Moving Iron Instruments
Moving iron instruments are the most common instruments used for measurements of electrical quantities. Ammeters, voltmeters are some of the examples of MI instruments. These instruments can measure current and voltage to a great accuracy. Still MI instruments are cheap as compared to any other type of a.c. instrument of the same accuracy.
There is a moving element made up of soft iron or high permeability steel. This moving element is situated such that it can move in the magnetic field produced by a stationary coil. The stationary coil is excited by the current or voltage being measured. The stationary coil becomes an electromagnet after it has been excited. The electromagnetic effect produced in the stationary coil causes movement in the moving vane. This moving vane is connected to a pointer which moves on a scale drawn. The opposing force for the vane is given by electromagnetic effect itself.
Classification of moving iron instrument
1. Attraction type
Construction of the attraction type moving iron instrument is as shown in the given figure. The moving iron, i.e. the disc of soft iron, is eccentrically mounted. Coil is situated around the disc. When the coil is excited it produces magnetic field. Due to magnetic field the moving iron moves from the weaker field outside the coil to the stronger field inside the coil. Thus moving iron gets attracted inwards and thus the name attraction type. The controlling torque is provided by the balance weights attached to the moving iron. Spring also can be used to provide controlling torque. Damping is provided by air friction in which aluminum piston is attached to the moving system and moves in a closed air damping chamber.
2. Repulsion type
In these type of instruments there two vanes present inside the coil in which one is fixed while other is movable. These both vanes gets similarly magnetized when coil is excited. Thus there is a force of repulsion between both vanes causing movement in movable vane. There are further two different designs of repulsion type moving iron instruments.
In radial vane type radial strips of iron are used as moving vanes. The fixed vane is attached to the coil and moving is attached to the moving spindle.
In coaxial vane type the fixed as well as moving vanes are the sections of coaxial cylinders as shown in above figure.
The controlling torque is provided by the springs or balancing weights (in vertically mounted instrument). The damping is provided by air friction damping same as in attraction type moving iron instruments.