Explanation of Errors in Moving Iron Instruments

Errors in Moving Iron Instruments

There are two types of error which occur in moving iron instruments. They are

1.       Errors with both A.C and D.C work:

              (a)     Hysteresis error

              (b)     Stray magnetic field error.

              (c)     Temperature error.

              (d)     Friction error.

2.       Errors with A.C work only:

              (a)     Frequency error. 

              (b)     Error due to reactance of the instrument coil. 

              (c)     Error due to eddy current.

              (d)     Error due to waveform.

      (A)    Hysteresis error is serious type of error in MI instruments when used on D.C circuits. The effect of this error is that the readings are higher when current increases than when it decreases.

This error can be reduced by employing vanes of mumtal (i.e. mumtal is magnetic material having low hysteresis loss and high permeability) and by working it over a low range of flux densities.

      (B)      Stray magnetic field error may be serious because of the weakness of the operating magnetic field. This error can be reduced by enclosing the movement in an iron case (or) shielding the instrument with the help of steel case.

      (C)     Temperature error is due to change of temperature that affects the instrument resistance and stiffness of the control spring.

      (D)    Friction error is due to the friction of moving parts this can be avoided by making torque-weight ratio high.

      (E)     Frequency Error change in frequency may be cause errors due to change of reactance of the operating coil and also to the changes in magnitude of eddy current set up in the metal parts of the instrument near the operating coil.

      (F)     Error due to reactance of the instrument coil the change in instrument coil reactance owing to change in frequency causes serious error in case of voltmeters.

    (G)    Eddy current error affects both ammeters and voltmeters equally. The frequency error may be compensated for by connecting a suitable capacitor C in the parallel with swamping resistance R (C = L/R², if frequency is not too high).

     (H)    MI instruments may be seriously affected by waveform both on account of the change in the form of flux waveform and in case of voltmeter, the effect of harmonic upon the inductance.