This is the note on the topic of Moving Iron Instrument or MI Instrument. About its Construction, Working, Advantages and Disadvantages. Poly Notes Hub lives to share an article on it.
Author Name: Arun Paul.
Moving Iron Instrument
There are two types of moving iron instrument –
- Moving Iron Attraction Type Instrument
- Moving Iron Repulsion Type Instrument
Construction of MI Instrument
Moving Iron Attraction Type Instrument
It consists of a fixed coil C and iron piece D. The coil is flat and has a narrow slot-like opening. The moving Iron is a flat disc, which is eccentrically mounted on the spindle. The spindle is supported between the jewel bearing. The spindle carries a pointer, which moves over a graduated scale. The number of turns on the fixed coil is dependent on the range of the instrument. For passing a large current through the coil, only a few turns are required. The construction is shown in the figure.
The controlling torque is provided by springs but gravity control may also be used for vertically mounted panel-type instruments. Damping torque is provided by air friction.
Moving Iron Repulsion Type Instrument
These instruments consist of two vanes one moving while the other is fixed. A fixed vane is attached to the frame of the coil. When current flows in the coil, vanes are magnetized with polarities induced on the same side. Due to the repulsion of like polarities the two magnets tend to force the magnets apart so that the moving system is deflected.
This force decreases as the distance between vanes increases. The controlling torque is provided by springs. Gravity control may also be used in vertically mounted instruments. The damping torque is produced by air friction as in the case of an attraction-type instrument. As operating fixed is weak, eddy current damping is not used.
For any direction of current, both the vanes get magnetized with the same polarities hence always producing force of repulsion. Hence these instruments can be used for both A.C and D.C measurement.
Working Principle of MI Instrument
When the current passing through the coil is proportional to the quantity to be measured, the coil becomes an electromagnet. This electromagnet attracts a soft iron piece towards it, thus producing deflecting torque. The soft iron piece is attached to the spindle, hence as iron gets attached, the spindle moves and hence pointer attached to the spindle also deflects over the graduated scale.
If the direction of the current is reversed, the magnetic field direction produced by the current-carrying coil will be reversed, and the magnetic field direction produced by the current-carrying coil will be reversed. But for any direction of the magnetic field, iron pieces will get attached to the magnet. Hence deflection torque is always unidirectional. Hence these instruments well suit A.C. as well as D.C. measurements.
Advantages
Below we listed some advantages of MI Instrument –
Disadvantages
Below we listed some disadvantages of MI Instrument –
- Scale is not uniform.
- Power consumption is very high.
- Resistance of the coil increases due to increases of temparature.
Conclusion | Diploma Engineering Notes | Poly Notes Hub
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