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

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

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Electrical Machines
Ward-Leonard system for controlling the speed of a DC motor is used where

 Very fine speed control over the whole range from zero to normal speed is required.
 The speed in both the direction is required.
 All of above.
 The accuracy in speed control is required.

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