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

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

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Electrical Machines
A DC shunt generator builds up to a voltage of 220 V at no load while running at its rated speed. If the speed of the generator is raised by 25% kepping the circuit conditions unaltered, then the Voltage to which the machine would build up will

 increase to 1.25 times 220 V
 increase to value lying between 220 V and 1.25 times 220 V
 increase to a value greater than 1.25 times 220 V
 not change and remain at 220 V

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