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 flux waveform is improved with reduction in harmonics
 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

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Electrical Machines
Armature reaction in a DC machine

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

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