Electrical Machines
To achieve spark less commutation brushes of DC generator are rocked ahead so as to bring them

 just ahead of magnetic neutral axis.
None of these
 in magnetic neutral axis.
 just behind the magnetic neutral axis.

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Electrical Machines
In a DC machine the armature mmf is

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

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