Electrical Machines
What is the necessity of copper brushes in a dc motors?

  None of these.
  To absorb the mechanical shocks produced during running condition.
  To increase the flux cutting by the rotor.
  To convert alternating supply current to direct current.

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Electrical Machines
Flashing the field of DC generator means

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

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