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

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Electrical Machines
The synchronous motor can be started by

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

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