Electrical Machines
Commutation in a DC machine may be explained as a process

 how alternate flux affects the main field flux.
  in which magnetic field is set up.
  by which electric current in a coil is reversed.
 by virtue of which emf is induced in the armature conductors.

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

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