Synchronous Motor : Concepts - 1

Introduction of Synchronous Motors:

A three-phase ac voltage is applied to the stator winding and a DC source supplies the rotor field. The rotor is assumed stationary. The stator field rotates at synchronous speed.

In order to develop a continuous torque, the two fields must be stationary with respect to each other. As we now appreciate, this occur only when the rotor is also turning at synchronous speed. It is only then when stator and rotor fields "lock in", hence the name synchronous motor. Figure shows this condition ; the south pole of the rotor will lock in with the stator north pole, and vice versa. There may be momentary fluctuations in speed, but on the average the speed is constant. If the average speed of the rotor is different from synchronous value, even by a small amount, the poles lose their "grip" and the machine will come to a standstill. The bond between stator and rotor poles is then lost, which is the essential criterion for the development of torque.

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