Question.

What are commutating (inter) poles? State purpose and location in DC generators and motors.

Answer.

They are small auxiliary magnetic poles placed between the regular main poles.

Usually there are the same number as there are main poles, but in small units there are sometimes half as many.

They are used to assist commutation and prevent sparking at the brushes.

They are always connected in series with the armature. In a generator the polarity of the interpole is the same as the main pole following it.

In a motor the polarity of the interpole is the same as the main pole behind it.

Question

Why is squirrel-cage windings used on main propulsion motors?

Answer.

Squirrel-cage windings are used on main propulsion motors of the 3-phase, synchronous type as a means of starting the motor and bringing its speed near the speed of the synchronism.

The torque of a 3-phase synchronous motor is obtained from the attraction of the electromagnetic poles of the rotor and the corresponding poles of the stator which rotate magnetically.

This rotating magnetic field is established in a 3-phase circuit because each phase becomes positive in succession and, also, negative in succession and the coils energized by the three phases have their polarities reversed successively.

Thus, with the stator coils arranged about the circular frame of a motor, the magnetic North flux produced by the coils moves successively from coil to coil around the frame and, correspondingly, the South flux tracks around with the North. The result is a rotating or revolving magnetic field.