MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/IIIsem... · continues to develop a single phase torque produced by its running winding alone. As it approaches synchronous speed,

EC 2201 – Electrical Engineering Page | 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI- 621213.

QUESTION BANK

DEPARTMENT: ECE SEMESTER – III

SUBJECT CODE: EC2201 SUBJECT NAME: Electrical Engineering

UNIT IV SYNCHRONOUS AND SPECIAL MACHINES

PART – A

1. What are the two types of synchronous machines? [NOV/DEC 2009]

i) Cylindrical rotor type

ii) Salient pole type

2. Give the applications of stepper motors. [NOV/DEC 2009]

i) Operation control in computer peripherals

ii) IC fabrications

iii) Robotics

iv) Tap drives

v) Floppy disc drives

vi) X – Y plotters

3. What is meant by synchronous impedance in alternator? [NOV/DEC 2009]

Voltage drop in an alternator under load is equal to IRa + j IXs.

Voltage drop = I(Ra + j Xs).

= IZs

Where Zs is known as synchronous impedance.

4. State application of hysteresis motor. [APRIL/MAY 2010]

i) Paper tape drives

ii) Magnetic tape drives

iii) Gramophones

iv) Recorders

5. Why should an alternator run always at synchronous speed? [NOV/DEC 2010]

A synchronous motor always runs at synchronous speed because of the magnetic locking

between the stator and rotor poles.

6. Write down the emf equation of alternator. [NOV/DEC 2010]


Induced emf per phase Erms = 4.44 Kp Kd f T volts

Kp = Pitch factor

Kd = Distribution factor

f = Frequency in Hz

= Flux per poles

T = Number of turns / phase

7. What is meant by hunting in synchronous motor? [NOV/DEC 2010]

When a synchronous motor is used for driving a fluctuating load, the rotor starts oscillating

about its new position of equilibrium corresponding to the new load. This is called hunting or

phase swinging. To prevent hunting dampers are damping grids are employed.

8. Name the different type losses in alternator. [NOV/DEC 2010]

i) Electrical loss (or) Copper loss (or) Ohmic loss

Armature copper loss = 3I2a Ra

ii) Magnetic loss (or) Core loss (or) Iron loss

There are two types

i) Hysteresis loss

ii) Eddy current loss

iii) Mechanical losses are

i) Friction loss ii) Windage loss

9. What are the different excitations of a synchronous motor? [NOV/DEC 2011]

i) Brushless Exciter

ii) Static Exciter

10. Specify the role of damper winding in synchronous motor. [MAY/JUNE 2011]

Most widely used method of starting a synchronous motor is to use damper windings. A

damper winding consists of heavy copper bars inserted in slots of the pole faces of the

rotor. These bars are short circuited by end rings at both ends of the rotor. These short

circuited bars form a squirrel cage winding. When three phase supply is connected to the

stator, the synchronous motor with damper winding will start as three phase induction

motor. As the motor approaches synchronous speed, the dc excitation is applied to the field

windings. The rotor will then pull into step with the stator magnetic field.

11. What is the principle of hysteresis motor? [NOV/DEC 2011]

It is a single phase motor whose operation depends upon the hysteresis effect (i.e.,

magnetization produced in the ferromagnetic material lags behind the magnetizing force)

and on the presence of continuously revolving magnetic flux.


12. State the advantage of rotating magnetic field system in synchronous machine.

[NOV/DEC 2011]

Relatively small amount of power required for field system can easily supplied to rotating

system using slip rings and brushes, more space is available in the stator part of the

machine to provide more insulation, it is easy to provide cooling system, stationary system

of conductors can easily be braced to prevent deformation.

13. What is the principle operation of reluctance motor? [NOV/DEC 2011]

Whenever a piece of ferromagnetic material is located in a magnetic field, a force exerted

upon the material, tending to bring it into the position of the densest portion the field.

The force tends to align the specimen of material so that the reluctance of the magnetic

path passing through the material will be at minimum.

14. Write down the relation between the speed and frequency. [NOV/DEC 2012]

Ns = p

120f

15. Define voltage regulation of an alternator. [NOV/DEC 2012]

The voltage regulation of an alternator is defined as the increase in terminal voltage when

full load is thrown off, assuming field current and speed remaining same.

Percentage of regulation 100V

VEo

16. Differentiate salient pole rotor and smooth cylindrical type synchronous machines.

Salient pole rotor Smooth cylindrical rotor

1. Large diameter and short axial length

2. Used for low speed alternators

3. Has projecting poles

4. Needs damper winding

5. Windage loss is more

1. Small diameter and long axial length

2. Used for high speed alternators

3. No projecting poles

4. Does not need a damper winding

5. Windage loss is less

17. List out the applications of synchronous motor.

i) Power factor correction

ii) Constant speed, constant load drives

iii) Voltage regulation of transmission lines


18. What is synchronous capacitor?

An over excited synchronous motor, running without any mechanical load, used specifically

for power correction is known as synchronous capacitor.

19. What is stepper motor?

A stepper motor is a digital actuator whose input in the form of programmed energization of

the stator windings and whose output are in the form of discrete angular rotation.

20. Define step angle

Step angle is defined as the angle through which the stepper motor shaft rotates for each

command pulse. It is denoted as (β )

i) β = [(Ns-Nr)/ Ns.Nr]x360o

Where Ns = no. of stator poles or stator teeth

Nr = no. of rotor poles or rotor teeth

ii) β = 3600/mNr

Where m= no. of stator phases

21. What are the advantages of salient type pole construction used in synchronous

machines?

They allow better ventilation, the pole faces are so shaped radial air gap length increases

from pole center to pole tips so flux distortion in air gap is sinusoidal so emf is also

sinusoidal.

21. Why are alternators rated in KVA and not in KW?

As load increases I2R loss also increases, as the current is directly related to apparent

power delivered by generator, the alternator has only their apparent power in VA/KVA/MVA

as their power rating.

22. What is different torques of a sync motor?

1. Starting torque

2. Running torque

3. Pull-in torque

4. Pull-out torque

23. What is the advantage in using stepper motor?

1. It can drive open loop without feedback

2. It requires little or no maintenance.

24. Give the applications of stepper motor?

1. Robotics


2. Computer peripherals

3. Facsimile machine

4. Aerospace

25. What is the advantage of reluctance machines?

1. Motor speed is constant

2. Simple construction

PART - B

1. Derive the expression for the induced emf of a synchronous generator. [NOV/DEC

2009]

Let Z = number of conductors or coil sides in series/phase

Z = 2T, Where T is the number of coils or turns/phase

P = Number of poles

f = frequency of induced emf in Hz

= flux/pole in webers

Kd = distribution factor β/2sinm

β/2msin

Kc or Kp = pitch factor or coil span factor 2/cos

Kf = form factor = 1.11

N = Rotor speed in r.p.m

For one revolution of the rotor each stator conductor is cut by a flux of P webers

d = P and dt = 60/N second

Average emf induced per conductor 60

ΦPN

N60

ΦP

dt

dΦ

We know that f = P

120fN(or)

120

PN

Substituting the value of N, we get average emf per conductor


volts2fΦp

120fX

60

ΦP

If there are Z conductors in series /phase, then

Average e.m.f/phsase = voltsz2f

RMS value of emf/phase = Average value/phase x Form factor

= z2f x 1.11

= 2.22 zf volts

If Kp and Kd are the pitch factor and distribution factor of the armature winding respectively,

then,

Erms/ Phase = 4.44 Kp Kd f Z volts

Sometimes, the turns (T) per phase rather than conductor per phase are specified.

In that case, the above equation becomes

Erms/ Phase = 4.44 Kp Kd f Z T volts

The line voltage will depend upon whether the winding is star connected or delta c

connected.

For star connected winding

Eline = phase/E3rms

Winding factor, Kw = Kd x Kp

For full pitch winding, Kp = 1

For concentrated winding, Kd = 1

2. Explain the construction and operation of a reluctance motor. [NOV/DEC 2009]

Let us consider a piece of magnetic material which is free to rotate and is placed in a

magnetic field as shown in fig.

A torque will act on the material shown fig (a) to bring it to the position shown in fig (b), so

as to produce minimum reluctance to the flux path.


(a) (b)

Fig. Reluctance torque developed on a magnetic material placed in magnetic field

Reluctance principle

Mechanical force is exerted on a sample magnetic material located in a magnetic

field. The force tends to act on the material in such way as to bring the material in to the

portion of the magnetic field that greater flux density. If the sample is irregularly shaped, it

will align in such a way as to produce minimum magnetic reluctance and consequently,

maximum flux density. Thus, particles of iron filling are aligned parallel to the field direction

in the presence of a magnetic field.

Construction

The construction of reluctance motor is same as that of single phase induction

motor. The motor has the main (running) winding and an auxiliary (starting) winding. In

general, the stator of single phase reluctance motor is similar to that of any one of the single

phase induction motor. The rotor of the reluctance motor is basically a squirrel cage rotor

with some teeth removed at the appropriate places such as to provide the desired number

of salient rotor poles.

Fig. Different

types of rotor laminations of reluctance motor

When the stator is connected to a single phase supply, the motor starts as a single

phase induction motor. At a speed of 75% of the synchronous speed, a centrifugal switch

disconnects the auxiliary winding and the motor continues to speed up as a single – phase

motor with the main winding in operation.When the speed is close to the synchronous

speed, the reluctance – torque developed is sufficient to pull the rotor to synchronous

speed.


Fig. Reluctance motor rotor

The motor works as a non – excited synchronous motor up to about 200 percent of the

full – load torque. At a load beyond 200 percent of the full load, the motor will continue to

work as a single phase induction motor. The direction of rotation of such motors can be

reversed in the same manner as done in a single phase induction motor.

Speed – torque characteristics

Fig. Speed – torque characteristics

The motor start at anywhere from 300 to 400% of its full – load torque, depending on

the rotor position of the unsymmetrical rotor with respect to the field windings. This is due to

the result of rotating magnetic field created by a starting and running winding. At 75% of

synchronous speed, a centrifugal switch opens the starting winding and now the motor


continues to develop a single phase torque produced by its running winding alone. As it

approaches synchronous speed, the reluctance torque is sufficient to pull the rotor into

synchronism with the pulsating single – phase field.

3. Explain the method of obtaining the voltage regulation of asynchronous generator

using EMF method. [NOV/DEC 2012]

This method involves the following steps.

I) Plot the open – circuit characteristic (O.C.C) from given data as shown in figure.

II) Plot short – circuit characteristic (S.C.C) from the data given by short circuit test.

Both these curves are drawn on a field current base.

Consider a field current If corresponding to this field current the open circuit voltage I s E1.

When the winding is short – circuited, the terminal voltage is zero. Hence it may be

assumed that the whole of this voltage E1 is being used to circulate the armature short

circuit current I1 against the synchronous impedance Zs.

E1 = I1Zs (or) Zs = E1/I1

III) To find the synchronous reactance

2

a

2

ssRZX

IV) After finding Ra and Xs vector diagrams for any load any power factor may be drawn.

Three cases are considered a) unity power factor b) Lagging power factor and

c)Leading power factor

Fig.O.C and S.C test curves of an alternator


Fig. a) Unity power factor

OC2 = (OA+AB)2 + BC2

i.e., Eo2 =(V+IRa)

2+(IXs)

(or) Eo = 2

s

2

a)(IXIRV

Fig. b) Lagging power factor

OC2 = (OA+AB)2 + (BD+DC)2

Eo2 =(Vcos +IRa)

2+(Vsin +IXs)2

Eo = 2

s

2

a)IX(VsinIRVcos


Fig. c) Leading power factor

OC2 = (OF+FD)2 + (BD - BC)2

Eo2 = (Vcos +IRa)

2 + (Vsin -IXs)2

Eo = 2

s

2

a)IX(VsinIRVcos

Of various methods for determination of regulation, this method gives result inexact.

The regulation obtained by this method is always higher than actual value and, therefore

this is called pessimistic method. However this method theoretically accurate for non –

salient pole machines with distributed field winding when saturation is not considered.

4. Briefly discuss the construction of the following: (i) Stator of synchronous machine (ii) Salient pole and cylindrical rotors. [NOV/DEC 2010]

The stator core assembly of a synchronous machine is almost identical to that of an

induction motor. A major component of the stator core assembly is the core itself, providing

a high permeability path for magnetism. The stator core is comprised of thin silicon steel

laminations and insulated by a surface coating minimizing eddy current and hysteresis

losses generated by alternating magnetism. The laminations are stacked as full rings or

segments, in accurate alignment, either in a fixture or in the stator frame, having ventilation

spacers inserted periodically along the core length. The completed core is compressed and

clamped axially to about 10 kg/cm2 using end fingers and heavy clamping plates.

Core end heating from stray magnetism is minimized, especially on larger machines,

by using non-magnetic materials at the core end or by installing a flux shield of either

tapered laminations or copper shielding.

http://electrical-engineering-portal.com/what-is-the-eddy-current


FIGURE - Magnetic skeleton (upper half ) and structural parts (lower half ) of a ten-

pole (720 rpm at 60 cycles) synchronous motor.

A second major component is the stator winding made up of insulated coils placed in

axial slots of the stator core inside diameter. The coil make-up, pitch, and connections are

designed to produce rotating stator electromagnetic poles in synchronism with the rotor

magnetic poles. The stator coils are retained into the slots by slot wedges driven into

grooves in the top of the stator slots. Coil end windings are bound together and to core-end

support brackets.

If the synchronous machine is a generator, the rotating rotor pole magnetism

generates voltage in the stator winding which delivers power to an electric load. If the

synchronous machine is a motor, its electrically powered stator winding generates

rotating electromagnetic poles and the attraction of the rotor magnets, operating in

synchronism, produces torque and delivery of mechanical power to the drive shaft.

Salient pole rotor

.


Fig. Salient pole rotor

Two types of rotors are used in the design of synchronous generators, the cylindrical

rotor and a salient-pole rotor. The rotor is rotated at the synchronous speed by a prime

mover such as a steam turbine. The rotor has as many poles as the stator, and the rotor

winding carries dc current so as to produce constant flux per pole.

The filed winding usually receives its power from a 115- or 230-V dc generator. The

dc generator may be driven either by the same prime mover driving the synchronous

generator or by a separate electric motor. The salient-pole rotor is used in low- and

medium-speed generators because the windage loss is small at these speeds. It consists of

an even set of outward projecting laminated poles. Each pole is dovetailed so that it fits into

a wedge-shaped recess or is bolted onto a magnetic wheel called the spider. The field

winding is placed around each pole, as indicated in Figure. The poles must alternate in

polarity.

Cylindrical rotors

Fig. 4-pole

cylindrical rotor.

The cylindrical rotor is employed in a 2- or 4-pole, high-speed turbo-generator. It is

made of a smooth solid forged steel cylinder with a number of slots on its outer periphery.


These slots are designed to accommodate the field coils, as shown in Figure. The

cylindrical construction offers the following benefits:

1. It results in a quiet operation at high speed.

2. It provides better balance than the salient-pole rotor.

3. It reduces the windage loss.

5. Explain the construction and operation of the following: (i) Reluctance motor (ii) Hysteresis motor. [NOV/DEC 2010]

Fig.Cross sectional view of a hysteresis motor

A hysteresis motor is a single phase synchronous motor without any projected poles

and DC excitation. The construction is either split phase type (or) shaded pole type. The

rotor of hysteresis motors are made with magnetic material of high hysteresis losses. i.e.,

whose hysteresis loop area is very large.

A ring of cobalt or chrome steel is mounted on a non – magnetic arbor made with

aluminum as shown fig. No winding is provided on this rotor. After giving supply to the

stator, the rotating magnetic field is produced which induces poles in the rotor. Since


hysteresis phenomena is dominant in rotor material, rotor pole axis lags behind the rotating

magnetic field. Therefore, the rotor poles get attracted towards the moving stator field poles

and hence rotor gets subjected to torque known as “Hysteresis Torque” which is constant at

all speeds. It is the principle of this motor.

Fig. Hysteresis loop

for material

(a)Stator poles induce poles on the (b) Torque due to residual magnetism

Rotor Of the rotor

The rotor pole strength remains constant, when the stator field axis moves forward.

Therefore higher is the hysteresis torque if higher is the receptivity. At the beginning the

rotor starts rotating due to the combined effect of hysteresis torque and torque due to eddy

current induced in the rotor. The rotor pull in to synchronism when the speed of the rotor is

at its synchronous speed, there is no induced emf in the rotor as the stator synchronously

rotating field and rotor are stationary with respect to each other.

Speed – torque characteristics


Fig.Speed –Torque characteristics

6. Write short note on MMF method of determining regulation of an alternator.[

NOV/DEC 2012]


This method is converse of emf method. In this method the effect of winding

impedance and armature reaction are equivalent to ampere – turns and hence this method

is call mmf method. The required data for calculation of regulation are obtained from the

open and short circuits test of the alternator.

From the OC and SC characteristics, field current If1 id determined to give rated

voltage V on no load, neglecting armature resistance drop and field current If2 is determined

to cause short circuit current, equal to full load current, on short circuit.

On short circuit, the field current If2 balances the impedance drop in addition to

armature reaction on full load. But since Ra is usually very small and xL is also small for low

voltage on short circuit, impedance drop can be neglected. P.f on short circuit is almost zero

lagging and the field ampere turns are used entirely to overcome armature reaction.

Therefore If2 gives demagnetizing ampere turns at full load. Now let the alternator supply full

load current at p.f of cos . Draw OA representing If1 to give full load rated voltage V (or

more exactly V+IRacos ) then draw AB at an angle (90 ) representing If2 to give full

load current on short circuit; +ve sign for lagging power factor and –ve sign for leading

power factor. Now find field current If measuring OB, which will give on open circuit an e.m.f

Eo, which can be determined from O.C.C.

The percentage regulation can be obtained from the following relation.

% Regulation = V

VE0

X 100

This method is also known as optimistic method since it gives values lower than

actual values. The reason of it is that the excitation to overcome armature reaction is

determined on unsaturated part of the saturation curve.

7. Explain the construction, working of any one type of stepper motor.[NOV/DEC 2011]


Construction

The single stack variable stepper motor has no permanent magnet either on the

rotor or the stator. It has salient pole stator and rotor. The stator has concentrated windings

placed over the stator poles, while the rotor has no windings.

The number of poles on the stator and rotor are different. This gives the motor to

have,

The self-starting capability.

The ability of bidirectional rotation

Consider

a single stack

variable reluctance stepper motor as shown below with three phases. In this figure, coils 1

and 1’ are connected in series to form a phase winding. This phase winding is connected to

a DC source with the help of a semiconductor switch S1 and electrical connection circuit of

VR stepper motor is shown below.

Similarly 2 and 2’, 3 and 3’ are connected to the same source through the

semiconductor switches S2 and S3. So, phase a consists of 1 & 1’ coils, phase b consists of

2 & 2’ and phase c consists of 3 & 3’.


The variable-reluctance (VR) stepper motor differs from the PM stepper in that it has

no permanent-magnet rotor and no residual torque to hold the rotor at one position when

turned off. When the stator coils are energized, the rotor teeth will align with the energized

stator poles. This type of motor operates on the principle of minimizing the reluctance along

the path of the applied magnetic field. By alternating the windings that are energized in the

stator, the stator field changes, and the rotor is moved to a new position.

The stator of a variable-reluctance stepper motor has a magnetic core constructed

with a stack of steel laminations. The rotor is made of magnetized soft steel with teeth and

slots. The relationship among step angle, rotor teeth, and stator teeth is expressed using

the following equation:

Ψ =

Where Ψ = step angle in degrees

Ns = Number of teeth on stator core

Nr = Number of teeth on rotor core

Figure shows a basic variable-reluctance stepper motor. In this circuit, the rotor is

shown with fewer teeth than the stator. This ensures that only one set of stator and rotor

teeth will align at any given instant. The stator coils are energized in groups referred to as

phases. In Figure, the stator has six teeth and the rotor has four teeth. The rotor will turn 30°

each time a pulse is applied. Figure (a) shows the position of the rotor when phase A is

energized. As long as phase A is energized, the rotor will be held stationary. When phase A

is switched off and phase B is energized, the rotor will turn 30° until two poles of the rotor

are aligned under the north and south poles established by phase B. The effect of turning

off phase B and energizing phase C is shown in Figure (c). In this circuit, the rotor has again

moved 30° and is now aligned under the north and south poles created by phase C. After

the rot or has been displaced by 60° from its starting point, the step sequence has

completed one cycle. Figure shows the switching sequence to complete a full 360° of

rotation for a variable-reluctance motor with six stator poles and four rotor poles. By

repeating this pattern, the motor will rotate in a clockwise direction. The direction of the

motor is changed by reversing the pattern of turning ON and OFF each phase.


Fig.Variable-reluctance stepper motor and switching sequence.

The VR stepper motors mentioned up to this point are all single-stack motors. That

is, all the phases are arranged in a single stack, or plane. The disadvantage of this design

for a stepper motor is that the steps are generally quite large (above 15°).

8. Explain the construction and working of salient pole alternator. [NOV/DEC 2010]

An alternator has 3 phases winding on the stator and DC field winding on the rotor.

The rotor carries field winding which is supplied with direct current through two slip rings by

a separate d.c source. Rotor construction is two types, namely,

1. Salient (or) projecting pole type

2. Non – salient pole (or) cylindrical type

Salient pole type

The rotor of this type is used almost entirely for slow and moderate speed

alternators; since it is least expensive and provides ample space for the field ampere –

turns. Salient poles cannot be employed in high speed generator on account of very high

peripheral speed and the difficulty of obtaining sufficient mechanical strength.


Fig. Salient pole rotor

The salient poles are made of thick steel laminations riveted together and are fixed

to rotor by a dove – tail joint. The pole faces are usually provided with slots for damper

windings. Those dampers are useful in preventing hunting. The pole faces are so shape that

the radial air gap increase from the pole Centre to the pole tips so that the flux distribution

over the armature is sinusoidal. The field coils are placed on the pole – pieces and

connected in series. The ends of the field windings are connected to a d.c source through

slip rings carrying brushes and mounted on the field structure.

The salient pole field structure has the following special features.

i) They have large diameter and short axial length.

ii) The pole shoes cover about 2/3 of pole pitch.

iii) Poles are laminated in order to reduce eddy current loss.

iv) They are employed with hydraulic turbines or diesel engines. The speed is from 120

t0 400 r.p.m


Fig. Cross sectional view of a salient pole alternator

Fig shows sectional view of salient pole alternator. The field magnets are

magnetized by applying 125 volts or 250 volts through slip rings. The field windings are

connected such that, alternator N and S poles are produced. The rotor and hence the field

magnets are driven by the prime mover. As the rotor rotates, the armature conductor is cut

by the magnetic flux. Hence an emf is induced in the armature conductors. As the magnetic

poles are alternately N and S pole, this emf acts in one direction and then in the other

direction. Hence alternating emf is induced in the stator conductors. The frequency of

induced emf depends on the number of N and S poles moving past an armature conductor

in one second. The direction of induced emf can be found by Fleming right hand rule and

frequency is given by

f = PN/120

9. Compare between synchronous and induction motor [MAY/JUNE 2013]

Sl.No Induction motor Synchronous motor

1. It is a self-starting motor It is a self-starting motor

2. Does not require DC excitation Require DC excitation

3. Speed can be controlled but to small extent Speed control is not possible

4. It always runs at less than synchronous speed It always runs at less at synchronous

speed

5. It operates at lagging power factor It can be operated under wide range

power factor both lagging and leading

6. Hunting does not present Hunting may be present


7. No need of damper winding Damper winding is needed

8. More simple and less cost More complicated and more costly

9. Employed for supplying mechanical load only Employed for supplying mechanical

load as well as power factor

improvement

10. Discuss the procedure for starting a synchronous motor. [MAY/JUNE 2013]

Three phase synchronous motor is not a self-staring motor. To start the synchronous

motor any one of the following method is used.

i) Starting by using a DC motor coupled to the synchronous motor

ii) Starting with the help of a small induction motor (Pony motor)

iii) Starting with help of Damper winding

i) By the use of DC motor

A DC motor is coupled to the shaft of the synchronous motor. The synchronous

motor is then excited and synchronized with the AC supply mains. Now the DC motor is

disconnected from the DC supply.

Fig. Starting of 3 phase synchronous motor by DC machine

In another option, the field of the DC machines is strengthened until the DC machine

acts as a DC generator. In this case, the synchronous motor will operate from AC the supply

and DC machine which was earlier using to start the synchronous motor will now act as a

load (DC generator) on it.

(ii) By using small induction motor


In this method, the synchronous motor is started with help of small induction motor.

Usually, the number of poles of the induction motor is kept two numbers less than the

number of poles of the synchronous motor. The small induction motor is first started from

AC supply main and then DC excitation from the exciter is given to the field windings of the

synchronous motor. Now the synchronous machine operates as a synchronous generator

and an alternating emf is generated in its armature winding. After this the alternator is

synchronized with the supply. After synchronizing, the supply to the pony motor is cut off

and now the 3 phase synchronous machine to work as a synchronous motor.

Fig. Starting of 3 phase synchronous motor by pony motor

iii) By using Damper winding (or) Started like Squirrel cage induction motor

The synchronous motor can be made a self-starting. Self-starting is made possible

by providing special type of winding, termed damper winding on rotor poles of the

synchronous motor.


Fig. Damper winding of the 3 phase synchronous motor

The damper winding consists of short circuited copper bars embedded in the pole

faces. As soon as 3 phase AC supply is connected to the stator winding, a rotating magnetic

field (RMF) is established. The rotating magnetic field interacts with the current produced in

the rotor cage and produces a torque to start the motor, just like three phase induction

motor. When the motor attains 95 % of the synchronous speed, the rotor winding is

connected to exciter terminals and the rotor is finally magnetically locked to the rotating field

of the stator and then motor runs at synchronous speed.

MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/IIIsem... · continues to develop a single phase torque produced by its running winding alone. As it approaches synchronous speed,

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