Moving Iron / (MI) Instruments | Attraction Type Moving lron Instrument | Applications

Moving Iron / (MI) Instruments :

Where ruggedness is more important than high degree of ccuracy, moving iron instruments are used.

Types of moving iron instruments:
1. Attraction type 2. Repulsion type

Attraction Type Moving lron Instrument:

This type of instrument is shown in Fig. 4.7.1. The basic principle of operation is that, a soft iron piece if brought near a magnet (either permanent magnet or electromagnet) gets attracted by the magnet.
Fig. 4.7.1 shows a fixed coil C and an oval shaped soft iron piece eccentrically mounted on the spindle. i
The spindle is supported between jewel bearings and it carries i the pointer. The instrument may be spring controlled or gravity controlled. Damping is provided by a piston moving in an air chamber.
This piston is attached to the main spindle. For such instrument, eddy current damping is not used because the presence of a permanent magnet required for damping i purpose would affect the operating field of the instrument.

When current to be measured is passed through the fixed coil C. it produces a magnetic field. Due to this field the soft iron piece gets attracted inside and the pointer deflects over ai calibrated scale.
The torque developed in this instrument is proportional to thei square of the current through the coil and therefore the scale is non uniform The scale is cramped at the beginning and open at the upper end of the scale.
The shape of the soft iron piece is so adjusted that, as far as possible a uniform scale is obtained.

The soft iron piece is always attracted, irrespective of the direction of the magnetic field of the coul. Therefore, the instrument can be used both for a.c. as well as dc.i measurements. i
The damping is provided by the piston attached to the spindle. This is air friction damping
The deflecting torque T, is proportional to the square of current passing through the coil.
(4.7.1)
So the deflection of the pointer is also proportional to square of the current. i
(4.7.2)
8
Due to this reason, the scale of this instrument is nonlinear. It is cramped at the beginning and open at the upper end of the scale.

Application :

This type of instruments can be used for the measurement of ac as well as dc quantities. This is because irrespective of thei direction of current, the deflection O will always be in the same direction. i
4.7.2