Electrical Machines
The armature mmf of a DC machine has

  triangular space distribution and is stationary in space.
 stepped distribution and rotates at the speed of armature.
  triangular space distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.

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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.
 All of above.
 Very fine speed control over the whole range from zero to normal speed is required.

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