Synchronous motors are not self-starting. Some additional devices should be used to start the motor. Basically, there are two methods for starting synchronous motors:
- Induction motor starting (Damper winding)
- Auxiliary motor starting.
Damper Winding Starting
Induction motor action will bring the motor to nearly synchronous speed. At synchronous speed, there is no relative motion between the poles of the air gap field and the pole face bars. No current is induced in the bars at synchronous speed, and no torque would be produced by them. However, the maximum speed developed on induction motor action is very close to the synchronous speed and the rotor falls into step when the dc field current is switched on.
- First, it protects the slip ring insulation from the high ac voltage induced in the field during starting.
- Second, the current circulating in the field winding would provide a small additional accelerating torque.
Auxiliary Motor Starting
When the speed of the synchronous motor almost equals the synchronous speed, it’s field winding is energized. The synchronous motor pulls into step and starts running at synchronous speed. However, in modern days this method of starting is hardly used. It is mainly the damper winding which is utilised for starting the synchronous motor.