Moving iron instruments

Moving iron instrument

Moving iron instruments or MI instruments are widely used in labs and switchboards. Most commonly these MI instruments are used in ammeter and voltmeter.

Working principle of MI instrument:

The working principle of moving iron instruments is when several soft iron pieces are caused to move by the magnetic field of a fixed coil excited by the current or voltage to be measured. The value of current or voltage is indicated by the position of the pointer when the deflection torque produced by the current is balanced by spiral springs.

Moving iron instruments are classified into two types:

  1. Attraction type instrument
  2. Repulsion type instrument

Working principle of attraction type MI instrument:

Attraction type Moving iron instrument

It works on the principle of attraction of a single piece of soft iron into the magnetic field. In an attraction type MI instrument, when the current is passed through the solenoid, a magnetic field is set up inside the solenoid. Then the iron piece gets magnetized which in turn deflects the pointer. The movement of the iron piece is always from a weaker magnetic field outside the coil into the stronger magnetic field inside the coil regardless of the direction of current flowing. This is because the iron piece tends to occupy the position of minimum reluctance.

Working principle of repulsion type moving iron instrument:

Repulsion type moving iron instrument

It works on the principle of repulsion of two iron pieces magnetized by the same magnetic field. When the current is passed through the two nearly placed iron pieces (one piece is fixed and the other is a moving iron piece). Both the pieces get magnetized and a repelling force occurred between both the pieces. The repelling force repels the moving iron piece which is connected to a pointer. The pointer comes to rest when equilibrium is attained between the operating repulsion force elements and controlling force.

The deflection torque produced in the moving iron instruments can be given as:

T = ½ I2 dL/dѲ


I = Current in the coil

L = Inductance of instrument

Ѳ = Deflection angle in radian

Attraction-type instruments usually have lower inductance than the corresponding repulsion-type instruments. Therefore, attraction type MI instruments voltmeter are more accurate over a wide range of frequencies and there is a greater possibility of using shunts with ammeters.

Advantages of Moving Iron instrument:

  1. These instruments can be used for both AC as well as DC circuits.
  2. These instruments are robust and simple in construction.
  3. These instruments are very cheap.

Disadvantages of Moving Iron instrument:

  1. The scale is not uniform
  2. These are affected by stray magnetic fields
  3. Hysteresis in iron causes error.
  4. Change in frequency and in waveform causes some serious errors.

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