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

 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.
 The speed in both the direction is required.

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Electrical Machines
Armature reaction in a DC machine

  makes an angle of 90° with the main pole axis.
 is in direct opposition to the main poles.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.

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