Electrical Machines
Armature reaction in a DC machine

  makes an angle of 90° with the main pole axis.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.
 is in direct opposition to the main poles.

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Electrical Machines
In DC machines, the interpoles are provided

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

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Electrical Machines
The introduction of interpoles in between the main poles improves the performance of a dc machine, because

 a counter-emf is induced in the coil undergoing commutation
 the interpoles produce additional flux to augment the developed torque
 the inequality of air-gap flux on the top and bottom halves of the armature is removed
 the flux waveform is improved with reduction in harmonics

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