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

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

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Electrical Machines
Armature reaction in a DC machine

 is in the same direction as the main poles.
 makes an angle with the main pole axis which is load dependent.
  makes an angle of 90° with the main pole axis.
 is in direct opposition to the main poles.

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