Electrical Machines
For standing a DC motor a starter is required because

None of these
  It starts the motor.
  it limits the starting current to a safe value.
  it limits the speed of the motor.

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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 changes to 1.5 times the original value
 changes to three times the original value
 remain the same as the original
 changes to 0.5 times the original value

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Electrical Machines
In a DC generator, the armature reaction results in

 demagnetization of centers of poles.
  magnetization of inter poles.
  demagnetization of leading & magnetization of trailing pole tip.
 magnetization of leading and demagnetization of trailing pole tip.

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