Electrical Machines
The rotor of a 3 - Φ wound induction motor is provided with

  heavy short-circuited end rings.
  single phase winding.
  heavy copper or aluminium bars placed in rotor slots.
  three phase winding.

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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)

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

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