Page 122 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

When the main winding is supplied from single phase AC supply it produces a field flux which induces EMF in the
shading ring. This EMF causes current to flow in the short circuited ring which produces flux. According Lenz’s law
this flux opposes the main produced by the main winding. Due to the opposition of flux maximum value of the flux
in the shading part of the pole lags behind the flux in the unshaded part of the pole. This shifting of flux produces
the same effect as the rotating magnetic field in a three phase motor. The direction of rotation of this motor cannot
be changed. Shaded pole have low starting torque and hence are used in small table fan, ceiling fan etc.

Commutator moto
Universal motor
The motor which operates on both AC and DC supply is called universal motor. If through a DC series motor
alternating current is passed it will develop a torque which is always unidirectional because the current in both
armature and field winding changes simultaneously. The field winding is connected in series with armature
winding. It has high torque and high speed. The speed is above the synchronous speed.

Repulsion motor
The principle of operation of repulsion motor is that like poles of two magnets repel and unlike poles attract each
other. It has two or four stator poles having stator windings would in the stator slots. A commutator is mounted on
the rotor shaft. Two brushes are placed in such a way that they make sliding contacts with the commutator. The
brushes are advanced by 20 degrees in the rotational direction, so as to meet the requirements of ac supply. The
rotor has no connection with the stator and it works as the secondary winding of a transformer while the stator
works as primary. When ac is applied to the stator then an emf is induced in the rotor winding due to mutual
induction. The brushes are short circuited to start the flow of induced current in the rotor. The induced current will
energise the rotor poles. The polarities of the rotor and stator poles will be the same and thus they will repel each
other and will set the rotor under rotation.

Types of repulsion motor
Repulsion start motor
1 Repulsion start –run motor
2 Repulsion induction motor

109

CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75