Electrical Machines
In DC machines, the interpoles are provided

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

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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 changes to 1.5 times the original value
 changes to 0.5 times the original value
 remain the same as the original
 changes to three times the original value

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