Short note: Starting synchronous motor

A synchronous motor produces torque only at synchronous speed, so it has no starting torque of its own. It must be brought near synchronous speed by an auxiliary means, and then the d.c. field is applied to pull it into step. The main methods are:

1. Damper (induction-motor) winding. Short-circuited bars are embedded in the pole faces and joined by end rings, like a squirrel cage. Reduced three-phase voltage is applied to the stator (autotransformer or star-delta starter) while the field winding is closed through a discharge resistance. The motor starts as an induction motor and reaches about 95 % of N_s. The d.c. field is then switched on and the motor pulls into synchronism. This is the most common method.
2. Using a small auxiliary (pony) motor. A small induction or d.c. motor is coupled to the shaft and brings the machine to synchronous speed. The synchronous machine is then synchronised to the supply like an alternator and the pony motor is disconnected. It is used where starting must be done off load.
3. Using a variable-frequency supply. The stator is fed through an inverter at low frequency, which is raised slowly, so the rotor field remains locked to the rotating field (starting from rest).
4. Using the d.c. exciter as a motor. The exciter, if fed from a d.c. source, can run the set up to speed.

 3-ph supply -> [autotransformer] -> stator
 Field: closed through discharge resistance while starting;
        switched to d.c. exciter at ~95% of N_s