Electrical Machines
For satisfactory parallel operation of two transformers which of the following conditions is most essential,

 percentage impedance of all transformers must be same.
 voltage ratio of all transformers must be same.
 MVA rating of all transformer inversely proportional to impedance.
  polarities of all transformers must be same.

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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 flux waveform is improved with reduction in harmonics
 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 the field of DC generator means

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

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