Electrical Engineering ⇒ Topic : Sources of Errors in Moving Iron Instruments

Sources of Errors in Moving Iron Instruments

The errors which may occur in moving-iron instruments can be divided into two categories viz..

(1) errors with both d.c. and a.c. work (2) errors with a.c. work only.

1. Errors with both d.c. and a.c. work

(1) Errors due to hysteresis. Since the iron parts move in the magnetic field, hysteresis loss occurs in them. 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 *mumetal and by working it over a low range of flux densities.

(2) Error due to stray fields. Since the operating field is comparatively weak (say 7 x 10^-3 Wb/m²), such instruments are readily affected by stray fields. This may give rise to wrong readings. This error can be reduced by enclosing the movement in an iron case.

(3) Error due to temperature. Change of temperature affects the instrument resistance and stiffness of the control spring.

(4) Error due to friction. Due to the friction of moving parts, a slight error may be introduced.

This can be avoided by making torque-weight ratio high.

2. Errors with a.c. work only. With the change in frequency, the impedance of the instrument coil changes. This will cause a change of current in the coil. This is particularly important in case of voltmeters since these are connected in parallel with the circuit. The indicated voltage V is given by:

where I_m is the meter cun-ent, R_m and L_m are the resistance and inductance of the coil of the meter and R_s, is multiplier. For high frequencies, the meter gives low readings and vice-versa. This error can be eliminated by connecting a capacitor of suitable value in parallel with the series resistance R_s of the voltmeter. The value of capacitor is given by C = L_m/R_s²