Electrical Machines
In DC machines, the interpoles are provided

 to counteract the reactance voltage and the cross-magnetizi
  to make commutation spark less.
  both (B) and (C).
 to counteract demagnetizing effect of armature mmf in commutating zone.

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Electrical Machines
Cogging of induction motor occurs at

 low voltage and when stator and rotor poles are equal.
 high voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are not equal.
 high voltage and when stator and rotor poles are not equal.

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