in the results o f routine inspections, and in the nature and rates o f failures; iv) The use to which the equipment is put - continuous, intermittent, stand-by, or emergency; v) Practical or operational restrictions, e.g. maintenance that can be performed only in dry-dock; vi) Intervals specified as part o f class, convention, administration and company requirements; vii) The need for regular testing o f standby arrangements. Inspections

Procedures for planned inspection routines should be written to include the following: i)

Acceptance criteria (e.g. pass/fail, tolerances);

ii) The use o f suitable measuring and testing equipment of the required accuracy; iii) The calibration o f the measuring and testing equipment to the appropriate standards; The following are examples o f the types o f inspection and test that may be employed: i)

Visual

ii) Vibration iii) Pressure iv) Temperature v) Electrical vi) Load vii) Water tightness Inspection methods

W here appropriate, checklists should be developed to ensure that inspection, test, and maintenance activities are performed in accordance with the procedures, and at the specified intervals. These checklists may be developed from manufacturers’ recommendations or specifications. Marine Electrical Technology

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Chapter 26 Permit-to-work systems Where appropriate, permit-to-work systems should be employed to ensure that inspections and maintenance activities are carried out safely. A well designed permit-to-work procedure will amount to a risk assessment, carried out before any hazardous activity is undertaken. As a result o f the assessment, controls will be imposed to eliminate or reduce the risks involved. These may include, among other things, an assessment o f the environment in which the work will take place and adjacent areas and compartments (especially for hot work), the isolation o f electrical circuits or the draining o f pipes and tanks, the provision o f appropriate and well maintained tools and equipment, the assignment o f qualified and experienced personnel, stand-by and emergency arrangements, 3. W h at Records Should Be K ept (and w hat use can we m ake of them ?) Records kept to demonstrate compliance with the company’s maintenance procedures, and their effectiveness, may be divided into two broad categories: A. E xternally-generated records Class records, reports and certificates Statutory records, reports and certificates Port State Control reports Reports o f vetting organizations B. Internally-generated records Records o f routine shipboard inspections Records o f maintenance work carried out Records o f the testing o f stand-by and other critical equipment Records o f the testing o f alarms and emergency shut-downs Superintendents’ visit and inspection reports Internal and third party audit reports Reports o f non-conformities, accidents and hazardous occurrences Records o f the implementation and verification o f corrective action Spare part requests, acknowledgements, delivery notes etc.

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Marine Electrical Technology

Maintenance and Troubleshooting As well as providing evidence o f compliance with procedures, the records generated by shipboard maintenance activities may also be seen as a database from which to extract valuable management information. For example, the appropriate analysis o f records o f inspections, defects, nonconformities and corrective actions may yield information that could lead to changes in inspection and planned maintenance intervals, thereby reducing unnecessary work and the frequency o f failures. The same analysis could permit the identification o f trends or repetitive problems that require further investigation and longer-term solutions. The proper filing and review o f non-conformities, reports o f accidents and hazardous occurrences, defect reports and spare-part requests permit the efficient control o f follow­ up and verification activities.

4. The Identification and Testing o f4Critical’ Equipment Clause 10.3 o f the ISM Code states, "The Company should establishprocedures in its SMS to identify equipment and technical systems the sudden operationalfailure of which may result in hazardous situations. The SMS shouldprovidefor specific measures aimed at promoting the reliability of such equipment or systems. These measures should include the regular testing ofstand-by arrangements and equipment or technical systems that are not in continuous use The list o f ‘critical’ equipment and systems will vary according to the type o f ship and the operations in which it is engaged. W hen the equipment has been identified, appropriate tests and other procedures should be developed to ensure its reliability. O n board any ship there may be equipment and systems the sudden operational failure o f which may result in hazardous situations and for which there may be no mandatory requirements. Measures aimed at promoting the reliability o f such equipment or systems should be provided. The testing and maintenance o f stand-by and infrequently used equipment should be part o f the company’s maintenance plan. The following are examples o f items to be subjected to inspection and test: i)

Alarms and emergency shutdowns,

ii) Fuel oil system integrity, iii) Cargo system integrity, iv) Emergency equipment (EPIRB, portable VHF, etc.), Marine Electrical Technology

1013

Chapter 26 v) Safety equipment (portable gas and CCh detectors, etc.), vi) (Pre-arrival and pre-departure tests of) emergency steering gear, generators, emergency fire pumps, telegraphs, etc, vii) Fire-fighting and life-saving equipment. C hecklist of P rincipal M aintenance System M anagem ent Controls S! No.

Control

■j

Do we receive prom pt and reliable inform ation about new and amended statutory, class, international and port state regulations, and about industry codes and guidelines?

2

Do we have controls in place to ensure com pliance with all applicable mandatory regulations, and to ensure that appropriate codes, guidelines and standards are taken into account?

3

Have the responsibilities and authority o f shipboard and office staff involved in inspection and maintenance activities been clearly defined?

4

Have inspection and maintenance activities been assigned to adequately qualified, trained and experienced staff?

5

Are controls in place to ensure that a ll applicable procedural and technical documents, o f the appropriate editions, are available where they are needed?

6

Have steps been taken to ensure that obsolete documents cannot be brought inadvertently into use?

j

8 g

10 14

| Yes

No

L.

| Do we have in place a system fo r the reporting and analysis o f defects, accidents and hazardous occurrences? Have the types and seriousness o f the defects and Incidents to be reported been clearly defined? j Do procedures exist fo r the implementation o f corrective action and | the verification of its effectiveness?

10

Do the inspection and maintenance records enable us to m onitor adequately the maintenance history o f the ship, its machinery and its equipment?

11

Have we established ail appropriate inspection intervals?

12

Have we defined inspection methods and the type and accuracy o f the inspection and measuring equipment to be used?

13

Have we established appropriate acceptance criteria?

Marine Electrical Technology

..........

Maintenance and Troubleshooting

SI No.

C o n tro l

14

Have we established all appropriate maintenance intervals?

15

Are sufficient inspection and maintenance records being kept to demonstrate compliance with company requirem ents and mandatory regulations?

16

Have we identified all equipm ent and technical systems, including stand-by and infrequently used item s, the sudden operational failure o f which may result in hazardous situations?

______ 17

Are appropriate perm it-to-work procedures in place to assess the risks involved in the inspection and maintenance activities, and to ensure that adequate controls are applied?

18

Is appropriately analyzed and summarized maintenance infonnation being provided fo r inclusion in the masters’ and the company’s reviews o f the effectiveness o f the management system?

Yes I No i

Unquote

26.1

The Basics of Maintenance

Maintenance can be performed and systems kept in a safe, reliable condition with a proper mixture of: 1. Common sense 2. Training 3. M anufacturers’ literature and compatible spare parts Maintenance and sensible troubleshooting are thus the lifelines o f any equipment. It is the watchword in any industry and at sea is part and parcel o f a seafarer’s life. It not only enhances operational reliability but also safety o f personnel and material. The environment and harsh conditions under which a ship has to sail needs no mention. Over a period o f time it has been realised that it is better to spend a little more on preventive maintenance than to lose die whole system. On the other hand, lethargy and stringent financial control where replenishment o f consumables and spares in due course o f tim e are concerned, leads to major breakdowns and unprecedented delays. It must also be remembered that some ships now operate with as few as 3 engineers on board and generally spend short periods in ports thus reducing the emphasis on maintenance. Foolproof systems have been devised and refined over the years. It needs very good knowledge o f the equipment in concern, foresight and commitment. Marine Electrical Technology

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Chapter 26 The demands for improvements in maintenance are classified under die title ‘Terotechnology’. This calls for enhancing o f performance and condition monitoring techniques in order to keep the equipment at their optimum levels o f efficiency. Such measures also enhance safety standards especially in vessels with unmanned machinery spaces (UMS ships). The following chart depicts the basic concept o f maintenance: Maintenance

I

>t

Breakdown or Corrective Maintenance

Planned Preventive Maintenance

l

1

Scheduled Maintenance (also includes Life Maintenance)

t

9

Carried irrespective of equipment condition

Depends upon the performance or physical state of the equipment

1 Based on calendar or running-hours

26.2

Performance / Condition Monitoring

Based on trend analysis and operating parameters

Planned Preventive Maintenance

This is a rigorously planned exercise and needs not only constant activity but also commitment by the staff concerned. M any times this type o f task is applied when no condition-monitoring task is identified or justified, and the failure mode is characterized with a wear-out region. The objective is to prevent a failure and consequent breakdown o f equipment / systems. Periodic maintenance leads to ‘planned preventive maintenance’ which in most cases cause forecasted shutdowns albeit for brief periods. This, though inconvenient at times, can in most cases alleviate the failure rate during prolonged operation and will surely contribute to the longevity o f a system’s life.

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Marine Electrical Technology

Maintenance and Troubleshooting Periodic maintenance in other words is the application o f specified routine maintenance that is generally stated by the manufacturer and executed after a specific calendar period or after a number o f running hours have elapsed. This could stem from the Restoration Task which is a scheduled task that restores the capability o f an item, at or before a specified interval (age limit) to a level that provides a tolerable probability o f survival to the end o f another specified interval. Certain advantages o f periodic maintenance are as follows: ☆

Fewer breakdowns and the consequent reduced downtime result in much higher levels o f operating efficiency.

■fr Maintenance is carried out at times most favourable to the plant. Effective utilisation o f the workforce on hand as activity can be planned. tV Replacement can be forecasted and executed without disruption o f activity. & Specialist advice can be obtained at opportune times. & Periodic maintenance requires the preparation o f specially designed charts, documents and many computer-based programs are available, which can even be monitored (via satellite) by the company headquarters ashore. This is generally under the purview o f the chief engineer. A maintenance schedule is a detailed version o f the manufacturer’s (and in some cases the ship owner’s) recommendations for periodic maintenance. It also contains points drawn from experience. The other possible guidelines are: >

Hours-run maintenance (e.g., 500 / 1000 / 2500 hourly routines)

>

Unscheduled and defect notes for immediate action. A defect log can also be maintained in the form o f a dedicated Equipment History Sheet, which is further kept in an Equipment History File

>

Work not completed in a period.

>

Work to be carried out in the next port.

Maintenance planning documents generally contain the following: S

The item o f equipment

S

Work instructions

S

Tools and spares required Marine Electrical Technology

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Chapter 26 S

Safety precautions to be adhered to

v'' A record o f work carried out S

Notes for future reference

26,2.1 E le c tric a l W ork P erm it

Past experience indicates that a majority o f electrical accidents can be avoided if proper safety precautions are taken before commencing any work. Some o f the factors that contribute to accidents are: a)

Lackadaisical attitude o f supervisors;

b)

Dearth o f knowledge;

c)

Over confidence or complacency; and

d)

Defective or sub-standard safety appliances

The work permit system is thus a safety procedure that is designed to overcome many o f these contributory factors. This document must have at least one duplicate copy o f the original that is kept as a record. As mentioned in article 2.6, a work permit is mandatory when personnel work on equipment operating at voltages greater than lkV . Sometime it is mandatory to even obtain an associated Electrical Isolation Certificate to declare where exactly circuit isolation and earthing has been applied before a permit can be authorized. A Sanction to Test safety certificate may also be required when an electrical test e.g. electrical insulation test, is to be applied; this is necessary as circuit earth may have to be removed / isolated during such testing. Such practice also helps to reduce the number o f accidents. It places a responsibility on the person issuing the permit, to ensure in writing that the equipment / line has been isolated and thus made dead before commencement o f work and m ust remain so till the completion of the work. In addition, the person issuing the permit has to necessarily assess the hazards that can arise while working; such hazards have to be overcome (before commencing any work) by taking the necessary precautions. Thereafter it is the responsibility o f the person (who is appointed to do such work) to ensure that all safety measures are taken. The acceptance o f the person is usually taken in writing by obtaining his signature on the work permit. On completion o f the work, the permit is to be returned to the issuing authority after the appointed person endorses in writing that the work has been completed and that all men and materials have been cleared o f the work place. The issuing authority will then cancel the permit. 1018

Marine Electrical Technology

Maintenance and Troubleshooting 26.2.1.1 Example of an Electrical Work Permit P e rm it N o .:_____________ D a te :_____________ S h ip :_____________

P la c e :_____________

This permit is to allow the following persons to work on the electrical circuit specified below under the stated conditions. It is cancelled in case of any imminent danger, emergency warning or breach of safety conditions. A copy of this permit shall be given to the person in command of the workfor control of the work area in accordance with the conditions and instructions as laid down in this permit. Copies are also kept in the Work Permit File and Displayed as Area Control Copy Area A uthorised:________________________________ _ Person in charge o f the work to be carried o u t:__________________ Person(s) appointed to do the w ork:__________________________ _ Person(s) to provide support:_________________________________ W ork to be carried o u t:________________________________ Additional permits if any: Perm it N o (s):________________________ General precautions to be observed:

1. All voltage has been cut-offbefore commencing the work: Yes/No Method of isolation: a) By removal of necessary fuses b) By dropping the circuit breaker c) _______________________ (any otherprecautions) 2. Particulars ofpoints where the equipment is earthed_________________. 3. Flammable gases and materials are removed from the vicinity of the work area: Yes /No 4. Precautions have been taken to ensure that the work area is not slippery, is stumble-free and not affected by the movementof the vessel: Yes /No 5. Danger/warning notices have been affixed at______________. Precautions fo r personnel involved:

1. Person is competent for the work involved: Yes l No 2. Personal clothing is dry: Yes / No 3. Wrist watch, metal bracelets and rings are removed: Yes/No 4. Clothing and foot wear are metal-free: Yes / No Special P recautions:_______________________________

Certified that we have fully understood the implications of the above entries and are aware of our responsibility Signature o f Receiver___________ Date: ________T im e____________

Signature o f Issuing O fficer______________ D ate:________ T im e ___________

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Chapter 26 Cancellation of the permit: I hereby certify that the work for which this permit w as issued is now suspended / completed and ail men, material and temporary earths are dear. All men under my charge have been warned that it is no longer safe to work on the equipment specified in the permit. Signature of permit holder_________ D ate :________ T im e _____________ The permit is hereby cancelled. Signature of issuing officer_______________ D ate:________ T im e _____________

i 26.3

Perform ance / Condition M onitoring

Although this seems to be a different concept, it is an offshoot o f preventive maintenance; it is also commonly known as ju st ‘Condition Monitoring’. This has been conceived to help prevent routine interference with equipment that is running satisfactorily. The system is also designed to help detect falling trends in perfonnance o f equipment. Condition-monitoring tasks should be chosen when a detectable potential failure condition is liable to exist before failure. Condition-monitoring tasks are also referred to as “predictive maintenance.” This may result in periodic checks namely: S

Insulation checks.

S

Vibration spectrum analysis / shock pulse monitoring to detect bearing deterioration in rotating equipment.

V

Performance checks by noting parameters under various circumstances and comparing these with standard (test-bench) data.

S

Temperature readings / thermography.

■S

Visual checks to detect leakage, wear, corrosion, etc.

V

Analysis o f lubricating media.

An example o f condition monitoring is where a motor may be checked regularly and a record kept o f this. Any deterioration can be seen and dealt-with before a breakdown occurs.

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M arine Electrical Technology

Maintenance and Troubleshooting 26.3.1

M ethods o f Checking R unning Motors

•

Visual inspection, such as current voltage, earthing, end play o f the motor, etc

•

Measurement o f insulation resistance and fitness o f connections

•

Monitoring vibration due to unbalance rotor, improper coupling or defective bearings

•

Temperature o f the motor

Note: A failure-finding task is a scheduled task used to detect hidden failures when no condition monitoring or planned-maintenance task is applicable. It is a scheduledjunction check to determine whether an item will perform its required function satisfactorily. Most of these items are standby or protective equipment; an item of this nature could be an overload relay or earth-fault detector. Guidelines for the maintenance o f some main systems / equipment are mentioned subsequently in this chapter. These will help to carry out specific jobs. It is also pertinent to mention that these serve only as a guideline. It would be prudent to follow dedicated maintenance manuals provided by suppliers of equipment on board a ship. Superintendents o f shipping companies are also competent to offer advice to engineers on board and in most cases work as a team to keep the ship both afloat and sailing!

26.3.2

Cold Checks with a Megger Extractfrom ABS Rulesfor Building and Classing Steel Vessels - 2012 Part 4 - Vessel Systems and Machinery Chapter 8 - Electrical Systems Section 3 - Electrical Equipment

Quote

3.15.2 Insulation Resistance Measurement Immediately after the high voltage tests, the insulation resistance is to be measured using a direct current insulation tester between: i) All current carrying parts connected together and earth; ii) All current carrying parts o f different polarity or phase, where both ends o f each polarity or phase are individually accessible. M arine Electrical Technology

1021

Chapter 26 The minimum values o f test voltage and corresponding insulation resistance are given in the table below. The insulation resistance is to be measured close to the operating temperature. If this is not possible, an approved method o f calculation is to be used. R a te d v o lta g e ,

M in im u m t e s t

M in im u m in s u la tio n

U n (V )

v o lta g e (V )

r e s is ta n c e (M Q )

Ur, < 2 5 0

2 Un

1

2 5 0 U n< 10 0 0

500

•j

1 0 00 U n^ 7 2 00

1000

Un / 1 0 0 0 + 1

7 2 0 0 U n5 1 5 0 0 0

5000

U n /1000 + 1

Unquote The M ega Ohm M eter commonly known as a “megger” is used to establish the condition o f insulation o f electrical equipment like generators, motors and similar power systems. However, extreme caution should be exercised when using these devices as they generate between 250V and 5000V.

26.3.2.1

Constructional Features o f an Analog M egger

Most ohmmeters utilize a battery o f relatively low voltage, usually nine volts or less. This is adequate for measuring resistances under several mega-ohms (MQ), but when extremely high resistances need to be measured, a 9 volt battery is insufficient for generating enough current to cause any electromechanical meter movement. Resistance is not always a stable (linear) quantity. This is especially true o f non-metals. While this is an extreme example o f nonlinear conduction, other substances exhibit similar insulating / conducting properties when they are exposed to high voltages. Obviously, an ohmmeter using a low-voltage battery as a source o f power cannot measure resistance at the ionization potential o f a gas, or at the breakdown voltage o f an insulator. If such resistance values need to be measured, nothing but a high-voltage ohmmeter will suffice. Necessity being the mother o f all inventions, the megger, an example o f which is shown in Figure 26.1, was thus invented. Remember when 1 — 0, R is considered to be infinity and vice versa.

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M arine Electrical Technology

Maintenance and Troubleshooting

Figure 26.1 - A 500V Portable Megger and its Basic Constructional Features A basic battery-operated high-voltage megger works differently. It has no hand-cranking mechanism and is capable o f generating a high-voltage, low-current output for insulation testing (o f circuits with high dielectric strengths) and in high-voltage applications. The numbered, rectangular blocks in Figure 26.2 are cross-sectional representations o f wire coils. These three coils move with the needle mechanism. *

Megger“ movement

Figure 26.2 - Cross-Sectional Representations of Wire Coils M arine Electrical Technology

1023

Chapter 26 There is no spring mechanism to return the needle to a set position. W hen the movement is not powered, the needle will randomly “float.” The coils are electrically connected as depicted in Figure 26.3

H ig h v o lta g e

2 3 — A V ----- HTTP------ 'TUP—

B lack T e s t le a d s

C urrent through coils 2 and 3; no current through c o il 1

Figure 26.3 Connection of Coils in a Megger

Figure 26.4 “Open Circuit” Indication

With infinite resistance between the test leads (open circuit), there will be no current through coil 1 and only through coils 2 and 3. When energized, these coils try to center themselves in the gap between the two magnetic poles, driving the needle fully to the right o f the scale where it points to “infinity” as depicted in Figure 26.4. Any current through coil 1 (through a measured resistance connected between the test leads) tends to drive the needle to the left o f scale, back to zero. The internal resistance values o f the meter movement are calibrated so that when the test leads are shorted together, the needle deflects exactly to the “0 Q” position. Because any variations in battery voltage will affect the torque generated by both sets of coils (coils 2 and 3, which drive the needle to the right, and coil 1, which drives the needle to the left), those variations will have no effect o f the calibration o f the movement. In other words, the accuracy o f this ohmmeter movement is unaffected by battery voltage: a given amount o f measured resistance will produce a certain needle deflection, no matter how much or little battery voltage is present. 1024

M arine E lectrical Technology

Maintenance and Troubieshooting The only effect that a variation in voltage will have on a meter’s indication is the degree to which the measured resistance changes with the applied voltage. So, if we were to use a megger to measure the resistance o f a gas-discharge lamp, it would read very high resistance (needle to the far right o f the scale) for low voltages and low resistance (needle moves to the left o f the scale) for high voltages. This is precisely what we expect from a good high-voltage ohmmeter: to provide accurate indication o f subject resistance under different circumstances.

26.3.2.2 Safety Features For maximum safety, most meggers are equipped with hand-crank generators for producing the high DC voltage (up to 1000 volts). I f the operator o f the meter receives a shock from the high voltage, the condition will be self-correcting, as he or she will naturally stop cranking the generator! Sometimes a “slip clutch” is used to stabilize generator speed under different cranking conditions, so as to provide a fairly stable voltage whether it is cranked fast or slow. Multiple voltage output levels from the generator are available by the setting o f a selector switch. Some meggers are battery-powered to provide greater precision in output voltage. For safety reasons these meggers are activated by a momentary-contact pushbutton switch, so the switch cannot be left in the “on” position and pose a significant shock hazard to the meter operator.

26.3.2.3 The Format o f a M egger Test Report _________________ Shipping Company Form No.:____

Place:___________________ D ate:_______

Megger Test Report M .V .__________ SNo.

Page _ o f _ M o to r D e ta ils

D a te M o to r L a s t \

O v e r h a u le d

O v e r lo a d S e ttin g (A m p s )

In s u la tio n (M O )

|

j

SI

|.

... M arine E lectrical Technology

1025

Chapter 26 26.3.2.4 Megger Reading (Sample) j V a lu e in M f2

E q u ip m e n t

E q u ip m e n t

V a lu e in M Q

12

Cargo Hold Ventilator No. 1

25

Engine Room Ventilator Starboard

200

Fresh W ater Pump

200

Hydrophore Pump

200

Steering G ear

20

Diesel Oil Separator

500

Steering Hydraulic Pump

20

Diesel Oil Transfer Pump

200

S e a Water Pump No. 1

20

Filter Pump

500

S e a Water Pump No. 2

50

Oily Water Separator

500

500

Galley Suction Ventilator

10 0

Lubricating Oil Pump

200

Galley Delivery Ventilator

100

Bilge Pump

30

Freezer Plant Compressor Starboard

15 0

Ballast Pump

20

Warm Water Boiler - Officer’s Mess

90

; Boiler Circulation Pump

500

Warm Water Boiler - Crew M ess

0.5

! Electric Welding Set

500

Warm Water Boiler - Galley

6

Galley Stove

2.5

Water Cooler - Officer's Mess

6

Lubricating Oil Separator

13 0

Potato Peeling Machine - Galley

2

Generator Starboard Generator Port

; Main Air Compressor No. 1

1

j

50

Note: Whenever insulationfor distribution of electric power in the accommodation is tested, ensure that the power supply to these circuits are isolated, sensitive loads are disconnected and then the cables /distribution boxes /fuse boards, etc., are checkedfor their insulation. 26.3.2.5

Earth Leakage Testers

These devices are equipped with three connection terminals, labeled “Line”, “Earth", and “Guard”. The schematic diagram is quite similar to the simplified version o f a megger with just two leads. Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. In order to measure the insulation resistance between a conductor and the sheath o f a twocore cable, we need to connect the “Line” lead o f the megger to one o f the conductors and connect the “Earth” lead o f the megger to a wire wrapped around the sheath o f the cable as shown in Figure 26.6. 1026

M arine Electrical Technology

Maintenance and Troubleshooting

In this configuration the megger should read the resistance between one conductor and the outside sheath. Or will it? If we draw a schematic diagram showing all insulation resistances as resistor symbols, what we have looks like the circuit in Figure 26.7.

Sheath

Figure 26.7 - An Equivalent Circuit Marine Electrical Technology

10 27

Chapter 26

Rather than just measure the resistance o f the second conductor to the sheath (RC2 -s)> what we will actually measure is that resistance in parallel with the series combination of conductor-to-conductor resistance (Rci-c2) and the first conductor to the sheath (Rci-s)I f we don’t care about this fact, we can proceed with the test as configured. I f we desire to measure only the resistance between the second conductor and the sheath (Rc2 -s)> then we need to use the megger’s “Guard” terminal. The wiring diagram is shown in Figure 26.8 and the schematic diagram is depicted in Figure 26.9. Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around it and not the current leaking through the first conductor’s insulation. Meggers are field instruments: that is, they are designed to be portable and operated by a technician on the job site with as much ease as a regular ohmmeter. They are very useful for checking highresistance “short circuit” failures between wires caused by wet or degraded insulation. They utilize high voltages, and are thus not as affected by stray voltages (voltages less than 1 volt produced by electrochemical reactions between conductors, or “induced” by neighbouring magnetic fields) as ordinary ohmmeters.

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Marine Electrical Technology

Maintenance and Troubleshooting

Figure 26.8 - Using All Three Leads Sheath

Figure 26.9 - Schematic Diagram for Figure 26.8 For a more thorough test o f a w ire’s insulation, another high-voltage ohmmeter commonly called a hi-pot tester is used. These specialized instruments produce voltages in excess o f 1 kV, and may be used for testing the insulating effectiveness o f oil, ceramic insulators, and even the integrity o f other high-voltage instruments. They are capable o f producing such high voltages and must be operated with the utmost care, and only by trained personnel. Marine Electrical Technology

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Chapter 26 It should be noted that high-potential testers and even meggers (in certain conditions) are capable o f damaging wire insulation if incorrectly used. Once an insulating material has been subjected to breakdown by the application o f an excessive voltage, its ability to electrically insulate will be compromised. Again, these instruments are to be used only by trained personnel.

26.4

Life Maintenance

A special case may arise for certain components associated with the equipment, when no inspection or repair is possible. For such equipment, life maintenance is the answer. Life maintenance o f short-life components can be arranged at scheduled intervals. This could also be termed as the “Discard Task ” as no maintenance is carried out during the pre-determined useful life span regardless o f its condition at the time.

26.5

Breakdown or Corrective Maintenance

This is sometimes seen as a simple and logical practice to let equipment run till a breakdown occurs, as the ship’s personnel are often hard-pressed for time; the dearth of manpower also leads to such practice. It is also known as a “Run-to-faiiure” management strategy that allows equipment item to run until a failure occurs and then repair is carried out.This is acceptable only if the risk o f a failure is acceptable without any proactive maintenance tasks. Such breakdowns must be reported to the chief engineer immediately. He will then be able to plan the course o f action, keeping in mind the operational requirements o f the ship. There are several shortcomings in this approach. They are as follows: @

A serious breakdown can lead to disastrous consequences.

IS

Delays are inevitable when several breakdowns occur simultaneously.

Specialist opinion and services may be needed, which causes further delay and unforeseen expenditure. It also reflects badly on the crew at times.

26.6

Troubleshooting

Shipboard equipment can malfunction for a variety o f reasons. Mechanical contacts and parts can wear out; wires can overheat and bum out; parts can be damaged by impact or abrasion; etc. Typically, whenever any equipment fails, there is a sense o f urgency to get it fixed and working again. In the shipping industry, defective equipment could also cause unexpected loss o f revenue and unprecedented loss o f time. 1030

Marine Electrical Technology

Maintenance and Troubleshooting There is a definite philosophy behind troubleshooting. It is the process o f analyzing the behaviour or operation o f a faulty circuit to determine what is wrong with the circuit. It then involves identifying the defective com ponents) and repairing the circuit. In other words, it is a logical approach in understanding the behaviour o f a system. As in the case o f condition monitoring, this also involves collection o f evidence, such as unusual sounds, vibration data, acrid smells, temperature variations, bum marks, etc. Use o f one’s basic senses would prove fruitful! The adequate use o f instruments and analysis o f the data that they display / log would also form a basis o f testing theories and assumptions, thus leading to precise fault identification and rectification. The following factors m ust be considered while troubleshooting: 26.6.1

System Knowledge

Understanding the basic operation o f a system leads to eliminating most common faults. It is often said that presence o f mind stems from knowledge gained. Troubleshooting is made easier when the main functional blocks can be identified and when correct operating procedures are followed. Using a logical, systematic approach to analyze the circuit’s behaviour is critical. There are several approaches in use. They may have different steps or processes but they have the following in common: they all approach problems systematically and logically thus minimizing the steps and ruling out trial and error. An alternate approach is explained in article 26.6.6. 26.6.2 System Configuration Locate all components, connections and the origin o f power supplies. Components as push buttons, contactors, various types o f switches, relays, sensors, motors, etc, are the ones that generally malfunction. We know that most electrical circuits control or operate mechanical systems and components; it also important to understand the mechanical aspects o f the equipment. You need to be able to determine how the circuit works under normal conditions and what effect changing one o f the circuit inputs has on the circuit operation. For example, what happens to the overall circuit operation when a push button is pressed, which relays energize, which lamps glow, does the pump start or stop, etc. You also need to be able to determine what effect a faulty component may have on the circuit operation. Marine Electrical Technology

1031

Chapter 26 26.6.3 System Parameters Study the normal operating parameters or operating ranges o f the system. More often, expectations don’t match with reality. 26.6.4

Test Equipment

Learn to use basic test equipment properly. It helps in expediting fault-finding. The following approach is advisable: 1.

Most defects turn out to be simple. Think first! Start with the basics unless you know the history o f the equipment. Do not apply complex, hypothetical theory that you do not understand.

2. Check system inputs. Don’t make assumptions. There maybe a situation when inadequate levels or values are available. 3.

Similarly check system outputs or outputs at various stages.

4.

In such situations, split the system into smaller sub-units and only check suspected blocks o r circuits. This helps in pin-pointing a defect.

5.

When a fault has been identified, more than h alf o f the work is done. Ascertain why it could have occurred and record the findings, corrective and preventive measures taken (preferably in the equipment’s respective history sheet); this will help in the future.

Various types o f instruments are available for testing electrical circuits. The ones you choose will depend on the type o f circuit and its components. A common test instrument which is invaluable to a troubleshooter is a multimeter. It is capable o f measuring voltage and resistance with some meters capable o f other measurements such as current, capacitance and frequency. A meter that is capable o f measuring current, voltage and resistance is also called an A VO meter (ampere volt ohm meter) You m ust be able to determine what type o f test instrument to use, when and where to use it, and how to safely take readings with it. The following procedure describes the method used to ensure that measuring equipment for critical and essential equipment is maintained, tested and calibrated within the manufacturer’s instructions and guidelines. The C hief Engineer / Superintendent are responsible for the calibration o f measuring equipment. 1032

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Maintenance and Troubleshooting 26.6.4.1 Procedures S

Pressure gauges, thermometers, voltmeters are for normal assessment purposes. They are replaced or repaired if they become defective.

S

Measuring instruments used by personnel are for checking purposes only. Instruments considered critical and essential for calibration are to be checked before use to confirm accuracy, and to be verified by calibration annually.

■ S Where there is a contract requirement, the measuring instrument used is to be regularly calibrated at the contract specified intervals. S

The Measuring instruments are to be calibrated by a specialist firm. The reference instrument used in the calibration shall itself be certified to be calibrated to nationally recognised standards or the basis used for calibration shall be documented.

■ / A record o f measuring equipment with dates when equipment was calibrated and certificates are to be retained by the Chief Engineer and recorded in the Maintenance reports. •/ I f measuring equipment is found to be defective or out o f the permissible limits during calibration, all equipment which has been tested with this instrument to be re-tested for accuracy. S

Calibration Certificates are to be kept for 10 years.

26.6.5

Understanding Howto Use Blue Prints and Diagrams

Some o f the key features to be determined from these are: *

How the circuit should operate

*

What kind o f features the circuit has

■

W hat voltages you should expect at various points on the circuit

■

Where components are physically located

■

How the components are actually wired together

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Chapter 26 26.6.6

The Alternative Approach to Troubleshooting

It is a proven process that is highly effective and reliable in helping to solve electrical problems. This approach differs from other troubleshooting procedures in that it is more o f a thinking process that is used to analyze a circuit’s behaviour and determine what is responsible for the faulty operation. This approach is general in nature allowing it to be used on any type o f electrical circuit. In fact, the principles covered in this approach can be applied to many other types o f problem solving scenarios; not ju st electrical circuits. This approach to troubleshooting comprises the following: 26.6.6.1

Preparefor the Task

Before you begin to troubleshoot any piece o f equipment, you must be familiar with safety rules and procedures for working on electrical equipment. These rules and procedures govern the methods you can use to troubleshoot electrical equipment (including your lockout / tag-out procedures, testing procedures etc.) and must be followed while troubleshooting. Next, you need to gather information regarding the equipment and the problem. Be sure you understand how the equipment is designed to operate. It is much easier to analyze faulty operation when you know the procedure. Operation or equipment manuals and drawings are great sources o f information and are helpful when available. I f there are equipment history records, you should review them to see i f there are any recurring problems. It would also help to have any documentation describing the problem i.e., a work order, failure report, or even your notes taken from a discussion with the user. 26.6.6.2

Observe

Most faults provide obvious clues to their cause. W ith careful observation and a little bit o f reasoning, most faults can be traced to the actual component with very little testing. When observing malfunctioning equipment, look for visual signs o f mechanical damage such as indications o f impact, chafed wires, loose components or parts lying in the bottom o f a cabinet. Look for signs o f overheating, especially on wiring, relay coils, and printed circuit boards. Don’t forget to use your other senses when inspecting equipment. The smell o f burnt insulation is something you w on’t miss. Listening to the sound o f the equipment operating may give you a clue to where the problem is located. Checking the temperature o f components can also help one to find problems but be careful while doing this, some components may be alive or hot enough to bum you. 1034

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Maintenance and Troubleshooting Pay particular attention to areas that were identified either by past history or by the person that reported the problem. A note o f caution here! Do not let these mislead you, past problems are just past problems, they are not necessarily the problem you are looking for now. Also, do not take reported problems as a fact; always check for yourself as far as possible. The person reporting the problem m ay not have described it properly or may have made his own incorrect and exaggerated assumptions. 26.6.6.3

D efin e th e P roblem A rea

It is at this stage that you apply logic and reasoning to your observations to determine the problem area o f the malfunctioning equipment. Often when equipment malfunctions, certain parts o f the equipment will work properly while others will not. The key is to use your observations to rule out parts o f the equipment or circuitry that are operating properly and not contributing to the cause o f the malfunction. You should continue to do this until you are left with only the part(s) that i f faulty, could cause the symptoms that the equipment is experiencing. To help you define the problem area you should have a schematic diagram o f the circuit in addition to your noted observations. Starting with the whole circuit as the problem area, take each noted observation and ask yourself, “What does this tell me about the circuit operation?” I f an observation indicates that a section o f the circuit appears to be operating properly, you can then eliminate it from the problem area. As you eliminate each part o f the circuit from the problem area, make sure to identify them on your schematic diagram (with a pencil). This will help you keep track o f all your information. 26.6.6.4

Id en tify P o ssib le C auses

Once the problem areas have been defined, it is necessary to identify all the possible causes o f the malfunction. This typically involves every component in the problem area(s). It is necessary to list (actually write down) every fault which could cause the problem no matter how remote the possibility o f it occurring. Use your initial observations to help you do this. During the next step you will eliminate those which are not likely to happen. 26.6.6.5

D eterm in e th e M o st P robable Cause

Once the list o f possible causes has been made, it is then necessary to prioritize each item as to the probability o f it being the cause o f the malfunction. The following are some rules of thumb when prioritizing possible causes. Although it could be possible for two components to fail at the same time, it is not very likely. Start by looking for one faulty component as the root cause. Marine Electrical Technology

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Chapter 26 The following list shows the order in which you should check components based on the probability o f them being defective: •

First look for components which bum out or have a tendency to wear out, i.e. mechanical switches, fuses, relay contacts, or light bulbs. (Remember, that in the case o f fuses, they bum out for a reason. You should find out why before replacing them.)

•

The next m ost likely causes o f failure are coils, motors, transformers and other devices with windings. These usually generate heat and, with time, can malfunction.

•

Connections should be your third choice, especially screw type or bolted type. Over time, these can loosen and cause a high resistance. In some cases this resistance will cause overheating and eventually will bum open. Connections on equipment that is subject to vibration are especially prone to coming loose.

•

Finally, you should look for is defective wiring. Pay particular attention to areas where the wire insulation could be damaged causing short circuits. Don’t rule out incorrect wiring, especially on a newpiece ofequipment

26.6.6.6

T est a n d R epair

Testing electrical equipment can be quite hazardous. The electrical energy contained in many circuits can be enough to injure or kill. Make sure you follow all safety precautions, rules and procedures while troubleshooting. Once you have determined the most probable cause, you must either prove it to be the problem or rule it out. This can sometimes be done by careful inspection however, in many cases the fault will be such that you cannot identify the problem component by observation and analysis alone. In these circumstances, test instruments can be used to help narrow the problem area and identify the problem component. There are many types o f test instruments used for troubleshooting. Some are specialized instruments designed to measure various behaviours o f specific equipment, while others like multimeters are more general in nature and can be used on m ost electrical equipment. A typical multimeter can measure AC and DC Voltages, Resistance, and Current. A very important rule when taking meter readings is to predict what the meter will read before taking the reading. Use the circuit diagram to determine what the meter will read if the circuit is operating normally. I f the reading is anything other than your predicted value, you know that this part o f the circuit is being affected by the fault. 10 36

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Maintenance and Troubleshooting Depending on the circuit and type o f fault, the problem area as defined by your observations, can include a large area o f the circuit creating a very large list o f possible and probable causes. Under such circumstances, you could use a “divide and eliminate” testing approach to eliminate parts o f the circuit from the problem area. The results o f each test provide information to help you minimise the problem area until the defective component is identified. Once you have determined the cause o f the faulty operation o f the circuit you can proceed to replace the defective component. Be sure the circuit is locked out and you follow all safety procedures before disconnecting the component or any wires. After replacing the component, you must test all features o f the circuit to be sure you have replaced the proper component and that there are no other faults in the circuit. 2 6 .6 .6 .7

F ollow -up

Although this is not an official step o f the troubleshooting process it nevertheless should be done once the equipment has been repaired and put back in service. You should try to determine the reason for the malfunction. •

Did the component fail due to age?

•

Did the environment the equipment operates in cause excessive corrosion?

•

Are there wear points that caused the wiring to short out?

•

Did it fail due to improper use?

•

Is there a design flaw that causes the same component to fail repeatedly?

Through this process further failures can be minimized. Many organizations have their own follow-up documentation and processes. Make sure you check your organization’s procedures. 26.6 .7

H igh V oltage (H V) E qu ipm ent T esting

First o f all, two people should always be present while working on any HV circuit or equipment. Essentially, one person should always be in a position to take necessary safety measures in case o f any contingency. For all HV equipments, the main criterion is to ascertain the status o f its insulation resistance. The insulation resistance can be measured and recorded between phases and between each phase and the earth. Marine Electrical Technology

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Chapter 26 After comparing historical values, we can predict the deterioration o f the equipment and the remaining useful life o f the same. According to regulatory requirements mentioned in article 26.3.2, the cold insulation resistance is measured with DC voltage produced by a high voltage megger o f at least IkV. A 500-volt megger cannot be utilised as it will not stress the insulation to the desired extent. However at 3.3kV and 6.6kV voltage levels, the recommended voltage for checking hot insulation resistance is 2.5kV and 5kV (i.e., 75% to 80%). The IR test is done for 1 minute. The minimum acceptable IR value for these systems is given by a simple thumb rule as (kV+1) M H, where kV is the voltage rating o f the equipment. For instance, i f the equipment is rated at 3.3kv then its minimum acceptable IR value is 4.3 M Q. Another measure o f healthiness o f the equipment is polarisation index (P.I.). The P.I. value is the ratio o f IR value for 10 min to the IR value for 1 min. The condition o f the insulation is dependent on temperature, humidity, surface condition and operating voltage. 26.6.7.1

L ive L in e T est

Before earthing high-voltage systems for complete safety during an IR test, it must be switched off, properly isolated and confirmed dead by an approved live line tester. The tester must itself be proven before and after such a test. This can be done by connecting the live line tester to a known high-voltage source. A live line tester can be either an external batteryoperated type or one with and internal self-test facility. 26.6.7.2

E arth in g D ow n

We are aware that high-voltage equipment m ust be earthed to the ship’s hull before commencing any work on a high-voltage switchboard and such equipment. This is considered essential during the maintenance o f high-voltage systems as the worker can be assured that the equipment and he him self will not experience any accidentally applied voltage because the earth connection bonds the circuit to the earth i.e. zero potential. There are two methods namely “Circuit Earthing” and “Busbar Earthing” 26.6.7.2.1

C ircu it E arth in g

As an example in the case o f a breaker, after the circuit is disconnected from the live supply, the outgoing feeder cable is disconnected by a manually-operated switch to connect the three phases to the earth. The permissive key is then released and the circuit breaker can be withdrawn to the “Test” position. It cannot be inserted back to the “Service” position until the earth has been removed and the key is normalised.

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Maintenance and Troubleshooting 26.6.7.3

B u sbar E arthing

At the time o f doing work on a section o f the HV busbar, it m ust be isolated from all possible sources o f electrical power. The possible sources can be generator incomers, bus tie breakers, transformer breakers etc. The transformer breakers can back feed power to the section of the busbar. Earthing down can be done at the bus section. In some cases, a special earthing circuit breaker can be temporarily used for earthing down a section o f the busbar as mentioned in the previous paragraph. For extra safety, additional earthing can be connected local to the work area, e.g. the terminals o f the HV motor, with portable earthing straps. While earthing down, always remember to make connections on earth (hull) side first and then to the equipment side. While removing the same, first remove the equipment-side connection and then the earth-side connections; this prevents the risk o f an electric shock. 26.6.7.4

P rocedu re to C arry-ou t an Insulation R esistan ce Test

1. M ake the earth connection for the circuit as mentioned above. 2. Connect the IR tester to the circuit. 3. Remove the earth connection for the circuit as mentioned above. 4. Apply the IR tester and record its readings. 5. Reconnect earth connection for the circuit. 6. Disconnect the IR tester.

26.7

Maintenance of Specific Equipment

26.7.1

G enerators

Regular inspection and correct maintenance o f both a.c and d.c. generators and their associated control gear is essential to prevent failure and inefficient operation. However, planned maintenance o f the emergency generator is to be carried out only while in port. The following guidelines will help: 'T Before you begin any maintenance, always ensure that the generator prime mover is shut down and that care is taken to prevent re-starting. A ‘Do Not Operate’ tally / board - slung / pasted conspicuously would be an added measure. Also ensure that the generator’s supply breaker is o ff (i.e., open), auto-start circuits are disabled and generator electric heaters are isolated. Marine Electrical Technology

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Chapter 26 ■ S All terminal boxes and wiring to the generator should be inspected for damage or frayed insulation and tightness o f terminal connections. Particularly check for signs o f a cable’s insulation deteriorating within terminal boxes. This may be due to ingress of oil and water. S

Check that the cooling air-ducting, air intake and exhaust openings and are not blocked and are free o f dirt and dust.

■ S Inspect and clean the generator rotor and stator windings by removing dust with a dry, lint-free cloth. S

Low-pressure, dry compressed air may be used to dislodge heavier dirt but be careful not to drive the dirt deeper into the windings (the air flow should be almost parallel to the winding).

S

An industrial-type vacuum cleaner is very effective for removing dirt from the windings. Use a rubber or plastic coated nozzle on the vacuum cleaner tube to prevent abrasive damage to the fragile winding insulation.

S

Alternator windings should be checked for signs o f oil or water contamination and any damage. Oil on the surface o f a winding’s insulation will reduce the insulation resistance and shorten its life. The oily deposit can be removed by washing the windings with special de-greasing liquids that are environmentally friendly / those that are not banned.

S

Minor abrasions to winding insulation can be repaired after cleaning, by the application o f a suitable air-drying varnish.

S

Rotor slip rings m ust be checked for even wear. The carbon brushes must be free in their holders or spring pressure will be too little, which could cause sparking.

^

Correct brush pressure can be checked by using a pull-type spring balance and compared with the manufacturer’s instructions. A ‘pull’ o f around 2-3 lbs. or 1-1.5 kg is usual (check the manufacturers instructions for adequate figures).

v'' The length o f the brushes should be at least 2cm. I f the brushes become too short, the reduced spring pressure will cause sparking at the slip rings / commutator. In most types, brushes have an engraved horizontal line closer to the pigtail-end. This indicates its minimum length. Replace brushes with the correct type and ‘bed’ them to the curvature o f the slip ring / commutator.

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Maintenance and Troubleshooting S

To bed the brushes, placing a thin strip o f fine, abrasive glass paper (not emery paper) over the slip ring / commutator with its cutting surface facing upwards and under the carbon brushes can achieve this. Pull the glass paper around the slip ring / commutator until the brush surface has the same contour as the slip ring / commutator. The last few passes o f the glass paper should be made in the normal rotor direction.

V Remove all traces o f carbon dust with a vacuum cleaner. S

With the help o f a feeler gauge, check to ensure that the air gap between the rotor and stator is satisfactory (according to the manual). This will determine the state o f the bearings. Record these values.

S

Generator excitation components viz. transformers, AVR components, rotating diodes, etc, must be kept free o f dirt, oil and dampness. Special contact grease is usually applied between the rotating diode connections to prevent electrolytic action occurring between dissimilar metals. Check such contacts for tightness but do not disturb them unnecessarily.

v' Remember to disconnect / short out any electronic circuit components and cards which may be damaged by a 500V insulation test. Consult the wiring diagrams and manufacturer’s instruction before testing these components separately. v' Measure the insulation o f the stator and rotor windings with respect to the earth and between stator phases i f the neutral point is available for disconnection at the terminal box. S

Record the insulation resistance values and note the prevailing temperature and humidity.

V Compare these with the previous test results. As mentioned in article 5.8, a minimum acceptable insulation resistance value is to be 1MQ. S

It is the historical trend o f the machine’s insulation resistance values that will give a better picture o f the insulation’s condition. Generators with very low insulation resistance values (< 0.5MQ) should be given a thorough cleaning and then dried out.

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Chapter 26 S

I f the insulation resistance has recovered to a reasonable value, which has become steady during the drying period, its windings should be covered with high-quality air­ drying insulating varnish.

^

Should the insulation resistance value remain low during a dry-out, the machine insulation needs to be completely re-impregnated or re-wound (a shore job).

S

If an alternator is to be idle for a considerable period, it should be ensured that the windings are suitably heated periodically in order to prevent internal condensation. In the case o f a rotating armature alternator, the brushes should be lifted off the slip rings to prevent pitting o f the metal by electrolytic action.

26.7.1.1

P recau tion ary M easures to b e Taken A fte r R epairs

After maintenance and repairs, no-load running checks should precede synchronising and loading. 1. Run in the engine (prime mover) without any load, otherwise known as “idling” 2.

Stop the engine after about 30 minutes, let it cool down and check the bottom-end bolts, bearings and other systems / auxiliaries.

3.

Restart the engine and load it partially - about 50% o f the generator’s full load for about 1 hour; continue to check all parameters and log (record) them adequately. Whilst the machine is on load, particularly check for excessive temperature rise and stability.

4.

Then slowly increase the load to about 80% o f the generator’s load; if this is satisfactory, a full-load trial may be carried out.

5. During the period which the engine is running, check all trips and cutouts; temperature and pressure simulators may be used to simulate abnormal conditions as the generator / engine may not reach these limits if all is well with the systems. If the machine runs “hot” then almost certainly internal condensation on its insulation would result when the machine cools down. As with all electrical equipment, dirt, overheating and dampness are detrimental! 6.

Synchronising trials may be carried out after it is ascertained that all parameters are stable and have been recorded.

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Maintenance and Troubleshooting 2 6 . 7.1.2

E xam ple o f a M on th ly S a fety C heck o fD iesel-G en era to r S ets Month o f____________ Particulars

D /G - 1

DIG-2

D /G - 3

Over-speed trip

Satisfactory (810 r.p.m .)

Satisfactory (805 r.p.m .)

Satisfactory (810 r.p.m.)

Over-load trip

Satisfactory

Satisfactory

Satisfactory

Lubricating oil trip

Satisfactory

Satisfactory

Satisfactory

Lubricating oil high temperature

Satisfactory

Satisfactory

Satisfactory

65° C

65° C

65° C

Exhaust high temperature

Satisfactory

Satisfactory

Satisfactory

580° C

580° C

580° C

Designed maximum power

550 kW

550 kW

550 kW

Maximum sea load

440 kW

440 kW

440 kW

Load sharing

Satisfactory

Satisfactory

Satisfactory

Governor

Satisfactory

Satisfactory

Satisfactory

Average daily lubricating oil consumption

15 litres

15 litres

15 litres

Date lubricating oil analysed

(date)

(date)

(date) i

(date)

(date)

(date)

(date)

(date)

(date)

Lubricating oil purified

(date)

(date)

(date)

A ll trips, etc., tried out

(date)

(date)

(date)

Date next due ................... .......... Lubricating oil changed

26.7.1.3

A uxiliary Engine: (Engine Make)

O vercom ing W inding C ontam ination in B rush less A ltern ators

Inspect the stationary and rotating winding periodically for cleanliness. The chief engineer or his appointed representative will supervise internal inspection o f the ship service generator. Never inspect internal generator components while the prime mover is operating or the generator is connected to the switchboard bus.

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Chapter 26 Always secure the prime mover fully and ensure that other power sources such as the emergency generator or shore power, cannot erroneously feed the generator being serviced. Textbook maintenance practices mention removal o f dirt by vacuuming and removal of grease and oil by wiping with lint-free rags. These methods rarely serve the purpose intended. Contamination prevention is the key. Inspect the generator prime mover for gasket and seal leaks. Check adjacent piping and deck plates for liquids and particles. Once the windings become contaminated, there is no thorough and safe method to clean the generator windings on board the vessel. The only effective remedy is the removal o f the generator, its complete disassembly, chemical cleaning, and baking. W hen contamination is found, use the megger to check the insulation values. Always disconnect the rotating rectifier, voltage regulator, and any other components that house semiconductors and sensitive components. Compare readings with the appropriate technical manual, with other known good generator readings, or against historical documentation. As per I.E. Rules (Rule 48), the test voltage for equipment o f different voltage ratings must be as follows: y

Low-voltage and Medium-voltage Equipment - 500 Volts for 1 minute

y

High-voltage up to 33 Kilo Volts - 1000 Volts for 1 minute

Remember that when a megger is being used to test the insulation o f

mi

alternator, the

resistance value o f a dry, clean winding will continue to rise as test potential is maintained, becoming fairly steady as the dielectric-absorption effect o f insulation stabilizes. 26.7.2

M ain C ircu it B reakers

Circuit breakers require careful inspection and cleaning at least once a year. Before working on circuit breakers, check the applicable technical manual carefully. Before working on shipboard circuit breakers, obtain the approval o f the Electrical Officer (and the Chief Engineer if the ship regulations require you to do so). Be certain to remove all power to the circuit breaker before working on it. Tag the switch that removes the power to the circuit breaker to ensure that power is not accidentally applied while working on i t

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Maintenance and Troubleshooting Once approval has been obtained, the incoming power removed, the switch tagged, and the technical manual checked, you may begin to check the circuit breaker. Manually operate the circuit breaker several times to be sure that the operating mechanism works smoothly. Inspect the contacts for pitting caused by arcing or corrosion. Under normal circumstances, replace the damaged or worn out circuit breaker as an assembly. Follow the contact-servicing procedure mentioned in this chapter, if the circuit breaker m ust be re-used. Generator circuit breakers and other large circuit breakers (600-6000A) on board a ship are usually o f the air break type. This means that the circuit-breaker’s contacts separate in air (Ashore, comparable circuit-breakers are often immersed in oil (OCBs) and larger circuitbreakers for high-voltage operation are either air-blast, or have a special gas filling or a vacuum-break). ACBs are mounted on special rails in the main switchboard cubicle and must be racked out and isolated from the bus bars for maintenance and testing. The ACB and its slide rails are usually mounted in a special cassette bolted into the switchboard cubicle and electrically connected to the bus bars. S

I f repair work demands that the ACB be completely removed from its cassette, then usually a special hoist or fork-lift is required for large, heavy-duty breakers.

S

In m ost cases the action o f withdrawing the breaker causes a safety shutter to cover the live bus-bar contacts.

S

Mechanical linkages in the circuit breaker are quite complex and should not be interfered-with except for maintenance and lubrication as specified by the manufacturer.

•/ The arc-chutes or arc-splitter boxes confine and control the inevitable arc to accelerate extinction. These must be removed and inspected for broken parts and erosion o f the steel splitter-plates. ■ S The main fixed and moving contacts are made o f copper (something o f a special arc resistant alloy or silver-topped) and m ost often silver-coated. v' Main contacts should not be scraped or filed. I f the main contacts suffer severe burning, they probably require re-alignment as specified by the manufacturer.

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Chapter 26 V Alignment o f contacts is checked by removing the arc chutes on ACBs to expose the contact assembly. On some high-voltage designs, the chute tilts forward to provide access. V Arcing contacts normally suffer burning and may be dressed by a smooth file as recommended by the manufacturer. x

Carborundum and emery should not be used - the hard particles can embed themselves in the soft copper contacts and can cause difficulty in future.

Various types of closing mechanisms may befitted: (a) Independent Manual Spring The spring charge is directly applied by manually depressing the closing handle. The last few centimetres o f handle movement releases the spring to close the breaker. Closing speed is independent o f the operator. (b) Motor-wound Stored-charge Spring A motor/gearbox unit charges closing springs. Spring re-charging is automatic following closure o f the breaker. Breaker closure is initiated by a push button. This may be a direct mechanical release o f the charged spring or it may initiate an electrical release via a solenoid latch. (c) Hand-woundStored-charge Spring This is simitar to (b) but is a manually charged closing spring. (d) Solenoid The breaker is closed by a d.c. solenoid energised from the generator or bus bars via a transformer-rectifier unit, contactor, push-button and sometimes a timing relay. CAUTION! C ircu it breakers sto re en ergy in th eir sprin gs in: (a)

S tore-ch arge m echanism s in th e closin g springs.

(b)

C on tact sprin gs a n d k ic k -o ff springs.

&

E xtrem e care m u st be ex ercised when handling circu it breakers w ith th e closin g sp rin g charged, o r w hen th e c ircu it breaker is m ade.

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Maintenance and Troubleshooting Iso la te d circu it breakers when b a ck ed -o u t ’f o r m aintenance sh o u ld b e le ft w ith th e clo sin g sp rin g d isch arged a n d in th e ‘o p en ’ (O ff) position . C ircu it breakers a re h eld in th e ‘clo sed ’ (in p o sitio n b y a m ech an ical latch ). The breaker is trip p ed b y releasin g th is latch allo w in g th e k ic k -o ff sp rin g a n d contact p ressu re to fo rc e th e con tacts open.

Tripping can be initiated by: (a)

Manual means (a push button with a mechanical linkage trips the latch).

(b)

A n under voltage trip coil (trips when de-energised). In some breakers there is a mechanical means o f by-passing this protection. This permits the operation o f the breaker even i f the breaker is withdrawn. However care must be taken to reset this after maintenance or repair.

(c)

An over-current / short-circuit trip device (trips when energised).

(d)

A solenoid trip coil (when energised by a remote switch or relay - such as an electronic over current relay).

26.7.2.1

In terlocks

Mechanical interlocks are fitted to ACBs to prevent racking out if they are still ‘made’. Care must be taken not to exert undue force i f the breaker does not move - otherwise damage may be caused to the interlocks and other mechanical parts. Dangers o f explosion and fire may also result from such actions. Electrical interlock switches are connected to circuitbreaker control circuits to prevent any incorrect sequence o f operations, e.g. when shore supply is connected through its dedicated breaker to a switchboard. The ship’s generator breakers are usually interlocked (Off) to prevent parallel running o f a ship’s generator and the shore supply. 26.7.2.2

R e-in sta llin g

Before installing any item that has been reconditioned, ensure that the C hief Engineer or the Electrical Officer has made a final inspection o f the component. When you have finished working on the circuit breaker, restore power and remove the tag from the switch that applies power to the circuit.

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Chapter 26 26.7.2.3

F u sed Isolators

Maintenance on fused isolators consists o f periodically checking the operating mechanism. S

Contacts must be inspected for damage and lightly greased with an electrical lubricant.

S

The interlock mechanism (if fitted) should also be examined for correct, safe operation.

26.7.2.4

M ain tenance o f a Vacuum C ircu it B reaker

A vacuum interrupter is sealed for life and does not require any replacement o f contacts for several thousands o f operations and about 50 operations on rated short circuits - depending upon its design features. The mechanism needs periodic lubrication as recommended by the manufacturer. The other parts need general cleaning and inspection. 2 6 .7 .2 .5

M ain tenance o f th e S F 6 C ircu it B reaker

During periodic maintenance, the gas sample from the SF6 circuit breaker is collected and tested for moisture and other impurities (as per IEC 376). The gas can be re-used after regeneration. At a suitable interval one interrupter per pole should be examined to establish the rate o f burning and erosion o f the contacts. Operating linkages should not be disturbed. 2 6 .7 .3

T ransform ers

Transformers are static items o f equipment, which are usually very reliable and troublefree. However, like all electrical equipment, transformers must be subjected to the usual maintenance checks: S

At regular specified intervals, transformers must be disconnected*, covers removed and all accumulated dust and deposits removed by a vacuum cleaner and suitable brushes. W indings must be inspected for any signs o f damage or over-heating. * In order to disconnect a transformerfollow these basic steps: a) Trip circuit breakers and take action toprevent their accidental resetting. b) Remove all thefusesfrom the power source. c) Disconnect the transformerfrom all power sources in the primary and secondary circuitry.

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Marine Electrical Technology

Maintenance and Troubleshooting d) Momentarily short-out transformer secondaries before connecting and disconnecting equipment (in order to kill any charge that would otherwise be retained in the supposedly 'dead' transformer), S

Winding continuity (low resistance) values are measured, noted and compared with each other for balance. Any differences in continuity readings will indicate winding faults such as short-circuits between turns.

S

The insulation resistance o f all windings must be measured both with respect to the earth and to the other phase windings.

S

The cause o f any low insulation resistance reading must be investigated and rectified. Cable connections must be checked for tightness.

S

Covers must be securely replaced and the transformers re-commissioned*. *

S 26.7.3.1

In order to prevent potentially high voltage and current levels, always connect a load to the secondary side o f the transformer before energizing the primary. The voltmeter is an excellent high-resistance load when connected with alligator clips.

All readings and findings should then be recorded for future reference. A d d itio n a l A ction s f o r W elding T ransform ers

1. Clean and tighten both the terminals (+ve & -v e) o f the machine 2.

Check cables and lugs for any wear and tear, repair and replace them if required.

3. Check the windings for continuity first with a multimeter; there will be some Ohmic value registered on each o f the windings. I f the multimeter indicates “0”, it may be implied that the winding is short-circuited; an infinite value could be the result o f an open circuit in the respective windings. 4.

Be careful when using a megger if need be; modem welding machines have printed circuit boards and components to control the welding currents.

5.

Switch on the system and log the open-circuit voltage. The voltage must be within the rated values (as stated by the manufacturer).

2 6 .7 .4 26.7.4.1

S

S ta rters an d M otor C on trol G ear E n closure

Check for accumulation o f dirt and rust. Any corroded parts m ust be cleaned and painted.

S Examine the starter fixing bolts and its earth-bonding connections - particularly _______ where high vibration is prevalent - e.g., steering flat, etc.__________________________ M arin e E lectrical Tech n olo gy

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Chapter 26 26.7.4.2

'f

C ontactors a n d R elays

Check for any signs o f over-heating mid loose connections.

'C Remove any dust and grease from insulating components to prevent leakage and subsequent breakdown by surface tracking. S

Ensure that the armatures o f contactors move freely.

■ S Remove any dust or rust from magnet faces which may prevent correct closing. 26.7.4.3

C ontacts

In high vibration areas, fixing bolts, earth-bonding connections and all terminal connections should be checked. Attention should be paid to all contacts likely to deteriorate due to wear, burning, and inadequate pressure, the formation o f a high-resistance film or becoming welded together. Faulty contacts are often indicated by over-heating when loaded. Different contact materials may need different treatment. Copper is widely used but is liable to develop a high resistance film, and copper contacts may become welded together if the contact pressure is low and the contents have to carry a high current. Copper is commonly used for contacts which have a wiping action when closing and opening, this action removing the film. Copper contacts are used on knife switches, laminated (brush) contacts o f regulators and other controllers, drum contacts, etc. Carbon and metalised carbon contacts are unsuitable for carrying high currents for long periods but, as they do not weld together, they are used for arcing contacts on some control gear. Pure silver and silver-alloy contacts tend to blacken whilst in service but the oxide film has a low resistance. Copper-tungsten (sintered compound), grey in colour, is used in the preparation o f contact facing. This material has a high surface resistance which resists heavy arcing and does not weld. Silver-tungsten (sintered) has similar properties to copper tungsten but has a lower contact resistance and is less liable to overheat whilst on continuous load. S

Examine them for excessive pitting and roughness due to burning.

S

Copper contacts may be smoothened with a fine file or fine glass paper; copper oxide, that acts as a resistance can be removed with the help o f glass paper.

S

The maintenance carried out on silver-plated contacts is very little but if cleaning is necessary, metal polish m ay be used.

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Maintenance and Troubleshooting y

A thin smear o f electric contact lubrication helps to prolong the life o f all contacts. Excessive quantities can lead to burning and pitting.

y Always replace both fixed and moving contacts in pairs. y Check and adjust spring-pressure if necessary. As far as possible, adjacent contacts must also have die same pressure. Check power and control (auxiliary) contacts for over-heating; lubricate moving contacts on fuse holders. N ote:

y

y

Carbon contacts should receive the same attention as copper contacts except that they do not need lubrication. Silver, Silver-alloy and copper-tungsten contacts do not require cleaning. As there is no need to remove surface-filmfrom pure silver contacts they may be usedfor light butt-contacts. Where some contacts have the appearance of pitting on both faces, this is sometimes referred to as being 'burnt in'. Some manufacturers recommend that the contacts, unless there is loss ofmaterial, are not dressed as this may destroy the contact area.

26.7.4.4

C onnections

•/ Check all power and control connecting leads for overheating and tightness. Brittle and fraying flexible leads must be replaced. 26.7.4.5

O ver C u rren t R elays

y Clean them, check the rating and test them by injecting calibrated currents to ensure tripping at optimum values 26.7.4.6

y

C on trol C ircuitry

Observe the sequence o f operation during starting, running and stopping.

y Check for excessive contact sparking. y Always check the operation o f emergency stop and auto start functions. 2 6 .7 .5

M otors

Periodic checks o f the following are to be carried out: 1) Load Current 2) Last Insulation Reading Marine Electrical Technology

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Chapter 26 3) CouplingAlignment 4) Bearing Condition (temperature rise, vibration or noise) 5) Starter and cable condition The following motors are considered important onboard a ship and need special attention; this list is not comprehensive and each ship will have its own list o f motors. Ventilation Fans for the Paint Room, Provision Room, Galley (Inlet and Exhaust), Accommodation, Deck, Engine Room, Boiler Room, Pump Room & Pumps for the following systems / equipment: M ain Sea W ater Cooling

Refrigeration Plant

Main Lube Oil

Air Conditioning Seawater

Fuel Oil Booster

Fresh Water Generator Ejection / Condensation

M ain Jacket Cooling

Deck Crane

Fuel Oil Valve Nozzle Cooling Pump

Oily Water Separator

Lube Oil Transfer

Bilges Discharge

Lube Oil Pump For Reduction Gear

Lathe, Drilling Machine, Grinder, etc.

Controllable Pitch Propeller

M ain Air Compressors

Turning Gear

Topping Up Compressor

Diesel Oil Separator

Boiler Forced D raft Fan

Lube Oil Separator No. 1 andN o.2

Boiler Feed water

General Service

Boiler Fuel Oil Booster

Fire M ain System

Boiler Oil Ignition

Ballasting

Steering Gear

Diesel Transfer

Main Hydraulic System

Fresh W ater Hydrophore

Hydraulic Motor For Anchor Winch

Sea W ater Hydrophore Forecastle Bilge The maintenance requirements for cage-rotor induction motors are very simple. This results in trouble-free service during its life span: ^

Keep insulation resistances high and contact resistances low.

S

Lubricate them correctly and maintain an even air gap.

S

Ensure proper internal and external hygiene.

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Marine Electrical Technology

Maintenance and Troubleshooting The m ost common cause o f induction motor failure is low stator insulation due to dampness. Open-ventilated motors are at a high risk, especially when they are not used for long periods. In such cases, anti-condensation heaters m ust be regularly checked and switched on to keep the motor dry. A regular cleaning routine is required to remove harmful deposits o f dust, dirt, grease, oil, salt and moisture from both inside and outside the motor. The external surfaces for continuously-run totally enclosed motors must be cleaned regularly. The heat generated in these motors is dissipated from the external surface. Therefore a thick layer o f dust forms a blanket and reduces the radiation o f h e a t Internal dust and dirt in open-ventilated motors must be regularly extracted. Their ventilation ducts must be cleaned too. This can be achieved by blowing them with absolutely dry air at a pressure o f not more than 1.75 bars. I f the pressure is any higher, then the dust will be forced deeper into the winding. The preferred alternative is to use a vacuum cleaner instead. The frequency o f cleaning will depend upon the area o f application. The next important aspect o f maintenance is to measure the insulation resistance regularly. In some cases, daily checks m ust be carried out and logged. Corrective action for low insulation m ust be taken at the earliest. Bearings m ust also be maintained and replaced at regular intervals as neglect in this case leads to most failures. In case a motor has been flooded with seawater, the following steps must be taken: S

The salt contamination m ust be removed by thoroughly washing it down with (preferably warm) clean, fresh water or i f possible, distilled water.

v' De-greasants must be used i f it is found that there was ingress o f oil, etc, S

Spirits or alcohol may be used to clean contacts and other sensitive parts.

S

Dry the motor with dry air and then switch on its heaters (if fitted) o r use powerful lamps i f possible, simultaneously keeping its inspection covers open to permit moisture to escape.

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Chapter 26 S

Alternately, the windings can be heated by current-injection from a welding set or from a special injection transformer. Be sure to inject currents well below the rated current o f the motor’s winding.

S

With the windings clean and dry and if the insulation remains high for a few hours at least, a fresh coat o f good-quality air-drying varnish may be applied to the windings.

S

2 6 .7 .6

The motor m ust be started on less than full load i f possible and its currents m ust be monitored for a few hours to confirm its satisfactory operation. M iscellaneous C on trol G ear

All moving contacts used in control gear have what is known as ‘wipe’ or follow-through; that is to say, i f the fixed part were to be removed, the moving part would follow on. In addition, particularly in the case o f contactors, a rolling or sliding action takes place for which there are two motives: firefly to provide a cleaning action to remove any oxide which may have formed, and thus ensure a good metal-to-metal contact and secondly to ensure that the point at which the contacts make and break and where roughening due to arcing occurs, is away from the operating position. Contacts that make and break infrequently may never need more than an occasional cleaning but those that operate frequently, such as lifts, winches, windlasses, cranes and capstans, may become so worn that not only would the contact pressure be insufficient but the wiping action also be lost, causing reduction o f contact pressure, and overheating. They must be examined and renewed periodically. Provided that the contact pressure is ample, a rough contact surface may have a lower contact resistance than one in a smooth, new condition, so a file should be used sparingly and only on badly burnt and pitted contacts. A thin film o f oil smeared-on with a lint-free cloth helps to reduce mechanical wear, but excess o f oil or grease encourages burning and pitting. Judicious greasing can considerably reduce mechanical wear on drum-type contacts. Silver-faced contacts and carbon contacts should not be lubricated. Copper contacts that have been closed for long periods tend to develop an oxide film, which may cause overheating. W hen opportunity permits, these contacts should be opened and operated several times to clean the contact surfaces. Magnet faces should be kept clean and free from grease or oil, and rust should be removed carefully w ith fine emery, making sure that metal particles are carefully removed and never deposited on contacts or insulation. 1054

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Maintenance and Troubleshooting See that the moving parts are free and without undue wear at the pivots, and that magnets are bedding properly. The faces o f a.c. magnets on no account are to be filed. 2 6 .7 .7

D eck C ranes

•S Inspect electrical contacts; service them as required S

Inspect slip rings / commutator and carbon brushes; service them as required

S

Check all limits

S

Check all controllers and brakes; service them as required

S

Overhaul the oil cooler blower motor

S

Clean the oil cooler

S

Send oil sample for analysis

S

Renew oil as required

26.7 .8

B atteries in G eneral

Chapter 21 has already established the fact that the heart o f any marine power system is the battery. It has a primary role as a power storage device, and a secondary one as a buffer, absorbing power surges and disturbances arising during charging and discharging. The battery remains the m ost misunderstood o f all electrical equipment. In a majority o f installations it is improperly selected and rated, with a resulting decrease in seaworthiness o f the vessel. For a system to function correctly, the power system must be able to provide power reliably and in an uninterrupted manner. The following are necessary: Battery securing arrangements must be proper; tighten holding-down brackets, if the battery is loose. The cell tops must be clean and dry. Battery connections must be checked and cleaned if needed. Check the tightness o f terminal nuts and apply a smear o f petroleum jelly to such connections in order to prevent corrosion. Be very careful when handling the battery electrolyte; use protective rubber gloves and goggles. Check electrolyte densities with a hydrometer, and keep a record o f the result.

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Chapter 26 ** Battery installations for both battery types (Ni-cad and Lead acid) are similar wherein the battery room should be well ventilated, clean and dry. a r Insulated spanners should be available for use on cell connections in order to prevent accidental short-circuiting o f the battery’s terminals. A short-circuit across the terminals o f ju st one cell o f a battery can cause a blinding flash with the probability of the cell being seriously damaged. This will also lead to an explosion in confined areas where H2 concentration levels are probably high (3% in enclosed spaces and 4% in open spaces). Both types generate hydrogen gas during charging, so smoking and naked flames must be prohibited in the vicinity o f the batteries. i3r Steelwork and decks adjacent to lead acid batteries should be covered with acidresisting paint and alkali resisting paint used near Ni-cad cells. ar Acid cells m ust never be placed near alkaline ones otherwise rapid electrolytic corrosion to metalwork and damage to both batteries is inevitable. For similar reasons, never use lead acid battery maintenance gear (e.g. hydrometer, topping up bottles, etc.) on an alkaline installation or vice-versa. Equipment with known battery problems should be checked more frequently until problem is solved. 26.7.8.1

L ead-acid C ells

The following important point should be kept in m ind to keep cells in good condition: ^

Discharging should not be prolonged once the minimum value o f voltage for a particular rate o f discharge is reached. Ensure that batteries are charged as soon as possible after discharging them. Allowing them to ‘sit’ will result in sulphation and permanent damage, as soon as 24 hours afterwards.

,ar The level o f the electrolyte should always be 10 to 15 mm above the top o f the plates, which must not be left exposed to air. Adding distilled water occasionally should make up for the loss due to evaporation o f water from the electrolyte, particularly in warm tropical climates. tar Since acid does not vapourise, none should be added.

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Marine Electrical Technology

Maintenance and Troubleshooting Charging often allows splatter around filling caps; sedimentation and dust build-up, together with moisture contributes to tracking between terminals. This results in discharge and “flattening”, which is very common. The battery terminals and metal supports should be cleaned down to bare metal, checked, tightened and covered with Vaseline or petroleum jelly. The acid and corrosion / grime on the battery top should be removed thoroughly with a cloth moistened with baking soda or ammonia water. The vent in the filling plug should be kept clean and open in order to prevent gases formed within, from buildingup a high pressure. 26.7.8.2

T roubleshooting B attery-P ow ered S ystem s

Troubleshooting battery-powered systems can become complex. Unlike many mechanical systems, numerous electrical problems can be identified with a good initial inspection. Burnt electrical components have a distinctive “electrical” smell, and charred wires and connections are readily identified. Once these areas are identified and corrected, further tests are needed to determine the reason for this condition. Check all connections, from the battery throughout the entire electrical system, regularly. All connections must be clean and tight. Vessels operating in an atmosphere that is normally “salty” are especially prone to oxidation. All units are prone to vibration, which together with oxidation account for a large percentage of electrical malfunctions. Any increase in resistance in the circuit reduces the current throughout the entire circuit. When current is reduced, the magnetic properties o f the circuit are reduced. Current as we know, is the quantity o f electrons (with their magnetic field) passing a point in the circuit in a period o f time. With fewer electrons, there is a reduction in the magnetic properties available to the circuit components. With a reduction o f electrons (and their magnetic influence), motors, solenoids, and other electrical components will function irregularly. Some o f the more obvious resistance increases are due to improper or dirty connections and corroded cable ends. To understand how a small amount o f additional resistance can reduce the capability of the electrical system, suppose that a resistance o f 1 ohm exists in a poorly made connection in a diesel engine starting system. The 24-volt battery-starting system normally provides 240 amps to a starting system with a resistance o f 0.1 f l The 24 volts must now supply a starting system with resistance o f 1.1 £2.

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Chapter 26 The additional 1 Cl resistance will consume power (power = amps x volts). The current will be reduced because the total resistance (Rt) is increased. The total amperage for the system is reduced as shown in the following equation: It = E* /Rt = 24 Volts / 1. i n = 21.8 amps. The 240 amps required to turn the starter motor has been reduced to ju st 21.8 amps. Thus the starter m otor will stall and may not even turn and engine cranking is drastically affected. 26.7.9 Explosion ProofEquipment Maintenance o f such equipment must not in anyway cause its operational safety to be de-graded (lower than its original certified state). This means that the maintenance must be carried out by a competent person. Any slip-shod method such as ‘lashing-up, refitting with wrongly rated components, not securing all bolts properly, etc, are absolutely forbidden. The inspection o f Ex-d (flame-proof) enclosures, switches, junction boxes, etc, requires meticulous care. The following is an example o f a guide to the inspection and maintenance points as applied to a flame-proof luminaire: 26.7.9.1

Corrosion

This will reduce the enclosure strength. In order to ascertain the extent o f corrosion, remove dirt, loose paint and surface corrosion with a wire brush. I f only the paintwork is deteriorating, then the enclosure should be painted to prevent further corrosion. 26.7.9.2 Bolts Ensure that there are no missing bolts. This is particularly important on flameproof luminaires because a missing bolt will invalidate not only the certification but could lead to a disaster Replacement bolts must be o f equivalent strength and dimensions, and are usually made o f high tensile steel. 26.7.9.3 Mountings Ensure that all mountings are secure. Corrosion and vibration are severe on ships and can cause premature failure. 26.7.9.4 Flame Paths Examine the flame paths for signs o f corrosion or pitting. If the flame path needs cleaning, this should be done with a non-metallic scraper and/or a suitable non-corrosive cleaning fluid.

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Maintenance and Troubleshooting 26.7.9.5

C em ent

Examine the cement / sealant used around lamp-glass assemblies both inside and outside. If it is eroded, softened or damaged in any way, advice should be sought from the manufacturer / competent authority regarding its repair. If the lamp-glass itself is damaged or cracked, then it must undoubtedly be replaced with a new one. 26.7.9.6

R e-assem blin g an E x -d (F lam e-proof) E n closu re

Always follow the manufacturer’s instructions. However the following points are also relevant: Lightly grease all flame-paths and threaded components with an approved form o f non­ setting silicone grease. Care must be taken to ensure that blind tapped holes are free from accumulated dirt or excessive grease which can prevent the correct closure o f flamepaths, or cause damage to the tapped components. Fitting a new lamp o f correct rating is preferred. ■$C Ensure bolts are not over-tightened as this can distort weather-proof gaskets, i f fitted, thereby allowing the ingress o f liquids and dust. Ensure that the luminaire is installed in accordance with the requirements o f the installation, particularly the classification o f the area (if it is hazardous) and that the correctly rated lamp is fitted. $