Electrical Machines
Copper brushes in DC machine are used

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

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Electrical Machines
Cogging of induction motor occurs at

 low voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are not equal.
 high voltage and when stator and rotor poles are not equal.
 high voltage and when stator and rotor poles are equal.

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