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 interpoles produce additional flux to augment the developed torque
 the inequality of air-gap flux on the top and bottom halves of the armature is removed

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Electrical Machines
Flashing of DC generator means

  increasing flux density by providing extra ampere-turns in field.
None of these
  creation of residual magnetism by a DC source.
 neutralization of residual magnetism.

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