Electrical Machines
Sparkless commutation can be achieved by employing

  high resistance carbon brushes.
  any one or more of the above methods.
 compensating winding.
  inter poles.

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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 flux waveform is improved with reduction in harmonics
 a counter-emf is induced in the coil undergoing commutation
 the interpoles produce additional flux to augment the developed torque

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