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

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

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 commutator action is not possible otherwise
 dc machines are comparatively low rating machines.
 it gives more uniform air-gap flux distribution
 it reduces field structure iron losses

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Electrical Machines
The coil spam

  may or may not be exactly equal to people pitch.
 none of above.
 must be exactly equal to people pitch.
 can never be equal to pole pitch.

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