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

 the flux waveform is improved with reduction in harmonics
 a counter-emf is induced in the coil undergoing commutation
 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
When the supply terminal of a DC shunt motor are interchanged

 The direction of rotation will reverse.
 The motor will run much faster in the same direction.
  The motor will run at its normal speed in the same direction as before.
 The motor will stop.

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