Synchronous motor construction is very similar to synchronous generator construction except that most motors are salient-pole machines.
Principle and Operation of Synchronous Motor
When an induction motor is connected to a three-phase supply, a rotating magnetic field is established; this induces currents in the rotor windings, thereby producing a torque. The rotor can never turn at synchronous speed since there must be a relative motion between the rotating magnetic field and the rotor in order that currents can be induced in the rotor circuit.
When the stator windings of a synchronous motor are excited with three-phase voltages, a rotating magnetic field is established as in the induction motor. However, in the synchronous motor, the rotor circuit is not excited by induction but by a source of direct current as in the AC generator. If the rotor is brought up to synchronous speed by some means, with the rotor poles excited, the poles of the rotor are attracted by the rotating magnetic field and the rotor continues to turn at synchronous speed. In other words, the rotor is locked into step magnetically with the rotating magnetic field. If for any reason the rotor is pulled out of step with the rotating stator flux, the attraction is lost and torque is no longer developed. Therefore, the synchronous motor develops torque only when running at synchronous speed. With the stator of a motor energized and the magnetic field revolving at synchronous speed, and with the rotor at a standstill, the rotor poles are first attracted in one direction and then the other, resulting in a net torque of zero. It is evident, then, the synchronous motor is not self-starting.
Effect of Load
In dc motors and induction motors, an addition of load causes the motor speed to decrease. The decrease in speed reduces the counter emf enough so that additional current is drawn from the source to carry the increased load at a reduced speed. In the synchronous motor, this action cannot take place, since the rotor is locked into step magnetically with the rotating magnetic field and must continue to rotate at synchronous speed for all loads.