Electrical Machines
In dc machine armature windings are placed on rotor because of the necessity for

 development of torque.
 electromechanical energy conversion.
 generation of voltage.
 commutation.

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Electrical Machines
Flashing the field of DC generator means

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

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