Moving Iron Instrument

There are indicating instrument which are used to measure current & voltage. In moving iron instrument the movable system consists of one or more pieces of specially- shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.

Types of Moving Iron Instrument

1. Attraction type moving iron instrument
2. Repulsion type moving iron instrument

1. Attraction Type Moving Iron Instrument :- It work on the principle that when a soft iron piece is placed in a magnetic field it appearance a force of attraction. The magnetic field is produced by the current following through the current coil of the instrument. The movement of soft iron piece due to magnetic force of attraction causes movement of moving system. A pointer is attached to the moving system moves in calibrated scale to indicate the quantity current & voltage being measured.

Deflecting torque is obtained by magnetic effect.

Controlling torque is provided by spring control method or gravity controlled method.

Damping torque is obtained by using air friction damping.

2. Repulsion Type Moving Iron Instrument :- The instrument work on the principle of repulsion of two magnetic iron pieces when two magnets are placed such that their similar pole face each other, a face of repulsion acts on them.

It has a fixed coil wound on a hollow cylindrical former. Supply current passed through this coil. Two soft iron pieces are placed along the axis of coil. Parallel to each other, one iron piece is fixed, while other one iron piece is fixed, while other one is provided on the movable spindle.

Deflecting torque is provided by the force of repulsion between two similarly magnetized iron pieces.

Controlling torque is provided by use of spring control or gravity control method.

Damping torque is provided by air friction damping.

Advantages of Moving Iron Instrument

1. They are reliable.
2. Simple in construction.
3. Cheap in cost.
4. Have high operating torque.
5. Can be used for DC as well as AC measurement.

Disadvantages of Moving Iron Instrument

1. The scale of these instrument are not uniform.
2. Power construction is higher for low voltage range.
3. Change in frequency also caused error in AC measurement.

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