Electrical Machines
A 240 V dc series motor takes 40 A when giving its rated output at 1500 rpm. Its resistance is 0.3 Ω. The value of resistance which must be added to obtain rated torque at 1000 rpm is

 2.2 Ω
 1.9 Ω
 5.7 Ω
 6 Ω

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Electrical Machines
Interpoles in a DC motor must be

 Shunt excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.
 Series excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.
 Shunt excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.
 Series excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.

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Electrical Machines
The compensating winding in a DC machine

 is located on commutating poles for improving commutation.
  is located on pole shoes for avoiding the flashover at the commutator.
 is located in armature slots for compensation of the armature reaction.
  is located on pole shoes to avoid the sparking at the brushes.

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