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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Electrical Machines
In a DC machine the armature mmf is

  stationary with respect to field poles.
  stationary with respect to armature.
  rotating with respect to field poles as well as armature.
 rotating with respect to field poles.

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