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

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

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Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at slip speed with respect to stator mmf.
  is at standstill with respect to stator mmf.
  rotates at synchronous speed with respect to rotor.
  rotates at the speed of rotor in the air gap.

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