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A compensated wattmeter carries an extra winding on the current coil former whose purpose is to

  • A.cancel the effect of the pressure coil current
  • B.increase the deflecting torque so that a smaller pressure coil current may be used
  • C.provide damping for the moving system without a permanent magnet being required
  • D.extend the range of the instrument by a factor of ten when it is switched in

Answer: A

The compensating winding has the same number of turns as the current coil and carries the pressure coil current in the opposite sense, so the flux due to that current is annulled. The instrument then reads the load power alone, whichever way it is connected.

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