Electrical Machines
Under commutation results in

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

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Electrical Machines
For satisfactory parallel operation of two transformers which of the following conditions is most essential,

 voltage ratio of all transformers must be same.
 MVA rating of all transformer inversely proportional to impedance.
  polarities of all transformers must be same.
 percentage impedance of all transformers must be same.

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