Explanation of Errors in Moving Iron Instruments
1. Errors with both A.C and D.C work:
- Hysteresis error
- Stray magnetic field error.
- Temperature error.
- Friction error.
2. Errors with A.C work only:
- Frequency error.
- Error due to reactance of the instrument coil.
- Error due to eddy current.
- 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.
Advantage of moving iron Instruments:
1.They are Simple in Construction.
2.The Stationary parts of the instruments are also simple.
3.These instruments posses high operating torque
4.These instruments can withstand overload for some moments
5.The main advantage is that these can be used both on a.c supply as well as d.c supply. Due to the above said advantage, these instruments have become most widely used in a.c circuit.
Disadvantage of moving iron Instruments:
1. The scale of these instruments are not uniform.
2. Power Consumption is higher for low voltage.
3. The error are due to hysteresis in the Iron of the operating system and due to stray magnetic field.
4. Change in Frequency also causes error in a.c measurements.
5. There is a Difference between d.c and a.c calibration because of inductance effect of the instruments.
Application of Moving Iron Instruments
These are Simple in construction and can be clearly manufactured. These type of Instruments are Generally used as an indicating instruments. These are very accurate on a.c and d.c if properly designed.
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