Electrical Machines
The 3 - Φ induction motors are, by a very considerable margin, the most widely used AC motors as

  they have very good speed regulation.
  these are cheaper in cost, simple and rugged in construction.
  they have maximum efficiency and maximum starting torque.
  they are capable of operation under range of leading and lagging pf.

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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 accuracy in speed control is required.
 All of above.
 The speed in both the direction is required.

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