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

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

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Electrical Machines
For an ideal transformer the winding should have

 maximum resistance on primary side and least resistance on secondary side
 no ohmic resistance on either side
 minimum resistance on primary side and maximum resistance on secondary side
 equal resistance on primary and secondary side

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