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 interpoles produce additional flux to augment the developed torque
 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

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Electrical Machines
Ward-Leonard system for controlling the speed of a DC motor is used where

 Very fine speed control over the whole range from zero to normal speed is required.
 The accuracy in speed control is required.
 The speed in both the direction is required.
 All of above.

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