Electrical Machines
A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 p.u. the supply voltage has to be approximately brought down to

  0.5 p.u
  0.25 p.u
  0.125 p.u
  0.75 p.u

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

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