Electrical Machines
In a DC generator sparking between brushes and commutator surface may be due to

  under commutation.
  any of the above.
  too rapid reversal of current.
 over commutation.

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Electrical Machines
In synchronous generator, inverted V curve is drawn between

 field current on Y-axis and armature current on X-axis
 field current on X-axis and power factor on Y-axis
 field current on X-axis and armature current on Y-axis
 field current on Y-axis and power factor on X-axis

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