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

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

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Electrical Machines
An alternator is feeding an infinite busbar. Its prime mover is suddenly shutdown. The alternator will

 continue to work as synchronous motor with same direction of rotation.
 continue to work as alternator but the direction of rotation will reverse.
 come to stand still.
 start working as induction motor.

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Electrical Machines
The synchronous motor can be started by

  any of the above methods.
  providing damper winding in its rotor circuit.
  coupling it with a DC compound motor.
  coupling it with a small AC induction motor called the pony motor.

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