Electrical Machines
Under commutation results in

  sparking at the middle of the brush.
  no sparking.
  sparking the trailing edge of the brush.
  sparking at the leading edge of the brush.

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Electrical Machines
For a given torque, reducing diverter resistance of a DC series motor causes

 decrease in speed but armature current remains the same.
 increase in speed but armature currnet remains the same.
 decrease in speed demanding less armature current.
 increase in speed demanding more armature current.

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