Electrical Machines
The interpoles in dc machines have a tapering shape in order to

 reduce the saturation in the interpole
 increase the acceleration of commutation
 reduce the overall weight
 economise on the material required for interpoles and their windings

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Electrical Machines
In a DC machine the armature mmf is

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

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