Question

why synchronous machine runs at constant speed

Answer 1

In the synchronous motor, rotor too have a DC excitation, so it produces a constant flux forming the electromagnetic poles on rotor which initially are stationary.

The stator is excited from a 3 phase supply which produces a rotating magnetic Flux in the air gap forming magnetic poles on stator. These are rotating at synchronous speed {Ns=120f/P} decided by the frequency and no. Of poles for which stator is wound.

As these stator poles and rotor poles (opposite) comes near each other, ( this is achieved by rotating the rotor near to synchronous speed or by simply using Damper winding, making it self starting) they attracts and magnetic locking happens. So rotor rotates at the speed decided by frequency and no. Of stator poles.

Even on loading, the speed is always constant but the torque angle (between stator and rotor magnetic poles) may vary. If load increased beyond certain limit, the rotor may fall out of synchronism and motor to haul.

Pardon me for grammatical mistakes.