The moving iron instruments or MI instruments are quite cheap in cost, simple in construction and very accurate at fixed power supply frequency. These instruments can be used on AC as well as DC. Therefore, these are widely used in laboratories and on switching panels. These instruments are used either as ammeters or voltmeters only. Moving iron instruments are of two types, namely;

• Attraction type moving iron instruments.
• Repulsion type moving iron instruments.

When a soft iron piece (vane) is placed in the magnetic field of a current-carrying coil, then the piece is attracted towards the center of the coil. This is because the piece tries to occupy a position of minimum reluctance. Thus, a force is exerted on the soft iron piece and deflection in the needle takes place.

Construction

A moving iron instruments consists of a stationary hollow cylindrical coil. An oval shaped soft iron piece is mounted eccentrically to the spindle to which a pointer (needle) is attached. The controlling torque is provided by gravity control method while damping torque is provided by air friction as shown in the figure.

Working

When the instrument is connected in the circuit, the operating current flows through the stationary coil. A magnetic field is set up and the soft iron piece is magnetized which is attracted towards the center of the coil. Thus the pointer attached to the spindle is deflected over the calibrated scale.

If the current in the coil is reversed the direction of magnetic field produced by the coil will reverse, also the magnetism produced in the soft, iron piece will reverse. Hence, the direction of deflecting torque remains unchanged. Thus attraction type moving iron instruments can be used on AC as well as on DC systems.

Since deflection θ is proportional to the square of current flowing through the coil, therefore, the scale of such an instrument is non-uniform, being crowded in the beginning. However, by choosing proper dimensions, shape and position of the soft iron piece (vane), it is possible to design and construct an instrument with a scale which is very nearly uniform over a considerable part of its length.

The basic principle of a repulsion type moving iron instruments is that the repulsive forces will act when two similarly magnetized iron pieces, are placed near each other.

Construction

It consists of a fixed cylindrical hollow coil which carries the operating current. Inside the coil, there are two soft iron pieces (rods or vanes) placed parallel to each other and along the axis, of the coil.

One of the rod or vane is fixed and the other is movable connected to the spindle. A pointer is attached to the spindle which gives deflection on the scale. The controlling torque is provided by spring control method while damping torque is provided by air friction.

Working

When the instrument is connected in the circuit, the operating current flows through the stationary (or fixed) coil. A magnetic field is setup along the axis of the coil, this field magnetizes both the pieces similarly i.e. both the pieces attain similar polarities. A force of repulsion acts between the two, therefore movable piece moves away from the fixed piece. Thus the pointer attached to the spindle deflects over the calibrated scale.

If the current in the coil is reversed, the direction of magnetic field produced by the coil is reversed. Though the polarity of the magnetized soft iron pieces is reversed but sill they are magnetized similarly and repel each other. Hence, the direction of deflecting torque remains unchanged. Thus, repulsion type moving iron instrument can be used on DC as well as on AC systems.

Since deflection θ is proportional to the square of current flowing through the coil, therefore, the scale of repulsion type moving iron instruments is non-uniform, being crowded in the beginning. However, by using tongue-shaped iron pieces, the scale of such instruments can be made uniform.

Advantages of Moving Iron Instruments

• The moving iron instruments are cheap, robust and simple in construction.
• These instruments can be used on both AC and DC.
• These instruments are reasonably accurate.
• The moving iron instruments possess high operating torque.
• These instruments can withstand overloads momentarily.

Disadvantages of Moving Iron Instruments

• The scale of moving iron instrument is non-uniform; crowded at the beginning, therefore, accurate readings are not possible at this end.
• These instruments are not very sensitive.
• Power consumption is quite high in the moving iron instruments.
• Errors are introduced due to change in frequency in case of AC measurements.

 
There are two types of errors which occur in moving iron instruments:

• Errors with both AC and DC.
• Errors with AC only.

Errors with both AC and DC

The following are the main errors in moving moving iron instruments, when these are used either on DC or AC.

Error due to hysteresis: Because of hysteresis in the iron parts of the operating system, the readings are higher for descending values but lower for ascending values. The hysteresis error is considerably reduced by using mumetal or permalloy which have negligible hysteresis loss.

Error due to stray magnetic fields: Since the operating magnetic field of the moving iron instruments in comparatively weak, therefore, stray fields (fields other than the operating magnetic field) affect these instruments considerably. Thus the stray fields cause serious errors. These errors can be minimized by using an iron case or a thin iron shield over the working parts.

Error due to temperature: The effect of temperature change on moving iron instrument arises mainly from the temperature coefficient of spring. With the change in temperature stiffness of the spring varies which causes errors. However, for voltmeters, both the temperature coefficient of spring and temperature coefficient of resistance of voltmeter circuit may balance each other.

Errors with AC only

Error in moving iron instruments due to change in frequency: The change in frequency produces a change in impedance of the coil and change in magnitude of eddy currents. The increase in impedance of the coil with the increase in frequency causes serious errors in case of voltmeters only.

However, this error can be eliminated by connecting a condenser of suitable value in parallel with the swamp resistance `r’ of the instrument. The impedance of the whole circuit of the instrument becomes independent of frequency if C = L/r², where C is the capacitance of the condenser.

Ranges

• Ammeters: From about 0 – 20 mA to 0 – 800 A maximum without the current transformer.
• Voltmeters: From about 0 – 1 V to 0 – 800 V maximum without the potential transformer.

A moving iron instrument can be used for ac or dc. But they are mainly used for AC measurements viz. AC currents and voltages. They also can measure DC quantities but their characteristics are inferior to that of permanent moving coil instruments. Therefore, in DC measurements permanent moving coil instruments are preferred and used.

Thanks for reading about moving iron instrument working principle.

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