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
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