Electrical Machines
Variation in speed by ward-Leonard control of DC motors occurs due to variation in

  Armature voltage.
  Armature current.
  Field excitaion.
  Supply voltage.

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

 the interpoles produce additional flux to augment the developed torque
 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

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Electrical Machines
Flashing the field of DC generator means

 increasing flux density by adding extra turns of winding.
  neutralising residual magnetism.
  making the magnetic losses forces parallel.
  creating residual magnetism by a DC source.

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