Moving Iron Instruments — Types, Working Principle & Applications
Moving iron (MI) instruments are among the most widely used measuring instruments in electrical engineering. They work on the principle of magnetic attraction or repulsion of iron pieces in a magnetic field produced by a current-carrying coil. Unlike PMMC instruments, MI instruments can measure both AC and DC quantities, making them versatile for industrial and laboratory use.
Working Principle of Moving Iron Instruments
Moving iron instruments operate on the fundamental principle that a soft iron piece, when placed in a magnetic field, gets magnetised and experiences a force. The magnetic field is produced by a fixed coil carrying the current to be measured. The deflection of the pointer is proportional to the square of the current (I²), which gives these instruments a non-linear (square-law) scale.
Where: I = current through coil, L = inductance, θ = deflection angle
Since the torque depends on I² (always positive regardless of current direction), MI instruments work on both AC and DC circuits. On AC, they read the RMS value of current or voltage.
Types of Moving Iron Instruments
There are two main types of moving iron instruments:
- Attraction type — operates on the attraction of a single soft iron piece into a magnetic field
- Repulsion type — operates on the repulsion between two similarly magnetised iron pieces
When used as an ammeter, the coil has fewer turns of thick wire (low resistance, connected in series). When used as a voltmeter, the coil has many turns of thin wire with a high series resistance (high impedance, connected in parallel).
Attraction Type Moving Iron Instrument
In the attraction type MI instrument, an oval-shaped soft iron disc is mounted on a spindle near the coil. When current flows through the coil, it produces a magnetic field that attracts the iron disc inward. The disc swings into the coil, and a pointer attached to the same spindle deflects over a calibrated scale.
Construction details:
- A fixed coil produces the magnetic field
- An oval-shaped soft iron disc (vane) is pivoted on a spindle
- A pointer is attached to the spindle
- Spring provides the controlling torque
- Air friction damping is used (not eddy current damping)
Attraction type moving iron instrument with air friction damping
Key point: Regardless of the direction of current through the coil, the iron disc is always magnetised such that it gets pulled inward. This is why MI instruments work on both AC and DC.
Why air friction damping? Eddy current damping requires a permanent magnet, which would interfere with the operating magnetic field and produce errors in readings. Therefore, an air chamber with a piston provides the necessary damping.
Repulsion Type Moving Iron Instrument
In the repulsion type MI instrument, two soft iron pieces (vanes or rods) are placed inside the fixed coil. One iron piece is fixed to the coil frame, and the other is mounted on the movable spindle. When current passes through the coil, both iron pieces get magnetised with the same polarity and repel each other, causing the movable vane to deflect.
Construction details:
- A fixed coil carries the current to be measured
- Two soft iron vanes — one fixed, one movable — are placed inside the coil
- The movable vane is attached to the spindle and pointer
- Spring control provides restoring torque
- Air friction damping is employed
Repulsion type moving iron instrument with air friction damping
(Always positive — both vanes have same polarity regardless of current direction)
The repulsion type is more commonly used in practice because it provides a more uniform scale and better sensitivity compared to the attraction type.
Torque Equation of Moving Iron Instruments
The general torque equation for MI instruments is derived from the energy stored in the magnetic field:
At steady deflection: Td = Tc (controlling torque)
½ × I² × (dL/dθ) = K × θ
θ = I² × (dL/dθ) / (2K)
Since deflection θ is proportional to I², the scale is non-uniform — it is cramped at the lower end and spread out at the higher end. Special shaping of iron vanes can partially linearise the scale.
Comparison — Attraction Type vs Repulsion Type
Advantages and Disadvantages
Advantages:
- Works on both AC and DC circuits
- Simple, robust, and cheap construction
- Can withstand overloads (no permanent magnet to demagnetise)
- Reasonably accurate for industrial measurements (accuracy class 1.0–1.5)
- Available in wide range (up to 250 A for ammeters, up to 750 V for voltmeters)
Disadvantages:
- Non-uniform (square-law) scale — cramped at lower readings
- Affected by stray magnetic fields (no shielding from permanent magnet)
- Hysteresis error when used on DC
- Frequency error on AC due to change in impedance and eddy currents in iron
- Lower precision compared to PMMC instruments
- Higher power consumption than PMMC
Applications
- AC ammeters and voltmeters in switchboards and panel boards
- Portable instruments for industrial measurements
- Power frequency (50/60 Hz) measurements in substations
- Laboratory measurements where both AC and DC capability is needed
- Protective relay circuits as indicating instruments
Frequently Asked Questions
1. Why can't eddy current damping be used in moving iron instruments?
Eddy current damping requires a permanent magnet. In MI instruments, this permanent magnet would create its own magnetic field that interferes with the operating field of the coil, causing errors in readings. Therefore, air friction damping (using a piston in an air chamber) is used instead.
2. Why do moving iron instruments have a non-uniform scale?
The deflecting torque in MI instruments is proportional to I² (square of current). This square-law relationship means the deflection is not linearly proportional to current, resulting in a cramped scale at lower values and a spread-out scale at higher values.
3. Can moving iron instruments measure DC current?
Yes. Since the deflecting force depends on I² (always positive regardless of current direction), MI instruments work on both AC and DC. However, on DC, they may suffer from hysteresis errors in the iron vanes.
4. What is the difference between PMMC and moving iron instruments?
PMMC instruments use a permanent magnet and work only on DC with a uniform scale and high accuracy. Moving iron instruments use an electromagnet (coil), work on both AC and DC, but have a non-uniform scale and lower accuracy. MI instruments are cheaper and more robust.
5. What type of control torque is used in MI instruments?
Spring control is generally used because gravity control requires the instrument to be kept strictly vertical, adds weight, and limits portability. Spring control allows the instrument to be used in any orientation.