ee029 electrical machines 3 th inst

7/25/2019 EE029 Electrical Machines 3 Th Inst

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

ELECTRICAL MACHINES 3Synchronous Machines

Theory

No: EE 029

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Electr ical Machines 3-Synchronous Machines

Theoretical Part

No.: EE 029

Edition: 2008Al l Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerst rasse 64 P.O.Box 36, A 4031 Linz / Aus triaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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ELECTRICAL MACHINES 3 - SYNCHRONOUS MACHINES

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................4

1

DEFINITION .................................................................................................................5

1.1 Operation as a generator .....................................................................................5

1.2 Operation as a motor ...........................................................................................6

1.3

Operation as a phase advancer...........................................................................6

2 CONSTRUCTION.........................................................................................................8

2.1 Rotor wound machines ........................................................................................8

2.1.1 Stator construction ...........................................................................................8

2.1.2 Rotor construction ............................................................................................8

2.1.3 Application........................................................................................................9

2.2 Stator wound machines ......................................................................................9

2.2.1

Stator construction ...........................................................................................9

2.2.2 Rotor construction ..........................................................................................10

2.2.3

Application......................................................................................................11

2.2.4 The stator.......................................................................................................11

2.2.5 Slots ...............................................................................................................12

2.2.6 Cooling the winding........................................................................................14

2.2.7

Rotor ..............................................................................................................14

3

EXCITATION..............................................................................................................17

3.1 Types of excitation .............................................................................................17

3.1.1

Separate excitation ........................................................................................17

3.1.2 Self-excitation with direct-current exciter generators .....................................18

3.1.3 Brushless excitation with rotating diodes .......................................................21

3.1.4

Self-excited compound synchronous generators...........................................23

4 SYNCHRONOUS GENERATORS.............................................................................25

4.1

Single-phase synchronous generators ..............................................................25

4.2

Three-phase synchronous generators ...............................................................26

4.2.1 Synchronous speeds......................................................................................28

4.3 Operating characteristics ...................................................................................29

4.3.1 No-Ioad operation ..........................................................................................29

4.3.2 Loading ..........................................................................................................30

4.4 Excitation of synchronous generators................................................................30


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4.4.1 Overexcitation ................................................................................................31

4.4.2

Underexcitation ..............................................................................................32

4.5 Power factor cos ...........................................................................................32

4.6 Terminal voltage ................................................................................................33

4.7 Paralleling arrangements ...................................................................................34

4.7.1

Methods for checking relative phase position ................................................36

5 SYNCHRONOUS MOTORS ......................................................................................39

5.1 Operation ...........................................................................................................39

5.1.1 The variation of the phase currents IL1, IL2, IL3in a cycle..............................41

5.1.2 Representation of the magnetic flux at 0 ......................................................42

5.1.3 Representation of the magnetic flux at 30 .................................................... 43

5.2 Starting a synchronous motor ............................................................................43

5.2.1 Methods used for starting synchronous motors .............................................44

5.3 Operating characteristics ...................................................................................45

5.3.1 No-load operation...........................................................................................45

5.3.2 Loading ..........................................................................................................46

5.3.3 The phase advancer ......................................................................................48

5.4 The identification of windings in synchronous machines ...................................49

5.5 Characteristics ...................................................................................................49

5.6 The advantage of synchronous motors..............................................................50

5.7

The disadvantages of synchronous motors ......................................................50

6 RELUCTANCE MOTORS ..........................................................................................51

6.1

Construction.......................................................................................................51

6.1.1 Stator..............................................................................................................51

6.1.2

Rotor ..............................................................................................................51

6.2 Functioning ........................................................................................................52

6.3 Characteristic .....................................................................................................52


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SYNCHRONOUS MACHINES

LEARNING OBJECTIVES

The trainee should . . .

. . . describe the constructional features of a synchronous machine.

. . . describe the operation of a synchronous motor using the rotating field concept.

. . . describe the principles of operation of a synchronous motor and a synchronous

generator.

. . . explain the operating characteristics of synchronous motors and synchronous

generators.

. . . name the advantages and disadvantages of the synchronous machine.

. . . name the four requirements for parallel connections and how they are measured.

. . . name and explain a few types of excitation for synchronous machines.


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S Y N C H R O N O U S M A C H I N E S

1 DEFINITION

Synchronous machines are rotating electrical machines which have a speed identical to

that of the rotary field (synchronous = moving in union). They are used mainly as

generators and less frequently as motors. Used as wattless machines (phase advancer),

they will improve the power factor, cos .

1.1 Operation as a generator

Generation of electrical energy in power stations is carried out by means of synchronous

generators.


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1.2 Operation as a motor

Synchronous motors are used for driving pumps or compressors. Synchronous motors for

single-phase alternating current are used for electric clocks and programmed sequential

circuits etc.

1.3 Operation as a phase advancer

When over excitation occurs, the synchronous machine will deliver lagging reactive power

to the supply system.


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When varying the excitation current IE , the reactive power Q and thus the power factor

cos will undergo variation. With over-excitation, synchronous machines will act like

capacitances, and with under-excitation they act like inductance.


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2 CONSTRUCTION

Synchronous machines are classified according to their construction:

- Rotor wound (External pole) machines.

- Stator wound (Internal pole) machines.

The winding referred to in the classification is the generating, or AC, winding.

2.1 Rotor wound machines

2.1.1 Stator construction

The stator is made of steel or cast iron and the magnetic flux is produced in it. The pole

shoes, with the windings arranged an them, are bolted onto the inner side.

2.1.2 Rotor construction

The rotor consists of Iaminated steel sheets. The winding is embedded in slots in the

laminations. lt is a three-phase winding. Collection of current is effected by means of slip

rings and brushes.


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External-pole machines are of little importance.

With high voltage, it is difficult to insulate the slip rings.

The centrifugal forces of rotating machinery parts prevent large size rotor wound

machines from being made economically.

2.1.3 Application

Rotor wound machines are used for rotary converters.

2.2 Stator wound machines

2.2.1 Stator construction

The stator is provided with a three-phase winding.


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2.2.2 Rotor construction

The rotor carriers the field winding, which is supplied with direct current via slip rings and

brushes.


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Large machines normally are of wound stator construction.

High voltages as well as high outputs are collected more easily at the stator than at the

rotor.

2.2.3 Application

Wound stator machines are used as generators in power stations and as motor for drives

at constant speed (e.g., pumps and compressors). They are also used to improve the

power factor in inductively-loaded supply systems, since they can take capacitive power.

2.2.4 The stator

The housing is made of cast iron, steel or an aluminium alloy. The surface is frequently

provided with cooling fins to aid the cooling process. The stack of laminations forming the

stator core is Iocated within the housing.

The stack is of 0.5 mm thick iron or steel laminations with insulating layers. The windings,

in most cases, consist of enamelled or silk-insulated copper wires.


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The stator winding is a three-phase AC winding.

lt is connected as either a star or delta circuit.

2.2.5 Slots

The winding is located in slots.

- Trapezoidal half-closed slot with a winding of enamelled (varnished) wire.

- Open slot with a winding of flat wire.


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- Half-closed slot with a winding of profiled wire.

- Open slot for shaped coils with a winding of flat wire.

- Half-open V-slot with a winding of enamelled wire.

- Slots and the winding design for large-size machines.


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2.2.6 Cooling the winding

Chemically pure water is used for cooling large machines. lt is brought directly to the

source of heat through hollow conductors. In this way an output is achieved which is up to

ten times greater than that obtained with machines of identical size using conventionalcooling systems.

2.2.7 Rotor

The rotor consists of the rotor shaft, the rotor body and the exciter winding. Rotor may be

divided into the following types according to their construction:

- Salient-pole rotor.

- Cylindrical rotors.

2.2.7.1 Salient-pole rotors

These are rotors with distinctive poles used for lower speeds often with a water turbine or

a large diesel engine as a prime mover.

In salient-pole synchronous machines, the field winding is located on poles which project

from the rotor shaft. The rotor is often provided with an additional short-circuited damping

winding which reduces transients during load surges or frequency fluctuations.


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2.2.7.2 Cylindrical rotors

In cylindrical rotors the field winding is located in radial slots. Cylindrical rotors are used

for high speeds (e.g., steam turbine as a prime mover).

In cylindrical synchronous machines, the field winding is located in the slots of a massive

rotor cylinder. The excitation current is usually supplied to the field winding via brushes

and slip rings.

In smaller machines, a rotor with a permanent magnet is used instead of the field winding.

Furthermore, claw-pole rotors may also be used in smaller machines with relatively low

speeds.

Using two claw-shaped rotor halves and one ring coil (excitation winding) several pole

pairs can be produced, as is the case with automotive alternators.


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2.2.7.3 Twelve-pole claw-pole rotor

With low-speed synchronous generators, the rotor shafts are usually vertical. The rotordiameter is as great as 7.5 m with most water turbines having speeds of 60 to 750

rev/min.

With high speed generators, the rotors are always arranged horizontally. Steam turbine

speeds are normally 3.000 rev/min. Rotor diameter maximum 1.20 m.


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3 EXCITATION

A number of systems of excitation derived from the generator output are used. Rectifier

circuits are more commonly used than separate DC generators.

3.1 Types of excitation

The different types of excitation are Iisted below:

- Separate excitation.

- Self-excitation with self-excited direct-current machines, and three-phase exciters with

static converters.

- Brushless excitation.

- Self-excited compound synchronous generators.

3.1.1 Separate excitation

This form of excitation is rarely used. lt is dependent on an outside power source.


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3.1.2 Self-excitation with direct-current exciter generators

The exciter (in most cases direct-current shunt-wound generators) have their rotors driven

by the shaft of the synchronous machine.

3.1.2.1 Self-excited exciter generator

An electronic regulating device (R) provided constant excitation current, or constant mains

voltage.


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1 synchronous generator

2 exciter

3 regulating device

4 transformer

3.1.2.2 Main exciter with an auxiliary exciter

Rapid variations of exciter voltages are impossible due to the high time constant of the

exciter windings.


2 exciter machine

3 auxiliary generator

4 regulating device

5 transformer

3.1.2.3 Static-converter excitation

Direct-current exciters may be replaced by three-phase exciters (50 to 400 Hz) .


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1 thyristor control device

2 regulator

3 three-phase thyristor

bridge network

4 three-phase bridge-

connected rectifier

In this type of static-current excitation, all three machines are mounted on one common

shaft. The auxiliary three-phase generator G1 is self-excited. The excitation current of this

machine is controlled by thyristors. The three-phase current which it supplies is rectified

and supplied to the field winding of the main exciter G2. Three-phase current supplied by

this generator is rectified in turn, and supplied to the field winding of synchronous

generator G3.

3.1.2.4 Static thyristor excitation

Voltage taken from a three-phase supply is rectified by a three-phase thyristor bridge

network and supplied to the synchronous generator via two slip rings.


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1 transformer


3 three-phase thyristor

bridge network

The rate of change of the excitation depends on the thyristors and is, therefore, very high.

A variation in the supply voltage also has an effect on the excitation of the synchronous

machine. The three-phase potential applied to the thyristor bridge must, therefore, be

correspondingly high to provide the required excitation in the event of a drop in the supply

voltage. Rotating direct-current exciters require intensive maintenance. Static converters

present fewer problems. Furthermore they have high rates of change of excitation and an

output that is largely unlimited.

3.1.3 Brushless excitation with rotating diodes

The actual regulation takes place in regulating device before the stator winding of the

exciter (a three-phase asynchronous machine). The three-phase synchronous machine is

driven in reverse rotation (relative to the rotating field).

When static, the slip will be 1, and this will increase as the rotational speed increases. The

rotating part of the synchronous machine contains the rotor, the rectifier bridge, the

protection system against excess voltages and the field winding.


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1 stator

2 rotor


4 thyristor

5 three-phase rectifier

6 ignition device

7 transformer for high-

voltage isolation

8 regulating device

9 stator

10 rotor

11 asynchronous generator

The three-phase current supplied from the rotor of the three-phase asynchronous

machine is rectified and supplied to the field winding. Should excessively high voltages be

induced in the field winding (e.g., from the stator winding in the event of a short circuit),

they will be reduced in one direction by the diodes, and in the other direction by the line

thyristor. At normal excitation voltage, the ignition device will not operate but if the

potential applied to the device (S) exceeds a preset value, the thyristor across the line is

given an ignition pulse and will conduct.

An excitation transformer with a rotary secondary winding may be used instead of the

three-phase asynchronous machine. Regulation at the primary side will be effected by a

three-phase regulator. Synchronous generators employing the above type of excitation

and voltage regulation may be called "brushless alternators".


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3.1.4 Self-excited compound synchronous generators

The excitation transformer consists of three respective windings per phase:

- The voltage-sensitive primary winding.

- The current-sensitive primary winding.- The secondary winding.

1 stator

2 rotor

3 choke coils

4 capacitors

5 excitation transformer

6 rotating parts

The phasor sum of the current-sensitive and the voltage-sensitive portions in the

secondary winding, produces the excitation current which is supplied via the rectifier

bridge to the field winding. With the synchronous generator running at no-load (stator

current being zero), the current-sensitive component will be zero. The required excitation

current is then supplied by the voltage-sensitive component. If a load were imposed, the

stator voltage would be reduced due to internal voltage drops. The current-sensitive

component, however, increases the excitation current so that the stator voltage remains

unchanged.


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This type of circuit arrangement, is called a "compound".

Load variations will adjust the excitation current rapidly.

Regulation is extremely fast.

When the synchronous generator starts up, the remanence of the rotor generates a low

voltage in the stator winding.

This low voltage causes the excitation transformer to produce a low excitation current.

The flow of excitation current produces an increase in the stator voltage. A resonant circuit

is formed by the capacitors and the choke windings. At the resonant frequency (at about

1/3 of the nominal rotating speed) resonance occurs, excitation builds up rapidly and the

full excitation current is finally reached.


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4 SYNCHRONOUS GENERATORS

Mechanical energy is supplied to the generator by rotating the armature. This can be

effected with water or steam turbines. Direct current flows through the exciter winding on

the rotor and generates a magnetic field. This magnetic field rotates with the rotor. The

rotating magnetic field cuts the stator coils and induces an electromotive force (EMF) in

them.

4.1 Single-phase synchronous generators

The rotating field induces a sinusoidal alternating voltage in the single-phase winding. The

instantaneous value of this voltage is proportional to the rate of change in the flux. In the

position of the rotor shown in the diagram, the rate of change of flux and thus the voltage

in the conductor loop are zero. The magnetic flux passing through the loop is at a

maximum.

As the armature continues to rotate clockwise, the flux passing through the loop

decreases. The rate of change of flux and the voltage in the loop increase. When the

North Pole of the salient pole rotor is in its bottom position, the induced voltage reaches its

maximum.


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One revolution of the rotor produces one cycle of the sinusoidal stator voltage (one

positive and one negative voltage half-wave). The angle of 360 electrical corresponds to

360 mechanical.

In order to obtain an alternating voltage of frequency 50 Hz, the rotational speed of a two-

pole machine (one pair of poles) must be 3.000 rev/min. If the rotor is provided with more

than one pair of poles, the speed will be correspondingly lower.

n1 = synchronous speed rev/min

f1 = mains frequency Hz

p = number of pole pairs

Single-phase alternators are rarely used.

With three-phase synchronous generators, three windings (U1 - U2, V1 - V2, W1 - W2)

are arranged in the stator and spaced at 120 relative to each other.

4.2 Three-phase synchronous generators

If the rotor is turned, its N and S poles will revolve past the three stator windings. The

rotating magnetic field includes a single-phase alternating voltage in each winding.


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VL1 = between U1 and U2

VL2 = between V1 and V2

VL3 = between W1 and W2

The wave forms for the voltages can be seen in the diagram below.

With one revolution of the rotor pole, the stator voltage VL1 / VL2 / VL3 generates a sine

wave (one cycle). Since the three windings are spaced at 120, the three voltages also

have a phase difference of 120. If a load resistance is inserted in each phase, there will

be a system of three currents each with a phase difference of 120 with respect to the

other two.

This system is called a "three-phase current system".


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Generation of three alternating voltages offset by 120.

Since the rotating field revolves at the same speed as the rotor (synchronously), thesegenerators are termed synchronous generators. In order to obtain the frequency of 50 Hz,

the rotating speed is 3.000 rev/min with one pole pair.

With a number of pole pairs, the rotating speed will be correspondingly lower.

Synchronous generators are used for power generation mainly in power stations and in

small generator sets (e.g., for emergency power generation). Turbo-generators used in

thermal power stations with cylindrical rotors, have high rotating speeds

(3.000 rev/min; 1.500 rev/min) and salient-pole rotor used in hydro-electric power stations

have low speeds (e.g., 55.5 rev/min at p = 54; 100 rev/min at p = 30).

4.2.1 Synchronous speeds

One pole pair 3.000 rev/min

Two pole-pairs 1.500 rev/min

Three pole-pairs 1.000 rev/min

Four pole-pairs 750 rev/min

Five pole-pairs 600 rev/min

Six pole-pairs 500 rev/min


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4.3 Operating characteristics

The induced voltage in a synchronous generator depends on the rotational speed and the

magnitude of the field current.

4.3.1 No-Ioad operation

The frequency corresponding to the rotational speed is usually fixed and determined by

the supply. At constant frequency, the voltage of the synchronous generator depends only

on excitation. As a result of the remanence in the pole wheel, a small voltage (remanent

voltage) in the stator is induced at zero excitation current.

lnitially the voltage rises linearly with the excitation current. As the excitation current

continues to increase, saturation occurs and after this the rise in stator voltage flattens off.

If the synchronous generator is connected to infinite bus bars (constant frequency), its

speed will remain constant and the load angle will vary with the electric output delivered.


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4.3.2 Loading

In generator operation, the rotor pole leads its no-load position by an amount called the

load angle.

The more the motor pole leads, the more effective power the generator delivers. This,however, is only possible with an increased torque of the prime mover (increased water

supply to the turbine). The electrical energy output of the generator is controlled by

regulating the power input to the turbine.

Should several generators deliver power into the same mains (parallel operation),

attention must be paid to the load distribution. The generator loads should be well

balanced. Their load angles , should be approximately equal.

4.4 Excitation of synchronous generators

Synchronous generators may be underexcited as well as overexcited. The simpiified

equivalent circuit of the synchronous generator consists of a voltage source Uowhich is

induced by the rotor, and a reactive impedance.

When the resistance Rimposes a load on a synchronous generator, the involved voltage

Uoforces the current Ithrough the reactive impedance and the resistance R.


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The voltage drop U across the load resistance R is in phase with current I. Voltage Uxs

across the reactive impedance leads the current Iby 90. Phasor addition of U and UXS

gives the induced voltage Ut.

Synchronous generators usually operate with normal excitation.

4.4.1 Overexcitation

If the generator is excited to a higher degree (over-excitation), the induced voltage will

rise. Current Iwill lag relative to the voltage Uat the terminals of the generator and the

synchronous generator will deliver reactive power to the supply system. The inductive

voltage drop UXSwill again Iead the current Iby 90. Phasor addition of Uand UXSgives

the induced voltage Uo.


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4.4.2 Underexcitation

With a smaller degree of excitation (under-excitation) the induced voltage Uo will be

reduced. Current Iwill lead voltage Uand the synchronous generator will absorb reactive

from the supply system. The phasor sum of Uand UXSgives Uo . The angle between U

and Uois the load angle .

Terminal voltage as well as the power factor may be influenced by excitation.

4.5 Power factor cos

Asynchronous motors require lagging reactive power as well as effective power. When

overexcited, synchronous generators deliver lagging reactive power which improves the

power factor of the supply system.


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With constant rated voltage and at constant speed, the excitation current may be utilized

to influence the power factor cos . Typical value of cos = 0.85 lagging.

4.6 Terminal voltage

The magnitude of the terminal voltage of generators depends on:

- The magnitude of the load.

- The type of the load.

- The exciter current.


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Since the winding of the generators (just like any other electrical conductor) have a

resistance, the flow of current resuits in an internal voltage drop, i.e., the voltage

measured at the terminals when supplying a resistive load is lower than the EMF

generated.

With an inductive load the voltage drops more steeply, since the armature field of the

generator acts against the magnetic field of the rotor pole. With a capacitive load on the

generator, the terminal voltage rises above the value of the EMF.

4.7 Paralleling arrangements

If a synchronous generator is connected in parallel with other synchronous generators

(e.g., to supply power to the same system), the following four conditions for

synchronization must be fulfilled:

- Identical voltage.

- Identical frequency.

- Identical phase sequence.

- Identical phase position.


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Two voltage meters or one dual voltage meter check that the voltages are equal.

A dual frequency meter checks that the frequencies are equal. The phase sequence

L1 - L2 - L3 is checked during commissioning by using a rotating field indicator.


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4.7.1 Methods for checking relative phase position

4.7.1.1 Synchronoscope

4.7.1.2 Dimmer circuit (lamps dim circuit)

The synchronism of both synchronous generators G1 and G2 is reached when the lamps

go out simultaneously, i.e., when the phase positions of both generator voltages are

identical.

The circuits may then be parallelled and a synchronizing force will then exist between the

two generators.

A generator which has been synchronized is "in step".


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When the Iamps are brightest, the two generator voltages have been displaced by 180.

The voltage at the Iamps is 2x VGen . The Iamps must, therefore, be suitable for twice the

generator voltage. A difference in frequency can be observed by the brightening and

dimming of the Iamps. The longer the cycle Iasts, the closer the frequencies of generators

G1 and G2 are.

When the lamps do not light up at the same time, the phase sequence (L1 - L2 - L3) is

incorrect. Two external Ieads must be exchanged.

A zero indicator may be used for the dimmer circuit. The graduations an the dial of a zero

indicator are widely spaced at the lower readings.

4.7.1.3 Brightening circuit (Iamps bright circuit)

Synchronism is reached when all lamps light up simultaneously at their brightest.

A brightening circuit is not used with three-phase generators. With single-phase circuits,

the moment for switching is readily observed.


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4.7.1.4 Rotating Iight circuit

When synchronism is reached, the lamp to be bridged is dark and the two other Iamps are

of equal brightness. If the speeds of the two generators are not identical, the Iamps of

such a circuit Iight up alternately, so that an apparent rotation of the light results. The

faster this rotation takes place, the higher the frequency difference between the

generators.

One can determine, from the direction in which the light rotates, whether the rotational

speed of the generator to be synchronized is too high or too low.

Synchronization must be precise or, the rotor will be jolted, particularly in large machines

this may cause instability and mechanical damage to the turbine coupling may result.

In power plants, paralleling of synchronous generators is usually accomplished with the

aid of synchronoscopes or by automatic devices.


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5 SYNCHRONOUS MOTORS

These are constructed identically to synchronous generators and require a magnetic

rotating field with which the rotor is in synchronism.

5.1 Operation

The magnetic fields of the coils are in synchronism with the coil currents. The current

waveform is sinusoidal, hence the flux produced by each coil varies sinusoidally in

magnitude.


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The magnetic needle takes up a position corresponding to the magnetic fields of the coils.

During one cycle, the magnetic field rotates by 360.

The rotating field produced by a three-phase system will be of constant magnitude during

rotation. lt is, therefore, called a circular or symmetrical rotating field.


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5.1.1 The variation of the phase currents IL1, IL2, IL3in a cycle

Assume that a current of maximum 1 A flows for each phase. With the aid of the

waveform diagram, the instantaneous values of the individual coil currents or of the

magnetic field of the coils can be determined.

Example:

When subdivided one period (360) into 12 parts, 1 part (30) will have a duration of

If the instantaneous currents IL1, IL2, IL3 are added together, the total is always zero.


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In the three-phase system, the three currents IL1, IL2andIL3 are out-of-phase (lag) by 120

per phase. The current IL2 has its maximum value 120 after the maximum value of IL1.

IL3 follows 120 after IL2 . The windings of the three-phase are spaced around the

circumference of the synchronous motor at 120 to each other. Winding V1 - V2 comes

120 after U1 - U2, and W1 - W2 follows 120 after V1 - V2. The current IL1flows through

winding U1 - U2, current IL2flows through V1 - V2 and current IL3through W1- W2. When

the three currents are displaced in time and are spaced by 120, a rotating field originates

in the machine.

At = 0, or 360, the currents IL1 = 0A, IL2= - 0.87 A, IL3= + 0.87 A.

5.1.2 Representation of the magnetic flux at 0


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Current IL2, negative at 0 flows out at V1. Current IL3, positive at 0 flows in at W1. The

resulting magnetic field N - S act downwards.

5.1.3 Representation of the magnetic flux at 30

Both positive currents flow in at 30, IL1 at U1 and IL3 at W1. Current IL2, negative at this

position, flows out at V1. The resulting magnetic field N - S will have rotated clockwise by

30 relative to its position 0. The individual magnetic field vary as their corresponding

currents and the resulting magnetic field rotates 360 with every complete cycle of current.

5.2 Starting a synchronous motor

If a synchronous motor is connected to a three-phase supply, it will not run up to speed.

The rotating field, in a two-pole motor, rotates at 3.000 rev/min. A stationary rotor would

have to be accelerated to such a speed within a short time. This is, however, not possible

due to the mass inertia of the rotor.


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As the North and South Pole of the rotating field move past the North and South Poles of

the rotor, the rotor poles are alternately attracted and repelled.

The torque acting on the rotor, which may be either stationary or rotating at a much slower

speed than the rotary field, acts first in the direction of the rotary field and then against it.

The rotor is, therefore, subject to an oscillating torque and cannot start to run. Starting

possibilities are:

- Use of starting motors.

- Self-starting as an asynchronous motor.

In order to enable synchronous motors to start-up, an additional damper winding is

installed within the rotor, as is the case with asynchronous machines (squirrel-cage rotor).

The synchronous motor runs up to speed as an asynchronous motor.

When the slip is sufficiently small at almost synchronous speed, the DC excitation is then

switched on.

The field current generates North and South Poles in the rotor. The poles of the rotating

magnetic field attract the poles of the rotor. The rotor is pulled along by the rotating field

(pulled into step), so that the "falling into step" takes place quickly and safety, the rotor

pole frequently receives impulse excitation (approximately 50 % higher field current for

1 - 2 seconds).

With asynchronous starting, a high alternating voltage is induced into the exciter winding.

With open circuit field windings, the winding insulation could break down. lt is thus

necessary to protect the field winding during the starting-up process.

5.2.1 Methods used for starting synchronous motors

- Short circuiting the field winding.

- Shunting the field winding with a low-value resistor.

- Controlling the field winding by varistors. Varistors become conductive when a

predetermined voltage has been reached.


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- Isolating the field winding by an electronic circuit. This circuit allows the field

current to flow when a predetermined voltage has been reached.

A synchronous motor may be mechanically driven by an external source until the

synchronous speed has been reached.

The rotor of the synchronous motor is then switched to a DC supply. When excitation is

switched on, the motor is pulled into step. With static-converter circuitry it is possible to

vary the frequency of the stator voltage from zero up to the desired rotational speed. the

synchronous motor thus runs up to synchronous from standstill.

Such a run-up is also called frequency starting.

5.3 Operating characteristics

Synchronous motors can deliver power to a shaft as normal motors. They may, however,

also be used as phase advancers (reactive power generators) to improve the power

factor, cos .

5.3.1 No-load operation

The magnetic rotating field is generated by the three-phase current supply. The fieldwinding is connected to a direct voltage source. The flow of direct current generates a

North Pole and a South Pole in the rotor pole. A firm magnetic coupling exists between

the rotating field of the stator and the exciting field of the rotor.

Both poles of the rotating field act on the poles of the rotor. North and South Poles attract

each other. Forces in the direction of rotation of the rotating magnetic field act on the

poles of the rotor. The rotor is pulled along by the rotating field. lt rotates synchronously.


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5.3.2 Loading

When the synchronous motor has been run up to speed, it operates synchronously. The

rotor pole rotates at the same speed as the rotating field. If the synchronous motor is put

under load, the poles of the rotor will lag for a certain distance behind the poles of the

rotating field. The angle of lag is called load angle . Load angle is measured in

electrical degrees.

As the load increases, the load angle will also increase. Each pole of the rotor takes up a

position in which it is pulled along by the leading pole of the rotating field and pushed by

the lagging pole.


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When the poles of the rotor are located between the poles of the rotating field, maximum

force acts on the rotor.

The load angle is then 90 electrical. In a two-pole machine the angle is also 90

mechanical. The maximum torque is called the pull-out torque. With a further increase of

the load angle, the force exerted by the leading pole of the magnetic field on the rotor pole

rapidly decreases. There is also a rapid drop in the torque. When the load angle rises

above 90 electrically, the synchronous motor falls out of step and stalls. When stalled,

the stator current is very high. The excess current relay will operate and the synchronous

motor must be restarted. If it is provided with a cage rotor, the motor will continue to run

asynchronously.

- 90 mechanical = 90 electrical (two-pole machine).

- 45 mechanical = 90 electrical (four-pole machine).

- 30 mechanical = 90 electrical (six-pole machine).

- 22.5 mechanical = 90 electrical (eight-pole machine).

If a supply voltage drops, the strength of the rotating magnetic field will also decrease.

The torque is reduced proportionally to the voltage. With half supply voltage, the torque is

halved. Synchronous machines are relatively insensitive to voltage fluctuations.


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5.3.3 The phase advancer

If an exciter voltage higher than the normal is applied to a synchronous motor, it will

become overexcited and deliver inductive reactive power to the supply. With lower

excitation (underexcitation) the motor absorbs inductive reactive power from the mains.With overexcited motors it is possible to reduce the phase angle in the supply system, i.e.

the improve the power factor cos .

If the exciter current is varied, the reactive power Q and power factor cos will vary.


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Large synchronous motors which operate overexcited and at no-Ioad are called phase-

advancing motors (synchronous compensators).

5.4 The identification of windings in synchronous machines

Old New

Stator winding U - X U1 - U2

V - Y V1 - V2

W - Z W1 - W2

Stator windings

Neutral point M N

Field windings J - K F1 - F2

5.5 Characteristics

Most synchronous motors have a pull-out torque which is twice as large as the rated

torque. A brief overload up to the double rated torque is, therefore, possible.


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Synchronous motors are insensitive to a great extent to voltage fluctuations. A portion of

the electrical energy supplied to them comes from a separate direct voltage source.

Efficiency is very high. The air gap is up to 15 mm, whilst in asynchronous machines it

varies from 0.3 to a maximum of 3 mm.

5.6 The advantage of synchronous motors

- Synchronous motors have a constant speed, a good power factor and high

efficiency.

- They have a relatively high overload capacity and are highly intensitive to

voltage fluctuations.

5.7 The disadvantages of synchronous motors

- Synchronous motors will not start up without outside help.

- They must have a direct current excitation.

- They have a constant speed, no speed control is possible.

- If overloaded they will stall and will have to be restarted.


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6 RELUCTANCE MOTORS

There are also synchronous motors without direct current excitation. These are permanent

magnet rotor motors and reluctance motors (reluctance = magnetic impedance).

6.1 Construction

6.1.1 Stator

The stator is the same as that found in synchronous machines.

6.1.2 Rotor

The rotor sheet stack has, at its periphery, as many recesses as there are poles in the

motor.

The magnetic reluctance of the rotor is not uniform at all points. There are zones of lower

and higher permeability. As a result of this, poles are formed.


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6.2 Functioning

The lines of force of the rotating field pass more easily through the rotor laminations than

through the larger air gap in the recesses. When the rotor reaches the approximate

synchronous speed it is pulled in by the rotating field (at an asynchronous speed of

approximately 90% of synchronous speed). For starting up, reluctance motors are

provided with a cage winding, as are most synchronous machines.

6.3 Characteristic

Due to the large air gap (resulting from the recesses), reluctance motors have a low

power factor.

Losses are relatively high. If the reluctance motor is overloaded by only a small degree it

will fall out of synchronism and continue to run as a cage rotor motor.


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EE 029

Electrical Machines 3-

Synchronous Machines

Theoretical Test


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EE 029


T E S T 1

1. State what is meant by the term "synchronous machine".

Name one application of the synchronous machine.

2. State the function of the brushes and slip rings on the wound rotor synchronous

machine.

3. State one advantage of the wound stator synchronous generator.

4. State the type of rotor that would be employed in a synchronous generator driven by:

A water turbine.

A steam turbine.

5. Name the type of windings used on the stator of the wound stator synchronous

machine.

State the two methods by which these windings may be connected together.

6. Draw a labelled sketch of one type of rotor for an wound stator machine and name the

type that you have drawn.

7. State the function of the short-circuited windings that may be provided in the rotor

poles of low speed machines.

8. Name two types of excitation systems that may be used for synchronous machines.9.

Name one electronic component used in synchronous machine excitation systems.

State two advantages of excitation systems using electronic components.

10. State the function of the DC supply essential for the operation of most synchronous

generators.


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EE 029


T E S T 2

1. Describe how the synchronous generator produces an AC voltage.

2. Name the shape of the wave-form produced by a synchronous generator.

3. A diesel engine is directly coupled to a sixteen-pole, 50 Hz synchronous generator.

Calculate the rotational speed of the diesel engine.

4. State at what intervals the phase-windings are spaced around the stator of a

synchronous machine.

5. Draw a diagram showing the construction of a simple three-phase generator.

6. A synchronous machine is running in parallel with a supply system. State how the

rotor pole reacts when:

- The machine is operating as a generator and the electrical output increases.

- The machine is operating as a motor and the load torque on the shaft increases.

7. A synchronous generator is driven by a steam turbine and is connected to a supply

system which is also supplied by other generators. State how the output of the

generator may be increased.

8. State the type of reactive power drawn from the supply system when a synchronous

motor is overexcited.

State why this is advantageous.


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9. A synchronous generator is operating in parallel with a supply system fed also by

other generators. State how the generator may be caused to increase its output of

lagging power.

10. State how the terminal voltage of a synchronous generator varies with:

- An increasing resistive load.

- An increasing inductive load.

- An increasing capacitive load.


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EE 029


T E S T 3

1. Name four essential conditions that must be established before a synchronous

generator can be connected in parallel with a supply system.

2. Name two instruments that may be used when preparing a synchronous generator for

paralleling.

3. Draw a labelled circuit diagram showing three lamps which will indicate when the

output of a synchronous generator has the same voltage and phase position as the

supply system.

4. State why synchronization should be carried out with care and precision.

5. Explain how a rotating magnetic field is produced by the stator of a synchronous

motor.

6. Explain why a synchronous motor will not start when connected to the supply.

7. Explain one method of starting a synchronous motor.

8. State why it is necessary to protect the field winding of a synchronous motor during

starting.

Name one method of doing this.

9. Name two advantages of the synchronous motor. Name two disadvantages of the

synchronous motor.

10. Draw a diagram of the rotor of a reluctance motor.


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EE 029


T E S T 1

( S o l u t i o n )

1. A synchronous machine is one that has the same speed of rotation as the rotating

field produced by the supply frequency. The synchronous machine can be a

generator, a motor or a power factor corrector.

2. The slip rings and brushes of the wound rotor synchronous machine connect the AC

parts of the machine to the supply and thus handle the major portion of the power

involved.

3. The wound stator machine has the advantage that the slip rings and brushes have

only to handle the relatively light DC power requirements of the exciting field.

4. The salient-pole rotor.

The cylindrical rotor.

5. Three-phase winding.

STAR or DELTA connection.

6.


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7. The short-circuited windings in the rotor poles of a salient-pole rotor are damping

windings and reduce any movement of the rotor relative to the rotating field during

load surges.

8. Separate excitation.

Self-excitation with self-excited direct-current machines, and three-phase exciters with

static converters.

Brushless excitation.

Self-excited compound synchronous generators.

9. Thyristor or diode.

Allows use of simple AC generators for excitation supply.

Allow excitation supply to be taken from output of main generator.

Makes systems fast acting by reducing time-constants.

10. The DC supply required by synchronous generators provides the flux by exciting the

field windings. The movement of the flux through the stator windings generators the

output voltage.


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EE 029


T E S T 2

( S o l u t i o n )

1. Each coil in the stator has a magnetic field produced by the rotor rotating past it.

The flux moves past the coil and generates a voltage in it. Flux due to the South Pole

will generate a voltage of one polarity and flux due to the North Pole will generate a

voltage of the opposite polarity. Thus an alternating voltage is produced.

2. The output wave-form of a synchronous generator is sinusoidal (a sine-wave).

3.

4. Phase windings are spaced 120 electrical around the stator of a synchronous

machine.

5.


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6. Rotor pole advances as generator output increase.

Rotor pole retards as motor torque increases.

7. When a steam turbine generator runs in parallel with other generators, its output is

increased by admitting more steam to the turbine.

8. When a synchronous motor is overexcited, leading reactive power is drawn from the

supply.

Most supply systems have to supply lagging reactive power and the leading reactive

power required by the overexcited synchronous motors reduces the phase angle

between supply current and supply voltage in the system. This improves the system

power factor and reduces current loading.

9. To increase the lagging reactive power output of a synchronous generator, the field

current would have to be increased.

10. Increasing resistive loading causes an increased voltage drop. Increasing inductive

loading causes a greater increase in the voltage drop.

Increasing capacitive loading causes a voltage rise.


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EE 029


T E S T 3

( S o l u t i o n )

1. Four conditions are essential for synchronization or paralleling:

- Same phase sequence.

- Same frequency.

- Same voltage.

- Same phase position

2. Synchronoscope; Voltmeter; Frequency meter; Phase indicator.

3.

4. Synchronization must be precise or the rotor will be jolted. In large machines this may

cause instability and mechanical damage to the turbine coupling may result.


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5. The three-phase supply system produces fluxes of varying polarity and magnitude in

the three stator windings. At one instant, one coil produces a North Pole and, as the

current reduces in this winding, the current in the next winding begins to produce a

North Pole. Thus the North Pole moves smoothly around the stator from winding to

winding. A South Pole is also produced 180 (electrical) behind the North Pole and

this also moves smoothly around the stator.

6. A synchronous motor rotor has a high inertia. The rotating field produced by a 50 Hz

supply will subject a stationary rotor to a torque that reverses 50 times each second.

The rotor cannot respond to this torque and, therefore, the motor cannot start.

7. The rotor of a synchronous motor can be run up to near synchronous speed, by

means of a separate DC motor or by incorporating a cage rotor into the synchronous

motor rotor. Once the machine is near synchronous speed, the field is energized and

the motor will then run at synchronous speed.

8. The rotating field will induce high voltages into the winding of a stationary rotor and

this may damage the insulation of the winding.

The rotor winding may be protected during starting by short-circuiting the winding with

a low value resistor, shunting the winding with a low value resistor, shunting the

winding with a voltage-sensitive resistor (varistor), or by using an electronic circuit that

will allow current to flow once a predetermined voltage has been reached.


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9. Advantages: High efficiency; High overload capacity; Insensitive to voltage

fluctuations; Can be operated at a range of power factors for the same loading.

Disadvantages: Expensive and complicated; Not-self-starting; No speed control

possible; Must be re-started once stalled.

10.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

ee029 electrical machines 3 th inst

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