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

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

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Electrical Machines
With the increase in speed of DC motor

 Back emf falls and line current increase.
 Both back emf as well as line current fall.
  Back emf increase but line current falls.
 Both back emf as well as line current increase.

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