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Electrical Machines

Electrical Machines
A commutator with a diameter of 50 cm rotating at 1000 rpm. For a brush width of 1.5 cm, find the time of commutation in ms.

  0.573.
 0.443.
 0.512.
 0.649.

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Electrical Machines
In case of IM the torque is

  inversely proportional to √slip.
  directly proportional to slip2.
  inversely proportional to slip.
  directly proportional to slip.

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Electrical Machines
A load of 900 lbs is to be raised with a lift car weighting 855 lbs at a speed of 110 feet per minute, the over balance being 40 % and percentage efficiency 45 %. Find the horse power of the motor without counterweight

 170.
 160.
 180.
  150.

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Electrical Machines
In DC generators, the cause of rapid brush wear may be

 severe sparking.
 any of above.
 imperfect contact.
 rough commutator surface.

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Electrical Machines
If Ke is eddy current constant, Kf is form factor, f is supply frequency, Bm maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by

 KefKf2Bm2.
 Kef2KfBm2.
 Kef2Kf2Bm.
  Kef2Kf2Bm2.

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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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