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ELECTRICAL MACHINES–II
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Unit-II Synchronous Motor
CONSTRUCTION OF THREE PHASE SYNCHRONOUS MOTOR
PRINCIPLE OF OPERATION
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Note: 1. The average torque exerted on the rotor of synchronous motor is zero. Hence
the synchronous motor is not self starting.
2. To obtain a continuous torque, the rotor should rotate at synchronous speed
given by expression 120
S
fN
P
3. Different power stages in a synchronous motor are as under
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EQUIVALENT CIRCUIT OF A SYNCHRONOUS MOTOR
TORQUE EQUATION
The gross torque developed by the motor 9.55 mg
s
PT
N N-m
Where sinbm
s
E VP
X
Net output of the motor then can be obtained by subtracting friction and windage i.e.
mechanical losses from gross mechanical power developed.
Pout = Pm – Mechanical losses
The shaft torque developed by the motor 9.55 outs
s
PT
N
Different toques in synchronous motor
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OPERATION ON INFINITE BUS BARS
The infinite bus represents a bus bar of constant voltage and frequency, which can deliver or
absorb active and reactive power without any limitations.
The fig. shows a synchronous machine which is to be connected to the bus bars with the help
of switch K. If the synchronous machine is running as a generator then its phase sequence should be
same as that of bus bars. The machine speed and field current is adjusted in such a way so as to have
the machine voltage same as that of bus bar voltage. The machine frequency should be nearly equal
to bus bar frequency so that the machine speed is nearer to synchronous speed.
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APPLICATIONS OF SYNCHRONOUS MOTOR
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SYNCHRONOUS CONDENSER FOR POWER FACTOR CORRECTION
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EFFECT OF EXCITATION ON ARMATURE CURRENT AND POWER FACTOR
(V AND INVERTED V CURVES)
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Comparison between synchronous and induction motor
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HUNTING IN SYNCHRONOUS MOTOR
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Use of damper winding to prevent hunting
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MERITS AND DEMERITS OF SYNCHRONOUS MOTOR
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STARTING METHODS OF SYNCHRNOUS MOTOR
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POWER INPUT TO SYNCHRONOUS MOTOR
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POWER DEVELOPED BY SYNCHRONOUS MOTOR
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PROBLEM: 01
PROBLEM: 02
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PROBLEM: 03
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PROBLEM: 04
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PROBLEM: 05
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PROBLEM: 06
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PROBLEM: 07
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PROBLEM: 08
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PROBLEM: 09
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PROBLEM: 10
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PROBLEM: 11
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PROBLEM: 12
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CURRENT LOCI FOR CONSTANT POWER INPUT
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CURRENT LOCI FOR CONSTANT EXCITATION
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CURRENT LOCI FOR CONSTANT MECHANICAL POWER DEVELOPED
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ANNA UNIVERSITY QUESTIONS
1
2
APR/MAY 2008
3
4
5
6
7
8
NOV/DEC 2007 9
10
11 Define pull-out torque in synchronous motor.
MAY/JUNE2009
12 Define hunting with respect to 3 phase synchronous motor.
13 Derive the expression for power delivered by a synchronous motor in terms of load
angle.
14 Explain the effect of changing excitation on armature current and power factor.
15 A synchronous motor having 40% reactance and a negligible resistance is to be
operated at rated load at (1) upf (2) 0.8 p.f. lag (3) 0.8 p.f. lead. What are the
values of induced emf? Indicate assumption made if any.
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16 Explain briefly the methods of starting a synchronous motor.
17
18
APR/MAY 2008
19
20
21
22
23
24
NOV/DEC 2008
25
26
27
28
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29
30
MAY/JUNE2009
31
32
33
NOV/DEC 2010
34
35
36
37
38
39
40
List the inherent disadvantages of synchronous motor?
When is a synchronous motor said to receive 100% excitation?
NOV/DEC 2010
41
42
Draw the equivalent circuit and phasor diagram of a synchronous motor.
Explain the significance of V and inverted V curves.
43
44
Discuss the methods of starting and procedure for starting synchronous motor.
A 3000 V, 3 phase synchronous motor running at 1500 rpm, has its excitation kept
constant corresponding to no load terminal voltage of 3000 V. Determine the power
input, power factor and torque developed for all armature current of 250 A if the
synchronous reactance is 5 ohm per phase and armature resistance is neglected.
45
46
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47
48
APR/MAY 2010
49
50
51
52
Discuss various starting methods of a synchronous motor.
Explain the V and inverted V curves of a synchronous machine.
NOV/DEC 2011
53 Explain the operating characteristics of a synchronous machine with
(i) Constant power input and
(ii) Constant Excitation
54
55
MAY/JUNE2013
56
57
58
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TWO MARKS - QUESTION AND ANSWER
1 State the characteristics features of Synchronous motor.
Synchronous motors possess the following three distinct characteristics features.
1. The motor is not inherently self starting.
2. The speed of operation is always in synchronism with the frequency of the AC voltage
applied to it irrespective of load conditions.
3. The motor is capable of operating at any desired power factor.
2 In what way synchronous motor is different from other motors?
All DC motors and AC asynchronous motors work on the same principle, viz, ”when a
current carrying conductor is placed in an uniform magnetic field, a force is developed in
the conductor”
Synchronous motor operates due to magnetic locking taking place between the stator and
rotor magnetic fields.
3 Name the methods of starting a synchronous motor.
Three phase synchronous motors are started by any one of the following three methods.
1. Using pony motors
2. Using damper winding
3. Using small DC machine coupled to it
4. As a slip ring induction motor
4 What is the effect on speed if the load is increased on a 3 phase synchronous motor?
From no-load to maximum load, the value of which is set by the excitation condition, the
speed of operation remain constant on a 3 phase synchronous motor operating on constant
frequency bus-bars.
5 How the Synchronous motor is made self starting.
By providing cage or damper winding in pole faces, the synchronous motor can be made to
self start.
6 At what load angle is power developed in a synchronous motor becomes its maximum
value?
Power developed in a synchronous motor is maximum when its load angle α is equal to the
impedance angle θ.
α = θ
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7 What is the significance of torque angle with reference to synchronous motor?
The torque/load angle α =0 on no load. Angle increases with increase in load condition.
When α = θ power developed is maximum.
When α =(180- θ) power input is maximum.
8
Draw the power angle diagram of a synchronous motor when the resistance is neglected.
9 Draw the V-curve and inverted V for a synchronous motor.
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10
What happens when the field current of a synchronous motor is increased beyond the
normal value at constant input?
11
Distinguish between synchronous phase modifier and synchronous condenser.
12 What is a synchronous condenser?
13 How the synchronous motor can be used as a synchronous condenser?
14 Write Different torques of a Synchronous motor.
1. Starting Torque 2. Running Torque 3. Pull-in Torque 4.Pull-out Torque
15 State the applications of synchronous motor.
1. Power factor correction 2. Constant-speed, constant- load drives
3. Voltage regulation
16 What is hunting in synchronous motor? How it can be minimized?
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Oscillations of the rotor about its new equilibrium position, due to sudden application or
removal of load is called swinging or hunting in synchronous motor.
By providing cage or damper winding in pole faces, hunting can be minimized.
17
Write the expression for power developed by synchronous motor.
18
Define pull-out Torque.
The maximum torque which the motor will develop without pulling out of step (or
synchronism) is called the pull-out torque.
19 List the advantages of synchronous motor.
1. These motors can be used for power factor correction.
2. They are more efficient than induction motor.
3. They may be less costly for the same output, speed and voltage ratings as compared
to induction motors.
4. They give constant speed from no-load to full load.
5. Electro-magnetic power varies linearly with the voltage.
20 List the disadvantages of synchronous motor.
1. They require DC excitation which must be supplied from external source.
2. They require collector rings and brushes.
3. They have a tendency to hunt.
4. They cannot be used for variable speed jobs as speed adjustment cannot be done.
5. They cannot be started under load. Their starting torque is zero.
6. They may fall out of synchronism and stop when overloaded.
21 What is meant by break away or break out torque?
It pertains to the ability of the motor to accelerate the load. It is called break way or break
out torque.
22 What is meant by pull in torque?
It pertains to the ability of the motor to pull-into synchronism when changing from
induction to synchronous motor operation.
23 Why a 3-phase synchronous motor will always run at synchronous speed?
The synchronous motor always runs at synchronous speed because there is an inter-locking
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action between stator and rotor fields of this motor.
24 What is meant by V curves of synchronous motor?
The curves drawn between armature current and field current for different constant loads
are known as V curves.
25 Why synchronous motor is not a self starting motor?
The torque acting on the rotor of a synchronous motor is not unidirectional but pulsating
one and due to inertia of the rotor, it will not move in any direction. So the synchronous
motor has got no self starting torque. Due to this reason it is not a self starting motor.
26 What is meant by inverted V curves of synchronous motor?
The curves drawn between power factor and field currents for different constant loads are
known as inverted V curves.
27 What are the causes of hunting?
The hunting is caused by change in load, change in excitation and cyclic variations of the
load torque.
28 What is the role of synchronous motor in a transmission line? How?
1. Synchronous motor acts as a voltage regulator in a transmission line.
2. When line voltage decreases due to inductive load, motor excitation is increases
thereby increasing its power factor which compensates for the line voltage drop.
3. When the line voltage increases due to line capacitive effect, synchronous motor
excitation is decreases, thereby making its power factor lagging which helps to
maintain the transmission line voltage at its normal value.
29 Name the parts of a synchronous motor.
1. A stator housing 3 phase winding.
2. A rotor having a number of N and S poles.
3. An exciter that supplies DC to the rotor pole.
4. A damper winding in the field pole shoes.
5. Slip-rings and brushes to inject DC supply to rotor.
30 When is a synchronous motor said to receive 100% excitation.
When Eb=V, synchronous motor is said to receive 100% excitation.
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