Electrical Machines
Copper brushes in DC machine are used

 where low voltage and high currents are involved.
 in both of above cases.
 where high voltage and low current.
None of these

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

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

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Electrical Machines
In a DC generator, the armature reaction results in

 magnetization of leading and demagnetization of trailing pole tip.
  magnetization of inter poles.
  demagnetization of leading & magnetization of trailing pole tip.
 demagnetization of centers of poles.

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