Short Answer

Moving iron instruments are electrical measuring devices that work on the principle of magnetic effect of electric current. They use a piece of soft iron which moves when current flows through a coil.

These instruments are mainly used to measure both AC and DC quantities like current and voltage. They are simple, strong, and widely used in electrical measurements.

Detailed Explanation:

Moving Iron Instruments

Meaning of Moving Iron Instruments

Moving iron instruments are measuring devices used in electrical engineering to measure current and voltage. They operate based on the magnetic effect produced when current flows through a conductor.

In these instruments, a coil carries current and produces a magnetic field. A small piece of soft iron is placed inside or near the coil. When current flows, the magnetic field attracts or repels the iron piece, causing it to move.

This movement is transferred to a pointer, which moves over a calibrated scale to show the value of the electrical quantity. The deflection of the pointer depends on the strength of the current flowing through the coil.

Moving iron instruments are widely used because they are simple in construction and can work with both alternating current (AC) and direct current (DC).

Types and Working

There are mainly two types of moving iron instruments: attraction type and repulsion type.

In the attraction type, a single piece of soft iron is attracted towards the coil when current flows through it. The magnetic field pulls the iron piece, causing movement of the pointer.

In the repulsion type, two iron pieces are used. One is fixed and the other is movable. When current flows through the coil, both iron pieces become magnetized with the same polarity, and they repel each other. This repulsion causes the movable iron piece to rotate and move the pointer.

The deflecting torque in moving iron instruments is proportional to the square of the current. This means the instrument can measure both AC and DC, as the direction of current does not affect the deflection.

A controlling torque is provided by springs, which balance the deflecting torque and bring the pointer to a stable position. Damping is also provided, usually by air friction, to reduce oscillations.

Moving iron instruments have a non-uniform scale because the deflection is not directly proportional to current. The scale is crowded at lower values and spread out at higher values.

These instruments are robust and can handle overloads better than moving coil instruments. However, they are less accurate and less sensitive compared to moving coil instruments.

They are commonly used as ammeters and voltmeters in power systems, especially where AC measurements are required.

Conclusion

Moving iron instruments are simple and reliable devices used to measure electrical quantities. They work on the magnetic effect of current and can measure both AC and DC. Although less accurate than some other instruments, they are widely used due to their durability and versatility.