Electrical Machines
DC generator will not build up voltage

 any one of above.
 if field resistance is more than critical resistance.
 due to wrong direction of rotation.
  if it has lost residual magnetism.

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

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

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