ATPL Electronics
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ATPL Electronics
© Atlantic Flight Training All rights reserved. No part of this manual may be reproduced or transmitted in any forms by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission from Atlantic Flight Training in writing.
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CHAPTER 1. Basic DC Terminology Introduction...................................................................................................................................... The Electrical Circuit........................................................................................................................ Current (I) ........................................................................................................................................ Electromotive Force (EMF).............................................................................................................. Potential Difference (PD) ................................................................................................................. Voltage (V)....................................................................................................................................... Resistance (R) ................................................................................................................................. Connecting Resistances in Series or Parallel in a DC Circuit .......................................................... Ohms Law ....................................................................................................................................... Loads............................................................................................................................................... Kirchhoff’s Laws .............................................................................................................................. Electrical Power (P) ......................................................................................................................... Electrical Work................................................................................................................................. Electrical Unit Prefixes..................................................................................................................... Typical Circuit Symbols ...................................................................................................................
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CHAPTER 2. Electrical Components Introduction...................................................................................................................................... Electrical Systems ........................................................................................................................... Electrical Circuit Faults .................................................................................................................... Busbars ........................................................................................................................................... Protection Devices........................................................................................................................... Reverse Current Circuit Breaker (RCCB) ........................................................................................ Switches .......................................................................................................................................... Electrical Generator ......................................................................................................................... Electrical Alternator ......................................................................................................................... Electrical Motor................................................................................................................................
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CHAPTER 3. Aircraft Batteries Introduction...................................................................................................................................... Lead Acid Battery ............................................................................................................................ Alkaline Battery (Nickel-Cadmium) .................................................................................................. Battery Venting ................................................................................................................................ Electrolyte Spillage .......................................................................................................................... Battery Capacity .............................................................................................................................. Battery Charging.............................................................................................................................. Thermal Runaway ........................................................................................................................... Battery State of Charge ................................................................................................................... Battery Condition Check .................................................................................................................. Emergency Use ............................................................................................................................... Connection of Batteries ................................................................................................................... Spare Batteries................................................................................................................................ Battery Compartment Inspection .....................................................................................................
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CHAPTER 4. Magnetism Introduction...................................................................................................................................... 4-1 Fundamental Laws of Magnetism.................................................................................................... 4-2 Classification of Magnetic Materials ................................................................................................ 4-5 Magnetic Flux .................................................................................................................................. 4-6 Flux Density..................................................................................................................................... 4-6 Reluctance....................................................................................................................................... 4-6 Permeability..................................................................................................................................... 4-6 Hysteresis........................................................................................................................................ 4-6 Saturation ........................................................................................................................................ 4-7 Magnetism Produced by Current Flow............................................................................................. 4-7 The Electromagnet ........................................................................................................................ 4-10 The Relay ...................................................................................................................................... 4-12 Electromagnetic Induction ............................................................................................................. 4-13
CHAPTER 5. DC Generator Systems Introduction...................................................................................................................................... 5-1 Basic Generator Theory................................................................................................................... 5-1 Simple AC Generator....................................................................................................................... 5-1 Conversion of AC to DC .................................................................................................................. 5-2 DC Generator System Architecture ................................................................................................. 5-4 DC Generator Construction ............................................................................................................. 5-4 Principle of Operation of a DC Generator ........................................................................................ 5-5 Types of DC Generator.................................................................................................................... 5-5 Voltage Regulator ............................................................................................................................ 5-7 Cut-out............................................................................................................................................. 5-8 Reverse Current Circuit Breaker...................................................................................................... 5-9 Busbars ........................................................................................................................................... 5-9 Power Failure Warning .................................................................................................................. 5-10 Ground Power ............................................................................................................................... 5-10 DC Generator System Fault Protection ......................................................................................... 5-11 Twin Engine DC Electrical System ................................................................................................ 5-11 Operation of DC Generators in Parallel ......................................................................................... 5-12 DC Load Sharing ........................................................................................................................... 5-13 Operation of an Equalising Circuit ................................................................................................. 5-13 Single Engine Aeroplane DC Electrical System............................................................................. 5-14 Operation of the Alternator ............................................................................................................ 5-15
CHAPTER 6. DC Motors Introduction...................................................................................................................................... 6-1 The Motor Principle.......................................................................................................................... 6-1 DC Motors ....................................................................................................................................... 6-2 Back EMF ........................................................................................................................................ 6-3 Direction of Rotation ........................................................................................................................ 6-4 Motor Speed Control........................................................................................................................ 6-4 Types of DC Motor........................................................................................................................... 6-5 Actuators ......................................................................................................................................... 6-7 Split-Field Series Motor ................................................................................................................... 6-8 Electromagnetic Brakes................................................................................................................... 6-9 Clutches......................................................................................................................................... 6-10 Instrument Motors.......................................................................................................................... 6-10 Architecture of a Starter/Generator System................................................................................... 6-10 Operation of a Starter/Generator System ...................................................................................... 6-11
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Inverters......................................................................................................................................... 6-13 Multiple Inverter Installations ......................................................................................................... 6-14
CHAPTER 7. Inductance and Capacitance Introduction...................................................................................................................................... Inductance ....................................................................................................................................... Self Induction................................................................................................................................... Inductors.......................................................................................................................................... Time Constant of an Inductor .......................................................................................................... Inductors in Series and Parallel ....................................................................................................... Capacitance..................................................................................................................................... Factors Affecting Capacitance......................................................................................................... Types of Capacitor........................................................................................................................... The Charging of a Capacitor............................................................................................................ Discharging of a Capacitor .............................................................................................................. The Time Constant of a Capacitor ................................................................................................... Capacitors in Series and Parallel in a DC Circuit.............................................................................
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CHAPTER 8. Basic AC Theory Introduction...................................................................................................................................... Advantages of AC over DC.............................................................................................................. Generating AC................................................................................................................................. Simple AC Generator....................................................................................................................... AC Terminology ............................................................................................................................... Relationship Between Radians and Degrees .................................................................................. Phase and Phase Angle .................................................................................................................. Phasor Representation ....................................................................................................................
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CHAPTER 9. Single Phase AC Circuits Introduction...................................................................................................................................... 9-1 The Effect of AC on a Purely Resistive Circuit................................................................................. 9-1 Power in an Ac Resistive Circuit ...................................................................................................... 9-1 The Effect of Ac on a Purely Inductive Circuit.................................................................................. 9-2 Power in an AC Inductive Circuit ..................................................................................................... 9-2 Inductive Reactance (Xl).................................................................................................................. 9-3 The Effect of Ac on a Purely Capacitive Circuit ............................................................................... 9-3 Power in an AC Capacitive Circuit ................................................................................................... 9-4 Capacitive Reactance (Capacitors Ac Resistance) ......................................................................... 9-5 Relationship Between Voltage and Current in Capacitive and Inductive AC Circuits ...................... 9-5 Resistive and Inductive (RL) Series AC Circuit................................................................................ 9-5 Resistive and Capacitive (RC) Series AC Circuit............................................................................. 9-6 Phase Shift ...................................................................................................................................... 9-6 Resistive, Inductive and Capacitive (RLC) Series AC Circuits......................................................... 9-6 Impedance (Z) in a Resistive, Inductive and Capacitive (RLC) Series AC Circuit ........................... 9-7 Resistive, Inductive and Capacitive (RLC) Parallel AC Circuit......................................................... 9-7 Impedance (Z) in a Resistive, Inductive and Capacitive (RLC) Parallel AC Circuit.......................... 9-7 Power in a Resistive, Inductive and Capacitive (RLC) AC Circuit.................................................... 9-8 Power Factor ................................................................................................................................... 9-8 AC Series Circuit Example ............................................................................................................ 9-10 AC Parallel Circuit Example........................................................................................................... 9-11
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CHAPTER 10. Resonant AC Circuits Introduction.................................................................................................................................... Series Resonant Circuit ................................................................................................................. Q Factor in a Series Resonant Circuit ........................................................................................... Parallel Resonant Circuit (Tank Circuit)......................................................................................... Q Factor in a Parallel Resonant Circuit.......................................................................................... Self Resonance of Coils ................................................................................................................ Use of Resonant Circuits ............................................................................................................... Tuning Circuits...............................................................................................................................
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CHAPTER 11. Transformers Introduction.................................................................................................................................... Construction and Operation........................................................................................................... Types of Transformers................................................................................................................... Transformer Rectifier Units............................................................................................................
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CHAPTER 12. AC Power Generation Introduction.................................................................................................................................... 12-1 Simple Three Phase Generator ..................................................................................................... 12-1 Star Connection............................................................................................................................. 12-2 Delta Connection ........................................................................................................................... 12-3 Advantages of Three Phase over Single Phase AC Generators ................................................... 12-3 Voltage and Frequency of AC Generators..................................................................................... 12-3 Phase Rotation .............................................................................................................................. 12-4 Faults on Three-Phase AC Generators ......................................................................................... 12-4 Generator Real and Reactive Load Sharing .................................................................................. 12-5 Types of AC Generator.................................................................................................................. 12-5 Brushless Three Phase AC Generator .......................................................................................... 12-7 Constant Speed Drive Unit ............................................................................................................ 12-8 Operation of the Hydro-Mechanical CSDU .................................................................................. 12-10 Protection of the Hydro-Mechanical CSDU.................................................................................. 12-11 Integrated Drive Generator .......................................................................................................... 12-12 Variable Speed Constant Frequency Power Systems ................................................................. 12-13 Auxiliary Power Unit..................................................................................................................... 12-13 Emergency Ram Air Turbine ....................................................................................................... 12-14
CHAPTER 13. AC Power Generation Systems Introduction.................................................................................................................................... Piston-Engine Frequency Wild AC System Architecture................................................................ Operation of a Piston-Engine Frequency Wild AC System ............................................................ Fault Protection in a Piston-Engine Frequency Wild AC System................................................... Twin-Engine Turbo-Propeller Frequency Wild AC System Architecture ........................................ Operation of a Twin-Engine Turbo-Propeller Frequency Wild AC System..................................... Fault Protection in a Twin-Engine Turbo-Propeller Frequency Wild AC System ........................... The Constant Frequency Split Busbar AC System ........................................................................ Operation of a Constant Frequency Split Busbar AC System........................................................ Regulation and Protection of Constant Frequency Units ............................................................... Faults on a Constant Frequency Split Busbar AC Generator System ........................................... Emergency Supplies...................................................................................................................... Battery Charger .............................................................................................................................
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Battery Power ................................................................................................................................ 13-8 Ground Handling Bus .................................................................................................................... 13-8 Constant Frequency Parallel AC System....................................................................................... 13-8 Operation of a Constant Frequency Parallel AC System ............................................................... 13-9 Reactive Load Sharing ................................................................................................................ 13-11 Real Load Sharing ....................................................................................................................... 13-11 Paralleling.................................................................................................................................... 13-11 Fault Protections in a Constant Frequency AC Parallel System .................................................. 13-12 DC Power Supplies...................................................................................................................... 13-13
CHAPTER 14. AC Motors Introduction.................................................................................................................................... 14-1 Stator-Produced Rotating Magnetic Field ...................................................................................... 14-1 Induction (Squirrel Cage) Motor..................................................................................................... 14-2 Two-Phase Induction Motor........................................................................................................... 14-5 Split-Phase Motor .......................................................................................................................... 14-5 The Synchronous Motor ................................................................................................................14-5
CHAPTER 15. Semiconductor Devices Introduction.................................................................................................................................... 15-1 Advantages and Disadvantages of Semiconductor Devices.......................................................... 15-1 Construction of a Semiconductor................................................................................................... 15-1 Doping ........................................................................................................................................... 15-2 P-Type Material ............................................................................................................................. 15-2 N-Type Material ............................................................................................................................. 15-3 P- N Junction Diode....................................................................................................................... 15-3 Use of Diodes ................................................................................................................................ 15-5 Bi-Polar Transistors ....................................................................................................................... 15-7 Operation of a PNP Bi-Polar Transistor ......................................................................................... 15-8 Operation of a NPN Bi-Polar Transistor......................................................................................... 15-8 Disadvantages of Diodes and Transistors ..................................................................................... 15-9 Transistor Applications .................................................................................................................. 15-9 Integrated Circuits ....................................................................................................................... 15-10 The Advantages and Disadvantages of Integrated Circuits ......................................................... 15-11 Types of Integrated Circuits......................................................................................................... 15-11
CHAPTER 16. Logic Circuits Introduction.................................................................................................................................... Number Systems ........................................................................................................................... Binary Representation ................................................................................................................... Basic Logic Gates.......................................................................................................................... Adder and Subtracter Circuits........................................................................................................ Digital Latch and Flip-Flop Circuits ................................................................................................
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CHAPTER 17. Computer Technology Introduction.................................................................................................................................... Analogue Computers ..................................................................................................................... Digital Computers .......................................................................................................................... Computer Architecture...................................................................................................................
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Input Devices................................................................................................................................. Central Processing Unit ................................................................................................................. Output Devices .............................................................................................................................. Storage Devices ............................................................................................................................ Operating Systems ........................................................................................................................ Programming .................................................................................................................................
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CHAPTER 18. HF and Satellite Airborne Communications Long Range Communications (Up to 4000 Km) ............................................................................ Short Range Communications (Up to 450 Km).............................................................................. Selective Calling (SELCAL) System .............................................................................................. Satellite Communications (SATCOM)............................................................................................ Satellite Aircom (SITA) ..................................................................................................................
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Chapter 1. Basic DC Terminology Introduction This chapter covers the basic Direct Current (DC) terminology, the connection of resistances in electrical circuits, and any associated laws. The Electrical Circuit An electrical circuit usually consists of a power source, a load, a switch and a conductor, which connects the components together.
The power source provides the force necessary to influence the flow of electrons around the circuit when the switch is closed, whilst the load is an electrical device that performs a useful function, eg. a lamp, a motor or a heating element. The switch makes and breaks the flow of current to the load, which only performs a useful function when current flows through it, whilst the conductor provides a low resistance path for the current to flow. In aeroplanes these conductors are usually formed from aluminium or copper or aluminium, or even the metal structure of the aeroplane. When the switch is closed, the force from the supply causes electrons to flow outside the source from the negative to the positive terminals, and is known as ‘Electron Flow’. Current flowing from the positive to the negative terminals outside the source is alternatively known as ‘Conventional Current Flow’.
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Current (I) Current is an indication of the flow of electricity and is measured in amperes (amps). This is the rate at which electrons flow in a conductor, such that when one Coulomb (C) or 6.25 x 1018 electrons pass a point in a conductor in one second, a current of one ampere is said to flow. Amperes = Coulombs
Seconds
Current in a circuit is measured by connecting an ‘Ammeter’ in line, or in series with the load, as shown below.
Electromotive Force (EMF) EMF is the force or pressure that sets electrons in motion, and is a natural result of ‘Coulomb's Law’, which states that like charges repel and unlike charges attract. Potential Difference (PD) Even though a circuit is open, and no current is flowing, a power source still has the potential for current flow. Thus whether a battery is connected in a circuit or not, a potential difference will still exist between its terminals.
Voltage (V) Voltage is the basic unit of electrical pressure and is measured in ‘Volts’, where one volt is the amount of pressure (EMF) that will cause one Coulomb of charge to move from one point to the other. A ‘Voltmeter’ is used for measuring voltage, and is connected across the load, or in parallel with the EMF or PD to be measured.
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Resistance (R) Resistance is measured in Ohms, where one Ohm is the amount of resistance that will allow one ampere of current to flow in a circuit to which one volt EMF is applied. Resistance opposes current flow and causes a reduction in the voltage. In doing so it produces heat, and power is consumed. Rubber and glass are examples of ‘Insulators’, and offer a great deal of opposition to the flow of electricity, ie. high resistance. These materials prevent conductors coming into contact with other objects, which could be harmed or damaged. Other materials such as silver and copper have very little opposition to current flow, ie. low resistance, and are known as ‘Conductors’. Alternatively materials, which offer some resistance to the flow of electricity, are known as ‘Semi-Conductors’. The resistance of a material at a constant temperature is affected by its:¾ ¾ ¾
Specific Resistance (ρ), the resistance offered by a cube of material at 0° C. Length (l). Cross Sectional area (A). R=
ρxL A
Resistors can have either fixed or variable values. An example of a variable resistor is a rotary switch, which is used to control* the intensity of a lighting circuit. Temperature also affects the resistance of a material. The resistance of most materials increases with increasing temperature, and exhibit a ‘Positive Temperature Coefficient (PTC)’. The resistance of a few materials, however, decreases with increasing temperature and exhibit a ‘Negative Temperature Coefficient (NTC)’. Generally most conductors have a ‘PTC’, whilst insulators and semi-conductors have a ‘NTC’. Another form of resistor is a ‘Thermister’, which is a NTC device, and is used for measuring temperatures in aeroplanes, ie. the higher the temperature the lower the resistance. Connecting Resistances in Series or Parallel in a DC Circuit Resistances can either be connected in series, in parallel, or in series-parallel combinations. When resistors are connected in series the same current flows through each of them, and the total opposition to current flow is thus equal to the sum of the individual resistances.
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Total Resistance (RT) = R1 + R2 + R3 + …… = 15 + 22 + 31 = 68Ω If the resistances are alternatively connected in parallel with each other, the current will flow along two or more paths, as shown on the next page-.
The greater the number of resistors connected in parallel the lower the overall resistance, and the greater the current flow from the supply. In a parallel circuit the supply voltage is common to all resistors, and the total resistance is calculated using the following method:-
1 = 1 + 1 + 1 R R R T 1 3 2
R
1 = 1 + 1 + 1 = 6 6 12 12 4
R
T
RT
=
12 6 = 2 ohms
In many circuits a parallel circuit is connected in series with one or more resistors.
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By firstly calculating the equivalent resistance in the parallel part of the circuit, and then adding this value to the series resistance enables the total resistance to be found. In the circuit shown above the total resistance is calculated as follows:Parallel Part of Circuit R
= R1 + R1 = 1 + 1 = 5 9 6 18 TP 2 1
1
Total Parallel Resistance (RTP) = 18 = 3.6Ω
5
(i)
Total Circuit Resistance
RT = RTP + R3 = 3.6 + 2.4 = 6 ohms Ohms Law Ohms law states that the current flowing in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance through which the current flows. Thus the higher the voltage the higher the current, and the higher the resistance the lower the current. Ohms Law may thus be stated by the following formulae:R = V ohms, V = IR volts or I = V amps I R Loads Loads are items of electrical equipment that have varying amounts of resistance, and are normally connected in parallel with the supply. Thus the amount of current being drawn from the supply will increase as more items of equipment are switched on. Kirchhoff’s Laws The first law states that the sum of the currents entering a junction must equal the sum of the currents leaving the junction.
The second law states that in a closed circuit the sum of the voltage drops always equals the supply voltage.
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In the circuit shown above a 10 volt battery is connected across a lamp, and as current flows through the circuit, a voltage drop will be developed across the lamp. The lamp will thus consume the same amount of energy as the battery provides, and the voltage drop across the lamp will equal the supply voltage.
If two identical lamps are connected in series, then the voltage drop across each will be the same, and the sum of the voltage drops will similarly equal the supply voltage. Electrical Power (P) Electrical power is the amount of work done in a specific time, and is the ability of an electrical device to produce work. Power is measured in ‘Watts’ (746 watts equals 1 horsepower). One Watt is also equal to one Joule per Second (J/s), which is the work done in one second by one volt of EMF, in moving one Coulomb of charge, ie. when one volt causes one ampere to flow, a power of 1 watt will be consumed. Power is represented by the following formulae:-
2 P=VI or I2R or VR . Electrical Work Electrical work is defined by the product of force x the distance an object moves under the influence of electrical power. Electrical Unit Prefixes For ease of usage and display, electrical units are normally divided into multiples and submultiples. Some of the most commonly used prefixes are as follows:Multiples Kilo Mega Giga Tera
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Sub-multiples
1 x 103 1 x 106 1 x 109 1 x 1012
Milli Micro Nano Pico
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Typical Circuit Symbols Components in electrical circuits are normally represented by the following symbols:-
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Intentionally Left Blank
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Chapter 2. Electrical Components Introduction Electrical circuits form an integral part of an aeroplane, and must be adequately protected. The flight crew must also be able to select and operate any equipment’s safely. Electrical Systems The possible electrical system layouts are:Single Pole or Earth Return System. This system is used on aeroplanes constructed from metal, where the airframe acts as a return path between the load and the power source.
This gives an overall reduction in the amount of wiring required and also gives a reduction in aeroplane weight. Dipole or Two-wire System. This system is used on aeroplanes, which are constructed from non-conductive or non-metallic materials.
In this system one wire connects the electrical supply to the load, whilst a second wire provides the return path from the load to the power source. This thus increases the aeroplanes overall mass. Ground (Earth). This is simply a zero or reference point within an electrical circuit and is the metal frame or chassis on which the various electrical circuits are constructed. On an aeroplane the metal airframe is called ground or earth.
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All voltages are measured with respect to the metal structure. In electrics, ground is important because it allows us to have both negative and positive voltages, with respect to the metal structure. For example if a 12 volt battery has a PD between its terminals of 12 volts, then it is not referred to as +12, or -12 volts, but simply as 12 volts. The ground reference allows us to express voltages as positive and negative with respect to ground. Remember ground is a reference point that is considered to be zero or neutral. For example if the positive terminal of a 12 volt battery is ground, the negative terminal is 12 volts more negative. It follows that the voltage at this terminal with respect to ground is -12 volts. Conversely if the negative terminal of the battery is connected to ground, then the other terminal of the battery will be +12 volts. Electrical Circuit Faults The following faults can occur in an electrical circuit:Short Circuit. This fault will occur in:an earth return system, if the live conductor touches the metal airframe, or in a di-pole circuit, if both conductors touch each other.
If a short circuit occurs due to a fault a low resistance path will be created across the supply. This will cause an extremely high current to flow, causing possible damage to the circuit and any associated wiring. It may even burn the cables, and cause a fire. Open Circuit. This type of fault will occur in:an earth return circuit if the conductor breaks, or becomes disconnected, or in a dipole circuit if either of the conductors becomes broken or disconnected.
If an open-circuit or break occurs in the conductor the load will become inoperative, just like opening a switch.
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Busbars A busbar is a current distribution point from which individual circuits take their power, and is simply a strip of metal, which is supplied with a voltage from the main power generating system or one particular element thereof. The busbars are also sub divided into vital, essential and non-essential, which indicates their power source or their importance in the overall system. The vital busbar or battery bus is supplied direct from the aeroplane battery, and supplies power to the vital systems that may be required in a crash situation, eg. fire extinguishers and fuel shut off valves. The essential busbars supply the systems required for the safe flight of the aeroplane, ig. Navigation lights and instrumentation, whilst the nonessential busbars supply the systems, which can be safely switched off in an emergency, eg. galley ovens. Protection Devices The following protection devices exist in an aeroplane electrical circuit:Electrical fuse. This protection device will open or break the electrical circuit when excessive current flows. This is because the magnitude of the current may ultimately damage either the circuit itself, or the system to which it is connected. A fuse is designed to form a weak link in an electrical circuit to protect the majority of the cable between the supply and the load against overheating and burn out. In its simplest form it consists of a strip or filament of low melting point metal, which is encased in a glass or ceramic envelope.
Fuses are rated in amperes, which is the maximum current they can carry without overheating and rupturing. They are located as near the supply (busbar) as possible, so that if an excessive current flows, due to a short circuit, the fuse will rupture protecting all of the cable to the load. In practice aircraft are required by law to carry spare fuses; minimum stocks of each type of fuse being 3 or 10%, whichever is the greater. Actions to be taken if a fuse ruptures in flight:¾ ¾ ¾
Switch off the circuit. Replace the fuse with one of the same value. Switch on the circuit
If the fuse blows again, switch off the circuit and do not attempt a further replacement. National and company regulations must be followed in this respect, but in either case the fault must be reported on landing. Current Limiters. These devices are used mainly to protect heavy-duty power distribution circuits and consist of a high melting point filament of tinned copper encased in a ceramic housing.
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The central portion of the filament in some types is wasted to form the fusible area. The time/current characteristics of the device allow a considerable overload current to flow in the circuit before rupturing occurs. Circuit breaker. This device has the same function as a fuse, but can be used to restore a circuit when it is reset. Like fuses, circuit breakers are also rated in amperes, and are fitted as close to the supply as possible. A circuit breaker is basically a switch, which can be opened (tripped) via a bi-metallic strip, as shown on the next page. If an overload current exists the bi-metallic strip will heat up and distort, causing the latch mechanism to be released. This will cause the main contacts of the circuit breaker to open, and a push-pull button to pop out. A white band will also be revealed, and indicates that the circuit breaker has tripped. To reset the circuit breaker the button that protrudes when it trips needs to be pushed in again. On modern aeroplanes circuit breakers are fitted in preference to fuses, and are referred to as ‘trip-free’, ie. they can not be reset whilst the fault still exists, regardless of whether the button is held in or not.
If a Circuit Breaker trips the following action should be taken:-
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¾ ¾ ¾ ¾
Switch the circuit off. Allow a period of approximately 20 - 30 seconds to allow the bi-metallic element to cool. Reset the circuit breaker. Switch the circuit on
If the circuit breaker trips again, switch off the circuit and do not attempt a further reset. In either case report the fault on landing. Circuit Breakers have the following advantages over fuses:¾ ¾
No spares have to be carried. They can be used as a switch, eg. when carrying out aeroplane maintenance.
A circuit breaker that can be physically held in against the fault is known as a Non-Trip Free Circuit Breaker. If this is allowed to happen it may cause severe damage to the aircraft wiring, and in extreme circumstances may even lead to a fire. Reverse Current Circuit Breaker (RCCB) Reverse current circuit breakers are used in DC power supplies to protect against short circuits and prevent extremely high currents flowing towards the power source. They operate at high speed, and are manually reset. Some of the more sophisticated types of RCCB have a separate thermal overload as an additional precaution against a forward current in excess of the power sources safe working capacity. Switches In aeroplane electrical installations, switches and relays principally perform the function of installing and controlling the operating sequences of circuits. Circuit breakers, though they control the flow of current to and within systems, are regarded as only circuit protection devices. In its simplest form, a switch consists of two contacting surfaces, which can be isolated from each other or brought together by a moveable-connecting link, called a ‘Pole’, and the number of circuits it controls is known as its ‘Throw’. Some examples of these are shown below.
If a switch has only one operating toggle it is known as a ‘Single Pole Switch’, but a switch where two or three toggles have been grouped together is known as a ‘Double’ or ‘Triple Pole Switch’. Switches which use 2 or 3 position switches, may be fitted with guards or latches to hold them in their normal operating positions with cover plates, spring loaded sliding guards or physical restraints, all of which have to be moved to operate the switch.
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The following types of switches are used on aeroplanes:Toggle switches (tumbler switches). extensively used.
These are general-purpose switches and are
They have simple ON/OFF functions and may be ganged, guarded or be 2 or 3 position devices, as shown on the next page.
Push switches. These switches are used for short duration operations. ie. when a circuit is to be completed or interrupted for a short duration. Other types are designed to close one or more circuits (through separate contacts) whilst opening another circuit. They may be designed for either push to make or push to break operation. Some contain small lamps which, illuminate legends. These are typically used in turbo-prop engine start and stop circuits, and operate either manually or electro-magnetically.
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In a typical start circuit the button is normally held latched in place until the start sequence is complete, whilst in a stop circuit the button usually operates the circuits which stop the fuel supply, remove electrical power from other engine systems and shut the engine down. Rocker button switches. These switches combine the action of both toggle and push button switches, eg. in a generator system a single switch may allow ‘ON, ‘OFF’ (selectable) and reset (spring loaded) selections to be made using the same switch.
Rotary switches. These switches are manually operated and are often used in place of toggle switches. A typical use is the selection of a single voltmeter between several busbars, generators or batteries.
Micro-switches. These are a special type of switch, and are the type most extensively used in aeroplanes. It is a switch in which the travel between make and break is in the order of a few thousandths of an inch.
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Activation of micro switches varies with the designs of the system, but are usually by a lever, roller or cam. They are used in various applications such as:¾ ¾ ¾ ¾
Landing gear systems to indicate the position to the indicator lights, Door warning systems, Power lever sequencing of system operation (arming of power augmentation systems) Nose wheel or main wheel weight on switches to ensure that systems do not operate on the ground.
Rheostats. These switches are used to alter the amount of current in a circuit by varying the overall resistance, eg. to vary the intensity of panel or flight deck lighting. They normally also have an ‘OFF’ position, to completely remove the current. Time switches. These switches are required to operate pre-determined controlled time sequences. They are usually linked to, and are controlled by an electric motor. For example the switching of power between the heater mats on propellers, or between pairs of propeller blades to achieve de-icing. In some sequences the time switch operations can be varied, which is done by a rocker or toggle switch, via a continuous operating time switch that selects power for different time sequences. Mercury switches. These switches are glass tubes in which stationary contacts and a pool of loose mercury are hermetically sealed, as shown on the next page. Tilting the tube causes the mercury to flow and close or open a gap, thus make or breaking a circuit. A typical application is in the torque motor circuits of Artificial Horizons where the gyro must be forced to, and be maintained in the vertical position.
Pressure switches. These switches are used to indicate high and low pressure in systems where pressure measurement is involved, eg. Hydraulic systems. They are usually linked to warning captions to indicate high or low pressure outside normal limits. Pressure switches
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are also installed in cabin pressurisation systems to indicate high differential pressure and cabin altitude above set limits. Thermal switches. These switches are used in systems where warning of excessive heat is required, eg. in engine fire and over-heat warning systems. Proximity switches. These switches are used in some aeroplanes to give warning of whether or not passenger doors, freight doors, etc. are fully closed and locked. They have certain advantages over micro switches in that they have no moving parts, which might break or malfunction. Bi-metallic switches. These switches are used in temperature sensitive areas where smaller devices than thermal switches may be required. Two different metals with different co-efficient rates of expansion are fastened together. The different metals cause the combined plates to bend and make or break contacts. They may be used in instruments, especially electronic instruments, to operate cooling fans that maintain internal temperatures within limits. Electrical Generator An electrical generator is a mechanical device that changes mechanical energy into electrical energy by using permanent magnets or electromagnets with moving conductors. Generally engine driven generators produce a voltage or EMF, which causes current to flow when the electrical circuit is completed. Electrical Alternator An electrical alternator is sometimes incorrectly referred to as a DC generator, because the alternating current output it produces is changed directly into DC within the alternator itself. Electrical Motor An electrical motor is an electrical or mechanical device that changes electrical energy back into mechanical energy. These are extensively used in many aeroplane electrical systems.
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Chapter 3. Aircraft Batteries Introduction All aircraft electrical systems include a battery, which is used to:¾
supply power to essential services in the event of generator failure.
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stabilise the power supplies during switching of transitory loads.
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supply power for engine starting.
Batteries are made up of a number of units called cells. Each cell consists of a series of negative and positive plates, which are immersed in a liquid known as electrolyte.
All cells and batteries store energy in a chemical form, which can then be released as electrical energy. The following basic types of cells exist:-
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Primary Cell. This type of cell is not rechargeable and only has a limited use in aircraft’s, where it is mainly used for emergency lighting.
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Secondary Cell. Batteries made up of secondary cells are rechargeable and are the type mainly used in aircraft’s. They are either of the lead-acid or NickelCadmium (Ni-Cd) / alkaline variety.
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Lead Acid Battery Each cell of a lead acid battery consists of positive plates of lead peroxide and negative plates of spongy lead, as shown below.
The plates are interleaved, and insulated from each other by plastic separators. An odd number of negative plates is used with one positioned either side of the positive plates to prevent buckling by evening out the thermal distribution. The complete structure is supported in an acid resistant casing containing an electrolyte of distilled water and concentrated Sulphuric acid, to a level just above the plates. Each cell is 2.2 volts fully charged and 1.8 volts fully discharged. In aircraft’s batteries of this type consist of either six cells (12 volts), or twelve cells (24 volts). When a battery is connected to an external circuit electrons in each cell are transferred through the electrolyte from the spongy lead to the lead peroxide and the net result of the chemical reaction is that lead sulphate forms on both plates. At the same time the electrolyte is diluted by the formation of water, which takes place during the chemical reaction. For practical purposes each cell is fully discharged when the ‘Specific Gravity (SG)’ or ‘Relative Density’ of the electrolyte falls from ‘1.27 SG (fully charged)’ to ‘1.1 SG (fully discharged)’, which equates to ‘2.2 and 1.8 volts’ respectively. Any change in the temperature of the electrolyte will also vary its specific gravity, so a correction must be made if the temperature
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is non-standard. The Specific Gravity of the electrolyte also determines its freezing point, and a discharged battery will freeze at a lower temperature than a fully charged battery. Batteries constructed from this type of cell must not be left in a discharged condition for extended periods of time since the Lead Sulphate will harden on the plates and cut down their active area. This process is known as ‘Sulphation’, which can drastically shorten the life expectancy of a battery. Lead acid batteries may be recharged by connecting the positive and negative terminals respectively, to the positive and negative terminals of a DC source of a slightly higher voltage than the battery. All of the fore-going reactions are reversed; the lead sulphate is removed from both plates, the positive plate is restored to lead peroxide, the negative plate is restored to spongy lead, and the electrolyte is restored to its original Specific Gravity (SG). Alkaline Battery (Nickel-Cadmium) Each cell of a nickel-cadmium battery in a fully charged condition consists of positive plates of Nickel Oxide and negative plates of pure Cadmium, as shown below.
The plates are interleaved and fully immersed in an electrolyte of dilute Potassium Hydroxide. The plates and electrolyte are placed in a stainless steel or plastic container.
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Each cell is ‘1.2 volts (fully charged)’ and ‘1.1 volts (fully discharged)’. Batteries of this type for use on an aircraft consist of either twenty cells (24 volts), or twenty-two cells ( 26 volts). During discharge the negative plates turn into ‘Cadmium Hydroxide’, and the positive plates turn into ‘Nickel Hydroxide’. The electrolyte in an alkaline cell has a Specific Gravity of 1.26, which remains constant, whether it is in a charged or discharged condition. Like lead-acid batteries alkaline batteries can be recharged by connecting the positive and negative terminals respectively to the positive and negative terminals of a DC source of slightly higher voltage than the battery. The chemical reaction is reversed, and the plates return to their former states; the negative plates to Cadmium, and the positive plates to Nickel Oxide. Battery Venting When charging batteries their temperature increases and volatile hydrogen gas is given off, which is safely vented to atmosphere by way of various systems. In each case however, a certain amount of distilled water is lost by evaporation, and it is therefore necessary to top the battery up to a specific level from time to time with distilled water. ¾
Lead-Acid Battery Venting. Lead-acid batteries are vented using one of the following methods:-
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Non-Spill Vent. This type of vent is most commonly used on small aircraft’s and allows the hydrogen gas to escape, whilst retaining the electrolyte.
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Cross-Flow Cell System. This system is used on larger aircraft’s, where cabin pressurisation air flows over the tops of the cells, and vents the battery to atmosphere.
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Alkaline Battery Venting. Alkaline batteries give off a mixture of hydrogen and oxygen gases towards the end of charging. Similar to lead-acid batteries different types of alkaline battery also exist:¾
Semi-Open Batteries. In this type of battery the cells are allowed to gas freely in order to avoid overheating, which can result from overcharging, and the gases given off during the chemical reaction are vented safely to atmosphere using a ‘cross-flow’ venting system. These batteries must also be topped up at regular servicing intervals with distilled water.
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Sealed Batteries. In these batteries, the cells are completely sealed and require no maintenance.
Electrolyte Spillage Any electrolyte spilled from a battery, normally due to heavy landings and severe turbulence, must be neutralised before it damages the aircraft structure. The neutralising agents for this purpose are as follows:¾
Lead-Acid Battery -
A solution of Bicarbonate of Soda.
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Alkaline Battery
A solution of Boric Acid.
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It is important that once the area is neutralised that copious quantities of fresh water are used to cleanse the area, and prevent corrosion setting in. Battery Capacity The capacity of a battery is a measured in ‘Ampere-Hours (Ahr)’, and is a measure of the total amount of energy that it contains. It is based on the maximum rated current in amperes, which will be delivered by a battery for a known time period until it has discharged to a permissible minimum voltage level, which varies according to the size and number of plates in each cell. The following definitions apply:¾
Rated Capacity. This is the manufacturers stated capacity that is usually stamped on the side of the battery, eg. 40 Ahr, which signifies that the battery is designed to last 10 hours when discharged at a 4 Ampere rate, or 1 hour when discharged at a 40 Ampere rate.
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Actual Capacity. This is the capacity of the battery, which is determined by a Capacity Test.
Batteries used in aircraft are normally removed, and capacity checked every 3 months in a specific battery charging bay, where the following process takes place:¾
Fully discharge the battery.
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Fully charge the battery.
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Discharge the battery at known amperage to a permissible minimum voltage level, and time how long it takes.
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Multiply the amperage by the time taken to obtain the ‘Actual Battery Capacity’.
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Compare this value against the batteries ‘Rated Capacity’. Eg.
Actual Capacity x 100 = 38 x 100 = 95% 40 Rated Capacity
Note: For continued use in aircraft this value must be 80%, or more. Battery Charging The following methods are used to charge the batteries whilst installed in the aircraft:¾
Constant Voltage. This method is used mainly on aircraft fitted with lead-acid batteries. The battery-charging rate is proportional to the difference between the battery and the generator voltage, which in aircraft using 12 Volt batteries is 2 volts, ie. the generator voltage is normally regulated at 14 volts.
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Pulse Charging. This method is used mainly on alkaline batteries, and aircraft using this method are fitted with a battery charger, which is supplied by Alternating Current (AC). This source is then rectified to provide a constant Direct Current (DC) of approximately 50 amps that continues flowing until the battery is nearly fully charged. The charger then goes into a pulse DC current mode to keep the battery topped up. A temperature sensor within the battery is normally designed to reduce or even stop the charging, if the battery starts to overheat.
Thermal Runaway Batteries are capable of performing to their rated capacity when the temperature conditions and charging rates are maintained within the values specified by the manufacturer. In the event of these values being exceeded ‘Thermal Runaway’ can occur, which causes violent gassing and boiling of the electrolyte. If this condition is allowed to continue the temperature of the battery will rise to such a level that it may melt, or even explode, and may cause damage to the aircraft structure. The reason for this effect is that when a battery exceeds a certain temperature its internal resistance drops, thus allowing a higher charging current to flow, and the battery temperature to rise. This effect is self-perpetuating, and in some aircraft’s, particularly those employing alkaline batteries, temperature-sensing devices are located within the batteries to provide a battery overheat warning on the flight deck, which indicates that the battery should be electrically isolated by the flight deck crew. Battery State of Charge The state of charge of lead-acid batteries can be found by : ¾
Measuring the terminal voltage.
¾
Measuring the specific gravity of the electrolyte.
The state of charge of alkaline batteries is however, not easily ascertained by these methods, and must therefore be assumed to be serviceable. Battery Condition Check An aircraft battery is a vital piece of equipment, and must therefore be checked for serviceability prior to flight. The check involves the following:¾
Check the battery ‘OFF’ load and note its voltage reading.
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Select a stipulated load and note the new voltage reading.
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¾
Compare both ‘ON’ and ‘OFF’ load readings, and ensure that the difference between the readings is within a set tolerance.
Emergency Use In an emergency, the aircraft batteries must be capable of maintaining a supply for a minimum period of time, according to JAR’s:¾
Main batteries. These batteries must last at least 30 minutes after total failure of the electrical generating system. (Refer JAR 25.1303).
¾
Emergency Lighting Batteries. These batteries must last for at least 10 minutes.
Connection of Batteries Batteries, which are connected together, must be of the same type, ie. Acid and alkaline batteries must never be mixed. Batteries may be joined together as follows:Series Connection. If three identical batteries are connected in series their voltages are added together, but their capacity remains the same as that of an individual battery, as shown below.
Parallel Connection. If identical batteries are connected in parallel their capacities are added together, but the voltage remains the same as that of an individual battery.
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Spare Batteries Some aircraft carry spare batteries, but no attempt should be made to change the batteries in flight. Battery Compartment Inspection Prior to flight the battery compartment should be checked as follows:¾
Check the batteries for security.
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Check the electrical connections.
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Check for any electrolyte spillage.
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Check the vent pipe for security and routing.
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Chapter 4. Magnetism Introduction Magnetism is so closely allied with electricity that without it the means of creating electrical power would be greatly reduced. A magnet attracts small pieces of iron or steel, and is surrounded by a magnetic field, which is made up of invisible lines of magnetic force, or magnetic flux. This is best demonstrated by sprinkling iron filings on a piece of paper placed over a magnet.
This illustrates that magnetism is concentrated at a magnet's extremities, called poles, and if it is freely suspended it will always align itself in a north-south orientation.
The north-seeking or red pole will always point north, and the south-seeking or blue pole will always point south. The earliest known form of magnetism was the Lodestone, which was a natural mineral found in Asia. It was found that if a piece of this ore was suspended horizontally by a thread, or floated on a piece of wood in water, it would likewise align itself in a north-south direction.
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This characteristic led to its use as a compass and the name Lodestone, meaning leading stone. This occurs because the earth itself is a huge magnet with it's own magnetic field
The fields interact with each other and the Lodestone aligns itself according to the fundamental laws of magnetism. Other than the earth itself, Lodestone is the only natural magnet; and all other magnets are produced artificially. For example an iron bar will become magnetised if it is repeatedly rubbed against a piece of Lodestone. Another type of magnet is the Electromagnet, which is produced when an electric current is passed through a conductor. Magnets are additionally classified by their shape, and can exist as horseshoe, bar or even ring magnets. Conversely a magnet can be demagnetised by:¾
heating it to a temperature known as its ‘Curie Point’.
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hitting it with a hammer.
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‘Degaussing’ it with an alternating magnetic field.
Fundamental Laws of Magnetism The fundamental laws of magnetism are as follows:-
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The line joining the poles is called the magnetic axis.
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Red or blue poles cannot exist separately.
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Like poles repel each other, and unlike poles attract, as shown on the next page.
Characteristics of Lines of Magnetic Flux Lines of magnetic flux have the following characteristics:¾
They have direction or polarity, and the lines of magnetic flux travel externally from the North Pole to the South Pole, as indicated in the following diagram.
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They always form complete loops, where each line of magnetic flux travels back through the body of the magnet to form a complete loop.
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They never cross each other; which is the reason why like poles repel, since lines of magnetic flux having the same polarity can neither connect nor cross.
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Thus when one field intrudes into another, as shown on the next page, the lines will repel, and the magnets will tend to move apart.
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They tend to form the smallest possible loops, which is the reason why unlike poles attract. Lines of magnetic flux having the same polarity will link up, as shown below, and the resulting loops will attempt to shorten by pulling the two magnets together.
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They can be distorted by interacting with other flux lines, as shown below. This is because the lines of magnetic flux pass through soft iron more readily than air, and at the same time the lines tend to contract to make the smallest possible loops. The iron bar is thus attracted towards the magnet, and strengthens its overall magnetic field.
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Classification of Magnetic Materials Theoretically all materials are affected to some extent by a magnetic field, and can be placed in one of the following categories:¾
Ferromagnetism. This is the property of a material that enables it to become a permanent magnet, ie. Ferromagnetic materials when placed in a magnetic field will develop a very strong internal field and will retain some of it when the external field is removed. The most common ferromagnetic substances are iron, cobalt, nickel, and alloys of these metals. Above the Curie temperature, thermal agitation destroys the domain structure and the substance becomes paramagnetic. In practice it is convenient to sub-divide ferromagnetic materials into two classes:¾
Hard Iron. This is a material which is difficult to magnetise, but once magnetised will retain it magnetism unless it is subjected to a strong demagnetising force. This is known as a ‘Permanent’ magnet.
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Soft Iron. This is a material, which is easily magnetised, but also easily loses it magnetism when it is not subjected to a strong magnetising force. This is known as a ‘Temporary’ magnet.
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Paramagnetic. This is the property of a material, which when placed in a magnetic field, will have an internal field stronger than that outside, and will thus slightly attract lines of magnetic force. However once the magnetic field is removed the magnetism will be destroyed by random thermal motion. Typical materials are platinum, manganese, and aluminium.
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Diamagnetic. This is the property of a material, which when placed in a magnetic field will have an internal field proportional to, but less than that outside, and will thus slightly repel lines of magnetic force. Typical materials are copper and bismuth.
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Magnetic Flux Magnetic Flux (φ) is produced by a force known as the ‘Magneto-Motive Force (MMF)’, whose magnitude is determined by the product of the current and the number of turns of wire that link together the magnetic circuit. Thus the greater the current, and the greater the number of turns, the greater the resulting flux. The unit of magnetic flux is the ‘Weber (Wb)’. Flux Density Flux density is the number of ‘Webers per square metre (Wb/m2)’, and is alternatively known as the ‘Tesla (B)’. Reluctance Reluctance is the opposition to magnetic flux, and is similar, in nature to resistance in an electrical circuit. It is the ratio of the Magneto-Motive Force (MMF) acting on a magnetic circuit, to the magnetic flux (Φ) being produced, ie. Reluctance = MMF
φ
Permeability Permeability (µ) is the ease by which a material will accept lines of magnetic flux and may be compared to conductance in an electrical circuit, which is the ease with which a material or circuit will allow current to flow. It is the ratio of B/H, where B is the induced magnetic flux, and H is the magnetising force. The table on the next page shows how the permeability of a material determines its characteristic. Material Bismuth Water Copper Air Aluminium Cobalt Nickel Iron
Permeability 0.999833 0.999991 0.999995 1.000000 1.000021 170 1000 7000
Characteristic
Action
Diamagnetic Diamagnetic Diamagnetic Paramagnetic Paramagnetic Ferromagnetic Ferromagnetic Ferromagnetic
Slightly Repelled Slightly Repelled Slightly Repelled Non-Magnetic Slightly Attracted Strongly Attracted Strongly Attracted Strongly Attracted
Hysteresis It is possible to take a iron ring, completely de-magnetized, and measure the value of flux density (B) with respect to increasing values of magnetizing force (H). This relationship is expressed by the curve OC.
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If the magnetizing force is reduced from this maximum value the flux density will follow the curve CD, and the flux remaining in the iron is called the ‘Remnant Flux’. To totally remove the remnant flux the magnetizing force needs to be reversed, which in this case is to a value of OE, whose value is called the ‘Coercive Force’. Further negative increases in H will cause B to grow in the reverse direction until saturation occurs, following the line EF. Decreasing the value of H, and subsequently increasing H in a positive direction completes a symmetrical figure, CDEFGC, which is termed the ‘Hysteresis loop’. The word ‘Hysteresis’ means to lag behind, and this is what happens to the flux density as it lags behind the changing values of the magnetising force. Saturation Saturation plays an important role in ferro-magnetic circuits, where the magnitude of magnetism being induced in a piece of iron is proportional to the current creating it. If the current is however increased beyond a certain point, no further appreciable increase in magnetism will occur, as the iron becomes fully saturated. This is a very important property, and is the principle on which a magnetic amplifier operates. Magnetism Produced by Current Flow When current flows through a conductor a magnetic field is produced around the conductor, and its magnitude is proportional to the current flow.
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The direction of the field depends on the direction of current flow, and the ‘Right Hand Grasp Rule’ is used to determine the direction of the field when a conventional current is flowing.
The thumb points in the direction of the current flow, whilst the fingers point in the direction of the magnetic field. In explaining some aspects of electromagnetism, it is also useful to show current flow in a third dimension, which can be done using two further symbols. If a wire is viewed from the end, the tail of the arrow will indicate current flowing into the wire, and a dot on the point of the arrow will indicate current flowing out of the wire, as shown on the next page.
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If two pieces of wire are placed side by side the resulting magnetic fields will act together, and will either attract or repel each other, which depends on the direction of the currents, as shown below. REPULSION
ATTRACTION
Currents flowing in opposite directions will produce opposing fields and will repel each other, whilst currents flowing in the same direction will produce fields, which add together, and attract each other. The magnetic field produced in a straight piece of wire is of little practical use, and has direction, but no north or South Pole. Unless the current is extremely high, the magnetic field
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will have little useful strength, but by shaping the wire into a loop its magnetic characteristics can be greatly improved.
The coil of wire, as shown above, will now possess the following characteristics:¾
The lines of flux will be closer together.
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The majority of lines of flux will be concentrated in the centre of the loop.
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North and south poles will be created at the ends of it, and it will assume the same magnetic characteristics as that of a permanent magnet, ie. lines of magnetic flux will emerge from the north pole, and return via the south pole, as seen below.
The Electromagnet The principle of an electromagnet is that when current passes through a loop of wire, a magnetic field is established, as shown above, and by increasing the number of turns in the wire, ie. by forming a coil, the individual fluxes will add together to produce a stronger magnetic field.
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This is known as a ‘Solenoid’, and the more current that flows through the coil, the greater the number of lines of flux. The strength of the magnetic field around a coil (electromagnet) thus increases with either an increase in current, or an increase in the number of turns. Another method of increasing the strength of the magnetic flux around a coil is to insert a bar of ferromagnetic material into it, ie. soft iron, as shown below.
This has the effect of concentrating the magnetic lines of flux because an iron core is much more permeable than air, and the polarity of a coil can be determined if the direction of current through the coil is known, using the ‘Right Hand Grasp Rule’.
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If the fingers of the right hand are wrapped around the coil in the direction of current flow, the thumb will point in the direction of the north pole. The Relay A relay uses the principle of the electromagnet (solenoid), and is typically used to remotely control a high current/voltage circuit using a low current/voltage circuit, as shown below.
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Electromagnetic Induction If relative motion exists between a conductor and a magnetic field an electromotive force (EMF) will be induced in the conductor, whose magnitude is determined by the following factors:¾
The strength of the magnetic field.
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The speed of the conductor with respect to the field.
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The angle at which the conductor cuts the field.
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The length of the conductor in the field.
These factors are all a natural consequence of ‘Faraday's law’, which states that the voltage (EMF) induced in a conductor is directly proportional to the rate at which the conductor cuts the magnetic lines of force. This principle is used in generators, and ‘Fleming’s Right Hand Rule’ can be used to establish the polarity of the induced EMF. This rule involves the thumb and the first two fingers of the right hand being placed at 90° to each other, as shown on the next page. The thumb points in the direction of motion of the conductor, the index finger points in the direction of the lines of magnetic flux, and the middle finger points to the positive end of the conductor. The middle finger also shows the direction of current flow, when an external circuit is connected across the two ends of the conductor.
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Chapter 5. DC Generator Systems Introduction Modern aircraft electrical power systems are extremely complex and varied. DC generator systems have now been mostly superseded by AC generator systems, although it is still necessary to understand the operation of a DC system. Basic Generator Theory All generators work on the principle of magnetic induction, and it is purely the method by which the resulting voltage (EMF) is converted which determines its type. An AC generator is a device, which converts mechanical energy into AC electrical energy using this principle, where a voltage (EMF) is induced in a conductor as it moves through a magnetic field.
The magnitude of the voltage produced is dependent on the following factors:¾ ¾ ¾ ¾
The strength of the magnetic field. The speed at which the conductor cuts the magnetic field. The length of the conductor within the magnetic field. The angle at which the conductor cuts the magnetic field.
The polarity of the induced voltage can be found using ‘Fleming’s Right Hand Rule’ for generators, which involves the thumb and the first two fingers of the right hand being placed at 90° to each other. The thumb points in the direction in which the conductor is moving, the first finger points in the direction of the magnetic field (N to S), and the second finger indicates the polarity of the induced voltage (+ Ve). The second finger also points in the direction in which conventional current is flowing in the conductor when it is connected across a load. Simple AC Generator In its simplest form an AC generator consists of a single loop of wire, which is mounted, so that it can rotate within a magnetic field. When the loop (Armature) is rotated an AC voltage is induced in it, which can be transferred easily to an external circuit by means of carbon
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brushes, which bear on 2 copper or brass slip rings that are connected directly to the loop. When the armature moves through 360°, or one revolution at a constant speed the output voltage and current rises to a maximum in one direction and back to zero, before reversing in polarity. It then rises to a maximum in the opposite direction, before again returning to zero. The paths plotted out by the voltage and current are in the shape of a sine wave.
The magnitude and polarity of the induced EMF is related to the actual position of the armature, as shown below.
Conversion of AC to DC The AC is converted to DC by replacing the slip rings with a commutator, which consists of 2 segments insulated from each other and connected to the ends of the loop, as shown on the next page. The commutator is a device, which is connected across the output in such a way that the connections to the load are reversed every time the polarity of the voltage in the loop changes thus maintaining the current to the load in the same direction. The load is connected to the loop by brushes, which bear on opposite sides of the commutator.
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When the loop is at 90° to the magnetic field no EMF is induced, and thus no current flows. With the loop in this position the brushes will be in contact with both segments of the commutator the loop will thus be short-circuited.
As the loop rotates the short circuit is removed; the left brush becomes connected to the down- going segment, whilst the right hand brush becomes connected to the up-going segment. The right hand brush is thus in contact with the segment, which is positive, since the current flows away from this side of the loop, and the left-hand brush is alternatively connected to the negative segment. As the brushes are always connected to the conductors moving in the same direction in relation to the magnetic field the output is always DC. The change over from one segment to the other takes place at the instants when the voltage induced in the loop is zero, ie. at positions A, C and E. The commutator is therefore a switching device, which reverses the direction of the current during alternate half-cycles in the output. To produce a smoother and more constant output voltage, as shown on the next page, by fitting additional wire loops and commutator segments.
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DC Generator System Architecture All aeroplane generator systems must be capable of supplying a constant voltage for varying engine speed and load conditions, which is achieved by varying the field strength (excitation) of the generator. The components of a basic single engined generator system are shown below:-
DC Generator Construction The construction of a typical DC generator is shown on the next page, and consists of the following components:¾ ¾
The Yoke. This is a cylinder of cast iron, which supports the pole pieces of the electromagnetic field. The Armature. This is driven by the aircraft engine and holds the windings in which the output voltage of the machine is induced.
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¾
The Commutator. The voltage induced in the armature is AC. commutator changes the AC voltage into DC voltage.
The
¾
The Quill Drive. This is a weak point, which is designed to shear and protect the engine, if the generator seizes.
¾
The Suppressor. This reduces radio interference, which may be caused by sparking between the brushes and commutator.
Principle of Operation of a DC Generator When the armature is rotated in the magnetic field a DC voltage is collected at the brushes, and if this voltage is applied to a load, a current will flow in the armature. This will produce a motoring torque in the generator, and this will act in opposition to the driving torque. This effect is noticeable on a car engine tachometer, when the headlights are switched on and off. When the lights are switched on more current is drawn from the generator, thus increasing the motoring effect, and slowing the engine down. Alternatively switching the lights off will reduce the load on the generator, thus reducing the motoring effect, and reducing the overall load on the engine. Types of DC Generator Three basic types of DC generator exist, with each differing in how the armature and field windings are electrically connected:Shunt Wound. In this arrangement the field windings are connected in parallel with the armature windings, as shown below.
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It is used in all aeroplane DC generators, and at a constant speed has a slightly falling voltage output with increasing load. Series Wound. In this arrangement the field windings are connected in series with the armature windings, as shown below.
This type of generator is however not used on aeroplanes, because at a constant speed it has a rising voltage output characteristic with increasing load. It is thus difficult to regulate the voltage output from this type of generator. Compound Wound. In this arrangement some of the field windings are connected in shunt, and some are connected in series with the armature windings, as shown below.
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This arrangement is used on larger more expensive types of aeroplanes, where it's output characteristics depend on the actual generator design, ie. the ratio of shunt to series windings. Voltage Regulator The voltage regulator is designed to maintain a constant generator output voltage for varying loads, and engine speeds. Many different types of voltage regulators are fitted in aeroplane generator systems, although the following are the most common types. Carbon Pile Voltage Regulator. A diagram of a typical carbon pile voltage regulator is shown on the next page. In this device two forces act on a pile of carbon discs fitted in series with the generator field coil. The first of these forces is due to a moveable iron armature, which is attached to a leaf spring, and holds the pile in compression, whilst the second force is due to an electromagnet, which tends to pull the pile apart. Any variation in the magnitude of pile compression will vary the resistance, and thus the excitation current being supplied to the generator field.
For example the regulator is set to control a generator to give an output of 28 volts DC, if the output voltage falls below 28 volts DC, the current through the electromagnet will reduce, thus causing the leaf spring to compress the pile. This in turn will cause a reduction in the
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piles resistance, and the amount of current to the field coil will increase, thus bringing the generator output back to 28 volts. The opposite will occur if the generator voltage output exceeds 28 volts. Transistorised Voltage Regulator. This type of regulator is a solid state device, and has the following advantages over the carbon pile voltage regulator:¾ ¾ ¾ ¾
Less maintenance Less Weight More reliable Little or no radio interference
Cut-out The DC generator in an aeroplane electrical supply system has to be protected from the battery voltage whenever the engine is shut down, or when its output alternatively fails. This is normally achieved by a ‘Cut-out’, which is fitted between the generator and the busbar. Many different types of Cut-out exist, of which the most common is the ‘Differential Current Cut-Out’, as shown on the next page. The main components in this device are a ‘Series (Current) Coil (DCO)’ that is wound physically on top of a ‘Differential (Voltage) Coil’, and which in turn controls the ‘Generator Line Contactor (GLC)’. The contacts in the cut-out are initially closed via the ‘Differential Coil’ when the generator output voltage is approximately 0.5 volts greater than that being already supplied by the battery. This in turn causes the GLC to close, and allows the generator to feed the busbar via the ‘Series Coil’. The resulting magnetic field produced by this coil then adds to that already being produced by the Differential coil, and helps to hold the GLC in its closed position.
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Conversely if the generator voltage output drops, for whatever reason, a reverse current will flow in the current coil, and a corresponding magnetic field will be produced. This field will act in opposition to the magnetic field being produced by the Differential coil, and will weaken the overall combined magnetic field. This will cause the GLC to open, and will stop the generator feeding the busbar. If the generator voltage subsequently returns to its normal output value, the generator will automatically feed the busbar again, as the DCO contacts close, via the Differential coil.
Alternatively if the engine is shut down or fails the Differential Cut-out will cause the GLC to open when the reverse current reaches a value of 20-30 amps. Reverse Current Circuit Breaker The Differential Cut-out does not provide complete protection to the generator system, since any short circuit on the outgoing side of the Differential Cut-out will not ‘open’ or ‘Trip’ the GLC. A ‘Reverse Current Circuit Breaker (RCCB)’ is thus fitted between the main contactor and the aeroplane busbars to provide complete protection. The RCCB is designed to operate at a very high speed if the reverse currents reach a value of approximately 500 amps, and will mechanically ‘lock’ itself out until reset. Some RCCB's are additionally fitted with auxiliary contacts, which are used to open the generator field and provide further protection against overload or fault conditions. Busbars Busbars are current distribution points and are usually standard rectangular sections of high conductivity copper or aluminium, which are categorised as follows.¾
Vital Busbar. This busbar is powered directly from the aeroplane battery, and is used for emergency undercarriage selection, and also to provide power for fire extinguishers and emergency lighting.
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Essential Busbar. This busbar supplies equipment, which is essential to ensure the safe flight of an aeroplane.
¾
Non-Essential Busbar. This busbar can be isolated (LOAD SHED) in emergency flight conditions, since the equipment they feed is of very low priority, eg. galley supplies.
Power Failure Warning All generator systems are fitted with a red warning lamp, which illuminates whenever the Generator Line Contactor is open, and the generator is no longer feeding the busbar. All warning lamps should be tested prior to flight, which in older types of aeroplanes is done by pressing the individual light, but on modern aeroplanes is automatically initiated whenever the electrical power is first switched on. Ground Power The battery on a modern aeroplane has a limited capacity, and is used only in emergencies, and for engine starting. On the ground the battery is only able to supply a minimum of services, and it is therefore necessary to provide ground supplies during servicing, or long holdover times. A typical Ground Power system is shown below.
In this system it is important that the aeroplane supplies (battery and generator) are disconnected whilst the ground supplies are connected to the aeroplane, and this is achieved by a short auxiliary pin in the ground power socket, which operates a ‘Hold-off Relay’ in the aeroplane electrical system. This is necessary because the ‘Ground Power Unit’s (GPU)’
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regulated voltage may not be identical to that of the aeroplane generators. For example if the ground power is too high, there is a risk of overcharging the aeroplane battery, and damaging electrical equipment, but if the ground power voltage output is alternatively too low, the first generator to come on line would feed into the GPU, and cause instability. DC Generator System Fault Protection A typical DC generator system is protected against the following faults:¾
Overheat. An overheat thermostat is fitted in most aeroplane generators, which will cause an overheat warning light to illuminate on the flight deck if the generators cooling air exhaust exceeds approximately 160°C. If this occurs the generator should be manually switched off.
¾
Seizure. If the generator seizes due to a mechanical fault, the aeroplane's engine may be damaged. A ‘Quill Drive’ is thus fitted between the engine and the generator, which is designed to shear if the generator seizes, and will automatically disconnect the generator from the engine.
¾
Over-Voltage. This condition is usually caused by a malfunction of the voltage regulator and may cause damage to the loads and battery if allowed to continue. An over-voltage sensor is thus fitted in the system, which will trip the generator off the busbar, and de-excite its field. One reset attempt is normally allowed, by ‘Recycling’ the system, ie. by switching the generator ‘OFF’, and then ‘ON’ again.
¾
Under-voltage. This is explained in the operation of the series coil in the Differential Cut-out.
¾
High Reverse Currents. Current Cut-out.
This is explained in the operation of the Reverse
Twin Engine DC Electrical System On a multi-engined aeroplane, a generator is normally fitted to each engine gearbox, and a typical twin engined turbo-propeller DC electrical system layout is shown below:-
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The generators are usually arranged in parallel with the loads so that:¾
There will be no break in the supplies if a generator fails.
¾
The system can handle the switching of high transient loads.
¾
The generators can share the loads equally to improve their life expectancy.
The main disadvantage of paralleling generators is that additional circuitry is required to ensure that both machines equally share the loads. Each generator is thus fitted with an ammeter so that the flight crew can regularly check that the load sharing is correct. Operation of DC Generators in Parallel Reference the diagram on the opposite page, two generators (G1 and G2), are fitted to the No.1 and No.2 engines respectively. When the No.1 engine is started the generator will rotate and will produce an output voltage, which will cause current to flow to the generator field coil via its own voltage regulator. After a short time its output will reach its regulated value, which the flight crew can check using the aeroplane's voltmeter, and sufficient current will flow in the Differential coil to close the differential relay. If the generator control switch is in the ‘ON’ position, the Generator Line Contactor (GLC) will subsequently close, thus allowing the generator to charge the battery, and feed the loads. The flight crew can confirm that the generator is feeding the busbar by the following methods:-
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¾
The dedicated generator warning light extinguishes.
¾
The generator ammeter reads the current being taken by the battery and loads.
After starting the No.2 engine the second generator can be brought onto the busbar in the same way as the No.1 generator. DC Load Sharing Whenever the generators are operating in parallel they must share the aircraft electrical load equally, and this is achieved by ensuring that their individual output currents are equal under all operating conditions. This is achieved by incorporating an ‘Equalising Circuit (Load Sharing Loop)’, as shown below, where the equalising coils are wound on the same core as the voltage coil in the voltage regulator. This circuit monitors the generator outputs and automatically adjusts the voltage regulators to ensure equal load sharing.
The flight crew can also check that any load sharing is equal, by referring to the individual generator ammeters. Operation of an Equalising Circuit If both generators are equally sharing the load, points X and Y as shown the previous diagram, will be at the same potential, because the volts drop across R1 and R2 will be the same. If the No.1 generator for example takes more than its share of load, the voltage drop across R1 will increase, and the voltage drop across R2 will decrease. This will cause point X to become more negative with respect to point Y, and current will flow from Y to X through the Equalising Circuit. The resultant magnetic fields associated with the current flowing in the equalising coils will thus reduce the overall magnetic pull on the No.2 Voltage Regulator, and increase the magnetic pull on the No.1 Voltage Regulator. The resistances of the Carbon Piles will therefore decrease and increase respectively This will in turn cause the output from
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the No.2 generator to increase and the output from the No.1 generator to decrease, which will continue until both generators are sharing the load equally. At this point X and Y will be at the same potential and no current will flow in the Equalising Circuit. The acceptable paralleled generator load difference varies between aeroplanes, but ideally with a 100 Amp load in a twin generator system each should carry 50 Amps, although a maximum differential of 60-40 may be acceptable. Note: the Equalising Circuit only operates when the generators are operating in parallel. Single Engine Aeroplane DC Electrical System Most modern single piston engined aeroplanes have a 14 volt DC electrical system, which consists of an ‘Alternator’ and battery combination, as shown below.
The Alternator is the primary electrical source when the engine is running and charges the battery. The battery provides the secondary power supply, which is used for initial engine start, and as an emergency power source. The engine mechanically drives the Alternator via a drive belt, and the resulting AC output is converted directly into DC by passing it through a ‘Bridge Rectifier Pack’, as shown on the next page. The Alternator is made up of field windings, which are wrapped around a number of pole pieces on a rotating shaft (rotor), and rotate within fixed windings (stator) that are arranged in a star configuration.
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Operation of the Alternator Two rocker type switches: a battery master switch, and an Alternator switch, control the Alternator, which are normally positioned next to each other, but are independently operated.
The battery master switch connects the battery to the busbar, whilst the Alternator switch controls the generators field excitation. With both switches in the ‘ON’ position current is initially passed to the rotor field winding, via brushes and slip rings, ie. ‘Separately Excited’. The resulting electromagnet induces a three phase alternating voltage in the stator windings as it rotates, at a frequency dependent on the rotor speed. The AC output is then converted electronically into DC before being supplied to the main busbar. The Alternator field winding is the supplied from the busbar, ie. ‘Self Excited’, via a voltage regulator, which maintains a constant voltage output, irrespective of the engines RPM, and load on the system. The output from the Alternator is monitored using one of the following Ammeter arrangements:¾
Zero Left Ammeter or Loadmeter. This type of ammeter is placed in the circuit as shown on the next page, and is used to measure the amount of current, or load being supplied by the Alternator. If the reading drops to zero in flight, it normally indicates that the Alternator has failed, and that the battery is alternatively supplying the loads.
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¾
Centre Reading Ammeter. This type of ammeter is placed in the circuit as shown below, and is used to measure the amount of current flowing to or from the battery, ie. the battery charge or discharge rate.
Following an engine start the pointer will normally be deflected to the right of centre, and this indicates that the battery is being charged. If the pointer is alternatively left of centre it shows that the battery is discharging, and helping to supply the necessary electrical power. If this occurs the flight crew should immediately reduce the electrical load on the system and conserve the battery power.
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Chapter 6. DC Motors Introduction A wide variety of components and systems depend upon mechanical energy, which is supplied by motors, and the exact number installed in any aeroplane depends on its complexity. Aeroplane electrical motors vary in size and complexity. Some of the typical applications of DC motors are given in the following table:Function
Equipment Actuators
Fuel ‘trimming’, cargo door operation, heat exchanger control flap operation and landing flap operation.
Control valves
Hot and cold air mixing for air conditioning and thermal deicing.
Pumps
Fuel delivery, propeller feathering and de-icing fluid delivery.
Flight Instruments and Control Systems
Gyroscope operation and Servo control.
In most of the above applications the motors and mechanical sections of the equipment are coupled together to form an integral unit. The power supply required for their operation is typically 28 volts DC and/or 26 volts AC, or constant frequency 115-volts AC. Many motors are also only operated for short periods of time during a flight, eg. between 15 and 90 seconds, and after operation at their rated load must be allowed to cool, in some cases for as long as 10 to 20 minutes, eg. a propeller-feathering pump motor. The construction of a DC motor is similar to that of a DC generator and many machines may be operated in either role, eg. a starter-generator. Continuously rated motors may be either fan cooled or blast cooled and, in the case of fuel booster pumps, which are immersed in the fuel, the heat generated during its operation is transferred from the sealed motor directly to the fuel. These motors tend to rotate at high speed, so a reduction gearbox is used as the intermediate transmission system, if they are being used to produce mechanical movements. The Motor Principle DC motors work in the opposite sense to that of DC generators, where instead of mechanically rotating the armature in a magnetic field to produce an electrical output, the armature is alternatively supplied with an electrical supply, thus converting electrical energy into mechanical energy. If a current carrying conductor is placed in a magnetic field, the field around the conductor will interact with the magnetic field and cause the conductor to move.
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On one side of the conductor the field will be strengthened, whilst on the other side it will be weakened, and the conductor will tend to move in the direction of the weaker magnetic field. The direction of the current in the conductor will thus determine the direction of motion, as shown on the next page.
The direction of this motion can be found using ‘Fleming’s Left Hand Rule’, by placing the thumb, the first finger, and the second finger at 90° to each other, as shown below.
The first finger points in the direction of the magnetic field (N to S), the second finger points in the direction in which the conventional current is flowing in the conductor, and the thumb points in the direction in which the conductor will move. DC Motors There is little difference between DC generators and motors, since they both consist of the same essential parts, ie. an armature, field windings, a commutator and brush gear. The armature and field windings are supplied from a common power source in most motors, and are also self-excited. In its simplest form a motor consists of a single loop of wire ‘PQ’, which is arranged so it can rotate between the pole pieces of a permanent magnet.
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The ends of the wire are connected to commutator segments, which are contacted by brushes to a DC power source. When current flows in the loop in the direction shown, a magnetic field is produced around the wire, which interacts with the main field, and produces a force. These forces act in opposition, and the resultant couple sets up a torque, which causes the loop to rotate in an anti-clockwise direction. When the loop reaches a position at 90° to the magnetic field the two halves of the commutator are shorted out, and no current flows in the loop. The loss of the field in the loop thus essentially stops the rotation, but the inertia of the loop continues to carry it through its vertical position. The action of the brushes on the commutator then reverses the polarity of the supply, and also reverses the direction of current in the loop. Due to the relative position of the field around the wire, and the main field at that instant, the resultant force causes the loop to continue moving in an anticlockwise direction. The process then repeats itself, and the loop will continue to rotate, as long as power is being supplied. Back EMF As the conductor (armature) of a motor moves through the magnetic field a voltage is induced in the conductor, similar to that involved in a generator, which according to Lenz’s Law oppose the motion producing it. The induced voltage or ‘Back EMF (EB)’ will thus oppose the supply voltage, and its direction can be established using Fleming’s Right Hand Rule. The magnitude of the back EMF is directly proportional to the speed of rotation of the conductor and the strength of the magnetic field, as shown below. (EB α N x Φ), where Φ = field strength and N = armature speed The back EMF never exceeds the supply input voltage and the difference between them is always such that current flows in the conductor. This is known as ‘Armature Current’, and the resultant torque produces motion. The relationship between the supply voltage and the back EMF is as follows:VSUPPLY = EB + IARA, where IA = armature current & RA = armature resistance
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For example if a DC motor with an armature resistance of 0.1Ω is supplied with a supply voltage of 200Volts and the back EMF is 199Volts, the armature current will be:VSUPPLY
= EB + IARA
200
= 199 + 0.1 x IA
IA
= 200 – 199 0.1
= 10Amps
Alternatively if the load on the motor increases it will slow down and the back EMF will reduce. The armature current will thus increase, as will the motor torque, in accordance with the following relationship, and the motor will return to its original speed. Torque α Φ x IA, where Φ = field strength and IA = armature current The force on each conductor (armature) and thus armature torque is proportional to the field strength and the armature current. As the speed of rotation increases the back EMF will similarly increase, and the armature current will reduce. Direction of Rotation The directions of the armature current and the main field in a motor, in accordance with Fleming’s Left Hand Rule, determine the way in which it will rotate. Reversing the direction of either will alter its direction of rotation, but if both are changed at the same time the direction of rotation will remain unchanged. Motor Speed Control Most DC motors run at a constant speed, but by varying the strength of the main field or armature current it is possible to vary the speed. Consider the ‘Normal’ operating speed of a motor to be the speed at which the motor will rotate when it is connected directly across the power supply, and any change in speed can be accomplished using one of the following methods:Armature Control. This is achieved by varying the magnitude of the main field using the following arrangement. IF 2)
M
SUPPLY
1)
If the resistance of the variable resistor is reduced it will cause the field current (IF) to reduce, thus weakening the main field, and increasing the speed of the motor. This is because the weaker field will produce a smaller back EMF, and the armature current will thus increase.
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This will increase the speed of the motor, which will increase the back EMF, thus restoring the balance between the applied EMF and the back EMF. Conversely if the resistance is increased it will cause IF to increase, thus strengthening the main field, and reducing the speed of the motor. Field Control. This is achieved by varying the magnitude of the armature current (IA) using the following arrangement.
VARIABLE RESISTOR
IA
M
SUPPLY
If the resistance of the variable resistor is reduced, it will cause the armature current (IA) to reduce and the motor to slow down. Conversely if the resistance is increased, it will cause IA to increase, and the motor will thus speed up. Types of DC Motor Like DC generators, DC motors used on aeroplanes, are classified according to the way in which the field excitation circuit is arranged. Series Motors. In series-wound motors, the field windings and the armature windings are connected in series with each other, as shown on the next page, so that the same current flows through both sets of windings. The windings consist of a few turns of heavy wire that have low resistance, which enables a series motor to be able to draw a large current on starting. This prevents the field strength increasing quickly, and gives the motor its principal advantages of high starting torque and good acceleration. A rapid build-up of back EMF induced in the armature also limits the current flow through the motor.
The speed load characteristic of a series wound motor is such that variations in mechanical load are accompanied by substantial speed variations; a light load will cause it to run at a dangerously high speed, and a high load will cause it to run at low speed.
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For a given motor:Torque α Φ x IA, where Φ = field strength and IA = armature current In a series motor the magnitude of the main field (Φ) is approximately proportional to the armature current (IA) up to full load, so that:Torque of a series motor is approximately proportional to (IA)2 The amount of driving torque will thus rapidly increase with increasing load, since any reduction in the back EMF will directly increase the armature current. It is thus important that this type of motor must be started on full load, and is the type used on aeroplanes as starter motors or actuators. Shunt Motors. In shunt-wound motors the field windings are made up of many turns of relatively thin wire, and are connected in parallel with the armature.
The resistance of the winding is high, but since it is connected directly across the power supply, the current through it will remain constant. These motors have a slightly falling speed load characteristic, and are thus used where a fairly constant speed is required, eg. in windscreen wipers, fuel pumps and rotary inverters. This is because a small voltage drop occurs across the armature (IA x RA) with increasing load. In a shunt wound motor the main field is virtually independent of the armature current, and the amount of torque being produced is directly proportional to the armature current up to full load. In this type of motor the majority of the current will flow through the low resistance armature, so the starting torque is small, compared to the series wound motor, since the main field is slow to build up. Shunt-wound motors must therefore be started on light or no load. Compound Motors. These motors utilise the principal characteristics of both series and shunt wound motors, but without the effects of some of their normally undesirable features.
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For example, a motor may be required to develop the high starting torque of a series wound motor, but without the tendency to over-speed when the load is removed. Another application may require a motor, which is capable of reducing its speed with increasing load, whilst still retaining the smooth speed control and reliability of a shunt wound motor when operating, ‘Off Load’. These and other requirements can be met by what is termed ‘Compounding’, or in other words, by combining both series and shunt field windings in the one machine, using one of the following arrangements:¾
Normal Compounding. In this arrangement a motor is biased towards the shunt-wound type, where the shunt winding produces approximately 60 to 70 per cent of the total flux, whilst the series winding produces the remainder. The desired characteristics of both series and shunt-wound motors are thus retained.
¾
Stabilised Shunt. In this arrangement the motor is also biased towards the shunt-wound type, and only has a relatively minor series winding. The purpose of this winding is to overcome the tendency of a shunt motor to become unstable when running at or near its maximum operating speed, when subjected to an increased load.
¾
Shunt Limited. In this arrangement the motor is biased towards the serieswound motor, and only has a minor shunt field winding incorporated in the field system. The purpose of this winding is to limit the maximum speed when running under ‘Off Load’ conditions whilst leaving the torque and general speed characteristics unaltered. Shunt limiting is applied only to the larger type of compound motors, eg. engine starter motors.
Actuators These are high-speed reversible series wound motors whose output is normally converted into a driving torque via a step-down gearbox. Motor actuators are self-contained units, which combine electrical and mechanical devices, that are capable of exerting reversible linear thrust over a short distance, or alternatively a reversible low-speed turning effort. The following types of actuators exist:Rotary Actuator. This type of actuator has a rotary movement, and is used mainly to rotate valves in the air conditioning and fuel systems.
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Linear Actuator. This type is driven directly from a reduction gearbox via a lead screw, which when rotated, extends or retracts a ram or plunger.
5)
B R A
EPICYCLIC REDUCTION LIMIT SWITCHES GEARING
4)
This type of actuator is capable of working against heavy loads, and is used to operate trailing edge flaps, trim tabs, and to move variable incidence tailplanes. Split-Field Series Motor In this type of motor the field winding is split into two separate electrical sections, thus establishing two independently controlled magnetic fields. One of the windings is used to control each direction of rotation, and is controlled by a single-pole double-throw switch. Both linear and rotary type actuators are equipped with limit switches to stop their respective motors when the operating ram or output shaft, as appropriate, has reached the permissible limit of travel. The switches are of the micro switch type, and are usually operated by a cam driven by a shaft from the actuator gearbox. In some cases, limit switch contacts are also utilised to complete circuits to indicator lights or magnetic indicators. For example consider the operation of an air conditioning duct valve, as shown in the diagram on the next page. If the switch is placed in the ‘Open’ position, current will flow in the ‘Open Field Winding’, and then through the armature winding. The two fields will interact, and the armature will rotate, which will cause the cams to rotate at the same time. These cams determine the position of two limit switches, which control the current through the field windings, and the position of magnetic indicators or lights that show the position of the valve.
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As soon as the motor starts to rotate ‘Limit Switch A’ breaks the circuit to the ‘Closed’ indicator and causes the light to go out. When the valve is in its fully open position the limit switches are arranged, so that ‘Limit Switch A’ completes the circuit to the ‘Close Field Winding’, whilst ‘Limit Switch B’ breaks the circuit to the ‘Open Field Winding’, and operates the ‘Open’ indicator. If the switch is then placed in the ‘Close’ position, current will flow through the ‘Close Field Winding’, and then through the armature. The two fields will interact, and the motor will rotate in the opposite direction, thus closing the valve. This occurs because the polarity of the field windings reverse, but the direction of the current through the armature remains the same, thus the resultant interaction of the fields will cause the armature to run in the reverse direction. When the valve is fully closed, the position of the limit switches will reverse, thus completing the circuit to the ‘Open Field Winding’, and operating the ‘Closed’ indicator. Electromagnetic Brakes Most actuators are fitted with electromagnetic brakes, as shown below, which are designed to prevent over-travel when the motor is switched off. The design of the brake system varies with the type and size of the actuator, but in all cases the brakes are spring-loaded to the ‘ON’ condition whenever the motor is de-energised, thus preventing the actuator overrunning.
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Conversely the brakes are immediately withdrawn whenever power is applied to the appropriate field winding, since the brake solenoid is connected in series with the armature. Clutches Friction clutches, that are usually of the single-plate type or multi-plate type, which depends on the size of the actuator, are also incorporated in the transmission systems to protect them against the effects of mechanical over-loading. Instrument Motors DC motors are not widely used in aeroplane instruments, but form the gyroscopic element in one or two types of turn-and-bank indicator. The motor armature together with a concentrically mounted outer rim forms the gyroscope rotor. The purpose of the rim is to increase the rotor mass and radius of gyration, thus increasing its rigidity. The armature rotates inside a cylindrical two-pole permanent magnet stator, which is secured to the gimbal ring. Current is fed to the brushes and commutator via flexible springs to permit gimbal ring movement. The rotor speed is kept constant by a centrifugal cut-out type governor, which consists of a fixed contact and a movable contact, that are normally held in the closed position by an adjusting spring. The contacts are fitted in series with the armature winding, and a resistor is connected in parallel with the contacts. When the maximum speed is attained, the centrifugal force acting on the movable contact overcomes the spring restraint, and causes the contacts to open. Current to the armature then passes through the resistor and reduces the rotor speed, until it resumes its nominal value. Architecture of a Starter/Generator System Some types of turbo-propeller aeroplanes utilise a single unit for starting the engine and supplying the aeroplane's DC power. This unit is called a ‘Starter/Generator’, and a typical system is shown below.
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It is basically a compound machine, which is coupled to the engine by way of a drive shaft and gear train. Operation of a Starter/Generator System When the engine start switch is operated, the following sequence of events takes place:-
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¾
The starter relay energises and the two batteries are connected in parallel supplying 24 volts to the starter motor. This reduces the initial starting current and torque, thus extending the life of the starter motor.
¾
When the engine reaches 10% RPM a speed sensor energises the paralleling relay (A), which causes the batteries to be momentarily connected in series, as shown below, thus supplying the starter motor with 48 volts.
¾
At 60% engine RPM the starter and paralleling relay are de-energised, which removes the power from the starter motor, and reconnects the batteries in parallel.
¾
When the engine is running correctly the generator control switch is operated, and the DC generator feeds the busbar.
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Inverters Certain electrical systems on aeroplanes require AC at a constant frequency, thus it is necessary to provide a means of producing a constant frequency supply on DC, and AC frequency wild powered aeroplanes. This is achieved via an inverter, of which the following basic types exist:Rotary Inverter. This type is basically a DC motor, which drives an AC generator on a common shaft, as shown below.
The motor drives the AC generator at a constant speed to give a constant frequency output, which is achieved by adjusting the field excitation of the DC motor, and the output voltage of from the AC generator is maintained by similarly adjusting its field excitation. This type of inverter has a DC input of 28 volts and produces a 3 Phase AC output of 115V at 400HZ. Most rotary inverters are only 50% efficient, and typically a 100 Watts DC input will produce a 50 VA AC Output.
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Static Inverter. This type differs from the rotary type in that it is constructed using solidstate transistorised circuitry. It is also more robust, more reliable, and requires less servicing. Static inverters cannot match the power output of rotary inverters, although most have an efficiency of approximately 70%. Multiple Inverter Installations A typical multiple inverter system is shown below, and is the type, which is commonly fitted on twin-engine turbo-propeller aeroplanes.
This system consists of three inverters, of which the No.1 and No.2 inverters, are of the same type, and supply normal constant frequency AC power, whilst the No.3 inverter is a smaller type, and is used to supply the essential AC loads in an emergency. Inverters cannot be operated in parallel, so it is thus necessary to devise a method by which each of the main inverters, No.1 and No.2, receive approximately the same running time. This is achieved in some airlines by using the No.1 inverter as the main one, and the No.2 inverter as the standby one on the outgoing journey, and vice versa on the return journey. Many inverters also have their outputs monitored for correct voltage and phase rotation. If either of these factors is incorrect the inverter will be automatically removed from the loads.
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Chapter 7. Inductance and Capacitance Introduction Inductors and capacitors commonly play a role in AC circuits, but they also possess important DC characteristics. Inductance Inductance is the property of a device or circuit, which opposes a change in current flow, or its ability to induce a voltage when there is a change in current flow. Every conductor displays the property of inductance, but in conductors of short length, the inductance value is so small that it can only be measured with very sensitive instruments. The unit of Inductance is the ‘Henry (H)’, where one Henry is the amount of inductance, which will induce an EMF of 1 volt into a conductor when the current changes at the rate of 1 ampere per second. Due to its magnitude the units; ‘milli-Henry (mH)’ and ‘micro-Henry (µH)’ are more commonly used in electronic applications. The symbol for inductance is ‘L’, thus if an inductance has a value of 15 milli-Henrys it can be written as:L = 15mH The following rules are instrumental to inductance:¾
When current flows through a conductor a magnetic field builds up around it.
¾
When a conductor is moved through a magnetic field an EMF is induced in it.
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Self Induction When power is first applied to a DC circuit the current rises from zero to its normal steady state value or the value computed by Ohm’s Law over a short period of time (transient time), during which a transient condition exists. In circuits containing only resistors, the transient condition exists only briefly, and can only be detected with sensitive instruments. If inductors or capacitors are also included in the circuit, the transient condition may be extended, so that it is more readily detected. This occurs due to ‘Self Induction’ because when current flows through a conductor, a magnetic field builds up around the conductor, and expands outwards from its centre. As the field moves outwards it cuts the conductor, thus inducing an EMF in it, which opposes the applied EMF (counter EMF), and the resulting induced current flows in the opposite direction to the original current.
If the current is then switched off the field will collapse inwardly and will induce an EMF in the conductor, which will cause an induced current to flow in the same direction as the original current. The direction of the induced current can be established using Fleming’s Right Hand Grip Rule. In some cases the induced EMF can be so high that if a circuit containing an inductor is open-circuited via a switch, violent arcing may take place across the switch. Inductors Every conductor has a certain value of inductance, but to be of any use they need to be wound in the form of a coil. In electronic circuits inductors that are used have a specific value of inductance. The inductance characteristics of an inductor can be increased by either increasing the number of turns on the coil, and/or by inserting a piece of permeable material into the coil, eg. soft iron.
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Time Constant of an Inductor The time taken for the current in an inductor to reach a steady value depends on the value of the inductance, and the value of any series resistance.
For a given value of resistance, the time required for the current to build to its maximum value, takes the form of an exponential curve, as shown on the next page, and is directly proportional to the value of the inductance. The higher the inductance, the more time it will take for the current to reach its maximum value. For practical purposes the growth or decay of the current is complete in 5 L , where R = resistance in Ohms, and L = inductance in R L Henrys. is known as the ‘Time Constant (T)’ of the circuit, which is measured in R seconds, and is the time taken for the current in an inductive-resistive circuit to rise to 63.2% of its maximum value when connected across a supply, or to fall 36.8% of its maximum value when disconnected from the same supply. Thus for a given value of inductance, the time taken will be inversely proportional to the resistance.
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These graphs show that the resistive voltage (VR) increases and decreases in line with the current (I), whereas the voltage drop across the inductor (VL) falls as the current rises, and vice versa.
Inductors in Series and Parallel Inductors can be connected in a DC electrical circuit in either series or parallel.
When connected in series, the inductance's are directly added together:Total Inductance (LT) = L1 + L2 + L3 When connected in parallel the reciprocals of the individual inductances are added together, and the reciprocal of the total gives the total inductance:-
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1 1 1 1 LT = L1 + L 2 + L3
Inductance calculations are thus similar to resistance calculations in a DC circuit. Capacitance Capacitance is the property of an electrical component, which enables it to store energy like a storage tank in an electrostatic field. The unit of capacitance is the Farad (F), where one Farad is the amount of capacitance that will store a charge of one Coulomb when an EMF of one volt is applied. The units of microfarad and pico-Farad are more commonly used. A device, which stores energy in this way, is called a ‘Capacitor (Condenser)’, which has the ability to store a quantity of electrons and release them at a later point. The number of electrons that a capacitor can store for a given applied voltage is a measure of its capacitance, ie. it acts as a reservoir and the total charge from empty to full depends solely on its capacitance and the voltage being applied. The formula, which expresses capacitance in terms of charge and voltage is thus:Q Capacitance(C) = V
where:
C = Capacitance in Farads Q = Charge in Coulombs V = Voltage in Volts
Factors Affecting Capacitance Capacitors in their simplest form consist of two metal plates separated by a non-conducting material called a dielectric.
Metal foil is often used for the plates, whilst the dielectric may be paper, glass, mica or another good insulator. Capacitors can exist in many forms, eg. the conductor of a cable could act as one plate of a capacitor, whilst the airframe of an aeroplane could act as the other plate, and the dielectric is the cable insulation that separates them. A capacitor has a certain amount of capacitance, so if the applied voltage is increased the charge will similarly increase, so that the ratio of the charge to the voltage will remain the same. The actual amount of capacitance is dependent on the physical shape and size of the capacitor, and varies according to the following formula:C= kA d where: k = type of dielectric A = area of the plates d = distance between the plates The capacitance of a capacitor is thus directly proportional to the dielectric constant or the area of the plates, and inversely proportional to the distance between the plates. Types of Capacitor Capacitors can be either fixed or variable, and the most common types are:-
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¾
Paper Capacitors. These are constructed of alternate layers of metal foil separated with similar strips of waxed paper, which act as the dielectric.
¾
Electrolytic Capacitors. These are formed by an electro-chemical process, and are mainly used where large values of capacitance are required. This is because their electrical capacity is high compared with their physical size, which is due to a very thin dielectric being used. Normal capacitors can be used in AC and DC circuits, but most electrolytic capacitors are only used in DC circuits. The polarity must be observed, since connecting them with the wrong polarity may cause damage and injury.
¾
Variable Capacitors. These consist of multiple plates, which are moved via a rotating shaft.
Note: All capacitors are rated not only by their capacitance value, but also by their maximum working voltage. This voltage rating must not be exceeded or the dielectric may break down and arcing may occur. To withstand higher voltages the thickness of the dielectric has to be increased, but the increased distance between the plates results in a lower capacitance, so the area of the plates has to be increased in order to maintain the same capacitance value. Capacitors that have a high voltage rating are thus physically larger than capacitors, which have the same capacitance, but operate at a lower voltage. The Charging of a Capacitor If a capacitor is uncharged the same number of free electrons will exist on both plates, and if a voltmeter is connected across the plates, it would read zero volts, as shown in the diagram below.
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If a DC voltage is subsequently applied to the plates of the capacitor, it will charge up until the potential across the plates is equal and opposite to the supply voltage.
When the switch is closed, the positive terminal of the battery will be connected to the upper plate of the capacitor, and the battery will attract the free electrons from the upper plate. This will leave the upper positive plate with a deficiency of electrons, and the negative lower plate with an excess of electrons. The positive plate will thus try to attract the electrons from the negative plate, but due to the insulator (dielectric) between the plates, no electrons will flow between them. The attraction of the positive charge on the upper plate will instead tend to pull electrons from the negative terminal of the battery to the lower negative plate, and the difference in potential between the plates will cause an electric field to build up in the dielectric between them. The capacitor will continue to charge until the potential difference between the plates equals the supply voltage.
When this occurs, no further current will flow, ie. current will only flow in the circuit whilst the capacitor is charging, and in a DC circuit will not pass through the capacitor. A good capacitor will retain a charge for a long period of time, and most capacitors can also be charged in either direction, by simply reversing the supply. If the supply is removed from the capacitor, the electrical charges on the plates will remain for a long time, which can pose a hazard to the unsuspecting, eg. high energy ignition units. However no dielectric has infinite resistance, so some of the charge will naturally leak away.
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Discharging of a Capacitor Theoretically all of the energy stored in a capacitor can be recovered. It follows that a perfect capacitor dissipates no power. It simply stores the energy, and later releases the energy. A capacitor is therefore only a temporary storage device.
Removing it from the supply and connecting it across a resistor can discharge a capacitor. This causes the current to flow until the capacitor is fully discharged, and its charge has been reduced to zero. The Time Constant of a Capacitor The length of time required for a capacitor to charge or discharge can be computed if certain circuit values are known, and exponential curves show how the voltage across the plates varies with time, as shown on the next page. The two factors, which effect the charge and discharge time, are the resistance (R) and the capacitance (C). R multiplied by C gives the time required for the capacitor to reach 63.2% of its full charge or 36.8% of its full charge value during discharge. This is known as its time constant (T), and is expressed as: T=RxC where:
T = time in seconds. R = resistance in Ohms. C = capacitance in Farads.
In practice the time taken for the capacitor to become fully charged or discharged is equal to 5CR Capacitors in Series and Parallel in a DC Circuit Like resistors and inductors, capacitors can also be connected in various combinations, as shown below.
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¾
Capacitors in Parallel. This increases the effective area of the plates, and thus increases the overall total capacitance. The formula for calculating the total value of capacitors connected in parallel is:-
CT = C1 + C2+ C3..... ¾
Capacitors in Series. This increases the overall thickness of the dielectric and the total capacitance therefore decreases. The formula for calculating the total value of capacitors connected in series is-: 1 1 1 CT = C1 + C2 +
1 C3
Notably if two capacitors of different values are in connected in series in a circuit the smaller capacitor will have a higher value across it rather than the larger one. To understand why this occurs, consider capacitance in terms of voltage: V=
Q C
Voltage is inversely proportional to the capacitance, so the smaller the capacitor, the higher its' voltage.
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Intentionally Left Blank
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Chapter 8. Basic AC Theory Introduction Alternating Current (AC) continually changes its polarity, and can vary in magnitude and direction. This differs from Direct Current (DC), which is usually a constant value, and flows in only one direction. The current in an AC circuit increases from zero to maximum, and back to zero again, before varying in the same manner in the opposite direction. The effect of an AC supply on resistors, inductors and capacitors also differs from that of a DC supply. Advantages of AC over DC AC is extremely versatile, and has the following advantages over DC:¾
AC can be simply and efficiently changed from one voltage to another using a ‘Transformer’.
¾
AC generators are simple in construction and lighter for the equivalent power output of a DC generator.
¾
AC can be easily and efficiently changed into DC using a ‘Rectifier’.
¾
The magnitude of alternating currents or voltages can be easily modified to carry or transmit information as ‘AC Signals’.
¾
AC can be easily converted into ‘Electromagnetic (Radio) Waves’, which can travel through space. This is possible because a conductor (aerial) that carries alternating current produces an electromagnetic field, which expands and collapses as the direction of the current changes. Thus if the current changes at a sufficiently high enough speed the magnetic field will radiate outwards in sympathy with the alternating current, and information will be transmitted from one place to another without the use of wires.
Generating AC An AC generator converts mechanical energy into AC electrical energy using electromagnetic induction, where a voltage (EMF) is induced in a conductor as it moves through a magnetic field. The magnitude of the voltage produced is dependent on the following factors:¾
The strength of the magnetic field.
¾
The speed at which the conductor cuts the magnetic field.
¾
The length of the conductor within the magnetic field.
¾
The angle at which the conductor cuts the magnetic field.
Similar to a DC Generator the polarity of the induced voltage can be found using Fleming’s Right Hand Rule, which involves the thumb and the first two fingers of the right hand being
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placed at 90° to each other, as shown on the next page. The thumb points in the direction in which the conductor is moving, the first finger points in the direction of the magnetic field (N to S), and the second finger indicates the polarity of the induced voltage (positive). The second finger also points in the direction in which conventional current will flow in the conductor when it is connected across a load.
Simple AC Generator In its simplest form an AC generator consists of a single loop of wire or ‘Armature’, which is mounted on a shaft, such that it can be rotated within a magnetic field. When it is rotated an AC voltage is induced in it, which can be easily transferred to an external circuit by means of carbon brushes that bear down on slip rings connected to the loop.
When the armature moves through 360°, or through one revolution at a constant speed the output voltage and current will rise to a maximum value in one direction and back to zero, before reversing in polarity. The voltage and current will then rise to a maximum value in the opposite direction, before again returning to zero. The paths plotted by the voltage and current are in the shape of a sine wave, whose magnitude and polarity are determined by the actual position of the armature as shown on the next page.
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AC Terminology The diagram below shows how the voltage output varies when the armature is rotated through 360°.
By convention the following terminology applies to the resulting sine wave:Cycle. This occurs when the armature of a basic AC generator rotates through one complete revolution (360°). Instantaneous Value. This is the value, which occurs at a specific instant in time. Peak Value. Two of these values occur during each cycle; one occurs during the positive alteration when the waveform reaches its maximum height (Positive Peak Value), and the second occurs during the negative alteration when the waveform reaches its maximum height below the zero line (Negative Peak Value). Peak to Peak Value. This is the overall magnitude of the sine wave between the two peaks. Peak to Peak = 2 x Peak Value
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Average Value. This is the value of the voltage or current that can be calculated by taking a large number of instantaneous values, either positive or negative, and taking their average value, or alternatively by using integral calculus. Either method shows that the average value of an alteration is 0.637 of its maximum peak value. Root Mean Squared Value RMS (Effective Value). This is the amount of power or heat, which will be dissipated by an AC of peak value 1 ampere compared to the amount of power or heat that would be produced by a DC of 1 ampere when flowing through an identical resistor.
The DC will make the resistor hotter than that compared to the AC because with DC the current is steady at I ampere whereas with AC 1 ampere is only reached at the peak of each half cycle. In practice the AC of peak value 1 ampere is only 0.707 times as effective in heating the resistor as a DC of 1 ampere. The effective value is thus equivalent to 0.707 of its peak value, or 0.707 amperes. RMS Value = 1 x Peak Value = 0.707 x Peak Value 2 Unless otherwise stated all values of voltage and current are given as RMS values. For example if the mains voltage is 240 volts RMS, its peak value will be approximately 339 volts. Most measuring instruments thus measure voltage and current as RMS values. Frequency. This is the number of cycles in one second and is measured in hertz. The frequency of an AC waveform is proportional to the speed at which the generator is driven, and the frequencies used in aviation range from just a few hertz to millions of hertz. The main electrical supply in a modern jet aeroplane is 200 volts 400 hertz, although in avionics the frequencies can be much higher, because high frequencies are required to carry information or intelligence. Also the higher the frequency, the easier it is to convert AC into electromagnetic waves, which can be transmitted over long distances. These higher frequencies must however be produced electronically rather than by a generator. Periodic Time. This is the time taken to complete one complete cycle, and is the reciprocal of frequency, ie. 1 Periodic Time = Frequency
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Relationship Between Radians and Degrees The phase relationship between AC voltages and currents are given in terms of either radians or degrees, as follows:-
π/2 π 3π/2 2π
= 90° = 180° = 270° = 360°
Phase and Phase Angle Consider two AC voltages having the same frequency, but having different magnitudes, as shown below.
Both waveforms cross the zero axis at the same time and are ‘in phase’ with each other. They also reach their maximum and minimum peak values at the same time. If the waveforms are alternatively displaced from each other, and cross the zero axis at different points they are ‘out of phase’, as shown below.
The maximum and minimum peak values also occur at different phase angles. By convention the angular difference between the two waveforms where they cross the zero axis and go positive is the ‘phase displacement’, or ‘phase angle’. In the above example V2 thus lags V1 by π/2 radians (90°) or V1 leads V2 by the same amount. The angular difference is also maintained throughout the waveform. If the waveforms are alternatively π radians (180°) out of phase, they are ‘anti-phase’ with each other, as shown below.
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Phasor Representation Any AC quantity that produces a sine wave output can be alternatively represented as a phasor, which is simply a vector representation that rotates at a constant velocity, as shown below.
The length of each phasor represents the amplitude of the waveform and its angle with respect to a given reference axis. In this example the phasors, V1 and V2, are both rotating at ω radians per second, and V1 is leading V2 by π/6 radians (30°). This can be more simply represented by using a phasor diagram, where V1 is taken as the reference phasor, as shown below.
By convention the reference phasor is placed in the 3 o’clock position and all other phasors rotate in an anti-clockwise direction with respect to it. In this case V1 is taken as the reference vector since it has a phase angle of zero and V2 is positioned π/6 radians (30°) behind it, ie. lags behind V1. Any number of voltages and/or currents can be drawn on the same phasor diagram provided that they are all of the same frequency, although in practice the anti-clockwise arrow is normally omitted from the diagram.
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Chapter 9. Single Phase AC Circuits Introduction The sine wave output produced by an AC circuit is dependent on whether a resistor, inductor or capacitor is connected in the circuit. The Effect of AC on a Purely Resistive Circuit When an AC supply is applied to a purely resistive component the current being produced will flow through the resistor in one direction and then the other, as shown below.
The current will vary in both amplitude and direction in accordance with the AC voltage, ie. they will both be in phase with each other. In other words the current is zero if the voltage is zero, and is maximum when the voltage is maximum. When the voltage changes its polarity, the current also changes its polarity. The voltage and current in a purely resistive AC circuit are thus in phase with each other, and can be represented on a phasor diagram as follows.
The value of the current flowing through the resistor at any given instant will depend on the voltage at that said instant, and the circuit resistance. This can be calculated by using Ohms Law, as in the case of a DC circuit, but when working with AC circuits, instantaneous values of voltage and current are seldom used in calculations. The effective (RMS) value is alternatively used. When calculating AC series and parallel resistive circuits, the same method should be used as in DC circuits. Power in an Ac Resistive Circuit In an AC resistive circuit, power is consumed by the resistive component in the form of heat, just as it is in a DC circuit. The power used in either a DC or AC circuit is measured in watts, where 746 watts = 1 horsepower. In DC circuit’s power equals voltage times current. The same relationship exists in an AC resistive circuit, where the power consumed by the resistor is the product of the current passing through the resistor, and the voltage across the resistor.
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The power consumed in a purely AC resistive circuit thus does useful work (True Power or Effective Power), which is measured in ‘Watts’. The Effect of Ac on a Purely Inductive Circuit When an inductor is supplied with an AC voltage a counter EMF is induced in the coil by the continually varying magnetic field.
This will result in a phase shift between the supply voltage and the current. The current will lag the voltage by π/2 radians (90°), or the voltage will lead the current by the same phase angle. This can be represented on a phasor diagram as follows.
Power in an AC Inductive Circuit The instantaneous power is given by multiplying the instantaneous values of voltage and current together.
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In the first quarter cycle the values of voltage and current are both positive quantities, thus producing positive power. In the second quarter cycle the value of the current is still positive, but the value of voltage is now negative, thus negative power is produced. This pattern will continue and will be repeated every half cycle of the waveform. The average power is thus zero, and a perfect inductor will thus dissipate zero real or effective power. The power produced is alternatively known as ‘Reactive Power’, and is measured in ‘Volts Amperes Reactive (VAR)’. Inductive Reactance (Xl) An inductive component (inductor) tends to oppose the change in current flow, and like a capacitor will also offer opposition to the flow of alternating current. The counter EMF induced in an inductor by the varying current will thus oppose the supply voltage. This opposition to current flow is called ‘Inductive Reactance (XL)’, which is directly proportional to the inductance of the inductor, and the frequency of the supply voltage, as shown below. Inductive Reactance (XL ) = 2ΠfL ohms where:-
f L XL
= frequency in hertz = inductance in henrys = inductive reactance in ohms
In an AC circuit, a inductor will have the same effect on current flow as a resistor. In a purely inductive circuit, the current in the circuit will be directly proportional to the applied voltage and inversely proportional to the inductive reactance. IL = V = V XL 2 ∏ fL If the supply frequency is increased the inductive current will decrease and vice versa. An inductive component may thus be damaged if the frequency is reduced. The Effect of Ac on a Purely Capacitive Circuit When an AC voltage is applied to a purely capacitive circuit, the capacitor will charge up in one direction, and then in the other.
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If the voltage and current is monitored, the current will not rise in phase with the applied voltage, as it does in a resistive circuit. In a purely capacitive circuit the current will reach its maximum value π/2 radians (90°) before the voltage across the capacitor reaches its maximum value. The voltage and current are therefore out of phase, and the voltage lags the current by π/2 radians (90°), or the current leads the voltage by the same angle. This can also be represented on a phasor diagram as follows.
Power in an AC Capacitive Circuit The instantaneous power is given by multiplying the instantaneous values of voltage and current together.
In the first quarter cycle the values of voltage and current are both positive quantities, thus producing positive power. In the second quarter cycle the value of the voltage is still positive, but the value of current is now negative, thus negative power is produced. This pattern will continue and will be repeated every half cycle of the waveform. The average power is thus zero, and a perfect inductor will thus dissipate zero real or effective power. The power produced, like in the case of an AC Capacitive circuit, is alternatively known as ‘Reactive Power’, and is measured in ‘Volts Amperes Reactive (VAR)’.
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Capacitive Reactance (Capacitors Ac Resistance) In an AC circuit the capacitor will constantly charge and discharge. This is due to the time lag, which exists and the voltage across the capacitor being in constant opposition to the supply voltage. This creates an opposition to current flow, ie. electrical resistance, which is known as ‘Capacitive Reactance (XC)’. Capacitive reactance is inversely proportional to the capacitance of the component, and the frequency of the applied voltage, as shown below. Capacitive Reactance (XC ) = where:-
f C XC
1 ohms 2Π fC
= frequency in hertz = capacitance in farads = capacitive reactance in ohms
In an AC circuit, a capacitor will have the same effect on current flow as a resistor. In a purely capacitive circuit, the current in the circuit will be directly proportional to the applied voltage and the capacitive reactance. IC = V = V x 2ΠfC XC If the supply frequency is increased the capacitive current will increase and vice versa. A capacitive component may thus be damaged if the frequency is increased. Relationship Between Voltage and Current in Capacitive and Inductive AC Circuits Depending on whether the circuit is inductive or capacitive, the acronym, ‘CIVIL’, acts as an aide memoir as to whether the current leads or lags the voltage.
C I V I L In a Capacitive (C) Circuit I before V (I leads V)
In an Inductive (L) Circuit V before I (V leads I)
This is particularly useful when dealing with series or parallel AC circuits. In series AC circuit’s, current is used as the reference phasor, and in parallel AC circuit’s, voltage is used. Resistive and Inductive (RL) Series AC Circuit When an AC voltage is applied across an RL circuit, and a voltage drop will take place across each component, and the same current will pass through both. Current is thus taken as the reference phasor in the phasor diagram.
The voltage drop across the resistor (VR) will be in phase with the current, and the voltage drop across the inductor (VL) will lead the current by π/2 radians (90°). The supply voltage (VS) can then be calculated using the vector sum of these voltages.
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Resistive and Capacitive (RC) Series AC Circuit When an AC voltage is applied across an RC circuit, and a voltage drop will take place across each component, and the same current will pass through both. Current is thus taken as the reference phasor in the phasor diagram.
The voltage drop across the resistor (VR) will be in phase with the current, and the voltage drop across the capacitor (VC) will lag the current by π/2 radians (90°). The supply voltage (VS) can then be calculated using the vector sum of these voltages, as in the case of the RL series circuit. Phase Shift The phase shift of a circuit is the angle between the voltage and current vectors. It is a function of the reactive and resistive components. In the case of a series RC circuit it can be X X expressed mathematically as tan φ = C , and for a series RL circuit as tan φ = L . R R Resistive, Inductive and Capacitive (RLC) Series AC Circuits In a RLC series circuit current is a common vector and the voltage drops across the resistor, the inductor and the capacitor are as shown below.
The voltage drop across the resistor (VR) will be in phase with the current, the voltage drop across the capacitor (VC) will lag the current by π/2 radians (90°) and the voltage drop across the inductor (VL) will lead the current by π/2 radians (90°). The vertical components, VL and VC, are in direct opposition to each other, so the resulting vertical component is thus (VL – VC). The supply voltage (VS) is found using Pythagoras, as follows. VS =
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Impedance (Z) in a Resistive, Inductive and Capacitive (RLC) Series AC Circuit Impedance is the total opposition to current flow in an AC circuit containing resistance and reactance. In a series AC circuit it is the vector sum of the inductive reactance (XL), capacitive reactance (XC), and resistance (R) as shown below.
Resistive, Inductive and Capacitive (RLC) Parallel AC Circuit In a parallel RLC circuit, voltage is the common vector, and the currents through the resistor (IR), the inductor (IL) and the capacitor (IC) are as shown below.
The current through the resistor (IR) will be in phase with the voltage, the current through the capacitor (IC) will lead the voltage by π/2 radians (90°) and the current through the inductor (IL) will lag the voltage by π/2 radians (90°). The vertical components, IL and IC, are in direct opposition to each other, so the resulting vertical component is thus (IL – IC). The supply current (IS) is found using Pythagoras, as follows. IS =
IR 2 + (IL − I C )
2
Impedance (Z) in a Resistive, Inductive and Capacitive (RLC) Parallel AC Circuit In a parallel AC circuit the reciprocal of impedance is the vector sum of the reciprocals of the inductive reactance (XL), the capacitive reactance (XC), and the resistance (R) as shown below. 2
1 = 1 + 1 − 1 Z XR XL X C
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Power in a Resistive, Inductive and Capacitive (RLC) AC Circuit The types of power which exist in an RLC circuit are shown below:-
True or Effective Power (Watts). This is the amount of power being consumed by the resistive component in an AC circuit. The unit of true power is the Watt. Reactive Power (VAR) (Wattless power). This is the power consumed by the reactive components. The unit of reactive power is volts-amperes reactive (VAR) Apparent Power (VA). This is found by measuring the voltage and current being applied to a circuit, and multiplying them together. The unit of apparent power is volt-amperes (VA), and most AC equipment is rated in VA. Apparent power (VA) is made up of the vector sum of true power (Watts) and reactive power (VAR). Power in an AC circuit can alternatively be represented as a triangle, as shown below. Apparent Power (VA)
Reactive Power (VAR)
Real or True Power (Watts)
Power Factor This is a means of indicating the amount of true power being consumed in an AC circuit when the apparent power (VA) is given. The formula is:Power Factor (Cos φ) =
True Power Apparent Power
For example, if the apparent power of a circuit is 1000 Volt-amperes and the generator has a power factor of 0.6, then the True Power will be 600 Watts, ie. the generator is only 60% effective. If the power factor is alternatively ‘1.0 or unity’, then the True Power would be
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1000 Watts. It is therefore important that the power factor is as close to unity as possible, although this is normally a fixed quantity, and cannot be altered.
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AC Series Circuit Example An AC series circuit with a supply voltage of 100 volts has a resistance (R) of 30Ω, a capacitive reactance (XC) of 60 Ω and an inductance reactance (XL) of 100 Ω, as shown below.
Calculate the:a) b) c) d) e)
Total impedance (Z). Supply current (I). P.D across each component (VR, VC and VL). True, reactive and apparent power. Power factor
Solution:-
S
α) Z = b) IS =
R
2
+ (X L − X C ) 2 =
S
30 2 + (100 − 60 ) 2 = 50 ohms (Ω)
VS = 100 = 2 amps Z 50
c) VR = I x R = 2 x 30 = 60 volts VC = I x XC = 2 x 60 = 120 volts VL = I x XL = 2 x 100 = 200 volts d) True Power
= VR x I
= 60 x 2 = 120 watts
Reactive Power = (VL – VC) x I = 80 x 2 = 160 volt-amperes reactive (VAR) Apparent Power = VS x I
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e) Power Factor =
True Power = 120 = 0.6 lagging Apparent Power 200
This is because the supply voltage is ahead of the supply current in the phasor diagram. AC Parallel Circuit Example An AC parallel circuit with a supply voltage of 100 volts has a resistance (R) of 30.3 Ω, a capacitive reactance (XC) of 10 Ω and an inductance reactance (XL) of 16.7 Ω, as shown below.
Calculate the:a) b) c) d)
Current through each component (IR, IL and IC). Total current (IT). True, reactive and apparent power. Power factor
Solution:-
a) IR = IC =
VS R VS XC
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=
100 33.3
= 3 amps
=
100 10
= 10 amps
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IL =
VS XL
b) IT =
=
100 16.7
= 6 amps
IR 2 + (IC − IL ) 2 =
c) True Power
3 2 + (10 − 6 ) 2 = 5 amps
= VS x IR
= 100 x 3 = 300 watts
Reactive Power = VS x (IC – IL) = 100 x 4 = 400 volt-amperes reactive (VAR) Apparent Power = VS x IT d) Power Factor =
= 100 x 5
= 500 volt-amperes (VA)
True Power = 300 = 0.6 leading Apparent Power 500
This is because the supply voltage is behind the supply current in the phasor diagram.
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Chapter 10. Resonant AC Circuits Introduction When a D.C. voltage is applied to a parallel circuit containing both inductance and capacitance, the capacitor will act like an open circuit, and the inductor like a short circuit. This means that XC will be infinite while XL will be zero. If a very low frequency AC is alternatively applied instead of DC and the frequency gradually increased XL will increase and XC will decrease. A point will eventually be reached where the value of XL is the same as XC. It follows that for any combination of L and C, there will be a frequency at which XL will equal XC. This is true whether the two components are connected in series or parallel. The condition where XL equals XC is known as ‘Resonance’, and the frequency at which this occurs is known as the ‘Resonant Frequency (fo)’. The resonant frequency can be calculated from the following formula:XL = XC 1 2Π fC 1 = 2Π LC
2πfL = fo
where: f = frequency in hertz, L = inductance in Henries and C = Capacitance in Farads
Series Resonant Circuit When current flows in a series circuit containing, a resistor, a capacitor and an inductor, a voltage will be developed across each component.
VS =
VR 2 + (VC − VL ) 2
At resonance the voltage drop across the capacitor will be equal and opposite to the voltage drop across the inductor, thus cancelling each other out.
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At resonance the voltage across the resistance (VR), will thus equal the supply voltage (VS). The capacitor and inductor therefore do not affect the supply, since they provide no opposition to current flow at resonance. The voltage across the individual reactive components can also be many times higher than the supply voltage. Similarly in terms of impedance (Z):-
Z=
R 2 + (X C − X L ) 2
Both the capacitive and inductive reactance is dependent on the frequency and both alters with changes in frequency, as shown below.
With increasing frequency XC reduces whilst XL increases, and vice versa. The value of Z similarly alters, and at resonance XC = XL, thus Z = R. Minimum impedance will thus allow maximum current to flow in the circuit when the resonant frequency is achieved. A series resonant circuit is alternatively known as an ‘Acceptor’ circuit, and is particularly useful in communication equipment because it increases the sensitivity of the receiver (RX). This is done by enabling signals of a given frequency to be magnified and separated from other signals. The range of frequencies over which it is selective is called the ‘Bandwidth’ of the resonant circuit, as shown below.
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Bandwidth
By convention the bandwidth of a series circuit is the separation between two frequencies either side of the resonant frequency, at which the output power falls to half its maximum value.
Q Factor in a Series Resonant Circuit The Q or magnification factor is very important in a series resonant circuit, and is defined as the ratio of the reactance to resistance.
Q=
XL R
or
XC R
This is the reason why the voltage across the reactive components can be very much larger than the supply voltage, because it magnifies the voltage by the factor of Q.
Parallel Resonant Circuit (Tank Circuit) In an ideal parallel resonant circuit containing only pure capacitance and pure inductance, XL will be equal to XC.
IL =
SUPPLY
Under these conditions an equal amount of energy would be firstly be stored in the capacitor in an electrostatic field, and then passed to the inductor, to be stored as an electro-magnetic field. This is known as the ‘Flywheel Effect’, and because there is no resistance in the circuit, the oscillation of energy between the capacitor and inductor would continue indefinitely. It follows that since no energy needs be replaced in the circuit, then none is drawn from the AC supply other than the initial amount of energy required to start the oscillation. The circuit therefore appears to the supply to be an open circuit. Practical parallel inductive-capacitive circuits however have resistance, and unlike the hypothetical circuit shown, which only stores energy, resistance dissipates it in the form of heat. In a practical tank circuit, the oscillation will therefore quickly die away unless the lost energy is replaced
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by the supply. If the resistance in the circuit is high, the oscillations will quickly damp out because the energy is rapidly dissipated. In a normal parallel RLC circuit the supply current (IS) can be established using a phasor diagram and Pythagoras’s Theorem, as shown below.
IS =
IR 2 + (IL − I C ) 2
At resonance the current through the capacitor will be equal and opposite to the current through the inductor, thus cancelling each other out.
At resonance the current through the resistance (IR), will thus equal the supply current (IS). The capacitor and inductor therefore affect the supply, since they provide maximum opposition to current flow at resonance. The circulating current in the inductor and capacitor can also be many times greater than the supply current at resonance. The impedance (Z) is thus maximum and the resultant current a minimum at resonance. Bandwidth
C U R R E N T
fO FREQUENCY
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A parallel resonant circuit is alternatively known as a ‘Rejector’ circuit, and is particularly useful in communication equipment because it increases the selectivity of the receiver (RX). This is done by enabling signals of a given frequency to be easily separated from other signals, by magnifying the supply current. The range of frequencies over which it is selective is called the ‘Bandwidth’ of the resonant circuit, as shown above. Often a parallel resonant circuit is too selective, and responds only to a very narrow band of frequencies. In these cases, connecting a relatively small value resistor across the tank circuit can increase the bandwidth.
Q Factor in a Parallel Resonant Circuit In a parallel resonant circuit the supply is applied directly across both C and L, so unlike a series resonant circuit, the current rather than the voltage is magnified by a factor of Q. This is determined by dividing the tank current by the source current, as shown below.
Q=
I Tank I Source
This is the reason why the current circulating around the reactive components can be very much larger than the supply current.
Self Resonance of Coils Every coil has a certain value of capacitance and therefore at some value of frequency the inductor (coil) will begin to self resonate.
Use of Resonant Circuits The characteristics of resonant circuits make them useful for filtering specific frequencies in electronic circuits, and in this capacity are known as ‘Filters’. The basic filters which exist are:-
Low-pass Filter. In this circuit an inductance coil is placed in series, and a capacitor is placed in parallel with the supply.
Low frequencies will pass easily through the inductance coil, but will be blocked by the capacitor, whereas at higher frequencies the reverse will occur. This is because the reactance of the components varies with frequency, and thus determines which component passes current more readily. At low frequencies the inductive reactance (XL = 2ΠfL) is low, whereas at higher frequencies the capacitive reactance (XC =
1 ) is low. A low-pass filter 2Π fC
with thus pass frequencies in the lower ranges, but will attenuate or reduce the current at frequencies in the higher ranges.
High-pass Filter. In this circuit a capacitor is placed in series, and an inductance coil is placed in parallel with the supply.
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Low frequencies will pass easily through the inductance coil, but will be blocked by the capacitor, whereas at higher frequencies the reverse will occur. A high-pass filter with thus pass frequencies in the higher ranges, but will attenuate or reduce the current at frequencies in the lower ranges.
Band-pass Filter. This filter consists of a series LC, and a parallel LC circuit, arranged as shown below.
In this arrangement the impedance of the series LC circuit remains high, except at or near the resonant frequency, whereas the impedance of the parallel LC circuit remains low until this frequency band is reached. The number of circuit components and their resistance also determines the bandwidth of this filter, ie. the greater the resistance the greater the bandwidth.
Band-reject Filter. This filter consists of a series LC, and a parallel LC circuit, arranged as shown below.
In this arrangement the impedance of the parallel LC circuit remains low, except at or near the resonant frequency, whereas the impedance of the series LC circuit remains high until this frequency band is reached. The resonant frequencies will thus be bypassed and blocked from reaching the output. The number of circuit components and their resistance also determines the bandwidth of this filter, ie. the greater the resistance the greater the bandwidth.
Tuning Circuits A filter may be used as a ‘Tuning Circuit’ if either a variable capacitor or inductor is used. A typical circuit is shown below.
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In this circuit a variable capacitor is used with a fixed resistor, but in other circuits a fixed capacitor is used with a variable inductor, which is altered using a moveable core. Tuning circuits usually have a high selectivity and only allow a narrow band of frequencies to pass, whilst rejecting all others. During its operation radio signals cut across the antenna and induce signals (currents) of various frequencies to pass through the primary (P) winding of the antenna coil to earth. The resulting electromagnetic waves induce an EMF in the secondary (S) winding of the antenna coil, and the variable capacitor (C). When the resonant frequency of the coil is reached a maximum voltage is developed across the capacitor, and a maximum voltage is applied to the emitter-base of the transistor. This voltage is the input signal to the transistor, which in turn amplifies the relatively weak signal being passed to the tuner. In other cases a series resonant circuit is used in the primary circuit, which only allows maximum current to flow in this section at the resonant frequency. This thus prevents unwanted frequencies from being induced in the secondary winding, and increases the systems selectivity.
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Chapter 11. Transformers Introduction Transformers are extremely versatile devices, and can be used to either, step up and step down AC voltages, or step up and step down AC current. They can also allow AC to pass and block DC.
Construction and Operation The most common type of transformer is the voltage transformer, which consists of two windings (primary winding and secondary winding). The windings are not electrically connected together, which is a safety feature in AC electrical circuits, but are wound on the same laminated soft iron core.
If an AC voltage is applied to the primary winding the resultant changing flux will link with the secondary winding. The changing flux will be concentrated by the iron core, and will cause an EMF to be induced in the secondary winding. The magnitude of the EMF is proportional to the ratio of the number of turns between the primary and secondary windings.
Turns ratio = Where:-
NP V = P NS VS
Vp = Primary voltage Vs = Secondary voltage Np = Primary turns Ns = Secondary turns
Voltage transformers are categorised depending on the ratio of the turns, and are represented by the following symbols.
If there are more turns on the secondary than the primary it is a ‘Step-up’ transformer, and if there are more turns on the primary than the secondary it is a ‘Step-down’ transformer.
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Transformers are also extremely efficient, ie. the amount of power in, is approximately equal to the amount of power out, and they are rated in volt-amperes (VA). The following relationship thus exists between the turns ratio, voltage and current. I N VP = P = S VS NS IP
where IS = Secondary Current IP = Primary Current If the voltage is stepped up, then the current will be stepped down. For example if a transformer has a turns ratio of 1:2, and inputs of 240volts and 5 amps, the outputs will be respectively:VS NS V = N P P
VS = 2 x 240 = 480 volts 1 IS N = P IP NS
IS = 1 x 5 = 2.5 amps 2 Transformers also consist of inductive components so it is important that they are operated at their correct frequency and voltage. Any under frequency condition will result in the primary current increasing, and the transformer overheating.
Types of Transformers In addition to voltage transformers the following types of transformer also exist:-
Three-Phase Transformer (Isolation Transfomer). This type is widely used on aeroplanes and consists of three individual isolation transformers, where the primary windings are connected together across a three-phase AC supply, as shown below.
STAR
DELTA
The secondary windings are also connected together and produce a three-phase output voltage of a value dependent on the supply, and the turn’s ratio between the three corresponding winding pairs, which are normally the same. The primary and secondary windings can be alternatively connected in a delta-star configuration, as shown on the next page, or connected in star-star or delta-delta, although this is dependent on the transformer's particular application.
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DELTA
STAR
Auto Transformer. This is a special type, since it has no electrical isolation between the primary and secondary windings. A single continuous winding is wound on a laminated iron core, where part of the winding is used as the primary, whilst the other part is used as the secondary, as shown below.
These transformers can be used to either step-up or step-down the applied voltage, depending on the winding configuration. In a step-down device the whole of the winding serves as the primary winding, whilst the lower half of the winding serves as the secondary winding. In this case there are fewer turns in the secondary than in the primary; so the voltage will be stepped-down, but the current will be stepped-up. This configuration is typically used to power aeroplane instruments where the voltage is stepped-down from 115volts 400Hertz to 26volts AC. The disadvantage of this format is that the full voltage will be placed across the load if the coil goes open circuit, since there is no voltage isolation between the two windings. Conversely in a step-up Auto Transformer the lower half of the coil is used as the primary, and the entire coil is used as the secondary. In this case the secondary has more turns than the primary, so the transformer will step-up the voltage and step-down the current. On aeroplanes this arrangement is typically used in windshield anti-icing systems. If the output from the Auto Transformer can be varied via a moveable tapping, as shown below, it is alternatively known as a ‘Variac’, and is typically used on the flight deck, to control the intensity of ultra-violet lighting.
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Current Transformer. This type differs from the voltage transformer because the primary circuit consists of a supply feeder cable, rather than a winding connected across a supply, as shown below.
In this arrangement the alternating magnetic field associated with the load current is linked to the current transformer secondary winding via a laminated soft iron core, through which the feeder (primary) passes. The secondary current is used to feed a meter, and typically registers the current flowing from an AC generator to the busbar or load. The secondary current can additionally be used to supply power meters and also to monitor the load sharing in an electrical circuit. In AC power generation systems this type of transformer can also be used as a sensor in a ‘Differential Protection Circuit’, as shown below.
7)
This system protects against line to line and line to earth short circuits on the feeder lines between the generator and the ‘Generator Circuit Breaker (GCB)’. Doughnut current transformers are placed around the feeder lines and secondary windings of each pair in series opposition, to ensure that the full output from the generator passes to the load. Under no fault conditions the currents at each end of the feeder lines will be equal, so the induced EMF’s will be in balance, and thus no current will flow to the ‘Differential Protection Relay’.
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If a difference in current of 30-40 amps exists a signal will flow to the protection relay, which will instantaneously trip the ‘Generator Control Relay (GCR)’ and the GCB, thus automatically disconnecting the generator from the system.
Transformer Rectifier Units A Transformer Rectifier Unit (TRU) is used to convert AC into relatively smooth DC, and an example of a simple TRU circuit is that which is used in a car battery charger, as shown below.
This device takes the mains 240 volts AC, and converts it to approximately 14 Volts DC to charge the battery. This is achieved by a transformer, which firstly steps down the AC voltage to a reasonable level, and then converts it via a bridge rectifier assembly into DC. Most large aeroplane AC generator systems have dedicated TRU’s, which operate on the same principle, although they are slightly more sophisticated, and a typical unit is shown below.
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The TRU that is fitted to an aeroplane is typically supplied with 200volts 400 Hz three-phase AC, which is stepped-down through a three-phase star-star, wound transformer and changed to 28 volts DC by way of a six-rectifier bridge assembly. The output from the TRU is then fed to the aeroplane’s DC busbars. Each TRU has the following basic protections:¾
Overheat. Most TRU's when they are operating are cooled by air from a thermostatically controlled cooling fan, but if the TRU overheats (150°-200°), due to fan or other failure, an warning light will be annunciated on the flight deck. The TRU should then be switched off, either manually or automatically.
¾
Reverse Current. When the TRU's are operating in parallel with some other power source, the failure of a rectifier in a TRU can cause a reverse current to flow into it, and may even cause a fire. Reverse current protection in the failed TRU is thus designed to sense the fault current when it reaches approximately 1 amp, and disconnect the TRU automatically from the DC bus bars.
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Chapter 12. AC Power Generation Introduction The majority of large modern aeroplanes now employ three-phase AC generators because they are more efficient than their DC equivalents, and the most powerful of these are called three-phase machines. The following explanation of three-phase circuits is based on a simple three-phase generator.
Simple Three Phase Generator A three-phase generator consists of two main parts, as shown below:(RED)
(YELLOW) (BLUE)
The rotor carries the electromagnetic field that is driven by the aeroplane engine, whilst the stator, carries three sets (pairs) of coils (phase windings). These windings are fixed to one another at angles of 120°, and the phases are AA1, BB1 and CC1 or coloured Red(RR1), Yellow(YY1) and Blue(BB1) respectively, where the A or Red phase is classified as the ‘Reference Phase’. As the rotor rotates it induces an EMF in each set of windings in turn, and produces a sine wave output from each, as shown below.
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At any instant the sum of the EMF’s or the currents in a balanced system will add up to zero. These windings supply the output of the generator, and are connected in either a ‘Star’ or ‘Delta’ configuration, as shown below. STAR
OUTPUT
Most aeroplanes similarly use three-phase AC motors with delta or star wound stators.
Star Connection In the star configuration one end of each phase winding is connected to a common point called the ‘Neutral (N)’ or ‘Star Point’, whilst the other end of each phase winding is connected to output terminals distributing AC power of different phases.
In this configuration the output voltages and currents are respectively:-
Phase Voltage(VP)
=
3 x Line Voltage(VL)
Line Current(IL)
= Phase Current(IP)
On most aeroplane generators the output voltages are:¾ ¾
Phase Voltage Line Voltage
= 115 VOLTS = 200 VOLTS
The vast majority of aeroplane AC generators are connected in the star configuration with the neutral point (N) connected directly to earth, which allows:¾ ¾
The generator to feed unbalanced loads. Easy access to the phase voltages.
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When a three-phase star connected generator is feeding a balanced load (ABC phases feeding the same current) the net current of all three phases is zero. In this case no current flows in the neutral line. When unbalanced currents feed the load the resultant of these currents will flow in the neutral line. If the currents being used are always balanced there is no need for a neutral point to be fitted. On aeroplanes, although desirable, it is not practical for the generator to feed balanced loads all of the time, so it is thus necessary on most generators to connect the neutral point to earth.
Delta Connection In the delta configuration the phases are connected in a triangular (delta) format, with no common or neutral point.
Unlike the star connection the phase and line voltages in the delta connection are the same:-
Line Voltage(VL) = Phase Voltage(VP) The line and phase currents however differ:-
Line Current(IL) =
3 x Phase Current(IP)
Advantages of Three Phase over Single Phase AC Generators Three phase AC generators are preferable to single-phase machines for the following reasons:¾
Less conductor weight is required for the transmission of a given power.
¾
They can produce a rotating magnetic field, which can be used to operate efficient three phase AC motors.
¾
Three-phase AC gives smoother rectification than single phase AC
Voltage and Frequency of AC Generators Adjusting the field excitation of an AC generator using a voltage regulator controls its voltage output. The output frequency of an AC generator is alternatively dependent on the rotational speed of the rotor, and the number of magnetic field poles, as shown in the following formula.
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Frequency(f) = NP / 60 where:-
N = Rotational Speed (RPM) P = Number of pole pairs on the rotor
Phase Rotation Three phase power supplies in an aeroplanes power system must have a positive phase sequence, ie. A.B.C, B.C.A or C.A.B. If any of the phases are crossed over, ie. A.C.B, C.B.A or B.A.C a negative phase sequence would exist and will result in three-phase motor running in the wrong direction.
Faults on Three-Phase AC Generators The two main faults, which can occur in the output phases and lines of an AC generator are earth, and open circuits. The diagram below shows how these faults would affect a star-connected generator.
The diagram below shows how these faults would affect a delta-connected generator.
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Generator Real and Reactive Load Sharing AC loads consume apparent power, which is measured in Volt-Amperes (VA), and so most AC machines are also rated in VA. On many large aeroplanes with 3 or 4 engines the generators are normally run in parallel, and must share the apparent power in terms of true power (Watts) and reactive power (Volt-Amperes Reactive, VAR). WATT/VAR meters, as shown below, are fitted to each generator system, and allow the flight crew to check that the load sharing between the generators is equal.
Types of AC Generator The basic types of aeroplane AC generators, which exist are:-
Salient Pole Three-Phase AC Generators. These are ‘Frequency Wild’ or ‘Brushed’ generators, which are mainly used on aeroplanes with turbo-propeller engines, eg. F-27, and generate frequency wild AC power. They consist of a rotor with electromagnets fitted to each salient pole, which alternate in polarity around the circumference of the rotor, and rotate inside a fixed three-phase stator, as shown below.
The outer shell of the machine holds the stator that consists of three fixed star-connected windings, and the generator is cooled by ram air.
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QUILL DRIVE
A typical three phase brushed AC generator, as shown above, would be rated at 22KVA with an output of 208 volts, and would supply a full load at this voltage through a frequency range of 280-400 Hertz. The generator frequency and output voltage vary with rotational speed, so cannot be used to operate circuits containing inductive and capacitive components. This type of generator can thus only be used to operate purely resistive circuits, such as the propeller de-icing system on turbo propeller aeroplanes, eg. the F27. During its operation some of the AC output is fed back to the voltage regulator via a three phase full rectifier pack, which provides a medium to low DC voltage, and self excitation of the generator, as shown below, although the majority is passed directly to the main AC busbars.
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The voltage regulator senses the output from the generator and automatically adjusts the excitation field for varying engine speed and load conditions. The battery is thus no longer required and is manually disconnected from the circuit via the control switch. A temperature sensor and a quill drive protect this type of machine. If the generator overheats it should be off-loaded or even switched off, and allowed to cool. The quill drive connects the generator to the engine, and is designed to shear if the generator seizes, thus protecting the engine. It is also designed to absorb any mechanical vibrations and produce a smoother output.
Brushless Three Phase AC Generator This is a highly sophisticated machine, and is used on large jet aeroplanes for generating constant frequency supplies. The brushless generator has the advantage over the brushed type, since it requires less maintenance and is also more reliable. It is driven by the aeroplane engine via a ‘Constant Speed Drive Unit (CSDU)’, which maintains a constant generator speed for varying engine speeds, and produces a constant frequency output of 400 Hz. This type of generator comprises of three individual parts as shown on the next page. A permanent magnet generator (PMG) initially induces a single-phase AC voltage into the pilot exciter when the rotor is driven via the CSDU. The AC voltage is then full-wave rectified and fed to the main exciter by way of the voltage regulator. As the three phase windings, which are mounted on a common drive shaft, are rotated within the field, a three-phase AC voltage is induced in the windings. The output is then rectified via a three-phase bridge rectifier circuit, which consists of six diodes that are mounted inside the drive shaft, and produces the main DC field. The temperature of the diodes is carefully controlled by ram air-cooling, which is directed down the centre of the shaft. The field coil is also fixed to the common drive shaft, and as it rotates it induces a voltage in the AC output windings. Some of the output is fed back to the voltage regulator and increases the output from the pilot exciter, which in turn increases the output from the main exciter. This sequence of events continues until the
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generator reaches its regulated AC output line voltage of 200volts and phase voltage of 115 Volts at 400 Hz.
Constant Speed Drive Unit The ‘Constant Speed Drive Unit (CSDU)’ is a mainly mechanical device, which is positioned between the aeroplane engine and the brushless AC generator. On older aeroplanes the CSDU and generator are normally separate items, as shown below, and the generator is air-cooled.
The CSDU is designed to keep the generator running at a constant speed, which is usually 8,000 RPM for varying engine speeds, and gives a constant output frequency of 400 Hz. One particular type of device is mechanically/hydraulically driven and consists of a self-
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contained oil system, as shown on the next page. A pump assembly provides high-pressure oil, which controls the pumping action of a pump/motor assembly via a centrifugal governor. The governor is a mechanical device, and is not sensitive enough to give the fine speed trimming required to control the frequency within close limits, 395-425 Hz. To achieve the required trim, an electromagnetic coil receives signals from the electrical system load controller, and modifies the position of the flyweights in the governor.
SERVO PISTON
The CSDU cylinder block is mechanically linked to the engine drive and as it rotates, the end of the pump pistons stroke against a stationary inclined ‘Pump Wobbler (Swash) Plate’, as shown below, thus producing a pumping action.
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The angle or inclination of this plate is controlled by a mechanical governor, which varies the hydraulic pressure to the two sides of a piston inside a control cylinder (servo mechanism). As the block rotates the end of the motor pistons also stroke against an inclined fixed angle ‘Motor Wobbler (Swash) Plate’ assembly, where an eccentric centre plate is sandwiched between two stationary plates. The centre plate is coupled to an output shaft, which drives the generator, and is free to rotate against ball bearings. The pressure being exerted on the motor pistons by the pump determines the rotational speed of the centre plate, and the higher the pressure the faster it will rotate. A typical analogy of this is a piece of soap on the side of the bath; where the harder you push, the faster it will tend to move away from you.
Operation of the Hydro-Mechanical CSDU If the throttle setting is decreased the engine speed will similarly decrease, thus rotating the casing of the CSDU slower, and decreasing the pumping action of the hydraulic pump. The engine output speed will now be slower than the required generator speed, and an ‘Overdrive’ condition will exist. The governor will sense this, and the angle of the swash plate will be increased, by oil being directed to the left hand side of the piston via the over-drive inlet port, thus increasing the stroke of the pistons. This increases the output pressure from the pump and forces the motor pistons to exert more force on the downhill side of the motor wobbler assembly. This causes the centre plate to rotate faster than the cylinder block, thus maintaining a constant generator speed. Conversely if the throttle setting is increased the engine speed will similarly increase, thus rotating the casing of the CSDU faster, and increasing the pumping action of the hydraulic pump. The engine output will now be faster than the required generator speed, and an ‘Under-drive’ condition will exist. The governor will sense this, and the angle of the swash plate will be decreased, by oil being directed to the right hand side of the piston via the under-drive inlet port, thus decreasing the stroke of the pistons. This decreases the output pressure from the pump and forces the motor pistons to exert less force on the downhill side
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of the motor wobbler assembly. This causes the centre plate to rotate slower than the cylinder block, thus maintaining a constant generator speed. When the engine output speed equals the required generator speed the oil pressure and oil flow within the hydraulic system will be such that the motor is hydraulically locked, ie. the cylinder block will be locked to the motor, and both will rotate together as a fixed coupling.
Protection of the Hydro-Mechanical CSDU To guard against mechanical failure, the oil pressure and temperature of the CSDU is monitored on the flight deck, as shown below.
If the CSDU fails mechanically it may cause an over-speed or under-speed (over frequency/under frequency) condition, and the reactive components in the aeroplane could be severely damaged. Sensors are thus fitted to detect any speed change, and will automatically disconnect the generator from the busbar via the Generator Circuit Breaker (GCB). Conversely if there is an indication of imminent failure, the 'CSDU Disconnect Switch’ can be manually selected by the flight crew. This operates a solenoid switch, as shown below, and allows the threaded pawl to engage with the course thread on the input shaft, thus separating the ‘Dog Tooth Clutch Mechanism’.
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This separates the drive between the engine and CSDU, and allows the generator to run down. Once the CSDU has been disconnected, it cannot be reset until the aeroplane is on the ground with its engine shut down, although the disconnect mechanism can be activated at any time. In order to prevent inadvertent CSDU disconnect the switches are normally guarded and locked with thin copper wire.
Integrated Drive Generator On modern aeroplanes the CSDU and generator are normally combined as one unit, which is known as an ‘Integrated Drive Generator (IDG)’, as shown below, and the generator is alternatively oil-cooled.
This is a much lighter and more compact unit.
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Variable Speed Constant Frequency Power Systems Variable Frequency Constant Frequency (VSCF) systems are fitted to some commercial jet aeroplanes, eg. Boeing 737-300, -400, and –500 variants. In this system no mechanical CSDU is fitted and the generators variable frequency output is converted into a constant frequency AC output of 400 Hertz, via solid state circuitry, as shown below.
VSCF systems are more reliable and offer greater flexibility than a typical CSDU and generator configuration. The generator is still driven directly from the aeroplane engine, but the control units of the VSCF system can be mounted virtually anywhere in the aeroplane, thus allowing for a more compact engine nacelle.
Auxiliary Power Unit The ‘Auxiliary Power Unit (APU)’ is a compact unit, as shown below, which is usually fitted in the tail section of an aeroplane, and provides electrical power (200 Volts 3 Phase 400 Hz) on the ground.
The APU can also be used to supply compressed air on the ground for engine starting, and electrical power in flight during an emergency.
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Most APU's have their own dedicated 24 volt battery for starting, or can alternatively be started from ground power, although the main aeroplane battery switch must be on to operate the APU control circuits. The APU can drive one or two generators, depending on the type of aeroplane, and these are the same type as those fitted to the main engines. The APU does not require a CSDU to maintain a constant frequency output, since the drive from the APU runs at a constant speed via a governor, and can be used up to 25,000 feet.
Emergency Ram Air Turbine In the case of total main electrical AC failure, a ‘Ram Air Turbine (RAT)’, as shown below, can be extended automatically or manually into the airstream.
A variable pitch propeller drives a hydraulic pump, which in turn drives an AC generator at a constant speed, and supplies 200 Volts 3 Phase 400Hertz for emergency loads. During the approach to landing the RAT may become inefficient, so the aeroplane batteries will take over, and will supply the necessary loads during the final approach. The RAT can additionally only be restored on the ground, and is also inhibited from deployment on the ground.
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Chapter 13. AC Power Generation Systems Introduction AC power supply systems can be split into the following categories: ¾ ¾
Frequency Wild AC Systems. These are used in small to medium sized aeroplanes ranging from small piston engined aeroplanes to large twin engined turbo propeller aeroplanes. Constant Frequency AC Systems. These are used on jet aeroplanes, and are either split busbar or parallel systems.
Piston-Engine Frequency Wild AC System Architecture A typical single-engined aeroplane uses a frequency wild electrical generation system.
In this system the AC generator (alternator) is driven directly from the engine via a fan belt, so the frequency output from the generator will be dependent, and proportional to the engine speed. Before being fed to the aeroplane loads the AC is changed directly into DC, via rectifiers inside the generator.
Operation of a Piston-Engine Frequency Wild AC System On initially switching the battery on, the busbar will be supplied with approximately 12 volts, and depending on the loads selected, the ammeter will read a discharge as a negative value. The under voltage lamp will also be illuminated indicating that the busbar is below 13.5 volts, and the output from the AC generator will be zero until the generator switch is placed in the 'ON' position. The generator field excitation will be supplied initially from the battery, but once the generator produces an output, it will become self-excited. When the generator output is approximately 14 volts DC the under-voltage warning lamp will go out and a
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charging current, indicated by the ammeter reading a positive value, will flow towards the battery. The advantages of this system over the older commutator generator method of producing DC are that:¾
There is no necessity for a cut out because no reverse current can flow into the AC generator.
¾
The output is taken from stationary stator windings and only a small current need to be transferred to the motor field by way of brush gear and slip rings.
¾
There is no need for a cast iron yolk to concentrate the field, so the AC generator is a lot lighter.
Fault Protection in a Piston-Engine Frequency Wild AC System The following faults protections exist in a piston-engined frequency wild AC system:¾
Over-Voltage. If an over-voltage occurs (15.5 volts approximately) the voltage regulator will break the field and lock it out, causing the under-voltage lamp to illuminate. One attempt to reset the system by switching the generator switch 'OFF' for a few seconds to break the lock, then switching it to 'ON' again may be made, which is often referred to as ‘Cycling’ the generator switch.
¾
Under-Voltage. If the generator under-volts or is switched off, the undervoltage warning lamp will illuminate, and will be automatically extinguished when the voltage returns to normal.
¾
Overheat. Some of these systems are fitted with an overheat thermostatic sensor. If an overheat condition occurs it is annunciated to the flight crew, who should manually switch the generator switch off, and allow it to cool. When the generator has cooled, the overheat warning annunciator will automatically reset itself.
Note that the indications on this system for an over-volt or under-volt condition are somewhat similar, so if this fault occurs, ‘cycling’ the generator switch can reset the system.
Twin-Engine Turbo-Propeller Frequency Wild AC System Architecture In this system the AC generators are fitted directly to each engine, and unless the engines run at a constant speed, the output frequency will vary (Frequency Wild).
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The output from each generator is normally 200 volt three-phase, and varies in frequency between 280 - 540 Hertz, which corresponds respectively to low and high engine RPM's. The generators in this system should not be run in parallel under any circumstance, so their AC output is normally used to feed heating elements only. This is because the elements are purely resistive, and are thus unaffected by changes in frequency. In some systems part of the frequency wild output is rectified in a Transformer Rectifier Unit (TRU), and provides an alternative DC supply. The DC supplies may also be paralleled provided that the voltages are matched.
Operation of a Twin-Engine Turbo-Propeller Frequency Wild AC System With the engine started and running the generator is initially excited by a separate power source, ie. the battery or ground power, as shown below.
Firstly switching the generator control switch to ‘RESET’, and thus closing the field relay achieves this. When the generator is producing an output part of it is fed back through the voltage regulator, and ‘Bridge Rectifier Pack’ to provide the generator field, thus providing
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self-excitation. Once the generator is operating at its regulated output voltage of 200 volts, the line-contactor will close, and the generator warning light will go out. Moving the control switch to the 'ON' position will subsequently de-excite the field relay, and will remove the source of the initial excitation current. The generator will now be fully self-excited, and the voltage regulator will continue to adjust the field excitation for varying speed load conditions.
Fault Protection in a Twin-Engine Turbo-Propeller Frequency Wild AC System The following faults protections exist in a twin-engined turbo-propeller frequency wild AC system:-:-
¾ Overheat. If the generator overheats due to inadequate cooling or overload, a
warning light will illuminate on the flight deck, and the generator should be manually switched off.
¾ Earth-Leakage.
If there is low insulation in the alternator system or loads a warning light will illuminate, and if this occurs the generator should be switched off.
¾ Under-Voltage. This fault normally uses the same warning light as that used to
indicate an earth leakage fault. The system voltmeter is thus used to discriminate between an earth leakage fault, and an under-voltage fault.
¾ Over-Voltage. If an over voltage occurs a sensing circuit will automatically de-
excite the generator and remove it from the busbar. One attempt is usually allowed to reset the system by cycling the control switch between ‘RESET’ and ‘RUN’.
¾ Differential Protection. This system is used to:¾ monitor line to line faults. ¾ monitor line to earth faults. ¾ ensure that the output current flowing from the generator is the same as that flowing to the loads and returning to the generator.
If one of the above faults exists the generator will be automatically de-excited, and will also be removed from the busbar. One reset may be attempted, but even if the system resets satisfactorily for the rest of the flight, the fault must still be reported on landing.
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The Constant Frequency Split Busbar AC System The electrical system shown below is typically used on a twin jet engine aeroplane whose AC power supply is 200 Volt 400 Hz three phase.
The power supply can be derived from four sources; two engine driven Integrated Drive Generators (IDG’s), an Auxiliary Power Unit (APU), and an external power receptacle. These sources should never be paralleled at any time. Under normal operation the generators will independently feed the left and right section loads of the electrical system. The loads being fed by these generators are normally indicated on ammeters fitted to each generator output. The APU is used to drive a third generator which can supply the electrical power necessary for ground operations, or act as a substitute for a failed engine-driven generator. External power can also be used instead of APU power on the ground, but not simultaneously.
Operation of a Constant Frequency Split Busbar AC System The circuit on the opposite page is shown in the power off condition. On most aeroplanes the APU is started by an electrical starter, which is supplied from its own dedicated battery, or from the aeroplane battery. When the APU is up and running, the generator is selected by the APU generator circuit breaker (GCB) to feed No.1 and No.2 main AC bus bars. The APU generator will then supply all of the aeroplane AC requirements, and the Transformer Rectifier Units (TRU's) will supply any DC requirements. If the No.1 engine is initially started and run up, its dedicated IDG will produce the correct output (200v 400 Hz three-phase) and it will feed the No. 1 main AC busbar. However before it can supply this busbar the APU power must be removed from the No.1 main AC busbar by opening the appropriate GCB, followed by the closing of the No.1 IDG GCB. The No.1 IDG will now feed the No.1 main AC busbar and the A.P.U. generator will continue to feed the No.2 main AC busbar. When the No.2 engine is up and running its IDG will alternatively feed the No.2 main AC busbar. The APU generator supply must however be firstly removed from the No.2 busbar before the IDG is allowed to feed it. At this point the APU is no longer needed to feed the electrical system, and is therefore shut down. Both engine driven IDG AC supplies will now operate independently of each other, and will be kept separated by the ‘Bus-Tie Breaker (BTB)’.
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If one of the IDG's fails the BTB between the two systems will automatically close, and the serviceable generator will feed both of the main AC busbars. If the APU is started again it will substitute for the failed generator and the BTB will open. The main aeroplane DC supply will be maintained by two TRU's (one for each IDG), as follows. ¾
The No.1 TRU will feed the DC essential busbar.
¾
The No.2 TRU will feed the DC non- essential busbar.
The TRU's are kept independent from each other by an ‘Isolation Relay’, but if either TRU fails, the Isolation Relay between the two sides will automatically close, and the serviceable TRU will feed both busbars.
Regulation and Protection of Constant Frequency Units Most of these systems have separate or combined solid-state regulation and protection units dedicated to each generator. The regulator is divided into the following parts:¾
A speed regulator, which senses the output speed or frequency of the IDG and adjusts the IDG to give a frequency output of between 380 - 420 Hz.
¾
A voltage regulator, which regulates the output voltage to 200 volts ± 5 volt by adjusting the IDG's field excitation.
A dedicated protection unit houses the circuitry, which detects any faults occurring up to, and including the busbars. Faults within this zone usually have time delays so that any faults occurring after the busbars will have time to trip the circuit breakers, or blow the fuses.
Faults on a Constant Frequency Split Busbar AC Generator System Some faults in a split busbar generator system will cause the IDG to de-excite and its related GCB to open, thus removing the IDG from its own busbar. These faults are as follows:¾
Over-Voltage. If this type of fault is allowed to persist it could cause serious damage to cable insulation and components.
¾
Differential Protection. This type of protection monitors the following faults:¾ ¾
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A line to line or line to earth fault, which normally occurs inside the IDG. If the current flowing to the busbar is different from the current flowing from the IDG.
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Differential faults are detected by current transformers, which sense an imbalance in current between the generator and the busbar. If one of the above faults exists the generator field will be automatically de-excited and the generator removed from the busbar ¾
Over-Frequency. If this fault is allowed to continue it may damage any capacitive circuits due to high currents.
¾
Under-Frequency. This fault will cause high currents and the overheating of any inductive circuits.
¾
Resetting. Many of the faults mentioned have a facility by which the system can be reset. One reset only is usually allowed, ie. the system is ‘Cycled’.
Other faults which might occur are:¾
Generator Overheat. If the generator overheats due to frictional heating or inefficient cooling, an overheat warning will be annunciated to the flight crew. If this occurs the system should be manually switched off.
¾
IDG Disconnect (CSDU Disconnect). The oil pressure and oil temperature of the IDG is monitored. If during a fault the oil pressure drops, accompanied with an oil temperature rise, the flight crew may elect to operate the IDG disconnect, but once this has been initiated, the system can only be manually reset on the ground with the engine stopped.
¾
Generator Bearing Failure. If an excessive clearance exists in the bearings of the engine, or APU generators, a bearing failure warning light will illuminate on the flight deck.
Emergency Supplies In the unlikely event that both IDG's and the APU generator fail AC can still be obtained from:¾
The aeroplane battery, which will automatically feed the AC essential busbar via a static inverter.
¾
A ‘Ram Air Turbine (RAT)’ can be automatically or manually dropped into the airstream to drive an AC generator, which will produce a constant frequency output for the AC essential busbar.
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If the emergency power supplies are selected it is normal to shed any non-essential loads, eg. galleys, in order to prevent overloading the remaining generators, which is known as ‘Load Shedding’.
Battery Charger Modern aeroplanes are fitted with battery chargers that are supplied from AC power supplies. These provide a DC supply to charge a battery in the shortest possible time, within certain voltage constraints, and without causing excessive gassing. The charger provides a DC current of between 45-50 Amps until the charge reaches completion. It will then revert to the pulse mode to prevent the battery voltage becoming excessive. Comprehensive protection circuitry is provided in the battery charger to give protection against:¾ ¾ ¾
Over voltage Overheating Battery disconnection
If the battery over-volts the battery charger will be automatically switched off, and can only be reset by a push-switch situated on the front of the battery charger. If the charger overheats it will be automatically shut down, but will reset itself when cooled. If the battery is disconnected the charger will not be able to be switched on.
Battery Power The batteries will supply secondary DC power. On most aeroplanes they will also feed essential DC, and through a static inverter essential AC for a period of 30 minutes or more. Some batteries are additionally fitted in non-pressurized areas in the fuselage, and are provided with electrically heated blankets to prevent freezing.
Ground Handling Bus The ground handling busbar is powered from either an APU generator or an external power unit. The busbar is powered automatically whenever external or APU power is available. This busbar is used mainly on the ground to power lights, and the refuelling system.
Constant Frequency Parallel AC System The constant frequency system is almost exclusive to three and four engine jet aeroplanes, and a typical system is shown on the next page. In older systems the AC generator and the CSDU are separate items, but on modern aeroplanes the two components are combined to form an IDG. In addition to the engine-driven generators an APU drives a generator, which is capable of supplying the aeroplane with power on the ground, and at altitudes up to approximately 35,000 ft. The APU may however experience difficulties in starting at altitudes above 25,000 ft. Some aeroplanes also have emergency ram air turbines, which can be deployed in an emergency. The generators fitted on each engine and are normally run in parallel. The system does however have the following advantages and disadvantages over Split Busbar AC System:¾
Advantages. When operating in parallel this system:¾
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provides a continuity of electrical supply.
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¾
¾
prolongs the generator life expectancy, since each generator is normally run on part load.
¾
readily absorbs large transient loads.
Disadvantages. The disadvantages of the system are that:¾
expensive protection circuitry is required since any single fault may propagate through the complete system.
¾
Parallel operation does not meet the requirements for totally independent supplies.
On the most aeroplanes only the engine-driven generators can normally be paralleled, but the APU or the ground power unit cannot be paralleled with the engine driven generators, or each other. Circuit interlocks will prevent this occurring in the case of incorrect system management.
Operation of a Constant Frequency Parallel AC System Once all of the above conditions have been satisfied, a ground power available light will come on. When 'ground power' is selected, the ground power breaker (GPB) will close and allow the ground power to feed the generator busbars. With the No.1 engine running its generator will be excited when the generator control relay (GCR) is closed, which will enable the generator to give an output (200v three phase 400 Hz.). On closing the generator switch, the external services breaker (ESB) will open, thus removing ground power, and the No.1 generator circuit breaker will close. This will allow the No.1 generator to supply the necessary aeroplane power. With the No.2 engine running, and its generator is producing the necessary output, it can be paralleled with the No.1 generator via the synchronizing busbars by closing the No.2 generator's GCB. The following conditions however must exist before paralleling can take place between two generators the:¾
voltages must be within tolerance.
¾
frequencies must be within tolerance.
¾
phase displacement must be within tolerance.
¾
phase rotation must be correct.
Once all of the above conditions have been satisfied the selecting the No.2 generator switch to 'ON', will cause the GCB. to close and the No.1 and No.2 generators to run in parallel. Both generators must share the real (Watts) and reactive (VAR) loads equally, and these are monitored on individual generator Watts/VAR meters on the flight deck.
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The No.3 and No.4 generators are paralleled using the same method as the No.1 and No.2. generators. When all of the generators are running the No.1 and No.3 generators will be
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kept separate from the No.2 and No.4 generators by a split system breaker (SSB). engine driven generator fails the SSB will automatically close.
If any
Reactive Load Sharing Reactive load sharing is achieved by a load-sharing loop, which will automatically adjust the excitation of the paralleled generator fields simultaneously, via their individual voltage regulators.
Real Load Sharing Real load sharing is achieved by a load-sharing loop, which adjusting the magnetic trim in the mechanical governor of the CSDU's simultaneously, via their load controllers.
Paralleling The following methods are used to parallel AC generators:-
Manual Paralleling is an old method of paralleling generators. To facilitate this method a lamp is fitted across the main contacts of the GCB. When both generators outputs are the same the lamp will darken and go out. When this occurs the engineer closes the on coming generators control switch. This is also known as the ‘Lamps Dark’ method of paralleling.
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Automatic Paralleling. When using the automatic paralleling method, the generator switch is selected to on at any time, and once the auto-paralleling circuits sense that both generators are ready for paralleling, the GCB will automatically close. Fault Protections in a Constant Frequency AC Parallel System The following fault protections exist in a parallel generator system:-
Over-Excitation (Parallel Fault). The over-excitation protection device operates whenever the excitation to the field of one of the generator increases. This is sensed when the over-excited generator takes more than its share of reactive load. The fault signal has an inverse time function which trips the BTB of the over-excited generator. The voltage regulator or reactive load-sharing circuit could cause this fault. Over-Voltage. The over-voltage protection device will operate whenever the system voltage exceeds 225 volts. It protects the components in the system from damage due to excessive voltages. This protection device operates on an inverse time function, which means that the magnitude of voltage determines the time in which the offending generator will be de-energized by tripping the GCR, and GCB. The GCR will de-energize the field, and the GCB will trip the generator off the busbar. The under-excitation protection device will Under-Excitation (Parallel Fault). operate whenever the excitation of one of the generator fields is reduced. This is sensed when the under-excited generator takes less that its share of reactive load, and a fault signal will cause the BTB to trip in a fixed time (3-5 sec). This type of fault could be caused by a fault in the:¾ ¾ ¾
Reactive load sharing circuit. Generator. Voltage regulator.
Under-Voltage. The under-voltage protection device will operate to prevent damage to equipment from high currents and losses in motor loads, which may cause over-heating and burn out. When this device operates it will trip the GCR and GCB in a fixed time (3 - 5 sec), resulting in the shut-down of that generator. Differential Protection. The differential protection device will operate in the same way as stated in the split busbar generator system. It will operate if any of the following faults exist:¾ ¾
A line to line, or line to earth fault. If the current flowing to the busbar is different from the current flowing from the generator.
The instability protection device is Instability Protection (Parallel Fault). incorporated in the system to guard against oscillating outputs from the generators, which may cause sensitive equipment to malfunction or trip off. This especially applies to autopilot and radio installations. If the system is operating in parallel, and the No.1 generator becomes unstable, the instability protection circuits in all generators will sense this and trip all of the BTB's. This will isolate the unstable generator from the other generators and the instability protection device will continue
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to operate, tripping its GCR and GCB. The generator, voltage regulator or CSDU may cause instability. The negative sequence voltage Negative Sequence Voltage Protection. protection device will detect any line to line or line to earth faults after the differentially protected zone, and will cause all the BTB's to trip.
Overheat. If A temperature sensor fitted in the generator senses an overheat, condition an overheat warning light will illuminate. This fault may be caused by overloading the generator on the ground (no ram air cooling), or by a blockage in the ram air cooling duct in flight. If this warning occurs the pilot should operate the GCR switch, which will cause the GCR and GCB to trip. Over-speed (Over Frequency). The over-speed device will operate if a fault occurs in the CSDU, which may cause the generator to exceed its specified frequency limits. If left unchecked this fault will damage the aeroplane capacitive loads. In older systems a pressure switch in the CSDU will detect this type of fault, but in modern systems frequency sensitive circuits detect it. If an over-speed condition occurs it will cause the GCB to trip, and will also put the CSDU into underdrive. Under-speed (Under-Frequency). An oil pressure switch in the CSDU senses underspeed of the CSDU. This will cause the GCB to trip, thus removing the generator from the busbar, and protecting the loads from an under-frequency. Time delays are fitted in the generator protection system to give the normal circuit protection devices, ie. circuit breakers and fuses, time to operate, rather than removing a generator from the system.
DC Power Supplies Primary aeroplane DC power supplies are derived from transformer rectifier units TRU's, which are supplied from the 200v AC busbars. The TRU's are normally run in parallel, although some systems have isolation relays installed, which are designed to separate the DC busbars during fault conditions.
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Chapter 14. AC Motors Introduction AC motors are mainly used on larger aeroplanes since they rely on a constant frequency supply. Motors are generally classified as follows:-
Large. Motors with an output of 3 KW or more, which are normally three-phase machines. Medium to Small. Motors in the range of 3 KW down to 50 W, which are mostly single-phase machines. Motors rated at less than 750 W are also referred to as ‘Fractional Horsepower (FHP)’ machines. Miniature. Motors rated at less than 50 W, which are used in instruments and servomechanisms. On aeroplanes these motors are either ‘Induction’ or ‘Synchronous’ machines
Stator-Produced Rotating Magnetic Field When a magnet is rotated within a three-phase stator a three-phase voltage is produced. If this process is reversed, ie. by connecting the three-phase supply to a three-phase stator, a rotating field will be produced, as shown below.
If the stator windings are symmetrically arranged as shown above the magnetic field produced will be of constant strength and will rotate at a uniform speed, which is dependent
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on the supply frequency. The magnetic field will thus rotate through one complete revolution during each complete cycle of the AC supply. For example if the supply has a frequency of 50 Hertz it will produce a rotating field of 50 revolutions per second or 3000 (50 x 60) revolutions per minute. Every 60° one set of poles will not generate a magnetic field, due to the distribution of the input currents, as shown above, whilst the other two will produce magnetic fields of equal strength, and the resultant field will act in the direction of the arrow. If a rotor is then placed in the centre of the rotating magnetic field, a magnetic field will be induced in it, which will lock onto the rotating outer field, and will turn with it.
Induction (Squirrel Cage) Motor The induction motor is one of the most widely used types of AC motor, which on aeroplanes is used to drive fuel pumps, actuators, and air conditioning. A typical machine is shown below.
The stator is almost identical to that of a three-phase AC generator, and when a three-phase AC supply is connected to the stator it will produce a rotating magnetic field, whose speed (synchronous speed) will be proportional to the frequency of the supply. The rotor consists of a cylindrical laminated-iron core having a number of copper or aluminium longitudinal bars, which are evenly spaced around its circumference. These bars are joined by end plates, and together form a ‘Squirrel Cage Rotor’.
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The rotating outer magnetic field cuts the stationary rotor and induces an EMF or voltage proportional to the rate of change of flux in the squirrel cage. The shorted bars offer little resistance and a large current flows in the bars, as shown on the next page. The passage of current through the bars results in a magnetic field being produced, which in turn interacts with the outer rotating magnetic field.
A torque now exists between the rotor and the stator magnetic fields. This causes the rotor to turn and accelerate in the direction of the stator field, as shown below.
When the applied torque equals the load torque, the motor will run at a speed slightly less than the stator field. The induction motor is thus an ‘Asynchronous’ machine, and possesses similar characteristics to that of a DC shunt wound motor, as listed below:¾
Slip Speed. This is the difference between the ‘Rotor Speed’ and the ‘Synchronous (stator) Speed’. Slip Speed = Synchronous Speed - Rotor Speed Synchronous Speed = 60 f P where f = frequency of supply(Hz), and P = number of pole pairs in stator.
¾
Reversal of Rotation. crossed over.
¾
Loss of a Phase. If this occurs when the machine is:¾
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This will occur if any two of the motor phases are
Running. The motor will continue to run at a reduced torque.
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¾
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Not running. The machine will not start, and fuses or circuit breakers will blow in the other two phases causing possible damage to the motor.
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Two-Phase Induction Motor A rotating magnetic field is produced in a two-phase induction motor stator by placing the windings 90° apart, as shown below.
One phase is the reference phase, and the other is the control phase. Thus by varying the phasing and the amplitude of the control phase currents, the direction of, and speed of rotation can be controlled. This type of motor is however not as smooth, nor as powerful as a three-phase machine, and is used mainly for autopilot servomotors, or fuel trim motors.
Split-Phase Motor This a Split-phase induction motor two windings; one capacitive and the other resistive, are both connected in parallel across a single-phase AC supply, as shown below.
The current in the capacitive winding will lead the current in the resistive winding by approximately 90°, and this is known as ‘Phase Splitting’. This type of motor operates like a two-phase AC motor, and is used to drive actuators.
The Synchronous Motor The stator in this type of motor is identical to that used in an induction motor, except the rotor in this machine alternatively carries its own magnetic field windings, which are supplied from a DC source. When the rotor is energised with DC it acts like a bar magnet, as shown on the next page, and tries to line itself up with the magnetic field being produced by the stator. The stator is fed with three-phase AC, and produces a rotating magnetic field, which the rotor follows. This type of motor is a single speed machine, where the actual speed is determined by the speed of the rotating field, ie. the frequency of the three-phase input. Due to the high inertia between the rotor and stator field, this type of motor does not normally start on its own accord. It therefore has to be started and run up to speed by a ‘Pony Motor’, which is usually an induction motor. When the speed of the driven motor nearly reaches that of the rotating field, it locks on to it, and continues to rotate in synchronism with the rotating field.
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Synchronous motors are used in situations where a constant speed is essential, eg. Gyroscopes.
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Chapter 15. Semiconductor Devices Introduction Semiconductors are used extensively in most items of aeroplane electronic equipment, with the three most common devices being diodes, transistors, and integrated circuits.
Advantages and Disadvantages of Semiconductor Devices The advantages and disadvantages of semiconductor devices are:-
Advantages. Components, which are made from semiconductor materials, are often referred to as solid-state components because they are made from solid materials. These components are more rugged than vacuum tubes that are made of glass, and also require heaters to operate them, which consume large amounts of power. Semiconductors are additionally much smaller, lighter, and are much cheaper than vacuum tubes. Disadvantages. Semiconductors are highly susceptible to changes in temperature, and can easily be damaged at high temperatures. Components manufactured from these materials must and thus highly sophisticated temperature control must be applied to. Solid-state devices are also sensitive to supply voltage polarity, and can be easily damaged if this is not correct. Construction of a Semiconductor A semiconductor is a material, which under certain conditions can act as either a conductor or an insulator. Silicon (Si) and Germanium (Ge) are both semiconductive elements, of which Silicon is the most popular. Each atom of Silicon has four electrons in the outer (valence) shell, as shown below, and does not readily gain or lose electrons. ELECTRONS NUCLEUS
NUCLEUS & OUTER ELECTRONS
+4 SIMPLIFIED ARRANGEMENT
Single atoms of Silicon are of little use, so they are grown into large crystals, which are then cut into wafers for the manufacture of electronic components. The Silicon atoms link up with neighbouring atoms to share electrons and a cluster of silicon atoms sharing outer electrons forms a matrix called a ‘Crystal’, as shown below.
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COVALENT BONDS
The four electrons in the outer shell of each atom are shared with the electrons from the adjoining atoms via ‘Covalent Bonding’, and will result in the valence shell of each atom in the crystal effectively holding eight electrons. These bonds are so strong that at absolute zero temperature (-273°C) there are no free electrons and the Silicon crystal assumes the properties of an electrical insulator. If the crystal of Silicon is subsequently heated or a voltage applied across it the covalent bonds will break down and its characteristics will be altered. The electrons will thus break away from the atom and will leave behind a hole in the atoms outer shell. The free electrons will then travel through the Silicon as negative electrical charges and as the electrons move from one atom to another, the holes appear as if they are moving from one atom to another in the opposite direction. The movement of holes and electrons thus forms the basis of a semiconductor.
Doping Silicon in its pure state is not particularly useful in electronics, so ‘Doping’ is carried out, where the silicon atoms are contaminated with other materials such as Phosphorous (P), or Boron (B), to give them useful electronic properties. This contamination leaves the Silicon atoms with incomplete outer valence shells and a ‘Hole’ is formed in the shell. The holes, which replace the missing electrons thus act as positive charges and attract any free electrons within the crystal.
P-Type Material If Silicon is doped with Indium it will produce a P-type material. Indium atoms only have 3 electrons in their outer shell (Trivalent), and is an ‘Acceptor Atom’. This results in vacant electron openings or ‘Holes’, which are positively charged, being left in the Silicon crystal, as shown below.
If a voltage is applied across P-type material, as shown below, the electrons within the crystal will tend to move towards the positive terminal of the battery and will jump into the available holes of the Indium atom near the terminal.
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An electron from an adjacent Silicon atom will then fall into the hole, and the hole will appear to move to another location. The electrons will thus move through the material from left to right, whilst the holes move in the opposite direction.
N-Type Material If Silicon is doped with Phosphorous it will produce a N-type material. Phosphorous atoms have 5 electrons in their outer shell (Pentavalent) and are known as ‘Donor Atoms’. Extra electrons, which are negatively charged, will thus be left floating around in the crystal, as shown below.
A N-type semiconductor contains many donor atoms that contribute free electrons, and these are free to drift through the material. The loss of an electron will thus leave the donor atoms with an overall positive charge and will form positive ions. Electrical current will therefore flow in the normal manner, due to the movement of the free electrons, but like P-type Silicon can also flow due to the migration of holes.
P- N Junction Diode Both P and N-type Silicon conduct electricity at different rates, depending on the amount of doping. Both types thus function as resistors, and will conduct in both directions. The N-type
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material contains mobile electrons and an equal number of positive ions, which provide an overall neutral charge. The P-type material similarly contains mobile holes and an equal number of negative ions. Each part is thus initially neutral. If a junction is made by joining a piece of P and N-type material together, electrons will only flow in one direction through the junction, from N to P.
Hole
When the two materials are placed together some of the free electrons in the N-type material will cross the junction and fill the holes in the P-type material close to the junction. As the free electrons cross the junction the N-type material becomes depleted of electrons in the vicinity of the junction and the holes in the P-type material become filled, thus depleting the holes near the junction. The region where the holes and electrons become depleted is known as the ‘Depletion Layer’.
This will leave the N-type material with an excess of positive ions and the P-type material with an excess of negative ions near the junction, thus the material close to the junction will be in a charged state. The N-side will thus be positively charged and the P-side negatively charged, which is known as a ‘Diode’. This is an electronic one-way valve and is represented by the symbol shown below.
The ‘Anode’ is the negative side of the diode, which is associated with the P-type material, and the ‘Cathode’ is the positive side, which is associated with the N-type material. If voltages, known as ‘Bias Voltages’ or currents are applied across a diode it will behave in a different manner, and will depend on the polarity of the power source. When the positive terminal is connected to the N-type material the diode will be ‘Reverse Biased’ and no current will flow, ie. it will be in a non-conducting state, as shown below.
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ELECTRO N
Cathode
Anode
Conversely if the negative terminal is connected to the N-type material the diode will be ‘Forward Biased’ and current will flow, ie. it will be in a conducting state, as shown above. If the diode is Reverse Biased the positive terminal will attract electrons in the N-type material away from the junction and the negative terminal will similarly attract the holes in the P-type material, thus increasing the thickness of the Depletion Layer, as shown below.
Conversely if the diode is Forward Biased electrons will be attracted from the N-type material across the depletion layer to the positive terminal and the holes will be attracted to the negative terminal, as shown below.
A Forward Biased diode therefore acts as a closed switch and a Reverse Biased diode as an open switch.
Use of Diodes Diodes in there basic forms are used for rectification (or conversion) of AC into DC, for example in a battery charger circuit, as shown below.
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4
1
3
2
The diodes offer an easy path for currents to flow in one direction and offer a high resistance path in the opposite direction. During the positive cycle (1) current flows through diodes 1 and 3, whilst diodes 2 and 4 are switched off and during the negative cycle the reverse occurs, producing a DC output. The following special types of diode exist:-
Zener Diode. This is a special type of diode, which consists of a reverse-biased Silicon P-N junction, and is represented by the following symbol.
Anode
Cathode
Unlike a conventional diode this type of diode is designed to operate normally when it is forward-biased, but is designed to also operate when high reverse currents are applied. When the reverse-bias voltage reaches a set value, typically 4 to 75 volts, depending on the design, when the Zener diode will breakdown, and ‘Thermal Avalanche’ will occur. When this occurs one electron will gain sufficient energy to knock others out of the valence band, and will cause a rapid increase in current flow through the diode, as shown on the next page.
Zener diodes are used to provide a fixed ‘Reference’ voltage over a range of input voltages and also for precisely regulating or stabilising the output from a power supply, as shown below.
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Variable Capacitance (Varicap) Diode. In this type of diode the depletion layer situated between the P-N junction acts like the dielectric in a capacitor, whilst the P and N materials act as its plates. When the diode is reversed biased the depletion layer widens and gives the affect that the plates of the capacitor have move further apart, thus reducing the capacitance value. Conversely if the reverse bias voltage reduces the capacitance value will increase. It is thus possible to vary the capacitance of this diode simply by altering the magnitude of the reverse bias voltage, which is the method that is commonly used in radio tuners using DC, rather than using a mechanical variable capacitor. A variable capacitance diode is represented by the following symbol.
Bi-Polar Transistors Transistors are made up of a sandwich of P and N-type materials. They can be used as relays, as switches or as variable resistors. The two configurations of bi-polar transistors are PNP and NPN as shown on the next page.
The three layers of a bi-polar transistor are the emitter, base and collector, where the arrowhead depicts the flow of conventional current. The base is extremely thin, and has
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fewer doping atoms than the emitter and collector. A very small voltage or current applied to the material in the centre of the sandwich (base) can thus control a much larger current flowing through the complete device, thus acting as an amplifier.
Operation of a PNP Bi-Polar Transistor If the transistor is reverse biased by connecting across two power sources the positive terminals of each will attract electrons in the N-type material away from the P-N junction, and the negative terminals will similarly attract the holes in the P-type material, as shown below.
This will thus increase the thickness of the ‘Depletion Layer’ between the different layers and the transistor will not conduct. For the transistor to operate the emitter-base junction has to be forward biased, whilst the collector-base is reverse biased, as shown below.
The positive junction of the emitter battery (Ve) will repel the holes in the P-type emitter towards the P-N or emitter-base junction and will cross through into the lightly doped N-type base. The majority of the holes (approximately 95%) do not combine with electrons in this region and pass directly to the P-type collector. The holes are then rapidly neutralised with electrons from the negative terminal of the collector battery and are swept away from the collector. For each hole, which is neutralised by an electron a covalent bond near to the emitter electrode will break down, and an electron will be released to the positive terminal of the emitter battery. This will in turn produce a hole, which will quickly move through the material from left to right. A small number of holes (approximately 5%) will also combine with electrons in the N-type base material and will be lost. The major charge carriers in a PNP bipolar transistor are therefore the holes, and a very small emitter-base current (Ib), will cause a large emitter (Ie) to collector (Ic) current to flow, but in all cases of operation:-
Ie = Ib + Ic Operation of a NPN Bi-Polar Transistor A NPN transistor will conduct if like the PNP transistor with the emitter-base junction forward biased and the base-collector junction reverse biased, which is achieved by reversing the battery polarity, as shown below.
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Electrons are repelled from the negative terminal of the emitter battery (Ve) and flow towards the positive terminal of the collector battery (VC). The electrons will thus be forced into the emitter junction, and since the P-region base is only lightly doped the majority of the electrons (approximately 95%) will diffuse through the base and reach the collector junction. A small amount of the electrons (approximately 5%) will however combine with the holes in the P layer and will be lost as charge carriers. For every electron, which leaves the collector one electron enters the emitter junction thus maintaining a continuous flow of electrons from left to right through the transistor. The major charge carriers in a NPN junction transistor are therefore the electrons.
Disadvantages of Diodes and Transistors Diode and transistors share several key features, eg. too much current will cause a transistor like a diode to become hot and burn out. This is because semiconductors have a negative temperature coefficient and can go into ‘Thermal Avalanche’ ie. one electron will gain enough energy to knock others out of the valence band, thus causing an increase in current flow through the transistor. If a transistor overheats it will also not operate properly, and engineers sometimes use a freezing spray to locate a failing component in a circuit. If the PNP transistor is to conduct, the emitter has to be connected to a positive voltage and the collector to a negative voltage. If the base is connected to a voltage, which is more positive than the emitter, a small current will flow into the base. The flow of current will then cause a large current to flow between the other two connections (emitter and collector).
Transistor Applications If the base of an NPN transistor is earthed (0 volts) no current will flow from the emitter to the collector, and the transistor will be switched off. The transistor will thus operate as an open switch, as shown below
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If the base emitters forward bias voltage is gradually increased, the emitter collector current, which is much higher, will follow the same variation as the smaller base current, and the transistor will act as an amplifier. This explanation applies to a transistor in which the emitter is the common connection for both input and output, which is known as a common emitter. Transistors can also be used in either the common base mode or the common collector mode.
Integrated Circuits Integrated Circuits (IC’s) are manufactured by combining transistors, diodes and resistors on a small piece of ‘Silicon’. The complete device is known as a ‘Chip’ and can contain a few, or many thousands of transistors.
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The Advantages and Disadvantages of Integrated Circuits The advantages of IC’s are that they:¾
are extremely small and light.
¾
consume little power.
¾
can operate at high speed.
¾
are extremely reliable.
The disadvantages of IC’s are that they:¾
are easily damages by high voltages or currents.
¾
can not be repaired.
The advantages however outweigh the disadvantages, and are thus extensively used in the aviation industry.
Types of Integrated Circuits IC’s are grouped into the following categories:-
Analogue (or Linear) IC. This type of IC is typically used in the manufacture of amplifiers, timers, oscillators and voltage regulators. They amplify or respond to variable voltages and produce outputs. Digital (or Logic) IC. This type of IC is typically used in the manufacture of microprocessors and computer memories. They normally respond to two discrete voltage levels (or Gates) representing 1’s or 0’s, and act as electronic switches to produce outputs.
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Chapter 16. Logic Circuits Introduction Logic gates are represented diagrammatically and their logic inputs are shown on a ‘Truth Table’. Logic gates also often have more than two inputs, which increases the decision making capability of a ‘Gate’ and also increases the number of ways of connecting one to another to form advanced ‘Digital Logic Circuits’.
Number Systems Decimal Number System. The decimal number system requires ten different numbers (0-9) and also requires ten discrete voltage levels. It then repeats itself by going into 10’s, 100’s and 1000’s etc. This system can be typically used to represent the position or ground speed of an aeroplane. Binary Number System. This system uses numbers that are to the base of 2, as shown below. 2
6
64
2
5
32
4
2
16
8
2
3
2 4
2
2 2
1
2 1
0
Binary Number Decimal Equivalent
In digital electronic applications, binary numbers are used as codes, which represent decimal numbers, letters of the alphabet, voltages and many other forms of information. For example a simple switch can be assigned a binary value 0 to the ‘OFF’ position and a Binary 1 to the ‘ON’ position. Alternatively the polarity of a DC switching circuit can be altered so that a (+) indicates a binary 1 and a (–) indicates a binary 0. An alternative method is to vary the mean voltage in a circuit, which can cause it to increase by a pre-set increment for a binary 1 and to decrease by a similar increment to achieve a binary 0. The latter method is the most common, and the voltages used for this purpose vary between manufacturers, but are normally in the range from + 5 volts to + 12 volts. They are also designed to use either positive or negative logic. ‘Positive Logic’ is where a Logic 1 voltage is more positive than a Logic 0 voltage, and ‘Negative Logic’ is where a Logic 1 is more negative than Logic 0 Other possible numbering systems are the:¾
‘Octal’ system in which the numbers are to the base 8.
¾
‘Hexadecimal’ system in which the numbers are to the base 16.
¾ Duodecimal system, which is based on the figure 12, eg. the clock, and is used on a daily basis.
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Binary Representation Digital computers are electronic units, and in electronics it is a relatively easy procedure to operate circuits in such way as to encode them in a binary format.
Basic Logic Gates The following basic gates exist:-
AND Gate. This type of gate is represented by two switches connected in series and requires two Logic 1’s (A & B) to produce an output (Q), as shown below.
OR Gate. This type of gate is represented by two switches connected in parallel and requires only one Logic 1 (A or B) to produce an output (Q), as shown below.
NOT Gate. A single switch represents this type of gate where the input signal (A) is inverted to provide an output (Q), as shown below.
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NAND (Not or Negated AND) Gate. This type of gate is represented by two switches connected in parallel and requires only one Logic 0 (A or B) to produce an output (Q), as shown below.
NOR (Not or Negated OR) Gate. This type of gate is represented by two switches connected in series and requires two Logic 0’s (A or B) to produce an output (Q), as shown below.
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EXCLUSIVE OR Gate. This type of gate is a combination of NOT and NAND gates and requires only one Logic 1 (A or B) to produce an output (Q), as shown below.
Adder and Subtracter Circuits ‘Adder’ circuits are used to add binary digits (1’s and 0’s) together and ‘Subtracter’ circuits are alternatively used to subtract binary digits. These circuits are thus used in computer systems to carry out basic arithmetic functions. When carrying out addition functions it is always necessary to carry a digit to the next adjacent higher order, eg. 011 + 100 = 111 or in decimal terms 3 + 4 = 7. Conversely in a Subtracter circuit it is necessary to borrow a digit from the next adjacent lower order column (if applicable), eg. 111 – 011 = 100 or in decimal terms 7 – 3 = 4. A ‘Half Adder’ circuit is capable of adding 2 digits but is unable to carry a digit to the next order, so it is necessary to join two Half Adder circuits together to form a ‘Full Adder’ circuit in order to satisfy this requirement. A Half Adder electronic circuit consists of a combination of ‘AND, OR and EXCLUSIVE OR’ Gates as shown below.
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Two Stage Adder Circuit A ‘Two Stage Adder’ electronic circuit similarly consists of a combination of ‘AND, OR and EXCLUSIVE OR’ Gates as shown below.
AB + CD
Example. The following table can be established using the above circuit, by inputting a series of 0’s and 1’s.
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A (21)
B (20)
0 0 1 1 1 1
1 1 0 0 1 1
+
C (21)
D (20)
0 0 0 1 1 1
0 1 1 0 0 1
16-5
= C12
(2 )
S1 (21)
S0 (20)
0 0 0 1 1 1
0 1 1 0 0 1
1 0 1 0 1 0
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Digital Latch and Flip-Flop Circuits These circuits both use a combination of logic gates, which are used to perform basic memory functions for computers and their peripherals. A typical latch circuit is the ‘RS Latch’ circuit as shown below, which retains the output signal even after the input signal has been removed.
The two inputs to the RS Latch circuit are S and R, whilst the outputs are Q and Q. If a binary 1 is inputted at S the latch memory will be ‘Set’ and will produce an output Q of 1, whilst Q will be outputted with a 0. Conversely if a binary 1 is inputted at R the latch memory will be ‘Reset’ and will alternatively produce the opposite outputs. A ‘Flip-flop’ circuit is similar to a Latch circuit, although the output will be changed if a ‘Trigger Pulse’ is applied to the circuit, as shown below.
Clock Pulse CP
This circuit has three inputs and two outputs, with the S and R inputs being identical to the Latch Circuit. The circuit switch time is however controlled by inputting a ‘Clock Pulse (CP)’, which will simultaneously change over the output signals, Q and Q at a specified time interval. This arrangement is particularly useful in computers when several memory circuits are being used simultaneously, since if the outputs changed out of sequence it may result in the entire memory may become invalid.
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Chapter 17. Computer Technology Introduction The modern aeroplane is highly dependent on the digital computer, and this piece of equipment governs almost every facet of its operation.
Analogue Computers Analogue computers are non programmable and deal with infinite continuous values rather than discrete ones. It uses digits from 0 to 9 and operates as a mechanical computer using a rotating gear or wheel to represent different values, eg. if the wheel is between 0° and 10° it represents 0 or between 11° and 20° it represents 1. The analogue computer thus suffers from friction between the moving parts and mechanical wear. The speedometer in a car is an everyday example of an analogue computer, since it is attached to a sensor that counts the revolutions of the road wheels and, using an assumed wheel radius, calculates the distance covered since the last reset. It adds this to the distance at the start of the run and indicates the total distance the car has covered since new. It also uses the distance per unit time to provide an indication of speed. The speedometer is thus a calculating machine, which uses a data input and by carrying out a calculation it converts the input into another form of information; speed via a moving needle and distance as a digital read out. Analogue computers are still widely and effectively used although they suffer from the following limitations and shortcomings:¾
They are specific to a particular role and a separate computer is required for different applications.
¾
They use moving parts.
¾
They tend to be bulky and heavy.
Digital Computers A Digital computer is also a calculating machine, but instead of using synchro and gears different voltages are used to represent the digits from 0 to 9. For example 0 – 0.9 volts would represent the digit 0 and a voltage from 1.0 – 1.9 volts would represent the digit 1 etc. This machine uses actual high-speed arithmetic to do the necessary calculations typically using a decimal number system. It is also possible to convert decimal values into digital values, or to convert analogue values into binary code. Everything that a digital computer does is based on one operation, which is represented by the ability to determine if a ‘switch’, or ‘gate’ is open or closed. That is, the computer can recognise only two states in any of its microscopic circuits, ie. an on/off, high voltage or low voltage, or in the case of numbers, 0 or 1. It is equally valid to reverse the process and produce an analogue value from a digital process using binary arithmetic. The speed at which the computer performs this simple act, however, is what makes it such an essential element of the modem technology aeroplane. Computer speeds are measured in megahertz, or millions of cycles per second. A computer with a "clock speed" of 133 MHz is capable of executing 133 million discrete operations every second.
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Digital computers are also normally integrated with other systems on an aeroplane, via signal-interfacing devices such as ‘Analogue-to-Digital (A/D) Converters’ and ‘Digital-toAnalogue (D/A) Converters’. The ‘Input Interface’ converts analogue data into a digital format and the ‘Output Interface’ converts the digital data into an analogue format. The processing speed of a Digital computer and its calculating power are further enhanced by the amount of data, which is handled during each cycle. If a computer checks only one switch at a time, that switch will only represent two commands or numbers. For example ‘ON’ would symbolise one operation, and ‘OFF’ would symbolise another. By checking groups of switches linked within a single unit simultaneously, the computer is able to increase the number of operations it can recognise during each cycle. For example, a computer that checks two switches at one time can represent four numbers (0 to 3) or can execute one of four instructions at each cycle, one for each of the following switch patterns: OFF-OFF (0); OFF-ON (1); ON-OFF (2); or ON-ON (3). When digital computers were first introduced they were capable of checking eight switches (binary digits) or ‘Bits’ of data during every cycle, or a ‘Byte’, which contains 256 possible patterns of ‘ONs’ and ‘OFFs’ (or 1's and 0's). A computer uses a standard information format that consists of a group of bits, or a ‘Word’, which equates to:¾ ¾ ¾
an instruction. part of an instruction. a particular type of datum, eg. a number, a character or a graphics symbol.
The pattern 11010010, for example, might be binary data (in this case, the decimal number 210) or it might tell the computer to compare data stored in its switches to data stored in a certain memory chip location. The total list of recognisable operations or patterns, which a computer is capable of, is called its ‘Instruction Set’.
Computer Architecture The physical components of a computer are known as ‘Hardware’ and a digital computer is not a single component machine, but is made up of the five distinct elements, as shown in the following diagram.
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The programmes used in a computer are alternatively known as ‘Software’.
Input Devices Input devices are the means by which a computer is fed with the information required for problem solving, and consist of the following typical hardware:¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾
Keyboard Scanner Touch sensitive screen Speech recognition Mouse Joy stick Data from sensors As long as the data is identifiable the computer’s processor will be able to recognise it, and will accordingly route it along the appropriate internal ‘Buses or Data Lines’. These form a network of communication lines that connect the internal elements of the processor, and also leads to external connectors linking the processor to the other elements of the computer system. The following types of CPU buses exist:¾
A ‘Control Bus’ consists of a line that senses input signals and another line that generates control signals from within the CPU.
¾
The ‘Address Bus’, is a one-way line from the processor that handles the location of data in memory addresses.
¾
The ‘Data Bus’, a two-way transfer line that both reads data from memory and writes new data into memory.
Central Processing Unit The Central Processing Unit (CPU) may consist of a single chip, or a series of chips that are able to perform arithmetic and logical calculations, and can also control the operations of the other system elements. A ‘Microprocessor’ is a miniature CPU chip, which incorporates additional circuitry and memory. CPU chips and microprocessors consist of the functional sections, shown below.
The CPU receives input data and uses that data to carry out specific instructions, from which an output is derived. Typical ‘Input’ data might be wind velocity and direction, or even the distance to run to a destination. The CPU then carries out calculations on this data using the following parts to give ‘Output’ data, such as TAS, or time to run to the next waypoint.
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Central Control Unit. This unit coordinates the functions being carried out in each section of the computer via a ‘Communication Link’ or ‘Data Transfer Bus’. The Control Unit decodes or reads the patterns of data being held in a designated ‘Register’, or temporary storage area, and keeps track of any instructions. The register also holds the location and results of these operations, and the control unit translates the pattern into an activity, such as adding or comparing. It also indicates the order in which individual operations use the CPU, and regulates the amount of CPU time that each operation may consume. Memory. This is normally divided into either ‘Volatile Memory’, which is lost whenever the computer loses power, and ‘Non-Volatile Memory’, which remains in the system until it is over-written with new data. The main types of internal memory are:RAM (Random Access Memory). This is Volatile memory and the data deposited in it is thus lost whenever the power is turned off, or alternative states are written in. ROM (Read only Memory). This is Non-Volatile memory and normally contains data that has been inserted on the chip during its manufacture. The ROM typically contains start-up details and mathematical formulae, which will be maintained even after the power has been switched off. Replacing the entire chip is the only way to change the instructions on a ROM. PROM (Programmable Read Only Memory). This form of memory is non-volatile, but unlike the ROM can be reprogrammed once only, with the chip still fitted in the aeroplane’s computer. EPROM (Erasable Programmable Read Only Memory). This type of memory is also non-volatile and can be reused indefinitely. It can be totally erased and then reprogrammed with the chip still fitted in the computer. Arithmetic and Logic Unit (ALU). This chip gives the computer its calculating capability, allowing both arithmetical and logical calculations using a combination of digital logic circuits. These circuits are used to make specific true-false decisions based on the presence of multiple true-false signals at the inputs, and the signals may be generated by either mechanical switches, or by solid-state transducers, which are combined together to form an ‘Integrated Circuit (IC)’ Output Devices The output devices enable the user to see the results of the computer's calculations or data manipulations. The most common output device is the video display screen, which is a monitor that displays characters and graphics on a ‘Cathode-Ray Tube (CRT)’, or television-like screen. A screen is usually small, and portable computers commonly use liquid crystal displays (LCD) or other forms of screen. Examples of such screens are the EFIS and ECAM displays on modern aeroplanes. The standard output devices include printers and modems. A modem links two or more computers by translating digital signals into analogue signals so that data can be transmitted via telecommunications. Outputs may also be in the form of signals that are sent to the operating devices, and are typically used to control the engines or Automatic Flight Control System on the aeroplane.
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Storage Devices Computer systems can store data internally (in memory) and externally (on storage devices). External storage devices, may physically reside within the computer's main processing unit, or external to the main circuit board. These devices store data as electrical charges on a magnetically sensitive medium such as an audiotape, or a disk coated with a fine layer of metallic particles, or alternatively as an imprint on a ‘Laser Readable Disk’. The most common external storage devices are called ‘Floppy’ and ‘Hard Disks’. Floppy disks can contain from several hundred thousand bytes to well over a million bytes of data, depending on the system. ‘Hard, or Fixed’,' disks cannot be removed from their disk-drive cabinets, which contain the electronics to read and write data onto the magnetic disk surfaces. Hard disks can store from several million bytes to a few hundred million bytes. ‘CD-ROM’ technologies, which use the same laser techniques that are used to create audio compact disks (CDs), also provide storage capacities in the range of several gigabytes (billion bytes) of data.
Operating Systems An operating system is a master control program, which is permanently stored in the memory. They interpret user commands and request various kinds of services, such as display, print, or copy a data file; list all files in a directory; or execute a particular program. Different types of peripheral devices, such as disk drives, printers, communications networks and so on, handle and store data differently from the way the computer handles and stores it. Internal operating systems are usually stored in ROM memory, and are developed primarily to co-ordinate and translate data flows from dissimilar sources, such as disk drives or coprocessors (processing chips that perform simultaneous but different operations from the central unit
Programming A program is a sequence of instructions that tells the hardware of a computer which operations to perform on the data. Programs can be built into the ‘Hardware’ itself, or they may exist independently as ‘Software’. In some specialised computers, the operating instructions are embedded in their circuitry; as in the Flight Management System (FMS). Once a computer has been programmed, it can only do as much, or as little as the software controlling. Software in widespread use includes a wide range of applications programmes and instructions to the computer on how to perform various tasks.
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Chapter 18. HF and Satellite Airborne Communications Introduction between:-
Air carrier operations ideally require uninterrupted communications
¾
Air Traffic Control to ensure a safe flow and separation from other traffic, and to be kept up to date with conditions along the route and at the destination. They are also essential in the event of any incident that might endanger the aeroplane or those on board.
¾
Communicating with the Company in respect of the business of transporting people or freight or with respect to maintenance related items.
¾
Providing assistance to other aviators in need of assistance.
In 1994 for example, 99% plus of such communications were achieved by voice, using either short range VHF, or HF for long distance communications. This situation has now dramatically changed, and information is now being digitized and routed over data links including satellites, with printouts available as required. The antenna map below shows the typical equipment installation in a modern jet transport aeroplane for communication purposes. Aerial Locations – Communications Only HF Couplers ATC 1 & 2
VHF 1
SATCOM
HF 1 & 2
ATC 1 & 2 VHF 3
VHF 2
Many equipment manufacturers produce communications and navigation equipment, so the following descriptions are typical of the many and various models that are available.
Long Range Communications (Up to 4000 Km) At present, when flying over 370 Km (200 miles) from land, aeroplanes use HF transceivers, which are linked with unreliable propagation characteristics. Such HF installations are usually duplicated, with one set used for ATC purposes, and the other for company messages. HF communications are also used in areas where VHF communications are not
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possible, eg. Sectors over the oceans, or over sparsely populated continents such as Africa. Because HF communications rely on skywave propagation it is essential that the correct frequency is used, ie. At night the frequency needs to be reduced to maintain the skip distance. A typical HF radio control panel is shown on the next page.
FREQUENCY DISPLAY HF - 2
HF - 1
4.675
8.891 TUNE AM
TUNE
USB LSB
AM
SQUELCH
OFF
FREQUENCY SELECTOR
USB
LSB
OFF
SQUELCH
FUNCTION SWITCH
TYPICAL HF RADIO CONTROL PANEL
The frequency range will cover the part of the spectrum between 2.8 MHz and 24 MHz (and very often between 2 and 30 MHz) in 1 KHz steps. There is also a facility for AM and LSB operations, however USB is the standard operating mode. In common with other receivers a squelch control cuts off background noise in the absence of ground transmissions. The power output is approximately 400 watts on voice peaks and gives ranges greater than 3700 Km (2000 nautical miles) in good conditions.
VHF 1
VHF 2
VHF 3
HF 1
HF 2
INT
PA
MARKER INT
DME
NAV 1
2 ADF
VOICE ONLY
RADIO
TYPICAL AUDIO SELECTOR PANEL
An audio selector panel is situated at each crew station and enables switching between the various radio devices.
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RADIO NO.1
RADIO NO. 3 ETC.
RADIO NO. 2
AUDIO SELECTOR PANEL AND INTERCOM AMPLIFIER
MICROPHONE NO. 1
SAMPLE OF SPEECH IS FED BACK TO HEADSET
MICROPHONE NO. 2
'SIDETONE' HEADSET NO. 1
HEADSET NO. 2
BLOCK SCHEMATIC - AUDIO SELECTOR PANEL
Short Range Communications (Up to 450 Km) Most communications when overland are effected on VHF frequencies, except possibly for remote sparsely populated desert or jungle areas, when HF will be used. Like the HF, operations are in a simplex mode, ie. Transmission and reception are not possible simultaneously. Commonly a VHF control unit displays two frequency readouts, and each is controlled by its own selector knob. A transfer switch is used to select one VHF frequency as active whilst the other is at standby, and a light over the frequency window will show which frequency is active. LIGHT INDICATES SELECTED TRANSCEIVER
FREQUENCY DISPLAY
VHF COMM
1 1 8 2 7
1 1 8 3 0 TRANSFER
SQUELCH
CONCENTRIC TUNING CONTROLS
VHF COMMS CONTROLLER
The radio also has a fine tuning (filter) facility and are also fitted with items such as:
Automatic Volume Control (AVC). This maintains the receiver output signal at a given strength, and automatically reduces the receiver gain if the signal becomes stronger. Automatic Frequency Control (AFC). This keeps the receiver tuned to the selected signal irrespective of any slight wandering of the transmitted frequency.
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Connection to microphone(s) and the headset/speaker is through individual crew member's audio selector facilities. The transceivers are remotely located and are connected to vertically polarized blade antennas. An aeroplane may be equipped with as many as three identical VHF transceivers, which operate in the frequency range 118 MHz to 137 MHz. The International Aeronautical Emergency frequency of 121.5 MHz also lies within this band. The frequencies are channelised at 8.33 KHz intervals. The type of transmission is AM and uses an output power in the order of 25 watts. The range is quasi-optical and is typically 407 to 460 Km (220 to 250 nautical miles) at jet cruising levels.
Selective Calling (SELCAL) System The Selcal System is designed to relieve the flight crew from continually monitoring the communication channels, and is operative on HF and VHF radios. A four-tone audio signal is transmitted by the ground station, and provided that the aeroplane radio is tuned to the same radio frequency, the four-tone signal will be routed to the decoder circuits of the Selcal unit, which will then over-ride the setting of the squelch control. When the transmitted tones match the pre-selected aeroplane tone combination (Selcal Code) the flight deck crew will be visually and audibly alerted by an intermittent light on the Selcal indication panel, and a twotone chime. HF 1 & VHF 1
HF2 & VHF 2 LIGHTS ILLUMINATE WHEN CALLED
SEL CAL
SEL CAL SELCAL NO. 2
SELCAL NO. 1
SELCAL INDICATOR PANEL
A SELCAL Code consists of a 4-letter group, eg, HMJE. A registrar of SELCAL codes makes an assignment from 10,920 combinations for use by the air carrier, who in turn assigns a 4-letter group, to each aeroplane in the fleet, for setting on the decoder unit. The system is not used on VHF air traffic channels, because immediate response to control instructions is essential. The system operates on two separate channels that may be switched to any one of the available transceivers.
Operational Check. System operation should be checked by calling the ground station, and requesting a SELCAL check using the code set in the decoder. Operation of both SELCAL units will be checked simultaneously if receivers are on and selected to the same frequency. An intermittent light and two-tone chime will indicate proper operation. Operation will continue until the SELCAL light cap is pushed or until a microphone is keyed to transmit on the appropriate HF or VHF system. Either action will reset the system for the next call. Satellite Communications (SATCOM) The deficiencies of VHF and HF over oceans and unpopulated areas may be overcome by the use of satellites for air/ground communications. An internationally owned co-operative called Inmarsat (International Maritime Satellite Organisation) maintains a number of geostationary satellites in orbit, which amongst other functions provide operational services, and passenger telephone facilities to aeronautical users. In support of the space segment there is a requirement for a number of ground stations linked to terrestrial communication networks. The British Telecom station at Goonhilly, in Cornwall, England, is one example.
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Groups of such stations band together to furnish near global coverage and they contract their services to the various airline users. This enables passengers to make directly dialed outgoing calls in flight from pay phones in the cabin. There is also a choice of voice or data, and the wide use of printers favours this data format. Other groups of ground earth stations support either Sita or Arinc, either of which can accept or distribute air traffic network (ATN) messages as well as company messages. Those airlines favouring the use of the Sita network (generally non-US carriers) will use ground earth stations located in California, Western Australia, France and Quebec, as shown below.
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Satellite Aircom (SITA) Messages to and from the satellites will be relayed to airline offices over the existing Sita network. US carriers favouring the Arinc network, will use another similar group of ground earth stations for the same function. It is the intention that both networks shall be mutually supportive. The flight crew may well be unaware of the service provider, since they merely log-on to the visible satellite at the start of a sector; and any subsequent receipt and dispatch of information is normally done automatically. The aeroplane to satellite link is accomplished on L-Band channels between 1530 MHz to 1660.5 MHz. The satellite/earth link is also able to use C-Band frequencies of 4000 MHz upwards via large steerable terrestrial dishes.
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To operate high speed data and digitized voice, a high gain directional antenna must be installed on the aeroplane. This antenna is steered electronically from a knowledge of satellite position and aircraft position derived from the aeroplanes' flight management computer. An option also exists to use low gain antennas, which are significantly cheaper than the high gain variety, but this precludes voice link-up and operation is restricted to lowspeed data transfer. Each aeroplane may be individually addressed by its 24 bit unique transponder mode S code, and is able to download FMC information, also engine/performance related information, on request.
LIMIT
OF C O
VERA GE
GEOSTATIONARY SATELLITE
EQUATOR
AREA OF NOT TO SCALE
COVERAGE
O LIMIT
F COV
E ERAG
LATITUDE LIMITATION OF GEOSTATIONARY SATELLITE
One shortcoming of the geostationary satellite is the inability to cover polar areas. The limit of cover is 81½ degrees north and south at sea level, which is increased by 2 or 3 degrees for high flying aeroplanes. A figure of 80° is most commonly quoted for the purposes of JAA examinations.
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