Synchronous motors used widely in the industry. Synchronous motors provide constant speed. The synchronous motor consists of a wound rotor and a stator. The stator winding is energized from the power supply. This sets up the rotating magnetic field. The rotor gets magnetized when the field winding is energized. During operation, the rotor is in synchronism with the rotating magnetic field of the stator. Hence, the name, synchronous machine.
The synchronous machine, however, is not self-starting. The synchronous machine has to be rotated to near the synchronous speed of the stator before it can "catch" the stator field and begin rotating on its own.
There are many different methods employed for Starting Synchronous Motors.
Pony Motor
The pony motor is an induction which drives the rotor of the synchronous motor. Once the speed reaches the synchronous speed, the field winding is switched on. The pony motor is then decoupled and the synchronous motor runs on its own.
Damper windings
Damper windings or amortisseur windings are special windings which are fixed on the salient pole of the rotor of the synchronous motor. These windings work in a similar manner to the squirrel cage winding in induction motor. Thus the synchronous motor starts as an induction motor. The rotor runs at a speed slightly lower than the synchronous speed. When the speed comes close to the synchronous speed, the field winding is switched on and the rotor gets locked to the stator magnetic field and the machine runs as a synchronous motor.
Starting using Variable Frequency
Synchronous motors which are electronically controlled can be started by supplying a reduced frequency to the stator winding. This generates a slowly rotating magnetic field in the stator. The rotor of the synchronous machine is able to follow this magnetic field. Once the rotor starts to rotate, the frequency is gradually raised to the power frequency. The synchronous motor can now run at the normal frequency.