Page 145 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The synchronous motor now operates as a synchronous generator and an alternating emf is generated in its
armature. Now this synchronous motor, operating as alternator is to be synchronised with the mains supply. After
synchronising, the supply to the pony motor is cut off and then the motor continuous to work as a synchronous
motor.
Starting as an induction motor

In this method the synchronous motors, which are provided with damping winding on the face of salient poles
of the rotor, is started as an ordinary squirrel cage induction motor. The damping winding acts as squirrel cage
winding which consists of thick copper bars placed in the slots made on the faces of the rotor poles and short
circuited by heavy copper rings at both the sides of the rotor.
To start the synchronous motor, the DC supply to the field winding is kept disconnected and the AC supply is
applied to the armature winding. The motor starts as a squirrel cage induction motor. When the motor rotates at
its maximum speed, the DC supply to the field is switched on. Then the rotor and stator poles get interlocked with
each other, because of the flux of rotating magnetic field is seeking the minimum reluctance path through the core
of the salient poles and the motor now acts as a synchronous motor.

However, two points should be noted:
1 At the beginning, when voltage is applied, the rotor is stationary. The rotating field of the stator winding induces
a very large e.m.f. in the rotor during the starting period, though the value of this e.m.f. goes on decreasing
as the rotor gathers speed. Normally, the field windings are meant for 110-V (or 250 V for large machines) but
during starting period there are many thousands of volts induced in them. Hence, the rotor windings have to
be highly insulated for withstanding such voltages.

2 When full line voltage is switched on to the armature at rest, a very large current, usually 5 to 7 times the full-
load armature current is drawn by the motor. In some cases, this may not be objectionable but where it is, the
applied voltage at starting, is reduced by using autotransformers. However, the voltage should not be reduced
to a very low value because the starting torque of an induction motor varies approximately as the square of the
applied voltage. Usually, a value of 50% to 80% of the full-line voltage is satisfactory. For reducing the supply
voltage, the switches S are closed and S are kept open. When the motor has been speeded-up, S are closed
2
1
2
and S opened to cut out the transformers.
1
Procedure for starting
While starting a modern synchronous motor provided with damper windings, following procedure is adopted.
1 First, main field winding is short-circuited.

2 Reduced voltage with the help of auto-transformers is applied across stator terminals. The motor starts up.
3 When it reaches a steady speed (as judged by its sound), a weak D.C. excitation is applied by removing the
short-circuit on the main field winding. If excitation is sufficient, then the machine will be pulled into synchronism.
4 Full supply voltage is applied across stator terminals by cutting out the auto-transformers.

5 The motor may be operated at any desired power factor by changing the D.C. excitation.

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CITS : Power - Electrician & Wireman - Lesson 76-85