Electrical Machines
The armature mmf of a DC machine has

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

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Electrical Machines
Commutation in a DC machine may be explained as a process

  in which magnetic field is set up.
 how alternate flux affects the main field flux.
 by virtue of which emf is induced in the armature conductors.
  by which electric current in a coil is reversed.

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