Question

Why is a synchronous motor not self stating

What are the various ways in which it can be started.

Rotors in synchronous motors are supplied directly with current from an outside source, unlike squirrel-cage and wound type rotors in which current is induced by the stator magnetic field.

Synchronous motors are constructed in the same way as alternators, with three-phase stator windings and rotors with salient poles.

Like alternators, they require a d.c. supply to the rotor poles from an exciter via slip-rings.

They are not self-starting.

Connection of a three-phase input to the stator will produce a rotating magnetic field, but the effect on the poles is of equal attraction and repulsion and rotor inertia will prevent movement in either direction.

Only if the rotor is brought up to synchronous speed like an alternator can the rotor poles and rotating magnetic field lock together.

The machine runs then at synchronous speed only.

There is no slip as with induction motors.

A pony motor can be coupled to run a synchronous motor up to speed, then with the excitation switched on the synchronous motor is synchronised and the driving induction (pony) motor shut down.

Another method of running up a synchronous motor employs solid copper bars permanently embedded in the rotor pole tips and short-circuited by rings to make it a temporary induction motor.

The synchronous machine is started direct on-line or by one of the low-current starting methods (e.g. auto-transformer start) as an induction motor.

At maximum speed the d.c. excitation is switched on for synchronisation.