What is Moving Iron Instrument?

What is Moving Iron Instrument?

Moving Iron Instrument Definition

A moving iron instrument is a type of measuring device that uses the magnetic properties of iron to measure electrical quantities.

Types of Moving Iron Instruments

There are two main types, attraction and repulsion, which function based on the magnetic interactions of iron pieces with a magnetic field.

Construction of Moving Iron Instrument

The basic construction of attraction type moving iron instrument is illustrated bellow

A thin disc of soft iron is eccentrically pivoted in front of a coil. This iron tends to move inward that is from weaker magnetic field to stronger magnetic field when current flowing through the coil. Older attraction moving iron instruments used gravity control, but modern versions now use spring control. Adjusting the balance weight achieves null deflection of the pointer.

Damping in these instruments is achieved through air friction, typically using a moving piston within an air syringe as illustrated in the figure.

Theory of Attraction Type Moving Iron Instrument

Suppose when there is no current through the coil, the pointer is at zero, the angle made by the axis of the iron disc with the line perpendicular to the field is φ. Now due current I and corresponding magnetic field strength, the iron piece is deflected to an angle θ. Now component of H in the direction of defected iron disc axis is Hcos{90 – (θ + φ) or Hsin (θ + φ). Now force F acting on the disc inward to the coil is thus proportional to H2sin(θ + φ) hence the force is also proportional to I2sin(θ + φ) for constant permeability. If this force is acting on the disc at a distance l from the pivot, then deflection torque,

Since l is constant.

Where, k is constant.

Now, as the instrument is gravity controlled, controlling torque will be

Where, k’ is constant.

At steady state condition,

Where, K is constant.