Electrical Machines
DC generator will not build up voltage

 due to wrong direction of rotation.
  if it has lost residual magnetism.
 any one of above.
 if field resistance is more than critical resistance.

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Electrical Machines
In a loaded DC generator, if the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will

 deteriorate with rise in terminal voltage.
 improve with rise in terminal voltage.
 deteriorate with fall of terminal voltage.
 improve with fall of terminal voltage.

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