Repulsion-type moving-iron instrument is shown in Fig. 9. Here there are two irons, one fixed (A) and the other mounted on a short arm fixed (B) to the instrument spindle. The two irons lie in the magnetic field due to a solenoid/coil C. When there is no current in the coil
Fig. 9. Repulsion-type moving-iron instrument.
the two iron pieces (moving one and fixed one) are almost touching each other and the pointer rests on zero position. When the current to be measured (or a definite fraction of it or proportional to the voltage to be measured) is passed through the solenoid, a magnetic field is set up inside the solenoid and the two iron pieces are magentised in the same direction. This sets up a repulsive force so moving iron piece, is repelled by fixed iron piece, thereby results in the motion of the moving iron piece, carrying the pointer. The pointer comes to rest in a deflected position when equilibrium is attained between the repulsive forces of the working elements and the controlling force.
Such instruments are commonly provided with spring control and air friction damping.
In commercial instruments, it is usual for the moving-iron B to be in the form of a thin curved plate and for the fixed iron A to be a tapered curved sheet. This construction can be arranged to give a longer and more uniform scale than is possible with the rods shown in Fig. 9.
Deflecting torque in moving-iron instruments :
In both the attraction and repulsion type moving-iron instruments it is found that for a given position of the moving system, the value of the deflecting torque is proportional to the square of the current, so long as the iron is working below saturation. Hence, if the current waveform is as shown in Fig. 10, the variation of the deflecting torque is represented by the dotted wave. If the supply
Fig. 10. Deflecting torque in a moving-iron instrument.
frequency is 50 Hz, the torque varies between zero and a maximum 100 times a second, so that the moving system (due to its inertia) takes up a position corresponding to the mean torque, where “mean torque oc mean value of the square of the current
k = a constant for a given instrument
I = r.m.s. value of the current.
Thus the moving-iron instrument can be used to measure both direct current and alternating current, and in the latter case the instrument gives the r.m.s. value of the current.
Owing to the deflecting torque being proportional to the square of the current, the scale divisions are not uniform, being cramped at the beginning and open at the upper end of the scale.
Note. For both types of instruments (attraction-type and repulsion type) the necessary magnetic field is produced by the ampere-turns of a current-carrying coil.
- In case the instrument is to be used as an ammeter, the coil has comparatively few turns of which win, so that the ammeter has low resistance because it is connected in series with the circuit.
- In case it is to be used as a voltmeter, the coil has high impedance so as to draw as small a current as possible since it is connected in parallel with the circuit. As current through the coil is small it has large number of turns in order to produce sufficient ampere-turns.