Electrical Machines
In DC generators brushes are used for

 collecting of voltage
 convert ac armature current in to dc
 collecting of current without any sparkings
 reduce eddy current loss

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Electrical Machines
Cogging of induction motor occurs at

 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.
 low voltage and when stator and rotor poles are equal.

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