rotated by some external means at a speed almost equal to synchronous
speed. And then the rotor is excited to produce its poles. At a certain
instant now, the stator and rotor unlike poles will face each other such
that their magnetic axes are near each other. Then the force of
attraction between the two, pulls both of them into the magnetic locking
Once magnetic locking is established, the rotor and stator poles
continue to occupy the same relative positions. Due to this, rotor
continuously experiences a unidirectional torque in the direction of the
rotating magnetic field. Hence rotor rotates at synchronous speed and
said to be in synchronism with rotating magnetic field. The external
device used to rotate rotor near synchronous speed can be removed once
synchronism is established. The rotor then continues its rotation at Ns due
to magnetic locking. This is the reason why synchronous motor runs only
at synchronous speed and does not rotate at any speed other than the
synchronous. This operation is shown in the Fig 1(a) and (b).
|Fig. 1 Unidirectional torque experienced by rotor|
It is necessary to keep field winding i.e. rotor excited from d.c.
supply to maintain the magnetic locking, as long as motor is operating.
Then drive the rotor by some external means like diesel engine in the
direction of rotating magnetic field, at a speed very near or equal to
Switch on the d.c. supply given to the rotor which will produce rotor
poles. now there are twp fields one is rotating magnetic field produced
by stator while the other is produced by rotor which is physically
rotated almost at the same speed as that of rotating magnetic field.
At a particular instant, both the fields get magnetically locked. The
stator field pulls rotor field into synchronism. Then the external
device used to rotate rotor can be removed. But rotor will continue to
rotate at the same speed as that of rotating magnetic field i.e. Ns due to magnetic locking.
Key Point :
So the essence of the discussion is that to start the synchronous
motor, it needs some device to rotate the rotor at a speed very near or
equal to the synchronous speed.