Electrical Machines
The air gap between stator and the rotor of the 3 - φ induction motor is kept

  air gap does not matter.
  depends on the material of stator and the rotor.
  as small as possible.
  as wide as possible.

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

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