Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

 to avoid over heating.
 to achieve a stronger magnetic field.
All of these
 to avoid locking of the armature.

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Electrical Machines
In a DC machine the armature mmf is

  stationary with respect to armature.
  stationary with respect to field poles.
  rotating with respect to field poles as well as armature.
 rotating with respect to field poles.

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

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

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