Advantage and Disadvantage of Moving Iron Instrument

The main advantage of Moving Iron Instrument is that it can measure AC as well as DC quantities. Though this instrument is responsive to DC, it suffers from hysteresis error which causes an appreciable error in the measurement of DC. Therefore, it is rarely used for DC applications. This is one of the main disadvantages of Moving Iron Instrument.  

Advantage of Moving iron Instrument

  • It is a universal instrument which can be used for the measurement of AC and DC quantities.
  • These types of instruments have high value of torque to weight ratio. Due to this error because of friction is quite low.
  • It is very cheap due to simple construction.
  • These instruments are quite robust due to its simple construction. Above all, there is no moving part in the instrument which carries current.
  • Accuracy of any instrument depends on its design and workmanship. These instruments can be designed to provide precision and industrial grade accuracy. A well designed moving iron instruments have a error of less than 2 % or less for DC. For AC, the accuracy of the instrument may be of the order of 0.2 to 0.3 % at 50 Hz.
  • These instruments can withstand large loads and are not damaged even under sever overload conditions.

Disadvantage of Moving Iron Instrument

  • These instruments suffer from error due to hysteresis, frequency change and stray losses.
  • The scale of moving iron instrument is not uniform like PMMC instrument. Its scale is non-uniform and cramped at lower end. This is the reason; accurate readings are not possible at lower range.
  • The calibration of these instruments should be done for both AC and DC. In fact, its calibration must be carried out at the frequency for which it is used in AC circuit. If it is used at 50 Hz, calibration must also be done at the same frequency i.e. 50 Hz.
  • Moving Iron Instruments are suitable for low frequency application. This is because at lower frequency, the eddy current error increases with square of frequency whereas at higher frequency this error is almost constant. Therefore moving iron instruments are not suitable for frequency above 125 Hz.
  • The reading of the instrument is affected by temperature variation. Increase in temperature decreases the spring stiffness and increases the resistance of coil.

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