Electrical Indicating and Test Instruments 1 Introduction 161

Moving Iron Meter

As well as measuring d.c. signals, the moving iron meter can also measure a.c. signals at frequencies up to 125 Hz. It is the least expensive form of meter available and, consequently, this type of meter is also used commonly for measuring voltage signals. The signal to be measured is applied to a stationary coil, and the associated field produced is often amplified by the presence of an iron structure associated with the fixed coil. The moving element

in the instrument consists of an iron vane suspended within the field of the fixed coil. When the fixed coil is excited, the iron vane turns in a direction that increases the flux through it.
The majority of moving-iron instruments are either of the attraction type or of the repulsion type. A few instruments belong to a third combination type. The attraction type, where the iron vane is drawn into the field of the coil as the current is increased, is shown schematically in Figure 7.4a. The alternative repulsion type is sketched in Figure 7.4b. For an excitation current, I, the torque produced that causes the vane to turn is given by

where M is the mutual inductance and is the angular deflection. Rotation is opposed by a spring that produces a backwards torque given by




Figure 7.4. Mechanisms of moving iron meters: (a) attraction type and (b) repulsion type.

At equilibrium, , and is therefore given by

The instrument thus has a square-law response where the deflection is proportional to the square of the signal being measured, that is, the output reading is a root-mean-squared (r.m.s.) quantity.
The instrument can typically measure voltages in the range of 0 to 30 volts. However, it can be modified to measure higher voltages by placing a resistance in series with it, as in the case of moving coil meters. A series resistance is particularly beneficial in a.c. signal measurements because it compensates for the effect of coil inductance by reducing the total resistance/ inductance ratio, and hence measurement accuracy is improved. A switchable series resistance is often provided within the casing of the instrument to facilitate range extension. However, when the voltage measured exceeds about 300 volts, it becomes impractical to use a series resistance within the case of the instrument because of heat-dissipation problems, and an external resistance is used instead.