Whenever a three phase supply is given to the stator, a rotating magnetic field is produces which is rotating at very high speed depending up on the no. of poles and frequency of supply. Due to change in flux w.r.t time an emf is induced in the rotor and torque in developed in anticlockwise direction (direction is taken acc. to fleming right hand rule)
Why Synchronous Motor is not a self starting?????
Now stator fields are rotating very fast i.e. at a speed Ns r.p.m. Due to inertia, before rotor hardly rotates in the direction of anticlockwise torque, to which it is subjected, the stator poles change their positions. Consider an instant half a period latter where stator poles are exactly reversed but due to inertia rotor is unable to rotate from its initial position. This is shown in the Fig.
At this instant, due to the unlike poles trying to attract each other, the rotor will be subjected to a torque in clockwise direction. This will tend to rotate rotor in the direction of rotating magnetic field.
But before this happen, stator poles again change their position reversing the direction of the torque exerted on the rotor.
As a result, the average torque exerted on the rotor is zero. And hence the synchronous motor is not self starting. So under any case, whatever may be the starting position of the rotor, synchronous motor is not self starting.