Electrical Engineering ⇒ Topic : Moving Iron Meter

MOVING IRON METER

It is classified into two types: (i) Attraction type and (ii) Repulsion type

Repulsion type is more commonly used. The basic principle is that when the current passes through a conductor coil, m.m.f. is set up which attracts the iron piece. This m.m.f. depends on the number of turns and current.

                                                                          figure (a)

         In attraction type, the iron piece inside the core of the coil is attracted. In repulsion type, there are two iron pieces inside the core - one is fixed and the other is movable as shown in Figure (a). They are magnetized with the same polarity. The repulsion of movable iron vane takes place. The spindle of pointer is attached with the movable vane. The force of attraction or repulsion is proportional to the square of the current being measured that is subjected to frequency and hysteresis effects. So, the pointer does not give the perfect square law. Since magnetization is independent of current, both ac and dc can be measured.Spring control and air friction damping are used. The torque of a moving instrument is:

                                                                                                                           .............. (1)

where T is torque in N-m, I is current to be measured in A, L is inductance in H, and θ is deflection angle in rad.

Currents from 0.1 A to 10 A and voltages from 50 V to 750 V can be measured by moving iron instruments.

Moving iron instruments are used as switchboard instruments.