Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at the speed of rotor in the air gap.
  is at standstill with respect to stator mmf.
  rotates at synchronous speed with respect to rotor.
  rotates at slip speed with respect to stator mmf.

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Electrical Machines
In DC machines, the interpoles are provided

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

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