EEC 123 Electrical Machine I Theory

UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II

NATIONAL DIPLOMA IN ELECTRICAL ENGINEERING TECHNOLOGY

ELECTRICAL MACHIENS I COURSE CODE: EEC 123

YEAR I- SEMESTER II THEORY Version 1: December 2008 1

TABLE OF CONTENT

Subject

Electrical Machines I

Year

1

Semester

2

Course Code

EEC123

Credit Hours

6

Theoretical

1

Practical

5

CHAPTER 1 : Magnetism …………………………………………....................WEEK1 1.1 Introduction ............................................................................................... 1 1.2 Concepts of Magnetism .......................................................................... 1 1.3 Types of magnets .................................................................................... 2 1.3.1 Permanent Magnet .................................................................. 2 1.3.2 Temporary Magnet ................................................................. 3 1.4 Electromagnetic Fields .......................................................................... 4 1.5 Magnetic Field Produced by a Coil ...................................................... 5 1.6 Induction ................................................................................................... 6 1.6.1 Induction Meaning ................................................................... 6 1.6.2 Self Inductance ...................................................................... 7 1.6.3 Mutual Inductance.................................................................. 8

CHAPTER 2 : DC Generator ………………………………….…………WEEK2 2.1 Introduction ............................................................................................. 2 2

2.2 The Basic Principle DC generator ....................................................... 2 2.2.1 The simplest AC generator .................................................. 6 2.2.2 The simplest DC generator................................................. 7 2.3 Constructions of DC Generator .......................................................... 2 2.3.1 Magnetic field structure ...................................................... 6 2.3.2 Armature structure ............................................................. 7

2.3.3 Commutator structure .......................................................... 6 2.3.4 Brush structure ..................................................................... 7 2.4 E.M.F Equation ....................................................................... WEEK3 2 2.5 Armature Reaction ................................................................................ 2 2.2.1 Shifting the Brushes .................................................................. 6 2.2.2 Compensating Windings and Interpoles .................................... 7 2.6 Classification Of Generators............................................................... 2 2.7 Voltage Regulation ................................................................ WEEK4 2 2.8 Generator Power Losses ..................................................................... 2 2.8.1 Copper Losses Losses ................................................................. 6 2.8.2 Eddy Current Losses ................................................................ 7 2.8.3 Hysteresis Losses

.................................................................. 7

CHAPTER 3 : DC Motor ………..………..... ............WEEK5 3.1. Introduction .........................................................................................26 3.2. Constructions and Operation Principle of DC Generator...........26 3.2.1 The dc motor torque ..................................................................26

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3.2.2 The dc motor torque .................................................................29 3.3. Back E. M. F...........................................................................................26 3.4. Types and characteristics of DC Motors ......................................26 3.4.1 Series DC Motor ........................................................................26 3.4.2 Shunt DC Motor ........................................................................29 3.4.3 Compound DC Motor ..................................................................26

3.4. Motor Nameplate ............................................................. WEEK6 37 3.4.1 Nameplate Terms ..................................................................37 3.4.2 Definition Nameplate ..........................................................37 3.5. Power Losses and Efficiency ...........................................................43

3.6. Starting Methods of DC Motor ..................................... WEEK7 45 3.6.1 Face –plate Starter ...........................................................46 3.6.2 Relay Starter .......................................................................48

3.7. Reversing the Rotation of DC Motor ............................ WEEK8 51 3.7.1. Reversing the Rotation of DC Series Motor ........................... 51 3.7.2. Reversing the Rotation of DC Shunt Motor ............................53 3.7.2. Reversing the Rotation of DC Compound Motor ....................54

3.8. Inspection and Maintenance of DC Motors ............... WEEK9 51

CHAPTER 4 : Single Phase Induction Motor…….WEEK10 4.1. Introduction .........................................................................................26 4

4.2. Construction of A.C single-phase induction motor .....................26 4.3. Types and characteristics of DC Motors ......................................26 4.2.1 Rotor ........................................................................................26 4.2.2. Stator ....................................................................................29 4.2.3. Frame enclosure .................................................................26 4.2.4. Fan ..........................................................................................26 4.2..5. Terminal ( connection ) box .............................................29 4.2.6 Centrifugal switch................................................................26

4.3. How Electrical Motor Work ...........................................................62 4.4. Operation Principle..............................................................................64

4.5. Motor Speed ....................................................................... WEEK11 67 4.5.1 Synchronous Speed ............................................................67 4.5.2 Rotor Speed .........................................................................68

4.6. Types of Single Phase Induction Motor ................... WEEK12 69 4.6.1 Split Phase Motors ..............................................................69 4.6.2 Capacitor Motors .................................................................72 4.6.3 Capacitor Run Motors..........................................................73 4.6.4 Capacitor Start Motors ......................................................75

4.6.5 Capacitor Start Capacitor Run Motors ........ WEEK13 77 4.6.6 Shadded Pole Induction Motors .......................................78

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4.6.7 Repulsion Motors ............................................... WEEK14 80 4.6.8 Universal Motors .................................................................. 81 4.7. Speed-torque characteristics of single-phase induction motor ....................................................................................83

4.8. power, losses and efficiency

.................................... WEEK15 84

4.8.1 Input power ............................................................................84 4.8.2 Kw to Hp Conversion ............................................................84 4.8.3 Motor Losses .........................................................................84 4.8.3.1 Core or Iron Losses................................................................... 86 4.8.3.2 Rotor Losses ............................................................................... 86 4.8.3.3 Stator Losses ............................................................................. 86 4.8.3.4 Friction and Windage Losses ................................................. 87 4.8.3.5 Stray Losses ............................................................................ 87

4.8.4 Efficiency ...............................................................................88 4.8.5 External speed control drives...........................................89 4.8.5.1 Direct drive ................................................................................. 89 4.8.5.2 Belt and pulley drives ............................................................... 89 4.8.5.3 Gear motors ................................................................................ 90 4.8.5.4 Gear drives .................................................................................. 90 4.8.5.5 Chain and Sprocket ....................................................................91

4.9. Nameplate information ................................................................... 91 4.10. Reversing the direction of rotation .............................................. 91 4.10.1 split-phase induction motor .............................................92

6

4.10.2 capacitor-run induction motor ........................................92 4.10.3 Very small induction motors ..........................................92 4.10.4 shaded-pole induction motors .........................................92 4.11. speed control .......................................................................................93 4.12. Applications .........................................................................................93

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This Page is Intentionally Left Blank

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Week 1 Introduction

Now, before we discuss basic electrical machine operation a short review of magnetism might be helpful to many of us. We all know that a magnet will attract and hold metal objects when the object is near or in contact with the magnet. 1.2 Concepts of Magnetism

A magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph shown in fig.(1-1). A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exist in threedimensions but are only seen in two dimensions in the image.

Figure(1-1) : The magnetic field surrounding a bar magnet

It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. The area where the exit poles are concentrated is called the magnet's north pole and the area where the entrance poles are concentrated is called the magnet's south pole.

9

Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane, the magnetic field is concentrated between the poles as shown in figure (1-2).

Figure(1-2) : Horseshoe magnet

The number of magnetic lines of force is a known as magnetic flux Φ. The flux has the weber (wb) as its unit, The number of magnetic lines of force cutting through a plane of a given area at a right angle is known as the magnetic flux density B. The flux density or magnetic induction has the tesla as its unit. One tesla is equal to 1 Newton/(A/m). From these units it can be seen that the flux density is a measure of the force applied to a particle by the magnetic field. T Types of magnets

There are two kinds of magnets permanent and temporary magnets.

1.3.1 Permanent magnet Permanent magnet will retain or keep their magnetic properties for a very long time. Permanent magnets are made by placing pieces of iron cobalt, and nickel into strong magnetic fields. Permanent magnets are mixtures of iron, nickel, or cobalt with

Figure(1-3) : natural magnet

other elements. These are known as hard magnetic materials. The natural form of a magnet is called a lodestone as shown in fig.(1-3), it contains iron. When man mixed the pure metals together ( ie. iron, nickel and cobalt ) we created an even stronger magnet which are the ones we use most today.

1.3.2 Temporary magnets Temporary magnets will loose all or most of their magnetic properties. Temporary magnets are made of such materials as iron and nickel. There are two essential methods for generating a magnetic field. Those two following methods:

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1- Magnetic material methods Magnetic material by stroking a permanent magnet onto a pure metal in one direction many times, soon it will become temporarily magnetized

(a)

as shown in fig.(1-4). Figure(1-4) : Generating magnetic material

2- Electrical currents methods Electrical currents can be used to make a magnet by getting a bar of iron and wrapping it with wires then run a current through the wires as shown in fig.(1-5). This arrangement is called a

(b

Figure(1-6): Magnetic field around the wire carried current

solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet.

Figure(1-5) :Generating electromagnet (Solenoid)

Electromagnetic Fields

Magnets are not the only source of magnetic fields. In 1820, Hans Christian Oersted discovered that the current in the wire was generating a magnetic field. He found that the magnetic field existed in circular form around the wire and that the intensity of the field was directly proportional to the amount of current carried by the wire as shown in fig.(1-6a) . A three-dimensional representation of the magnetic field is shown in fig.(1-6b).

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There is a simple rule for remembering the direction of the magnetic field around a conductor. It is called the right-hand rule. If a person grasps a conductor in ones right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field as shown in fig. (1-7).

Magnetic Field Produced by a Coil

A loosely wound coil is illustrated in figure(1-8) below to show the interaction of the magnetic field. The Figure(1-7): Right-hand rule magnetic field is essentially uniform down the length of the coil when it is wound tighter.

Figure(1-8): Magnetic Field Produced by a Coil

The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. Coiling a current-carrying conductor around a core material that can be easily magnetized, such as iron, can form an electromagnet. The magnetic field will be concentrated in the core. This arrangement is called a solenoid. The more turns we wrap on this core, the stronger the electromagnet and the stronger the magnetic lines of force become.

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Inductance Induction Meaning

Faraday noticed that the rate at which the magnetic field changed also had an effect on the amount of current or voltage that was induced. Faraday's Law for an uncoiled conductor states that the amount of induced voltage is proportional to the rate of change of flux lines cutting the conductor. Faraday's Law for a straight wire is shown below. Figure(1-9): Induction in wire

Induction is measured in unit of Henries (H) which reflects this dependence on the rate of change of the magnetic field. One henry is the amount of inductance that is required to generate one volt of induced voltage when the current is changing at the rate of one ampere per second. Note that current is used in the definition rather than magnetic field.

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Self-inductance

When induction occurs in an electrical circuit and affects the flow of electricity it is called inductance, L. Self-inductance, or simply inductance is the property of a circuit whereby a change in current causes a change in voltage in the same circuit as shown in fig.(1-10).

The mmf required to produce the changing magnetic flux (Φ) must be supplied by a changing Figure(1-10): Self inductance current through the coil. Magnetomotive force generated by an electromagnet coil is equal to the amount of current through that coil (in amps) multiplied by the number of turns of that coil around the core (the unit for mmf is the amp-turn). Because the mathematical relationship between magnetic flux and mmf is directly proportional, and because the mathematical relationship between mmf and current is also directly proportional (no rates-of-change present in either equation), the current through the coil will be in-phase with the flux waveform as shown in fig.(1-11):

Figure(1-11): Current, flux and voltage waveform

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Mutual-inductance

When one circuit induces current flow in a second nearby circuit, it is known as mutual-inductance. The image to the right shows an example of mutual-inductance as shown in fig.(1-12). When an AC current is flowing through a piece of wire in a circuit, an

i1

electromagnetic field is produced that is constantly growing and shrinking and changing direction due to the constantly changing current in the wire. This changing magnetic field will induce electrical current in another wire or circuit that is brought

e2

close to the wire in the primary circuit. The current in the second wire will also be AC and in fact will

Figure(1-12): Mutual inductance

look very similar to the current flowing in the first wire. An electrical transformer uses inductance to change the voltage of electricity into a more useful level. In nondestructive testing, inductance is used to generate eddy currents in the test piece.

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Week 2 2.1

Introduction

A generator does not create energy. It changes mechanical energy into electrical energy. Every generator must be driven by a turbine, a diesel engine, or some other machine that produces mechanical energy. For example, the generator (alternator) in an automobile is driven by the same engine that runs the car. Engineers often use the term prime mover for the mechanical device that drives a generator. To obtain more electrical energy from a generator, the prime mover must supply more mechanical energy. For example, if the prime mover is a steam turbine more steam must flow through the turbine in order to produce more electricity. 2.2

The Basic Principle DC generator

A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. This principle is explained as follows: Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor. • The amount of voltage generated depends on: 1. The strength of the magnetic field 2. The angle at which the conductor cuts the magnetic field 3. The speed at which the conductor is moved 4. The length of the conductor within the magnetic field. • The polarity of the voltage depends on: 1. The direction of the magnetic lines of flux. 2. The direction of movement of the conductor.

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To determine the direction of current in a given situation, the left-hand rule for generators

is

used.

This

rule

is

explained in the following manner. Extend the thumb, forefinger, and middle finger of your left hand at right angles to one another, as shown in fig.(2-1). Point your

thumb

in

the

direction

the

conductor is being moved. Point your forefinger in the direction of magnetic flux (from north to south). Your middle finger will then point in the direction of current flow in an external circuit to

Figure(2-1): Left-hand rule for generators.

which the voltage is applied.

2.2.1 The simplest AC generator The simplest generator that can be built is an ac generator. Basic generating principles are most easily explained through the use of the elementary ac generator. For this reason, the ac generator will be discussed first. The dc generator will be discussed later. A simplest generator fig.(2-2) consists of a wire loop placed so that it can be rotated in a stationary magnetic field. This will produce an induced e.m.f

(

electromotive force) in the loop. Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf.

17

Figure (2-2): The simplest generator.

The pole pieces (marked N and S) provide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop. The loop of wire that rotates through the field is called the armature. The ends of the armature loop are connected to rings called slip rings. They rotate with the armature. The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these brushes. The simplest generator produces a voltage as shown in fig.(2-3)

Figure (2-3): Output induced voltage of a simplest generator during one revolution. 18

2.2.2 The simplest DC generator A single-loop generator with each terminal connected to a segment of a twosegment metal ring is shown in fig.(2-4). The two segments of the split metal ring are insulated from each other. This forms a simple commutator. The commutator in a dc generator replaces the slip rings of the ac generator. This is the main difference in their construction. The commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses. Through this process the commutator changes the generated ac voltage to a pulsating dc voltage as shown in the graph of fig.(2-4). This action is known as commutation.

Figure (2-4) : Effects of commutation.

For the remainder of this discussion, refer to fig.(2-4), parts A through D. This will help you in following the step-by-step description of the operation of a dc generator. When the armature loop rotates clockwise from position A to position B, a voltage is induced in the armature loop which causes a current in a direction that deflects the meter to the right. Current flows through loop, out of the negative brush, through the meter and the load, and back through the positive brush to the loop. Voltage reaches its maximum value at point B on the graph for reasons explained earlier. The generated voltage and the current fall to zero at position C. At this instant 19

each brush makes contact with both segments of the commutator. As the armature loop rotates to position D, a voltage is again induced in the loop. In this case, however, the voltage is of opposite polarity. 2.3

Constructions of DC Generator

Fig.(2-6), views A through E, shows the main component parts of dc generators. (1) Magnetic field structure views A, B (2) Armature structure views C (3) Commutator structure views D (4) Brushes structure views E.

\ Figure(2-6) : The main parts of DC generator

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2.3.1 Magnetic field structure A magnetic field structure acts like the simple generator's magnet. It sets up the magnetic lines of force. It is electromagnets poles to create the lines of force in most generators. The electromagnetic field poles consist of coils of insulated copper wire wound on soft iron cores, as shown in fig.(27). The number of field poles commonly are two or four poles, some

small

generators

have

permanent magnets. Figure (2-7) : Four-pole generator

2.3.2 Armature structure The armature contains coils of wire in which the electricity is induced. It acts like the loop of wire in the simple generator. The coils for the armature and field structure are usually insulated copper wire wound around iron cores. The iron

Figure (2-8) : Rotor of a dc motor

cores strengthen the magnetic fields as shown in fig.(2-6) views C and in fig.(2-8)

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2.3.3 Commutator structure The commutator converts the AC

into

a

DC

voltage as

discussed before It also serves as a means of connecting the brushes to the rotating coils. In a simple one-loop generator, the commutator is made up of two semicylindrical pieces of a smooth material,

usually

separated

by

conducting copper,

mica insulation

Figure (2-9) : Connection of commutation with the end of armature coils

material, as shown in fig.(2-6) views D and in fig.(2-9). Each half of the commutator segments is permanently attached to one end of the rotating loop, and the commutator rotates with the loop. The segments are insulated from each other.

2.3.4 Brush structure The brushes structure is consist of brush holder, brush spring and brush as shown in figs.(2-6) views E and (2-10).

The brushes usually

made of carbon or graphite, rest against the commutator and slide along the commutator as it rotates. This is the means by which the brushes make contact with each end

Figure (2-10) : The brushes structure and its connection with commutation

of the loop. Each brush slides along one half of the commutator and then along the other half. The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact with one of the ends of the loop. Since the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the commutator. The brushes are positioned on opposite sides of the commutator; they 22

will pass from one commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. Every time the ends of the loop reverse polarity, the brushes switch from one commutator segment to the next. Fig.(2-11) shows the entire DC generator with the component parts installed. The cross sectional drawing helps you to see the physical relationship of the components to each other

Figure(2-11) : The cross-sectional of DC generator

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Week 3 2.4 E.M.F Equation The principle of DC generator is already been explained in 2.2 section. Whenever a

conductor is moved within a magnetic field as shown in fig.(2-12) that the conductor cuts across magnetic lines of flux, voltage is generated (e.m.f) in the conductor. The magnitude of voltage generated (e.m.f in volt) depends on The strength of the magnetic field (flux density β in Tesla or wb/m2), the angle at which the conductor cuts the magnetic field (angle of conductor θ relative to magnetic field), the speed (velocity) at

Figure(2-12): Right-hand rule for e.m.f

which the conductor is moved (V in m/s) , and the length of the conductor within the magnetic field(the effective length L in m).

e.m.f = β L V sin θ where, e.m.f = Induced electromotive force (voltage generated) in V or (volts)

β = Flux density of the magnetic field in Tesla or wb/m2 L = Length of conductor V

= Velocity of conductor in magnetic field in meter per second(m/s)

θ

= The angle between the magnetic field direction and the conductor

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Example 2-1 Calculate the e.m.f generated in a conductor of active length 20cm. When moves with a velocity of 15 m/s in the magnetic field of flux density 300mT at the following cases: (a) Conductor perpendicular to magnetic field (b) Conductor at angle of 30o relative to the magnetic field Solution (a) e.m.f = β L V sin θ

e.m.f = (300×10-3) × (20×10-2) ×15× (sin 90o) = 0.9 volts (b) e.m.f = β L V sin θ

e.m.f = (0.3×10-3) × (20×10-2) ×15× (sin 30o) = 0.45 volts

Example 2-2 A conductor of length 50cm, is moved at 10 m/s at right angles to a magnetic field. If the flux density of the field is 0.3 wb/m2. Find the induced e.m.f in conductor Solution

e.m.f = β L V sin θ e.m.f = (0.3) × (50×10-2) ×10× (sin 90o) = 0.9 volts = 1.5 V

2.5 Armature Reaction

From previous study, you know that all current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a

25

shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called armature reaction. You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole dc generator, shown in fig.(2-13). View A of the figure shows the field poles and the main magnetic field.

Figure (2-13) : Armature reaction.

The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. The symbols within the circles represent arrows. The dot represents the point of the arrow coming toward you, and the cross represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot. The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the coil is shown in view B of fig.(2-13). This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field. Now you have two fields - the main field, view A, and the field around the armature coil, view B. View C of fig.(2-13) shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short-circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing 26

1) The brushes must be shifted to the new neutral plane. 2) Used compensating windings or interpoles 2.5.1 Shifting the Brushes

In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes. The practice of shifting the brush position for each current variation is not practiced except in small generators. 2.5.2 Compensating Windings and Interpoles

In larger generators, other means are taken to eliminate armature reaction. for this purpose fig.(2-14). The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again.

Figure (2-14) : Compensating windings and interpoles.

Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels 27

the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.

2.6

Classification Of Generators When a dc voltage is applied to the field windings of a dc generator, current

flows through the windings and sets up a steady magnetic field. This is called field excitation. This excitation voltage can be produced by the generator itself (This is called self-excited generator) or it can be supplied by an outside source, such as a battery(This is called separately-excited generator). Self-excitation is possible only if the field pole pieces have retained a slight amount of permanent magnetism, called residual magnetism. When the generator starts rotating, the weak residual magnetism causes a small voltage to be generated in the armature. This small voltage applied to the field coils causes a small field current. Although small, this field current strengthens the magnetic field and allows the armature to generate a higher voltage. The higher voltage increases the field strength, and so on. This process continues until the output voltage reaches the rated output of the generator. Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections series-wound, shuntwound (parallel), and compound-wound. compound-wound generators are further classified as cumulative-compound and differential-compound. these last two classifications are not discussed in this book.

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Classification of DC Generators

separately-excited DC generator

Self-excited DC generator

Types of DC Motors

compoundwound

shunt-wound

series-wound

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Week 4 2.7 Voltage Regulation The regulation of a generator refers to the voltage change that takes place when the load changes. It is usually expressed as the change in voltage from a no-load condition to a full-load condition, and is expressed as a percentage of full-load. It is expressed in the following formula:

where EnL is the no-load terminal voltage and EfL is the full-load terminal voltage of the generator. Example 2-3 Calculate the percent of regulation of a generator with a no- load voltage of 462 volts and a full-load voltage of 440 volts ? Solution:

No-load voltage EnL = 462 V Full-load voltage EfL= 440 V

NOTE: The lower the percent of regulation, the better the generator. In the above example, the 5% regulation represented a 22-volt change from no load to full load. A 1% change would represent a change of 4.4 volts, which, of course, would be better.

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2.8 Generator Power Losses In dc generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the armature, are considered as losses and may be defined as follows: 1) Copper loss, or I2R in the winding 2) Eddy current loss in the core 3) Hysteresis loss (a sort of magnetic friction)

2.8.1 Copper Losses The power lost in the form of heat in the armature winding and field winding (if its found) is known as copper loss. Heat is generated any time current flows in a conductor. Copper loss is an I2R loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its crosssectional area. Copper loss is minimized in armature and field windings by using large diameter wire.

2.8.2 Eddy Current Losses The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called eddy currents. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss. Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Fig.(2-15), view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Fig.(2-15), view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to crosssectional area.) The currents in the individual pieces of the laminated core are 31

so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core. As you can see, eddy current losses are kept low when the core material is made up of many thin sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The laminations are insulated from each other by a

Figure (2-15) : Eddy currents in dc generator armature cores.

thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small. Most generators use armatures with laminated cores to reduce eddy current losses.

2.8.3 Hysteresis Losses Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.

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Week 5 3.1 Introduction Motors change electric energy into mechanical energy. Direct current motors and generators are constructed very similarly as explain in the previous chapter. They function almost oppositely at first because a generator creates voltage when conductors cut across the lines of force in a magnetic field, while motors result in torque-- a turning effort of mechanical rotation. Simple motors have a flat coil that carries current that rotates in a magnetic field. The motor acts as a generator since after starting, it produces an opposing current by rotating in a magnetic field, which in turn results in physical motion. 3.2 Constructions and Operation Principle of DC Generator

Motors change electric energy into mechanical energy. Direct current motors and generators are constructed very similarly described earlier in the previous chapter. They function almost oppositely at first because a generator creates voltage when conductors cut across the lines of force in a magnetic field, while motors result in torque a turning effort of mechanical rotation. Simple motors have a flat coil that carries

current that rotates

in a

Figure(3-1): Simple motor

magnetic field as shown in fig.(3-1). The motor acts as a generator since after starting, it produces an opposing current by rotating in a magnetic field, which in turn results in physical motion. This is accomplished as a conductor is passed through a magnetic field, then the opposing fields repel each other to cause physical motion. The left hand rule can be used to explain the way a simple motor works fig.(3-2). The pointer finger points in the direction of the magnetic field, the middle finger points in the direction of the current, and the thumb shows which way the conductor will be forced to move. 33

Figure(3-2): Left hand rules

DC motor has a rotating armature in the form of an electromagnet. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the DC motor going in the proper direction. See the diagrams shown in fig.(3-3).

(a)

(b) (c) Figure(3-3) : Diagrams that explains the operation of a DC motor.

a) A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. b) The armature continues to rotate.

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c) When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

3.2.1 The dc motor torque When the conductor is bent into a coil, the physical motion performs an up and down cycle. The more bends in a coil, the less pulsating the movement will be. This physical movement is called torque, and can be measured in the equation:

T = kt Ф Ia where : T = Torque in (Newton- meter) kt = Constant depending on physical dimension of motor Ф = Total number of lines of flux entering the armature from one N pole in (wb/m2) Ia = Armature current in (A)

3.2.2 Back E. M. F. While a dc motor is running, it acts somewhat like a dc generator. There is a magnetic field from the field poles, and a loop of wire is turning and cutting this magnetic field. For the moment, disregard the fact that there is current flowing through the loop of wire from the battery. As the loop sides cut the magnetic field, a voltage is induced in them, the same as it was in the loop sides of the dc generator. This induced voltage causes current to flow in the loop. this current direction opposite to that of the battery current. Since this generator-action voltage is opposite that of the battery, it is called "Back emf." (The letters emf stand for electromotive force, which is another name for voltage.) The two currents are flowing in opposite directions. This proves that the battery voltage and the back emf are opposite in polarity. At the beginning of this discussion, we disregarded armature current while explaining how back emf was generated. Then, we showed that normal armature current flowed opposite to the current created by the back emf. We talked about two opposite currents that flow at the same time. However, this is a bit oversimplified, as you may already suspect. Actually, only one current flows. Because the back emf can never become as 35

large as the applied voltage, and because they are of opposite polarity as we have seen, the back emf effectively cancels part of the armature voltage. The single current that flows is armature current, but it is greatly reduced because of the counter emf. In a dc motor, there is always a counter emf developed. This counter emf cannot be equal to or greater than the applied battery voltage; if it were, the motor would not run. The back emf is always a little less. However, the back emf opposes the applied voltage enough to keep the armature current from the battery to a fairly low value. If there were no such thing as back emf, much more current would flow through the armature, and the motor would run much faster. However, there is no way to avoid the back emf.

3.3 Types and characteristics of DC Motors There are three basic types of dc motors: (1) Series motors (2) shunt motors (3) compound motors They differ largely in the method in which their field and armature coils are connected.

3.3.1 Series DC Motor In the series motor, the field windings, consisting of a relatively few turns of heavy wire, are connected in series with the armature winding. Both a diagrammatic and a schematic illustration of a series motor is shown in fig.(3-4). The same current flowing through the field winding also flows through the armature winding. Any increase in current, therefore, strengthens the magnetism of both the field and the armature.

Figure(3-4) : Series DC motor

Because of the low resistance in the windings, the series motor is able to draw a large current in starting. This starting current, in passing through both the field and 36

armature windings, produces a high starting torque, which is the series motor's principal advantage. The speed of a series motor is dependent upon the load. Any change in load is accompanied by a substantial change in speed. A series motor will run at high speed when it has a light load and at low speed with a heavy load. If the load is removed entirely, the motor may operate at such a high speed that the armature will fly apart. If high starting torque is needed under heavy load conditions, series motors have many applications. Series motors are often used in aircraft as engine starters and for raising and lowering landing gears, cowl flaps, and wing flaps.

3.3.2 Shunt DC Motor In the shunt motor the field winding is connected in parallel or in shunt with the armature winding. See fig.(3-5), The resistance in the field winding is high. Since the field winding is connected directly across the power supply, the current through the field is constant. The field current does not vary with motor speed, as in the series motor and, therefore, the torque of the shunt motor will vary only with the current through the armature. The torque developed at starting is less than that developed by a series motor of equal size.

Figure(3-5) : Shunt DC motor

The speed of the shunt motor varies very little with changes in load. When all load is removed, it assumes a speed slightly higher than the loaded speed. This motor is particularly suitable for use when constant speed is desired and when high starting torque is not needed.

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3.3.3 Compound DC Motor The compound motor is a combination of the series and shunt motors. There are two windings in the field: a shunt winding and a series winding. A schematic of a compound motor is shown in fig.(3-6). The shunt winding is composed of many turns of fine wire and is connected in parallel with the armature winding. The series winding consists of a few turns of large wire and is connected in series with the armature winding. The starting torque is higher than in the shunt motor but lower than in the series motor. Variation of speed with load is less than in a series wound motor but greater than in a shunt motor. The compound motor is used whenever the combined characteristics of the series and shunt motors are desired.

Figure(3-6) : Compound DC motor

Like the compound generator, the compound motor has both series and shunt field windings. The series winding may either aid the shunt wind (cumulative compound) or oppose the shunt winding (differential compound). The starting and load characteristics of the cumulative compound motor are somewhere between those of the series and those of the shunt motor. Because of the series field, the cumulative compound motor has a higher starting torque than a shunt motor. Cumulative compound motors are used in driving machines which are subject to sudden changes in load. They are also used where a high starting torque is desired, but a series motor cannot be used easily.

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In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor. These two tend to offset each other and the result is a practically constant speed. However, since an increase in load tends to decrease the field strength, the speed characteristic becomes unstable. Rarely is this type of motor used in aircraft systems.

Figure(3-7) : Composite of the characteristic curves for all of the DC motors.

A graph of the variation in speed with changes of load of the various types of dc motors is shown in fig.(3-7).

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Week 6 3.4 Motor Nameplate

Motor nameplates are provided by virtually all manufacturers to allow users to accurately identify the operating and dimensional characteristics of their motors years after installation.

3.4.1 Definition Nameplate The nameplate is usually a metal plate, secured by a pair of screws or rivets, and is generally located on the side of the motor. (Expert maintenance technicians will tell you that the nameplate is always located on the side of the motor where the nameplate is most difficult to read!) The following cryptic information will usually be stamped into the nameplate (stamping is used because it doesn't wear off as ink tends to do. Unfortunately, the lack of contrast can make it difficult to read. Sometimes, a little bit of dirty oil or grease applied to the nameplate and then wiped "smooth" puts the dark substance into the indentations of the stamped letters and allows for easier reading.).

3.4.2 Nameplate Terms 1) Motor Manufacturer 2) Mod. (Model), Tp. (Type), or Cat. (Catalog) 3) Ser. (Serial Number) 4) HP (Horsepower) or KW (kilowatts) 5) RPM (Revolutions per Minute) 6) V (Volts) 7) ARM. (Armature) 8) FLD. (Field) 9) A (Amps) 10) Fr (Frame) 11) Enc. (Enclosure) 12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)

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1) Motor Manufacturer

This is the trade name of the company which manufactured the motor. If you are lucky, the company's home city, and perhaps even an address and/or telephone number will be on the nameplate. 2) Mod. (Model), Tp. (Type), or Cat. (Catalog)

Some companies distinguish between a Model number and a Type number. (I don't know why). In any event, this is the key number that you need if you want to contact the manufacturer. 3) Ser. (Serial Number)

Serial numbers are important because they often contain "date codes". This is information which helps the manufacturer determine when the motor was manufactured. Since many motors have multiple revisions through their lifecycle as the manufacturing process (hopefully) improves, this helps determine which set of drawings to use and lets the technical people at the manufacturer help you quicker and more accurately. 4) HP (Horsepower) or KW (kilowatts)

If you are using an American made motor or an older English or Canadian motor, it will probably be rated in Horsepower. European and Asian motors are usually rated in kilowatts -- unless they have been designed for export to the American market. Rule to remember: 1 HP = 3/4 KW (more precisely 746 watts). Second rule to remember: Volts x Amps = Watts. 5) RPM (Revolutions per Minute)

The number of times each minute that the shaft turns on its axis. This is rated at the Hertz listed. Typical values are 1750, 1450, 3450, etc. If more than one speed is listed,

this indicates a multi-speed motor. Note that AC inverter drives can change the speed of a motor from its rated speed.

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6) V (Volts)

The operating voltage of the motor. DC motors will have numbers such as 24, 48, 90, 180, or other voltage, and will usually say "VDC". 7) ARM. (Armature)

This is the maximum voltage which can be applied to the armature of a DC motor. Typical values are 90 or 180 VDC. An amperage will often be listed. 8) FLD. (Field)

This is the voltage which should be applied to the field of a DC motor. Typical values are 100, 150, 200 VDC. An amperage will often be listed. 9) A (Amps)

The amount of current consumed by the motor. 10) Fr (Frame)

The physical dimensional standard to which the motor adheres. This is critical when it is necessary to locate a mechanical replacement for an old motor. NEMA motor frames have been established for decades to allow motors from various manufacturers to replace each other. For example, a foot-mount NEMA 56 frame motor has the same mounting dimensions no matter which manufacturer has built it. NEMA refers to the National Electrical Manufacturers Association. NEMA is part of the IEC. The IEC is the International Electrotechnical Commission. Although the IEC includes Japan and the United States of America among its members, the IEC is essentially a European Community standards association. IEC standards are heavily influenced by VDE - the German electrical standards association.

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11) Enc. (Enclosure) This is the degree of protection offered by the enclosure. Common terms are TEFC, TEBC, TENV, ODP, TEAO, etc. TEFC A TEFC enclosure on a motor means "Totally Enclosed, Fan Cooled". This motor is probably the most commonly used motor in ordinary industrial environments. It costs only a few dollars more than the open motor, yet offers good protection against common hazards. The motor is constructed with a small fan on the rear shaft of the motor, usually covered by a housing. This fan draws air over the motor fins, removing excess heat and cooling the motor.

The enclosure is "Totally Enclosed". This ordinarily means that the motor is dust tight, and has a moderate water seal as well. Notice that TEFC motors are not secure against high pressure water. For these applications, consider the "wash down" motor, which is usually designed to withstand regular washing, such as found in a food processing facility. In addition, the TEFC motor is not "Explosion-proof", nor is it capable of operation in "Hazardous Environments". TEBC

A TEBC enclosure on a motor means "Totally Enclosed, Blower Cooled". TEBC motors are most commonly used for variable speed motors combined with variable speed drives of some sort. Sometimes these motors are rated as "Inverter duty" or "Vector duty". They are considerably more expensive than similarly rated TEFC motors. The motor is constructed with a dust tight, moderately sealed enclosure which rejects a degree of water. A constant speed blower pulls air over the motor fins to keep the motor cool at all operating speeds. Notice that this motor is not suitable for used in "washdown" or "Hazardous" environments.

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TENV A TENV enclosure on a motor means "Totally Enclosed, Not Ventilated". TENV motors are used in a wide variety of smaller horsepower variable speed applications. It is particularly effective in environments where a fan would regularly clog with dust or lint. The motor is constructed with a dust-tight, moderately sealed enclosure which rejects a degree of water. The motor radiates its entire excess heat through the body of the motor: Hence, the TENV motor has extra metal and extra fins to allow radiation of this heat. The TENV motor is commonly built with special high temperature insulation, since the motor is designed to run hot. As such, care should be taken to avoid human contact with the body of the motor, as well as contact between inflammable objects and the motor. Notice that this motor is not suitable for use in "washdown" or "Hazardous" environments. ODP An ODP enclosure on a motor means "Open, Drip Proof". ODP motors are relatively inexpensive motors used in normal applications. The construction of an ODP motor consists of a sheet metal enclosure with vent stamped to allow good air flow. The vents are designed in such a way that water dripping on the motor will not normally flow into the motor. A fan is mounted on the motor's rear shaft to pull air through the motor to keep the motor cool. The ODP motor is relatively inexpensive, but care should be taken not to use the motor in applications where the TEFC motor is required. TEAO A TEAO enclosure on a motor means "Totally Enclosed, Air Over". TEAO motors are designed to be used solely in the airstream of the fan or blower which they are driving. As such, they are very low cost, but of limited application. TEAO motors are constructed with a dust-tight cover and an aerodynamic body. They rely upon the strong air flow of the fan or blower which they are driving to cool them. TEAO motors are not suitable for use in "Hazardous" environments.

NEMA Enclosure Standard 250 NEMA enclosure standards represent an enclosure's ability to protect against the external environment. The following represent brief summaries of the NEMA standard. some examples of NEMA Enclosure Standard 250

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1- Type 1

Intended for indoor use primarily to provide a degree of protection against (hand) contact with the enclosed equipment. Sometimes known as a "finger-tight" enclosure. This is the least costly enclosure, but is suitable only for clean, dry environments. 2- Type 2

Intended for indoor use primarily to provide a degree of protection against limited amounts of falling dirt and water.

3- Type 3

Intended for outdoor use primarily to provide a degree of protection against windblown dust, rain, and sleet; undamaged by ice which forms on the enclosure. 4- Type 3R

Intended for outdoor use primarily to provide a degree of protection against falling rain and sleet; undamaged by ice which forms on the enclosure. This is the most common outdoors enclosure. 12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation) When facing the motor from the shaft end, this is the direction of rotation of the motor (if the motor is unidirectional). 3.5 Power Losses and Efficiency

Losses occur when electrical energy is converted to mechanical energy (in the motor), or mechanical energy is converted to electrical energy (in the generator). For the machine to be efficient, these losses must be kept to a minimum. Some losses are electrical, others are mechanical. Electrical losses are classified as copper losses and iron losses; mechanical losses occur in overcoming the friction of various parts of the machine. Copper losses occur when electrons are forced through the copper windings of the armature and the field. These losses are proportional to the square of the current. They are sometimes called I2R losses, since they are due to the power dissipated in the form of heat in the resistance of the field and armature windings.

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Iron losses are subdivided in hysteresis and eddy current losses. Hysteresis losses are caused by the armature revolving in an alternating magnetic field. It, therefore, becomes magnetized first in one direction and then in the other. The residual magnetism of the iron or steel of which the armature is made causes these losses. Since the field magnets are always magnetized in one direction (dc field), they have no hysteresis losses. Eddy current losses occur because the iron core of the armature is a conductor revolving in a magnetic field. This sets up an e.m.f. across portions of the core, causing currents to flow within the core. These currents heat the core and, if they become excessive, may damage the windings. As far as the output is concerned, the power consumed by eddy currents is a loss. To reduce eddy currents to a minimum, a laminated core usually is used. A laminated core is made of thin sheets of iron electrically insulated from each other. The insulation between laminations reduces eddy currents, because it is "transverse" to the direction in which these currents tend to flow. However, it has no effect on the magnetic circuit. The thinner the laminations, the more effectively this method reduces eddy current losses.

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Week 7 3.6 Starting Methods of DC Motor If we apply full voltage to a stationary DC motor, the starting current in the armature will be very high and we run the risk of a. Burning out the armature; b. Damaging the commutator and brushes, due to heavy sparking; c. Overloading the feeder; d. Snapping off the shaft due to mechanical shock; e. Damaging the driven equipment because of the sudden mechanical hammerblow.

All dc motors must, therefore, be provided with a means to limit the starting current to reasonable values, usually between 1.5 and twice full-load current. One solution is to connect a rheostat in series with the armature. The resistance is gradually reduced as the motor accelerates and is eventually eliminated entirely, when the machine has attained full speed.

3.6.1 Face-plate starter Fig.(3-8) shows the schematic diagram of a manual face-plate starter for a shunt motor. Bare copper contacts are connected to current-limiting resistors R1, R2, R3, and R4. Conducting arm 1 sweeps across the contacts when it is pulled to the right by means of insulated handle 2. In the position shown, the arm touches dead copper contact M and the motor circuit is open. As we draw the handle to the right, the conducting arm first touches fixed contact N. The supply voltage Es immediately causes full field current Ix to flow, but the armature current / is limited by the four resistors in the starter box. The motor begins to turn and, as the emf (Eo) builds up, the armature current gradually falls. When the motor speed ceases to rise any more, the arm is pulled to the next contact, thereby removing resistor R1 from the armature circuit. The current immediately jumps to a higher value and the motor quickly accelerates to the next higher speed. When the 47

speed again levels off, we move to the next contact, and so forth, until the arm finally touches the last contact. The arm is magnetically held in this position by a small electromagnet 4, which is in series with the shunt field.

Figure (3-8) : Manual face-plate starter for a shunt motor.

If the supply voltage is suddenly interrupted, or if the field excitation should accidentally be cut, the electromagnet releases the arm, allowing it to return to its dead position, under the pull of springing 3. This safety feature prevents the motor from restarting unexpectedly when the supply voltage is reestablished.

3.6.2 Relay starter Today, electronic methods are often used to limit the starting current and to provide speed control as the following. The most important component of a motor starter is the magnetic relay, or sometimes called a magnetic contactor (depending the size). The relay is an electromechanical device that contains a coil of wire, a mechanical contactor, and a spring mechanism. The spring mechanism is used to hold the contactor in its "NORMAL" state, which is the state of the device when the coil is deenergized. When the coil is energized, the current flowing through it sets up a magnetic field. The magnetic field generated by the coil then pulls the contactor to its "ENERGIZED" state. When the coil is turned off, the spring pulls the contactor back to its normal state again.

The contacts on the contactor can either be open or closed when the coil is deenergized. If the contacts are closed when the coil is deenergized, they are called 48

normally closed contacts. If they are open when the coil is deenergized, they are called normally open contacts. When the coil is energized, the contacts change state. In other words, when the coil is energized, the normally closed contacts open, and the normally open contacts close.

Overload sensors have normally closed contacts associated with them. Overload devices can be either magnetic or thermal. Thermal overloads contain two parts, the heater strip and the contacts. The heater strip senses the armature current, and when the current becomes excessive, the heater actuates the contacts.

The

contacts in turn secure the motor to prevent damage. Magnetic overloads operate similarity, except the contacts are actuated magnetically due to an increase in magnetic flux when the current is excessive.

Timer relays can be one of two types, Time On (TON) or Time Off (TOF). A time on relay is one where the time delay is associated with the "ON" state, and a time off relay is one where the time delay is associated with the "OFF" state. For example, when a TON relay is energized, the timing mechanism starts. After the delay, the TON contacts change state. When a TON relay is deenergized, the contacts change state immediately. With a TOF relay, the opposite is true. When the TOF relay is energized, the contacts change state immediately.

When the TOF relay is

deenergized, the time delay mechanism starts. After the time delay, then the contacts change state. Most starters are of the TON variety, however, there is one TOF starter in this laboratory. The difference between TON and TOF are more important when programmable controllers are studied later in this course. A simple D-C motor starter is shown below in fig.(3-9). MOTOR CIRCUIT CONTROL CIRCUIT

Figure (3-9) : Simple D-C Motor Starter and Controller 49

The circuit consists of two major sections, the motor circuit and the control circuit. The control circuit is usually fused from the motor circuit (not shown) to protect from shorts. The motor circuit contains the power to the shunt field, and to the armature circuit. The armature circuits contains the main line contacts (labeled "M"), the starting resistor (labeled Rs), the overload sensor (labeled OL), and the motor armature. The motor circuit is the "high current" circuit that handles the current applied to the motor directly.

The control circuit consists of the start switch, stop switch, overload contacts, M-coil, and the timer (T-coil). The control circuit is the "low current" circuit that does not handle any power directly applied to the motor. The operation of the circuit follows what's called relay logic, or sequential logic.

When the motor is turned off, the four M contacts are open, the start switch (normally open) is open, the stop switch (normally closed) is closed, the overload contact is closed, and the T contact is open. With the T contact open, full starting resistance is inserted in the armature circuit.

To start the motor, the start button is pressed. This completes the circuit to the M-coil, and the M-coil energizes. When the M-coil energizes, the magnetic field generated by the coil changes the state of the M-contactor. When this occurs, all four M-contacts close. The two M-contacts in the armature circuit close which start the motor with full starting resistance applied. The M-contact across the start switch closes sealing the start switch, and the last M-contact closes energizing the timer.

The sealing M-contact is necessary to keep the motor running after the start push button is released. If the sealing contact was not there, as soon as the start push button was released the M-coil would deenergize, the M-contacts would all open, which would stop the motor. All motor starters using push buttons will have some kind of sealing circuit across the start switch.

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The last M-contact energizes the timer. After the timer is energized, the time delay starts. After a certain amount of time is allowed for the motor to build up speed, and CEMF, the timer contacts change state. When this occurs, the T-contact closes, which shorts the starting resistance. After the T-contact closes, the motor is operating at base speed.

To stop the motor, the stop switch is pressed. When the stop switch is opened, the M-Coil deenergizes. When the M-coil deenergizes, all four M-contacts open. The two M-contacts in the armature circuit open removing power from the armature, stopping the motor. The M-contact around the start switch opens, resetting the sealing circuit. The fourth M-contact opens deenergizing the T-coil timer. The timer coil deenergizes and its contactor immediately changes state, opening the T-contact. Note there is no time delay associated with the timer when it's turned off. The time delay applies only when the timer coil is energized. When the T-contact opens, full starting resistance is reapplied to the armature circuit

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Week 8 3.7 Reversing the Rotation of DC Motor 3.7.1 Reversing the Rotation of DC Series Motor The direction of rotation of a series motor can be changed by changing the polarity of either the armature or field winding. It is important to remember that if you simply changed the polarity of the applied voltage, you would be changing the polarity of both field and armature windings and the motor's rotation would remain the same.

Figure (3-10) : DC series motor connected to forward and reverse motor starter.

Since only one of the windings needs to be reversed, the armature winding is typically used because its terminals are readily accessible at the brush rigging. Remember that the armature receives its current through the brushes, so that if their polarity is changed, the armature's polarity will also be changed. A reversing motor starter is used to change wiring to cause the direction of the motor's rotation to change by changing the polarity of the armature windings. Fig.(3-10) shows a DC series motor that is connected to a reversing motor starter. In this diagram the armature's terminals are marked Al and A2 and the field terminals are marked Sl and S2. 52

When the forward motor starter is energized, the top contact identified as F closes so the Al terminal is connected to the positive terminal of the power supply and the bottom F contact closes and connects terminals A2 and Sl. Terminal S2 is connected to the negative terminal of the power supply. When the reverse motor starter is energized, terminals Al and A2 are reversed. A2 is now connected to the positive terminal. Notice that S2 remains connected to the negative terminal of the power supply terminal. This ensures that only the armature's polarity has been changed and the motor will begin to rotate in the opposite direction. You will also notice the normally closed (NC) set of R contacts connected in series with the forward push button, and the NC set of F contacts connected in series with the reverse push button. These contacts provide an interlock that prevents the motor from being changed from forward to reverse direction without stopping the motor. The circuit can be explained as follows: when the forward push button is depressed, current will flow from the stop push button through the NC R interlock contacts, and through the forward push button to the forward motor starter (FMS) coil. When the FMS coil is energized, it will open its NC contacts that are connected in series with the reverse push button. This means that if someone depresses the reverse push button, current could not flow to the reverse motor starter (RMS) coil. If the person depressing the push buttons wants to reverse the direction of the rotation of the motor, he or she will need to depress the stop push button first to de-energize the FMS coil, which will allow the NC F contacts to return to their NC position. You can see that when the RMS coil is energized, its NC R contacts that are connected in series with the forward push button will open and prevent the current flow to the FMS coil if the forward push button is depressed. You will see a number of other ways to control the FMS and RMS starter in later discussions and in the chapter on motor controls.

3.7.2 Reversing the Rotation DC Shunt Motor The direction of rotation of a DC shunt motor can be reversed by changing the polarity of either the armature coil or the field coil. In this application the armature coil is usually changed, as was the case with the series motor. Fig.(3-11) shows the 53

electrical diagram of a DC shunt motor connected to a forward and reversing motor starter. You should notice that the Fl and F2 terminals of the shunt field are connected directly to the power supply, and the Al and A2 terminals of the armature winding are connected to the reversing starter. When the FMS is energized, its contacts connect the Al lead to the positive power supply terminal and the A2 lead to the negative power supply terminal. The Fl motor lead is connected directly to the positive terminal of the power supply and the F2 lead is connected to the negative terminal. When the motor is wired in this configuration, it will begin to run in the forward direction. When the RMS is energized, its contacts reverse the armature wires so that the Al lead is connected to the negative power supply terminal and the A2 lead is connected to the positive power supply terminal. The field leads are connected directly to the power supply, so their polarity is not changed. Since the field's polarity has remained the same and the armature's polarity has reversed, the motor will begin to rotate in the reverse direction. The control part of the diagram shows that when the FMS coil is energized, the RMS coil is locked out.

Figure (3-11) : Diagram of a shunt motor connected to a reversing motor starter. 54

Notice that the shunt field is connected across the armature and it is not reversed when the armature is reversed.

3.7.3 Reversing the Rotation of DC Compound Motor Each of the compound motors can be reversed by changing the polarity of the armature winding. If the motor has interpoles, the polarity of the interpole must be changed when the armature's polarity is changed. Since the interpole is connected in series with the armature, it should be reversed when the armature is reversed. The interpoles are not shown in the diagram to keep it simplified. The armature winding is always marked as A1 and A2 and these terminals should be connected to the contacts of the reversing motor starter.

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Week 9 3.8 Inspection and Maintenance of DC Motors Use the following procedures to make inspection and maintenance checks: 1) Check the operation of the unit driven by the motor in accordance with the instructions covering the specific installation. 2) Check all wiring, connections, terminals, fuses, and switches for general condition and security. 3) Keep motors clean and mounting bolts tight. 4) Check brushes for condition, length, and spring tension. Minimum brush lengths, correct spring tension, and procedures for replacing brushes are given in the applicable manufacturer's instructions. 5) Inspect commutator for cleanness, pitting, scoring, roughness, corrosion or burning. Check for high mica (if the copper wears down below the mica, the mica will insulate the brushes from the commutator). Clean dirty commutators with a cloth moistened with the recommended cleaning solvent. Polish rough or corroded commutators with fine sandpaper (000 or finer) and blow out with compressed air. Never use emery paper since it contains metallic particles which may cause shorts. Replace the motor if the commutator is burned, badly pitted, grooved, or worn to the extent that the mica insulation is flush with the commutator surface. 6) Inspect all exposed wiring for evidence of overheating. Replace the motor if the insulation on leads or windings is burned, cracked, or brittle. 7) Lubricate only if called for by the manufacturer's instructions covering the motor. Most motors used in today's airplanes require no lubrication between overhauls. 8) Adjust and lubricate the gearbox, or unit which the motor drives, in accordance with the applicable manufacturer's instructions covering the unit. When trouble develops in a dc motor system, check first to determine the source of the trouble. Replace the motor only when the trouble is due to a defect in the motor 56

itself. In most cases, the failure of a motor to operate is caused by a defect in the external electrical circuit, or by mechanical failure in the mechanism driven by the motor. Check the external electrical circuit for loose or dirty connections and for improper connection of wiring. Look for open circuits, grounds, and shorts by following the applicable manufacturer's circuit testing procedure. If the fuse is not blown, failure of the motor to operate is usually due to an open circuit. A blown fuse usually indicates an accidental ground or short circuit. The chattering of the relay switch which controls the motor is usually caused by a low battery. When the battery is low, the open circuit voltage of the battery is sufficient to close the relay, but with the heavy current draw of the motor, the voltage drops below the level required to hold the relay closed. When the relay opens, the voltage in the battery increases enough to close the relay again. This cycle repeats and causes chattering, which is very harmful to the relay switch, due to the heavy current causing an arc which will burn the contacts. Check the unit driven by the motor for failure of the unit or drive mechanism. If the motor has failed as a result of a failure in the driven unit, the fault must be corrected before installing a new motor. If it has been determined that the fault is in the motor itself (by checking for correct voltage at the motor terminals and for failure of the driven unit), inspect the commutator and brushes. A dirty commutator or defective or binding brushes may result in poor contact between brushes and commutator. Clean the commutator, brushes, and brush holders with a cloth moistened with the recommended cleaning solvent. If brushes are damaged or worn to the specified minimum length, install new brushes in accordance with the applicable manufacturer's instructions covering the motor. If the motor still fails to operate, replace it with a serviceable motor.

57

Week 10 4.1 Introduction

Single phase induction motors are used in residential and commercial applications. Where three-phase power is unavailable or impractical, it's single-phase motors to the rescue. Though they lack the higher efficiencies of their three-phase siblings, single-phase motors, correctly sized and rated can last a lifetime with little maintenance. Single-phase AC motors are as ubiquitous as they are useful, serving as the prime industry and in the home. Knowing how to apply the various types is the key to successful design. Eighty percent of operating motors in the world are AC single phase induction motors. They are used in applications with power requirements of 10 horsepower or less. In this chapter, single-phase motors, their constructions, types, principle of operations and speed control will be detailed. Also the power and efficiency will be discussed. 4.2

Construction of A.C single-phase induction motor

Figure (1) shows the construction of a single-phase induction motor.

Figure(4-1) : The construction of single-phase induction motor

There are two main parts of a single-phase induction motor are : a) Rotating part, called the rotor. b) Stationary part, called the stator.

58

4.2.1 Rotor The rotor is the rotating part of the electric

motor.

Motors contain either a squirrel cage or wound rotor. Like the stator, rotors are constructed of a core wound with soft wire, but with the addition of a shaft and bearings. The shaft and bearings

Figure(4-2) :rotor

are supported by end caps, which allows the rotor to turn see fig. (4-2 ).

4.2.2 Stator The stator is the immobile portion of an electric motor. A stator is made of pairs of thin sections of soft iron, called slotted cores. The cores are wound with insulated copper wire. Each of these wound cores has two magnetic poles as shown in fig. ( 4-3 ). When

an

electrical

source

is

Figure(4-3) :stator

connected to the wires, they function as electromagnets. The stator can have several sets of windings. These include start windings, run windings, and windings for variable voltage operation.

4.2.3 Frame enclosure The enclosure is also designed to dissipate heat from current flow in the windings, friction in the bearings, and other sources. Without heat dissipation, the insulation around motor windings deteriorates, causing short circuits and motor failure. Motor frames differ according to the size and type of the motor. Motor enclosures fall primarily into either open or totally

Figure(4-4) : Frame enclosure 59

enclosed categories fig.(4-4).

4.2.4 Fan Some motors have their own cooling fans to blow air over the enclosure. Cooling fans can be located inside or outside the enclosure fig.(4-5). Without fans, motors cool themselves by conduction of heat to the surrounding air. Figure(4-5) : Fan

4.2.5 Terminal ( connection ) box The

conduit

box

houses

the

electrical

connection points from the motors internal windings to an electrical power supply. Another important part in the construction is the centrifugal switch. It used to disconnect the starting winding after the rotor speed Figure(4-6) : Terminal ( connection ) box

has reached a predetermined speed see fig.(4-6 ).

4.2.6 Centrifugal switch A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating shaft, most commonly that of an electric motor or gasoline engine. The switch is designed to activate or de-activate as a function of the rotational speed of the shaft fig.( 4-7-a ).

Figure(4-7.a) : Centrifugal switch

Centrifugal switches typically serve as a means of turning ON or OFF circuit functions depending on motor speed. The most widespread use of such switches is as a start winding cut-out for single-phase

fractional-

horsepower motors. In some clothes driers, the switches can also be found controlling dryer heating elements, allowing the dryer to switch on Figure(4-7.b) : Centrifugal switch 60

only when the drum motor is up to speed.

The basic operating principle of the switch is to use a speed-sensing mechanism that consists of a conical spring steel disc that has weights fastened to the outer edge of a circular base plate. Fingers on the spring attach to an insulating spool that rides free of the shaft see fig. (4-7 b,c,d )

Figure(4-7.c) : Centrifugal switch operation

Figure(4-7.d) : Centrifugal switch Open &closed position

61

4.3 How electric motor work?

Electric motors function on the principle of magnetism; where like poles repel, and unlike poles attract. In a simple motor, a freeturning permanent magnet is mounted between the prongs of an electromagnet fig.( 4-8 ). Since magnetic forces travel poorly through air, the electromagnet has metal shoes that fit close to the poles of the permanent magnet. This creates a stronger more stable magnetic field. (The electromagnet functions as the stator, and the freeturning magnet is the rotor.) Fluctuating polarity

Figure(4-8) : a simple- motor

in the electromagnet causes the free-turning magnet to rotate. The poles are changed by switching the direction of current flow in the electromagnet.

The direction of current flow can be changed in one of two ways. The stator in an AC motor is a wire coil, called a stator winding as shown in fig. ( 4-8-a ). It's built into the motor. When this coil is energized by AC power, a rotating magnetic field is produced.

Induction motors are equipped with

Figure(4-8a) : stator of A.C motor

squirrel rotors see fig. ( 4-8 –b ), which resemble the exercise wheels often associated with pet rodents like gerbils. Several metal bars are placed within end rings in a cylindrical pattern. Because the bars are connected to one another by these end rings, a complete circuit is formed within the rotor.

Figure(4-8b) : rotor of A.C motor

62

Consider this close-up of a 2-pole stator and one of its rotor bars as shown in fig. ( 4-8. c ). Alternating current flowing in the stator causes the poles to change rapidly, from north to south and back again. If the rotor is given a spin, the bars cut the stator lines of

Figure(4-8c) : 2-pole stator and one rotor

force. This causes current flow in the rotor bar. This current flow sets magnetic lines of force in circular motion around the rotor bars. The rotor lines of force, moving in the same direction as those of the stator, add to the magnetic field and the rotor keeps turning see fig. ( 4-8 d ).

Figure(4-8d) : single-phase motor

4.4 Operation principle

The most common method of starting a single phase motor combines a capacitor and auxiliary winding or start circuit. A schematic view shows an auxiliary starting winding, a capacitor, and a centrifugal switch. The auxiliary winding is actually a second winding in the motor

Figure(4-9):induction motor

see fig. (4-9 ).

63

AC single phase induction motors are classified by their start and run characteristics. An auxiliary starter winding is placed at right angles to the main stator winding in order to create a magnetic field. The current moving through each winding is out of phase by 90 degrees see fig. (4-9 .a ). This is called phase differential.

After

the

motor

has

reached

approximately 75% of operating speed, the

Figure(4-9a) : phase shift between winding

auxiliary winding is disconnected from the circuit by a centrifugal switch.

When current is applied to the motor, both the run winding and the start winding produce magnetic fields. Because the start winding has a lower resistance, a stronger magnetic field is created which causes the motor to begin rotation. Once the motor reaches about 80 percent of its rated speed, a centrifugal switch disconnects the start winding. From this point on, the single phase motor can maintain enough rotating magnetic field to operate on its own. The

Figure(4-9b) : induction motor characteristics

graph shows a typical torque/speed curve for auxiliary starting on single phase motors fig.( 4-9 b).

64

There are a variety of starting methods used in the different single phase motor types. These are covered in more detail in this chapter what these starting methods all have in common is the ability to produce a rotating magnetic field using the input power that is applied to the motor fig. (4-9.c ). Figure(4-9c) : phase-relationship in split-phase motor

65

Week 11 4.5 Motor speed 4.5.1 Synchronous speed There are two ways to define motor speed. First is synchronous speed. The synchronous speed of an AC motor is the speed of the stator's magnetic field rotation. This is the motor's theoretical speed since the rotor will always turn at a slightly slower rate. The other way motor speed is measured is called actual speed see fig. (4-10 ). This is the speed at which the shaft rotates. The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed. A motor's synchronous speed can be

Figure(4-10) : motor speed

computed using this formula: synchronous speed equals 120 times the operating frequency, divided by the number of poles.

Where : Synchronous speed ( Ns ) in

r.p.m

Supply frequency

( f ) in

Hz

Number of poles

( P ) in

poles

Example : (1) A 6-pole, single-phase induction motor is fed from a 50 Hz. Calculate the Synchronous speed ?

Solution : Synchronous speed = 120 × f / P

r.p.m

= 120 × 50 / 6 = 1000 r.p.m

4.5.2 rotor speed and slip speed

66

The difference between rotor speed ( nm ) and synchronous speed ( ns ) is called the ' slip speed ' nslip = nsyn – nm

r.p.m

where : nslip

= slip speed ( r.p.m )

nsyn

= synchronous speed ( r.p.m )

nm

= motor ( rotor ) speed ( r.p.m )

4.5.3 Slip The slip speed expressed as a function of n slip is called ' slip '.

Slip (S) = nsyn - nm / nsyn Example : (2) A 4-pole, single-phase induction motor is fed from a 50 Hz supply and has a rotor speed of 1425 rpm. Calculate the slip and percentage slip? Solution : Synchronous speed = 120 × f / P

r.p.m

= 120 × 50 / 4 = 1500 rpm Slip (S) = nsyn - nm / nsyn = 1500 – 1425 / 1500 = 0.05٪ percentage slip ( S٪ ) =

slip ×100٪

=

0.05 ×100٪

=

5٪

4.6 Types of single-phase induction motors AC single phase induction motors are classified by their start and run characteristics. An auxiliary starter winding is placed at right angles to the main stator winding in order to create a magnetic field. The current moving through each winding is out of phase by 90 degrees fig.(4-11). This is called phase differential. After the motor

Figure(4-11) :start and run characteristics

has reached approximately 75% of operating

67

speed, the auxiliary winding is disconnected from the circuit by a centrifugal switch. The most commonly used types of induction motors are : 4.6.1 Split phase motors Simply constructed split phase motors are among the least expensive. They're widely used on easy starting loads of 1/3 horsepower or less. Washing machines, tool grinders and small fans and blowers are among the applications that use these motors. Split phase start motors are equipped with a special set of stator windings or starting purposes fig(4-12 ). They are called start windings or start pulls. These start windings are

Figure(4-12):split phase motor

made of smaller wire than the run windings. Because these wires are smaller, they offer less resistance and provide higher current flow. Accordingly, the start pulls are first to become magnetized when the power is applied. The current flow through the start winding begins after power is applied to the motor by 20 degrees or so fig.(4-12. a ).

Current is induced in the rotor as the run pulls establish a stronger magnetic field. The interaction of the induced current in the rotor and the magnetic field causes the rotor to turn one quarter turn. The current induced in the rotor perpetuates its motion as speed increases and the start pulls are no longer needed. At about 75% of

Figure(4-12 a) start and run

operating speed the centrifugal switch opens

winding current

disconnecting current to the start winding see fig. (4-12 b ).

Split phase motors have moderate to low starting torque. 100 to 125 percent of full load and high starting current. Sizes range

Figure(4-12b): Split phase motor winding

68

from 1/20 to 3/4 horsepower as shown fig. ( 4-12.c ).

Split phase motors draw 6 to 8 times normal current when starting. They usually operate on single voltages. Split phase motors have lower starting torque and are less expensive because they use no capacitors in the start winding circuit.

Figure(4-12c): Starting torque winding current

The split phase motor is most widely used, for "medium starting" applications fig.( 4-12.d ). The split phase motor has a start and run winding. Both windings are energized when the motor is started. When the motor reaches about 75% of its rated full load speed, the starting winding is

Figure(4-12 d) speed-torque characteristics

disconnected from the circuit by an automatic switch.

Applications This motor is excellent for medium duty applications and where stops and starts are somewhat frequent. Popular applications of split phase motors include: fans, blowers, office machines and tools such as small saws or drill presses where the load is applied after the motor has obtained its operating speed.

69

Week 12 4.6.2 Capacitor motors Some single phase motors utilize a capacitor installed in series with one of the stator windings fig.(4-13). A capacitor is an electrical device which can rapidly build up an electrical energy supply that can be used to create more current flow in the motor's windings. When input

Figure(4-13) :Capacitor motors

power is applied to the motor, the capacitor becomes charged up almost instantaneously.

The capacitor's energy helps create current flow in the start winding before the run winding gets any current flow. This difference in timing, called "phase differential", creates a rotating magnetic field in the stator fig.(4-13a,b). This stronger magnetic field induces more current into the rotor causing it to rotate quicker. The end result is a motor with the ability to start

This technique is widely used for motor applications like air conditioners and compressors. All the capacitor motors discussed in this section operate in essentially in the same manner.

The advantages that capacitor motors have

Figure(4-13a):Capacitor motors

over split phase motors are : •

They produce more starting torque, and

•

They use less current while running at steady speeds

70

Capacitor motors vary in size ranging from small motors of fractional horsepower to motors up to 10 horsepower. Torque and voltage ratings of a motor determine the rating of the capacitor. The voltage rating of a capacitor must always

Figure(4.13b):Capacitor motors

meet or exceed the voltage requirements of the motor in which it is used.

4.6.3 Capacitor run motors One type of capacitor motor is the capacitor run or permanent split capacitor motor fig.(4-14). These are used in instances where low starting torque is needed as in air conditioner Permanent split motors found in sizes up to three horsepower are economical and easily customized. This type of motor is similar to the split phase motor with the exception being that the current to its start winding is not switched off during motor operation. In normal split phase motors this current is turned off after starting. A small

Figure(4.14) :Capacitor run

capacitor within the start circuit of the capacitor run motor remains functional throughout start and operation of the motor. Permanent split capacitor motors cost less than those with switching system. They provide greater starting torque and better running characteristics than split phase motors. Capacitor run motors make good replacements for shaded pole motors. In this role they're more efficient and require lower current levels than shaded pole motors. For these reasons they're effective in fans with low starting torque requirements.

71

Characteristics Because of its improved starting ability, the capacitor start motor is recommended for loads which are hard to start. The motor has a capacitor in series with a starting winding and provides more than double the starting torque with one third less starting current than the split phase motor see fig.(4-15).

Figure(4.15) :motor characteristics

Applications It has good efficiency and requires starting currents of approximately five times full load current. The capacitor and starting windings are disconnected from the circuit by an automatic switch when the motor reaches about 75% of its rated

full load speed. Special applications include: compressors, pumps, machine tools, air conditioners, conveyors, blowers, fans and other hard to start applications.

4.6.4 Capacitor start motors Capacitor start / induction run motors are similar in construction to split phase motors. The major difference is the use of a capacitor connected in series to start windings to maximize starting torque. see fig.(4-16) The capacitor is mounted either at the top or side of the motor. A normally

Figure(4-16) :capacitor start motor

closed centrifugal switch is located

72

between the capacitor and the start winding. This switch opens when the motor has reached about 75 percent of its operating speed. Capacitors in induction run motors enable them to handle heavier start loads by strengthening the magnetic field of the start windings. These loads might include refrigerators, compressors, elevators, and augers. The size of capacitors used in these types of applications ranges from 1/6 to 10 horsepower. High starting torque designs also require high

Figure(4-16 b) :capacitor start motor –starting torque

starting currents and high breakdown torque. Capacitor start / induction run motors typically deliver 250 to 350 percent of full load torque when starting see fig.(4-16.b ). Motors of this design are used in compressors and other types of industrial, commercial, and farm equipment. Capacitor start induction run motors of moderate torque values are used on applications that require less than 175 percent of the full load. These are used with lighter loads like fans, blowers, and small pumps.

73

Week 13 4.6.5 Capacitor start capacitor run motors Capacitor start / capacitor run motors are more efficient and require less running current than motors with start capacitors only. These motors have two capacitors in series with the main stator winding see fig. (4-17). Start capacitors have a high capacity while the run capacitors do not. One optimizes starting torque while another optimizes running characteristics. Throughout

Figure(4-17) :Capacitor startCapacitor run Induction motor

both starting and operation all the windings in the motor remain energized. At operating speed, the switch disconnects the start capacitor and turns on the run capacitor to maintain the motor's performance. Optimum levels of both starting torque and running characteristics are achieved with this design. Capacitor start / capacitor run motors are used over a wide range of single phase applications primarily starting hard loads. They are available in sizes from 1/2 to 25 horsepower.

4.6.6 Shaded-pole induction motors The simplest and least expensive type of single phase motor is the shaded pole motor. Fig (4-18) shows the construction of this motor. Due to low starting torque, its use is limited to applications that require less than 3/4 horsepower, usually ranging from 1/20 to 1/6 horsepower. Figure(4-18) :shaded-pole induction motor construction

74

Shaded pole motors use no starting switch. The stator poles, see fig.(4-18a,b) are equipped with an additional winding in each corner called a shade winding. These windings have no electrical connection for starting but use induced current to make a rotating magnetic field.

Figure(4-18 a ) :main and shaded winding

The pole structure of the shaded pole motor enables the development of a rotating magnetic field by delaying the buildup

of

magnetic

flux.

A

copper

conductor isolates the shaded portion of the pole forming a complete turn around it. In the shaded portion, magnetic flux increases but is delayed by the current induced in the copper

shield.

Magnetic

flux

in

the

unshaded portion increases with the winding current forming a rotating field.

Figure(4-18 b ) :shaded-pole induction motor structure

Rotor torque initiates as the magnetic field sweeps across the face of the pole between the unshaded and shaded portions. The rotor is highly resistant in order to maximize the torque. Shaded pole motors function best with low torque applications and usually rate less than 1/10 horsepower. They should never be used to replace single phase motors. 75

Shaded pole motors are best suited to low power household application because the motors have low starting torque and efficiency ratings. Some compatible applications include hair dryers, humidifiers and timing devices.

76

Week 14

4.6.7 Repulsion motors The repulsion-induction motor is a combination of a repulsion motor and a squirrelcage induction motor. This motor is always a 2pole configuration. The stator winding is identical to the run winding of a 2-pole split-phase or capacitor-start motor. The rotor is nearly identical to a universal series motor armature, with the exception of having a greater number of windings

Figure(4-19) :repulsion induction motor

(in most cases) and no connection to a power source. The brushes are connected to each other directly, in order that they may complete a circuit through windings within the rotor see fig.(4-19). The closed-loop circuits in the rotor are effectively the short-circuited secondaries of a transformer, where the motor's field windings are the primary coil. The currents induced in the rotor create a magnetic field which repels that of the field winding (Lenz's law). This repulsion is what gives the motor it's torque. Rotation happens because the brushes are offset 15 or so degrees from the field poles, so that the repulsive forces are pushing on the rotor somewhat tangentially to it's rotation axis (see the schematics below). In addition to this repulsion motor setup, the rotor also has buried within it a squirrel cage winding. As the repulsion-induction motor comes up near synchronous speed (3600 RPM on 60Hz), the squirrel-cage winding is responsible for most of the torque, and the repulsion effect diminishes.

77

4.6.8 Universal motors The universal motor is a rotating electric machine similar to a DC motor but designed to operate either from direct Fig.(4-20) single-phase universal motor current or single-phase alternating current. The stator and rotor windings of the motor are connected in series through the rotor commutator. Therefore the universal motor is also known as an AC series motor or an AC commutator motor. The universal motor can be controlled either as a phase-angle drive or as a chopper drive. This type of motor is identical in principle to the DC series motor fig(4-20a,b), but a few modifications have been made to optimize the motor for AC use: The cores of the field poles are made from stacks of laminated sheet metal punchings like you find in transformers, instead of solid iron. This is to reduce the eddy-current losses in the core. In addition, the slots of the armature are slanted slightly to reduce AC buzzing and give the motor uniform starting

Figure(4-20 a) : rotor of universal motor

characteristics regardless of the armature's initial orientation relative to the field coils. Shown here are the armature and field coils of a typical universal motor. This motor happens to be from a vacuum cleaner, but the design is common to siren motors as well.

78

The name "universal" is derived from the motor's compatibility with both AC and DC power. Among the applications using these motors are vacuum cleaners, food mixers, portable drills, portable power saws, and sewing machines. These motors seldom exceed

Figure(4-20 b) : stator of universal motor

one horsepower. In most cases, universal motors reach little more than a few hundred rpm under heavy loads. If the motor is run with no load, speed may approach up to 15,000 rpm. This can result in serious heat damage to the motor's components. Universal series motors differ in design from true induction motors. They have series wound rotor circuitry similar to that of DC motors. The rotor of a universal series motor is made of a laminated iron core with coils around it. The ends of the wire coils connect directly to the commutator.

Figure(4-20 c) : universal motor diagram

79

Electric current in the motor flows through a complete circuit formed by the stator winding and rotor winding fig.(420c,d). Brushes ride on the commutator and conduct current through the rotor from one stator coil to the other. The rotor current interacts with the magnetic field of the stator causing the rotor to turn. As long as an electrical current is present in the rotor coils, the motor continues to run. Figure(4-20d) : universal motor diagram

4.7 Speed-torque characteristics of single-phase induction motor

Figure(4-21) : speed torque characteristics

80

Week 15 4.8 power, losses and efficiency 4.8.1 Input power The electrical power input in kilowatts for a single phase motor is calculated by multiplying the voltage measured at the motor, by the amperage measured at the motor, then multiplying this product by the power factor of the motor, and dividing the result by 1,000.

4.8.2 Kw to Hp Conversion Electric utilities use their meters to measure the input power of a motor fig.(4-22). Input power is the power consumed by a motor in operation. It's typically measured by electric utilities in terms of kilowatts. Kilowatts can be converted to horsepower by dividing the number of kilowatts by a constant of 0.746 Figure(4-22) : power

For example: To convert an input power of 9 kilowatts to units of horsepower, divide 9 kilowatts by 0.746. The result is 12.06 horsepower.

4.8.3 motor losses Motor loss refers to the consumption of electrical energy not converted to useful mechanical energy output. Every AC motor has five aspects of power loss as shown in fig.(4-23 ). Combined, these five types of energy loss constitute the total power loss of a motor.

81

Power loss comprises energy converted to heat and dissipated from the motor frame. One of the functions of a cooling mechanism is to alleviate power losses. Motor design alterations that diminish any of these losses contribute to the enhancement of motor efficiency. Reduction of energy losses always improves a motor's efficiency .

Figure(4-23) :motor losses

4.8.3.1 core or iron losses Core or iron losses are comprised of the energy required to magnetize the laminated core and current losses from magnetically induced circulating currents inside the laminated core. Core losses make up about 25 percent of the total losses fig.(4-24). Core or iron losses can be reduced by utilizing higher quality steels with low core loss characteristics found in high grade silicon steel, using thinner gauges of steel, and designing longer cores to reduce

Figure(4-24) : motor core

operating flux density.

82

4.8.3.2 rotor losses Rotor losses are due to the heating effect of current flow in the rotor fig.(4-25). Rotor losses are proportional to the current squared and multiplied by rotor resistance in Ohms. As current flow in the rotor increases, power loss, as Figure(4-25) : rotor

well, increases. Rotor losses account for about 25 percent of total motor losses. Rotor losses

diminish with the use of higher grade steel and larger conductor bars with increased cross sectional area, which lower the resistance of the rotor.

4.8.3.3 stator losses Stator losses are due to the heating effect of current flow through resistant stator windings fig (4-26). Stator losses are proportional to the current squared and multiplied by winding resistance in ohms. As current flow in the stator increases, so does power loss. Stator losses account for approximately 35 percent of total motor losses.

Figure(4-26) :stator

Reduction of stator losses is possible with the use of high grade copper and larger conductors with increased cross sectional area. This lowers the resistance in the motor windings, reducing stator losses.

4.8.3.4 friction and windage losses Friction and windage losses comprise bearing friction, wind friction, the motor's cooling fan load, and any other source of friction or air movement in the motor. These losses are often appreciable in large and high speed totally enclosed fan cooled motors. Friction and windage losses typically make up about 5 percent of total

83

efficiency loss. Friction and windage losses are less problematic with the use of high quality bearings and lubricants, and improved fan designs.

4.8.3.5 stray losses Stray losses are other losses in addition to core, stator, rotor and frictional losses. They are primarily due to leakage induced by load current, design flaws and manufacturing variables. Stray losses make up about 10 percent of total motor losses. Optimizing motor design and enforcing strict quality control largely diminishes the extent of stray load loss.

4.8.4 Efficiency Electric motors are not 100% efficient. Upon conversion of input power into output power, some of the energy consumed is displaced as heat. This amounts to energy lost in its conversion from electrical to mechanical energy fig.(4-27). The amount of energy lost in this manner, the difference between the input and output power, determines the motor's efficiency. Electric motors are not 100% efficient. Upon conversion of input power into output power, some of the energy consumed is displaced as heat. This amounts to energy lost in its conversion from electrical to mechanical energy. The amount of energy lost in this manner, the difference between the input and

Figure(4-27) :motor efficiency

output power, determines the motor's efficiency.

84

Also, motor efficiency is a measure of the effectiveness with which a motor converts electrical energy to mechanical energy output to drive a load. It is defined as a ratio of motor power output to source power input. The difference between the power input and power output comprises electrical and mechanical losses.

4.8.5 External speed control drives 4.8.5.1 Direct drive With the use of direct drive systems, fig.(4-28) very few power losses occur. The

direct drive offers the most efficient transfer of power from motor to load of all drive types. Direct drive motor and load shafts connect by a coupling. Where the motor and load shafts are misaligned, or if a motor's speed is not controllable, direct drives function poorly. A flexible coupling, to correct this,

Figure(4-28) :direct drive motor

allows slight misalignment while minimizing the transmission of adverse thrust to motor bearings. As an added advantage, direct drives require very little maintenance.

4.8.5.2 belt and pulley drives

Figure(4-29) :belt and pully drives 85

A belt drive has at least two pulleys, fig.(4-29). Connected to the motor shaft is the drive pulley. The driven pulley connects to the load shaft. A belt joins these pulleys, transferring power from the motor to the load. Belt and pulley drives are low in cost, and capable of speed variation through alterations in pulley size. This type system is not as efficient as a direct drive, since wear and loosening of the belts results in wavering efficiency. Because of this, these systems require heavy and frequent maintenance.

4.8.5.3 gear motors A gear motor is a combination of a standard motor with a matched gear driven transmission fig. (4-30). This combination of a constant speed motor with a gear transmission

functions

to

provide

an

application with the quality of adjustable speed. A number of application factors must be considered in properly sizing a gear motor for a particular application. These include: the load type, motor type, the coupling, and other specific requirements.

Figure(4-30) :gear motors

Gear motors are convenient and efficient since the motor shaft is coupled directly to the gear shaft eliminating belts, chains or other speed reducers. This provides for an optimally matched system with higher efficiency than with a motor and gear transmission purchased separately.

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4.8.5..4 gear drives Certain drives have shafts placed closely together to transmit large amounts of power. These drives use gears, fig.(431). Geared drives are more expensive than others but are nearly as efficient as direct drives. As long as gears remain well lubricated, they do not wear out as quickly as belt-pulley or chain-sprocket systems.

Figure(4-31) :gear drives

Other than regular lubrication, gear drives require very little maintenance.

4.8.5.5 chain and sprocket Chain and sprocket drives resemble belt and pulley systems, fig.(4-32). They are, however, capable of transmitting more power, since metal chains don't slip as pulley belts do. Chain and sprocket systems cost more than belt and pulley systems, but are more efficient. Drive efficiency diminishes rapidly as the chain

Figure(4-32) :chain and sprocket

and sprocket components wear-out, so these systems require a significant amount of maintenance

4.9 Nameplate information's The motor nameplate contains specific information about the motor, fig.(4-33). Motors are required to be shipped with a nameplate. The National Electrical Code requires specific items: •

the manufacturer's name, model and serial number;

•

rated voltage and full load

Figure(4-33) :nameplate information

amperage; 87

•

rated frequency;

•

phase;

•

rated full load speed;

•

rated temperature rise or insulation class and rated ambient temperature;

•

duty rating;

•

rated horsepower; and

•

design code letter. Additional information is sometimes provided on these items:

•

service factor,

•

enclosure type,

•

frame size,

•

connection diagrams,

•

unique or special features.

4.10 Reversing the direction of rotation 4.10.1 split-phase induction motor In order to reverse the direction of rotation of split-phase motors, we have interchange the leads of either the auxiliary winding or the main winding. However, if a single-phase motor is equipped with a centrifugal switch, its rotation cannot be reversed while the motor is running. If the main winding leads are interchanged, the motor will continue to run in the same direction.

4.10.3 capacitor-run induction motor The direction of rotation can be changed while the motor is running because both windings are in the circuit at all times.

4.10.4 very small induction motors The direction of rotation can be reversed by using a double throw switch.

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4.10.5 shaded-pole induction motors for shaded-pole induction motors, there is no easy way to reverse their direction of rotation. To achieve reversal, it is necessary to install two shading coils on each pole face and to selectively short one or the other of them.

4.11 speed control The speed control of a single-phase induction motor may be controlled by using one of the following techniques : 1.

changing the number of poles.

2.

changing the applied terminal voltages.

3.

varying the stator frequency.

In practical design involving fairly high-slip motors, the usual approach to speed control is to vary the terminal voltage of the motor. This may be done by on of the following methods: • using an autotransformer to adjust the line voltage. • Inserting a resistor in series with the motor stator circuit. • Using a solid-state control circuit.

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4.12 Applications Type of motor

Torque

Applications -Refrigeration compressors -loaded conveyor belts -reciprocating pumps

Split-phase induction motor Capacitor motors

High starting torque

Universal motors

-portable hand drills -power saws -rowters -portable hand jointers -planers -centrifugal pumps

shaded-pole motors

normal starting torque

-machine tools

low starting torque

-fans -tape-recorder

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