Different Starting Methods of Three Phase Synchronous Motor:
As synchronous motor is not self starting, different methods of starting are as follows:
1) By using an Induction (Pony) motor
2) By using a DC Machine / Source
3) By using Damper windings
1) By using an Induction (Pony) motor: A small induction motor (pony motor) is directly coupled with the synchronous motor. The number of poles of the induction motor should be less than the synchronous motor. First supply is given to the pony motor, when it rotates the rotor of the synchronous motor near to the synchronous speed, the main switch and DC switch of synchronous motor are closed. The rotor poles of synchronous motor are pulled into synchronism. After that, supply to the pony motor is disconnected and it can be de-coupled from the synchronous motor shaft.
2) By using a DC Machine / Source: A DC machine is coupled to the synchronous motor. The DC machine works like a DC motor initially and brings the synchronous motor near to synchronous speed. The main switch and DC switch of synchronous motor are closed. The rotor poles of synchronous motor are pulled into synchronism. Once it is achieved, the DC machine can be operated like a DC generator and DC power generated can be supplied to the rotor of the synchronous motor.
3) By using Damper Windings: In this method, the motor is first started as an induction motor and then starts running as a synchronous motor after achieving synchronous speed. For this, damper windings are used. Damper windings are additional windings consisting of copper bars placed in the slots in the pole faces. The ends of the copper bars are short-circuited. These windings behave as the rotor of an induction motor. When 3 phase power is supplied to the motor, the motor starts running as an induction motor at a speed below synchronous speed. After some time DC supply is given to the field winding. The rotor gets pulled into synchronism and starts running at constant speed as a synchronous motor.