MOVING IRON INSTRUMENTS (Types & Working principle) - ELECTRICAL ENCYCLOPEDIA

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MOVING IRON INSTRUMENTS (Types & Working principle)

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In this article we will study about the MOVING IRON (MI) INSTRUMENTS.

-Also check about Permanent magnet moving coil Instruments (PMMC)

TYPES OF MOVING IRON (MI) INSTRUMENTS

There are two types of moving iron instruments namely , attraction type moving iron instruments and repulsion type moving iron instruments.
The operation of attraction type moving iron instruments depends upon the attraction of single piece of iron in a magnetic field.

While in the case of repulsion type moving iron instruments, the operation depends upon the repulsion of two pieces of iron which are magnetised by the same magnetic field.
For both types of instrument, the magnetic field is produced by the current carrying coil.

In case ,if the instrument is used as ammeter ,then the coil is provided with the fewer turns of thick wire so that ammeter has low resistance.

In case to be used as voltmeter , the coil has high impedance so that it may draw as small current as possible since it is connected in parallel with the circuit.

ATTRACTION TYPE MOVING IRON (MI) INSTRUMENTS

It is well known that if a piece of unmagnetised soft iron is brought near the ends of current carrying coil, then it would get attracted towards the coil. This phenomena is used in this type of instrument.

In attraction type moving iron instruments, an oval-shaped disc made up of soft iron is pivoted on the spindle near the coil. When the current to be measured is passed through the coil, then coil produces a magnetic field and attracts the oval-shaped disc, hence disc will swing into the coil. A pointer is also fixed to the same spindle on which oval-shaped disc is mounted. So during the passage of current through the coil, the pointer will also get deflected. Amount of deflection of pointer depends upon the magnetic field produced due to coil which in turn depends upon the current passing through the coil.Pointer will show the reading on the calibrated scale.

Remember that whatever may be the direction of current through the coil, the iron disc would always be magnetised in such a way that it is pulled inwards.
Hence , such instruments can be used for measurements of direct as well as alternating currents.

Generally air friction damping is provided in such instruments because Eddy current damping could not be applied because in such damping permanent magnet are required and hence these magnets can affect the deflection of pointer and also affects the reading of instruments.
So an air chamber is provided with the instrument for air friction damping as shown in the figure
Attraction type moving iron instrument
with Air friction damping

Control torque can be provided by either spring control or gravity control but generally spring control method is used because gravity control suffers from many disadvantages like keeping the instrument always in vertical position , increases the weight of the instrument etc.

REPULSION TYPE MOVING IRON (MI) INSTRUMENTS.

It consists of a fixed coil inside which two soft iron rods/pieces are placed. One of them is fixed while the other is movable. When the coil to be measured is passed through the coil, it will set up it's own magnetic field which magnetises the two iron rods with the same magnetic polarity. Hence due to same polarity the two rods will repel each other.Pointer will show the reading on the calibrated scale.

The force of repulsion in the instrument is directly proportional to the square of the current passing through the coil.

Similar to the attraction type moving iron instruments, whatever may be the direction of the current through the coil, the two rods will be magnetised similary and will repel each other. 

In this instrument also, eddy current damping cannot be provided because in such damping permanent magnet are required and hence these magnets can affect the deflection of pointer and also affects the reading of instruments.
So an air chamber is provided with the instrument for air friction damping as shown in the figure.
Repulsion type moving iron instrument
with Air friction damping

Control torque can be provided by either spring control or gravity control but generally spring control method is used because gravity control suffers from many disadvantages like keeping the instrument always in vertical position , increases the weight of the instrument etc.

CONCLUSION

Moving iron instruments are cheaper and reliable.
It can be used with both a.c. and dc circuits.
But they cannot be calibrated with higher degree of precision as well as they have non-uniform or uneven scale. And eddy current damping cannot be provided.


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