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

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

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Electrical Machines
The armature mmf of a DC machine has

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

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