Electrical Machines
In a DC machine the armature mmf is

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

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Electrical Machines
Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

 2, 3 and 4
 1, 2, and 3
 1, 2 and 4
 1 and 3

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