What is a Moving Iron Instrument & Its Working

The moving iron meter was initially formulated by an Australian engineer  Drexler in the year 1884. These moving iron meters act in response to both the AC and DC currents. This iron component includes a movable vane that is connected to the pointer and consists of an immovable plane and this is enclosed by a coil. The instrument’s deflection has a direct relation to twice the current value. Subsequently, those meters generally consist of a non-linear scale, whereas the iron components are normally varied in order to get back the scale to a linear shape. So, this a brief on moving iron meters history. Let us have a detailed discussion on what is a moving iron instrument, its working, types, principle, and uses.

What is a Moving Iron Instrument?

The device where the moving iron is utilized for calculating either the current or voltage flow is termed as the moving iron instrument. This device operates on the principle that iron is placed in close to the magnet and it attracts this. This attractional force is based on the magnetic field strength. And this magnetic field is stimulated by the electromagnet where its ability is based on the current’s magnitude that flows across it. The basic moving iron instrument diagram is shown as below:

Moving Iron Instrument
Moving Iron Instrument


The basic construction of the moving iron instrument is explained below.

Here, for the construction of the device either a plate or a sheet of soft iron is employed as the movable component for the device. This sheet is placed in the way so that it has free movement in the stationary coil’s magnetic field. The conductor element creates the stationary coil and this is stimulated either by current or voltage for which magnitude has to be calculated.

The device makes use of the stationary coil to function as an electromagnet. This electromagnet is only the transient magnitude where its magnetic field ability either enhances or lessens along with the current’s magnitude that flows across it.

Working Principle of Moving Iron Instrument

These devices make use of either aluminum constructed wire or a static copper coil in order to function as an electromagnet when there is a current passage all through the instrument. The capacity of the magnetic field is stimulated by the electromagnet and it has a direct relation to that passage of current.

Either the plates of the iron sheets that pass across the coil enhance the static coil inductance (The inductance is the conductor’s factor that enhances its EMF when there is variable movement through the inductor). There will be an attractional force for the electromagnet to attract the iron sheet.

The sheet that is passed through the coil attempts to absorb the least reluctance path where reluctance is the magnetic property that obstructs the current flow. The sheet which is passed via the coil will get repulsion force where this is created by the electromagnet. This force enhances the ability of the coil’s inductance. With this, the reluctance and inductance properties are inversely related. This is a moving iron instrument operation.


There are mainly two types of moving iron instruments where those are:

  • Attraction type
  • Repulsion Type

Attraction Type Moving Iron Instrument

In this type, the iron plate is attracted towards the stronger field from the weaker field is termed as the attraction type of instrument. Here, the static coil in the device is in a flat shape and it has a small opening. The movable element is considered as the iron core’s flat disc. The flow of current across the static coil generates the magnetic field and this has the attractional force to attract the coil.

The iron plate will get deflect from the low to the high magnetic field and this deflection capacity has a direct relation to the current flow magnitude which flows through the iron plate. In a brief, it is defined that an iron coil will be attracted.

This kind of device makes use of a spring where creates torque regulation. The coil’s deflection is minimized by the piston made of aluminum metal and this is attached to the movable coil.

Repulsion Type Moving Iron Instrument

This device is constructed with two iron sheets where one is static and the other sheet is movable. These iron plates will be magnetized when there is current flow across the static coil and this created repulsion force in between the plates. Due to this repulsion force, the movable coil begins to start away from the static plate.

The spring in the device will provide regulation movement of the torque. The air friction stimulates the damping torque where obstructs coil movement. These kinds of moving iron instruments come under the category of the non-polarized device where means that the device is to be set free from the current path that flows the device. So, it is employed for both the direct and alternating current.

Differences in Attraction and Repulsion Types

The attraction type of moving iron device will posses minimal inductance when compared with the repulsion type device and because of this voltmeters are completely precise over extended frequency levels and so there exists enhanced scope of employing shunts with the ammeters. Whereas repulsion devices are mainly applicable for economical purposes and with these instruments linear scale can be easily achieved

Deflecting Torque

In order to find the expression for deflecting torque of a moving iron instrument, it can be achieved with the consideration of energy relations when there happens minimal enhancement in the supply of current to the device. In view of this, there will be a minimal amount of deflection ‘ds’ takes place.

So, let us assume that ‘Td’ is the deflection of torque. And the mechanical effort is Td*ds. In addition to this, there will also be a variation in the stored energy where results in the variation in inductance value.

Consider that the device has I as initial current, L as inductance, and deflection ‘s’. When the amount of current is augmented by a factor ‘di’, then the deflection varies by ‘ds’ and inductance varies by ‘dL’. So as to impact the current enhancement, there has to be an increment in the level of applied voltage which is shown as

e = d/dL (LI)

On solving the energy equations, the deflecting torque is given as

Td = ½ l2 (dL/dϴ)

At the final equilibrium position, Tc becomes Td and then

Kϴ = ½ l2 (dL/dϴ)

With this, the deflection is ϴ = ½ (l2/K) (dL/dϴ)

With this, it can be proved that torque deflection is proportional to the twice RMS of the functional current. So, the torque deflection is unidirectional which means that it operates in a single direction independent of the current polarity.

Advantages and Disadvantages

The advantages of moving iron instruments are:

  • These devices are neutral to the current flow path and so employed for both the alternating and direct current applications.
  • Errors caused by friction are very minimal because the proportion of torque and weight is more. The proportion is more as the current-carrying section is static and movable sections are less weighed.
  • As the MI devices require minimal turns than that of permanent magnet moving coil devices, these are less costly.
  • Also, the devices have more robustness due to their simple design.

The disadvantages of moving iron instruments are:

  • The moving iron instrument scale is not uniform and so the results are not precise.
  • As because of stray, hysteresis, and frequency losses, there might be a scope of happening critical errors in the device.
  • In the device, there will be waveform errors because the deflection of the torque has no direct relation to twice the current value.

So, this is all about the concept of MI devices. This article has provided a clear description of the moving iron instrument, its working principle, types, construction, benefits, and limitations. In addition, also know about what are the applications of moving iron instruments?

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