Electrical Machines
In a DC machine

 current and emf in armature are unidirectional
 current and emf at the terminals are alternating.
 emf in armature conductors and at the terminals is alternating.
 current and emf in armature are alternating.

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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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