#6 Moving Iron Instrument

                                       MOVING IRON 

• Moving-iron instruments are generally used to measure alternating voltages and currents. 

• In moving-iron instruments the movable system consists of one or more pieces of soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.

Components 

• Moving element: a small piece of soft iron in the form of a vane or rod. 

• Coil: to produce the magnetic field due to current flowing through it and also to magnetize the iron pieces. 

• In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity. 

• Control torque is provided by spring or weight (gravity). 

• Damping torque: the damping device consisting of an air chamber and a moving vane attached to the instrument spindle. 

• Deflecting torque produces a movement on an aluminum pointer over a graduated scale.

Types

• Attraction (or single-iron) type • Repulsion (or double iron) type

Attraction type

Working Principle

• The basic working principle of these instruments is very simple that a soft iron piece if brought near the magnet and gets attracted by the magnet. 

• Construction: It consists of a fixed coil C and moving iron piece D. 

• The coil is flat and has a narrow slot like opening. The moving iron is a flat disc which eccentrically mounted on the spindle.

• The spindle is supported between the jewel bearings. The spindle caries a pointer which moves over    a graduated scale. 

• The controlling torque is provided by the springs. 

• The damping torque is provided by the air friction. 

• A light aluminium piston is attached to the moving system. It moves in a fixed chamber. The chamber is closed at one end.

Repulsion Type

• These instruments have two vanes inside the coil, the one is fixed and other is movable. 

• When the current flows in the coil, both the vanes are magnetised with like polarities induced on the same side. 

• Hence due to repulsion of like polarities, there is a force of repulsion between the two vanes causing the movement of the moving van. 

• Types: 

i) Radial vane type and 

ii) Co-axial vane type

Repulsion (or double iron) type

Radial Vane                      Co-axial Vane

Radial vane type:The two vanes are radial strips of iron. 

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• The fixed vane is attached to the coil. 

• The movable vane is attached to the spindle and suspended in the induction field of the coil. 

• The needle of the instrument is attached to this vane.

Co-axial vane type: In this type of instruments the fixed and moving vanes are sections of co-axial cylinders.

Advantages

• The instruments are suitable for use in AC and DC circuits. 

• The instruments are robust, owing to the simple construction of the moving parts.

• The stationary parts of the instruments are also simple. 

• Instrument is low cost compared to moving coil instrument.

• Torque/weight ratio is high, thus less frictional error.

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