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
The armature mmf of a DC machine has

  trapezoidal space distribution and is stationary in space.
 stepped distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.

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