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.
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