Exam Question

(a) A Synchronous motor is not self-starting because, at the start of the motor, the average torque on the rotor is zero. This is because when a DC supply is applied to the stationary rotor, the unlike poles try to attract each other, causing the rotor to be subjected to an instantaneous torque in one direction. However, the rotor's inertia prevents it from rotating, and as the stator poles continue to rotate, the direction of the torque on the rotor changes. This cycle continues, resulting in an average torque on the rotor of zero, so an external force is required to bring the motor up to the synchronous speed.
Ways to start a synchronous motor:
Pony Motor  
A smaller auxiliary motor (the "pony motor"), either AC or DC, is mechanically coupled to the synchronous motor. The pony motor accelerates the synchronous motor to a speed slightly above synchronous speed. Once this speed is reached, the pony motor is disconnected, and the synchronous motor's field is energized, allowing it to lock into synchronism with the AC supply.