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

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

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Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at synchronous speed with respect to rotor.
  rotates at the speed of rotor in the air gap.
  is at standstill with respect to stator mmf.
  rotates at slip speed with respect to stator mmf.

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