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To take care of change in frequency of the A.C current, while using a moving iron type instrument

  • To take care of change in frequency of the A.C current, while using a moving iron type instrument a condenser of suitable value is used in parallel with the swamp resistance.
  • A moving iron type instrument is used to measure AC voltage or current, and it works on the principle of the magnetic deflection of a movable iron vane.
  • The deflection of the vane is proportional to the RMS value of the applied AC voltage or current.
  • However, the inductance of the instrument changes with the frequency of the AC current.
  • At higher frequencies, the inductance of the instrument increases, which can cause the instrument to under-read the true value of the AC voltage or current.
  • To compensate for this, a shunt capacitor is connected in parallel with the instrument, which provides a low impedance path for high-frequency signals, bypassing the inductance of the instrument and allowing the vane to respond accurately to changes in the AC signal.
  • Therefore "To compensate for the effect of inductance at high frequencies, a capacitor of suitable value is connected in parallel with a moving iron type instrument used to measure AC voltage or current."