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Eet3082 binod kumar sahu lecture_29
1. Electrical Machines-II
6th Semester, EE and EEE
By
Dr. Binod Kumar Sahu
Associate Professor, Electrical Engg.
Siksha ‘O’ Anusandhan, Deemed to be University,
Bhubaneswar, Odisha, India
Lecture-29
2. 2
Learning Outcomes: - (Previous Lecture_28)
To understand the basics of Synchronous Motor.
To know the working principle of poly-phase Induction Motor for its
performance comparison with Synchronous Motor.
3. 3
Learning Outcomes: - (Today’s Lecture_29)
Working Principle of Synchronous Motor.
To understand various methods of starting a Synchronous Motor.
4. 4
Working Principle of Synchronous Motor (Contd...): -
We have already discussed the various features of three-phase synchronous machine. It has a
three phase armature winding generally placed in stator and a field winding kept in the rotor.
In alternator, the synchronous machine receives mechanical power input from its prime
mover and a DC electrical input for field excitation. The mechanical energy is converted
into electrical energy. So alternator can also be called as a synchronous generator.
When the armature winding of the synchronous machine is connected to a 3-phase AC
supply and its field winding to a DC supply, the 3-phase AC electrical input is converted
into mechanical energy and the machine is said to be running as a synchronous motor.
Synchronous motor converts electrical energy into mechanical energy running at
synchronous speed irrespective of the loading condition.
The motor speed varies momentarily only at the instant of load variation.
5. 5
Working Principle of Synchronous Motor (Contd...): -
Synchronous motor works on the principle of the magnetic locking.
When two, unlike poles, are brought near each other, if the magnets are strong, there
exists a tremendous force of attraction between those two poles. In such condition, the
two magnets are said to be magnetically locked.
If now one of the two magnets is rotated, the other also rotates in the same direction
with the same speed due to the force of attraction i.e. due to magnetic locking.
When the stator/armature winding of the synchronous motor is connected to a 3-phase
supply, the current through the 3-phase winding stator winding produces a rotating
magnetic field which rotates at synchronous speed.
120 2
,s s
f f
N rpmor n rps
P P
6. 6
Working Principle of Synchronous Motor (Contd...) : -
Figure 1: Torque produced is clockwise. Figure 2: Torque produced is anticlockwise.
X
.
X
.
XXX
...
X
R1
Y2B2
Y1
B1
R2
R
Y
B
ir
iy
ib
Te
N1
S2
S1
N2
X
X
.
XXX
...
R1
R2
Y1
Y2
X
B1
B2
.
X
.
XX
.
R
Y
B
ir
iy
ib
Te
N2
N1S1
S2
7. 7
Working Principle of Synchronous Motor (Contd...): -
So, for the 2-pole machine, with 50 Hz supply, the speed of the rotating magnetic field
will be 3000 rpm or 50 rps. This effect is similar to the physical rotation of two poles
(N1 & S1) with a speed of 3000 rpm.
When the field winding on the rotor is excited with DC supply, it also produces two
poles (N2 & S2) which are stationary.
In figure-1, north pole of rotor (N2) is attracted by the south pole of stator (S1) and
south pole of rotor (S2) is attracted by the north pole of the stator (N1). So, the
electromagnetic torque is developed in clockwise direction.
8. 8
But within a small time interval (half of the time period) i.e. 10 msec or 0.01 sec, (for 50 Hz
supply) the stator poles (N1 & S1) will rotate by 1800 electrical i.e. they interchange their
positions, as the direction of current through stator coil automatically get reversed.
Due to inertia, before rotor hardly rotates in
clockwise direction, the stator poles change
their positions as shown in figure 2.
Now, the north pole of the rotor (N2) is
repelled by the north pole of the stator (N1),
and south pole of the rotor (S2) is repelled by
south pole of the stator (S1). So, the rotor
experiences a torque in anticlockwise
direction which is opposite to that in figure 1.
9. 9
So, the net torque exerted on the rotor is zero. Due to this, the synchronous motor cannot start
on its own.
Practically it is not possible for stator poles to pull the rotor poles from their stationary position
into magnetic locking condition.
This is because the speed with which rotating magnetic field is rotating, is so high that it is
unable to rotate the rotor from its initial position, due to the inertia of the rotor.
Therefore, a synchronous motor is not self starting.
So, to develop unidirectional torque, before connecting the field winding to a DC source, the
rotor must be rotated at synchronous speed or near synchronous by some means, so that the
relative velocity between the synchronously rotating stator field and rotor is zero.
Since, both rotor and stator field now are stationary with respect to each other, DC excitation
to field winding creates rotor poles which magnetically interlocks the stator poles.
11. 11
Different methods of starting a Synchronous Motor: -
We have seen that synchronous motors are not self starting. Before exciting it’s field
system, the rotor must be rotated at or nearer to synchronous speed by some means.
This can be accomplished by the following three methods:
i. Auxiliary motor starting (using pony motors): - Suitable for no load or light
load.
ii. Induction motor starting (using damper winding): - Suitable for no load or light
load.
iii. Synchronous induction motor starting: - Suitable for starting even with full load
condition.
12. 12
(i) Auxiliary Motor Starting: -
The purpose of the auxiliary motor is to bring the synchronous motor speed, near to its
synchronous speed. The auxiliary motor may be an Induction motor or a DC motor.
After bringing the main motor speed almost equal to its synchronous speed, the armature
winding of synchronous motor is also energised from 3-phase supply.
Now, when the field winding of synchronous motor is connected to DC source, the field
poles get magnetically locked with those produced by armature winding. As a result of this,
main motor starts running as a synchronous motor at synchronous speed.
Auxiliary motor (Induction motor/DC motor) can now be disconnected from three-phase
supply/DC supply or isolated from the main motor.
13. 13
Sometimes an induction motor with two poles, fewer than the synchronous-motor poles, is
used as an auxiliary motor. This induction motor runs the main motor above or equal to its
synchronous speed. After this, the induction motor is switched off and the synchronous
motor armature is switched on to 3-phase AC supply and field winding is energised from DC
supply.
If the main synchronous motor has 12 poles its synchronous speed is (120 x f)/P = 6000/12
= 500 rpm. If the auxiliary motor (induction motor) has also 12 poles, it cannot rotate the
synchronous motor at 500 rpm as induction motor always runs at a speed less than
synchronous speed.
So, if we take an auxiliary motor (induction) motor having 12-2 = 10 poles, its synchronous
speed will be 600 rpm. So this induction motor can run at a speed less than 600 rpm i.e. at
500 rpm which is the synchronous speed of the synchronous motor having 10 poles.
14. 14
If the synchronous motor is coupled with a DC machine, then dc machine is first run as a
DC motor. The main motor, now made to operate as a synchronous generator, is
synchronized with the 3-phase supply system.
If the DC motor is now switched off, the main motor starts running as a synchronous motor.
The disadvantage of this method of starting is that the motor can’t be started under load; in
case it is desired to do so, the auxiliary-motor rating has to be larger, thus increasing the
cost of the set. In view of this, the auxiliary-motor starting is used only for unloaded
synchronous motors.
Note: - During starting, the field winding of the synchronous motor is shorted by means of an
external resistance. Current through the field winding produces an additional torque just like in
induction motor, which helps the rotor to accelerate at a faster rate.
17. 17
(ii) Induction Motor Starting: -
In order to make the motor a self-starting synchronous motor, the amortisseur or
damper winding is embedded in the slots in the rotor pole faces. This winding is short-
circuited at both ends by metal rings. Thus the damper winding is exactly similar to the
squirrel cage winding of a 3-phase induction motors.
When armature is excited from 3-phase supply, a rotating magnetic field is established.
This rotating field and the damper winding develop induction motor torque, and rotor is
therefore, accelerated up to about synchronous speed.
The synchronous motor with damper bars in its rotor pole shoes, runs as a squirrel cage
induction motor, from standstill up to near its synchronous speed.
18. 18
If the field winding is now energised from a DC source, the rotor and stator poles will
magnetically lock together provided the rotor poles just approaching the stator poles
are of opposite polarity.
Once the rotor starts rotating at synchronous speed, no emf would be induced in
damper bars.
22. 22
(ii) Synchronous Induction Motor: -
The rotor of synchronous induction motor is similar to the rotor of wound-rotor
induction motor. At the time of starting, high resistance is inserted in the rotor circuit in
order to develop high starting torque as in a wound rotor induction motor.
As the motor speeds up, the external resistance is gradually reduced to zero.
Zero external rotor resistance would cause the rotor to rotate at a speed very near to
synchronous speed. At this time, rotor short-circuit is removed and rotor winding is
switched over to DC supply.
The rotor poles thus created would be magnetically locked with the stator poles and the
motor would start running as a synchronism motor.
