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 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
 the interpoles produce additional flux to augment the developed torque

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Electrical Machines
The speed of a DC series motor is

 Proportional to field current.
 Inversely proportional to the armature current.
 Proportional to the square of the armature current.
 Proportional to the armature current.

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