Electrical Machines
Copper brushes in DC machine are used

 where high voltage and low current.
 where low voltage and high currents are involved.
 in both of above cases.
None of these

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Electrical Machines
The synchronous motor can be started by

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

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