Electrical Machines
In a DC machine the armature mmf is

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

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

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

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