Page 790 - Basic Electrical Engineering

result, more current will be drawn by the motor. If, however, a synchronous
machine has to start as a motor from its standstill condition, three-phase
supply has to be given to the stator windings and dc supply has to be given to

the field winding. The principle of working of a synchronous motor and the
method of starting are discussed as follows.

10.15.2 Principle of Working of a Synchronous Motor

The stator has a three-phase winding which is fed from a 50 Hz three-phase
supply. When a three-phase supply is provided to the stator winding, a

rotating magnetic field rotating at synchronous speed is produced. The
process is same as that described in the case of the three-phase induction

motor. In a three-phase induction motor the rotor had a closed winding and
the rotor got its excitation through electromagnetic induction. In the case of

the synchronous motor, the field windings placed on rotor slots are provided
with dc excitation through the brush and slip-ring arrangement. Thus, two

magnetic fields are produced; one rotating at a synchronous speed, N and the
s
other produced by the field winding. The rotor having the field system should

start rotating at the same speed as the rotating magnetic field, N . The reason
s
is that two magnetic fields will always try to align with each other. However,

due to its inertia, the rotor will not pick up speed. That is why a three-phase
synchronous motor is not self-starting. To make it self-starting, a squirrel-

cage winding is made on the pole faces so that the rotor will start rotating as
an induction motor first, without having the field windings excited.