Electrical Machines
The rotor speed of an induction motor is given by

  Ns = 120f / p × ( 1 - s ).
None of these
  Nr = 120f / p × (1-s).
  Ns = 120f /p × ( 1 - r).

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Electrical Machines
A 3-φ alternator has negligible stator resistance. A short circuit test is conducted on this alternator. At a particular speed, a field current of If1 is required to drive the rated armature current. If the speed of the alternator is reduced to half, the field current required to maintain rated armature current

 Would be equal to If1.
  Cannot be predicted due to insufficient data.
 Woud be equal to 2If1.
 Would be equal to If1/2.

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