MOVING IRON INSTRUMENTS

As we know that there are two types of measuring instruments named moving iron and moving coil which we have discussed in  article. In this article, we will discuss moving iron instruments. These instruments are widely used in laboratories and switchboards at commercial frequencies. Because these are cheaper in cost, robust in construction and can be manufactured with the required accuracy.

What is Moving Iron Instrument

A soft iron piece when attracted or repelled by the magnetic field a coil is moves. A pointer is attached with the moving piece. Then shows deflection on a calibrated scale. These instruments are simple cheap and reasonably accurate and can be used for ac/dc measurement. Hence if finds use in practical fields.

There are two general types of moving instruments, which are

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

Attraction type operate on the principle of attraction of a single piece of soft iron into a magnetic field. Repulsion type operate on the principle of repulsion of two adjacent iron pieces which is magnetized by the same magnetic field. Repulsion type of measuring instruments are more sensitive. Because in these instruments large operating torque is developed. This torque is developed by having two iron elements positioned close together inside the field coil where the magnetizing effect is maximum.

 Attraction type

The earliest and simplest form of attraction type moving iron instrument uses a solenoid and moving oval-shaped soft iron pivoted eccentrically as shown in Fig. To this iron, a pointer is attached so that it may deflect along with the moving iron over a graduated scale. The iron is made of sheet metal specially shaped to give a scale as early uniform as possible. The moving iron is drawn into the field of the solenoid when current flows through it. The movement of the iron is always from the weaker magnetic field outside the coil into the stronger magnetic field inside the coiƂ.

Attraction type moving iron instrument

When the current to be measured is passed through the solenoid Then a magnetic field is set up inside the solenoid, which magnetizes the iron. Thus, the iron is attracted into the coil causing the spindle and the pointer to rotate. Such instruments normally have spring control and pneumatic damping. In Fig. gravity control attraction type instrument, which has been now set aside by spring control instruments owing to their merits over gravity control instruments.

It has a scale cramped at the lower end and greatly expanded at the upper end. As the operating torque is quite low when the moving iron is just entering the solenoid and increases rapidly as iron is drawn further in. However, it does permit the development of large working torque. It is still used in some instruments requiring high operating torque.

Repulsion Type

In this type of instrument, there are two irons. Curved iron of soft iron or of mu-metal is fixed to the inside of the bobbin former. Another curved iron is mounted on a spindle which passes axially through the solenoid. The two irons lie in the magnetic field produced by a solenoid. When there is no current in the solenoid the two irons, moving one and fixed one. Both are almost touching each other and the pointer rests on zero position.

measuring instruments
Repulsion type moving iron instrument

When the current to be measured is passed through the solenoid, which is wound with insulated copper wire on a cylindrical nonmagnetic former. A magnetic field is set up inside the solenoid and the two irons are magnetized in the same direction. This sets up a repulsive force so moving iron is repelled by fixed iron, thereby results in the motion of the moving iron carrying the pointer.

The pointer comes in a rest in a deflected position when equilibrium is attained between the repulsion force of the working elements and the controlling force. Control of the movement is either by hair-spring, when the instrument is to be employed in any position. Or by gravity, when the instrument is to be operated only vertically.

Damping used is air friction (pneumatic) one. Eddy current damping is not possible because of the presence of permanent magnet needed for such purpose would affect the deflection.

Scale of Moving Iron Intrument

The repulsion of iron is proportional to the square of the current. So the scale is uneven, being crowded at the lower values and wide spread at the higher values. The iron may be so shaped and arranged that the scale is spread out to some extent at the lower values of current.

Errors in moving iron instruments

 The errors usually occurring in moving iron instruments are friction, temperature, stray magnetic field, hysteresis, frequency and wave form. The first four errors affect the instrument on AC and DC. While last two affect only on ac.

Stray Magnetic Field

Stray magnetic field error may be serious if not guarded against because of the weakness of the operating magnetic field in case of moving iron instruments. This error is minimized by the following methods:

  • By shielding the instrument with the help of steel case.
  • By sheet steel lining to the case.

Hysteresis Error

Hysteresis error is a serious type of error in moving iron instrument specially when used on a  circuit. This error may be reduced by the following methods:

  • By making the iron part short. So that they demagnetize themselves.
  • By choosing a value of flux density in the iron or by using a mu-metal.
  • By permalloy having negligible hysteresis.

Frequency Change

Change in frequency may cause due to change of reactance of the operating coil. It is also due to the changes in magnitude of eddy current set up in the metal part of the instrument near the operating portion. The change in instrument reactance owing to change in frequency causes serious error in the case of voltmeters. The eddy current error affects both the measuring instruments equally. The frequency error may be compensated for the connecting a suitable capacitor in parallel with the swamping resistance R.

Waveform

Moving iron instruments may be seriously affected by waveform. It affects on both account of the change in the form of flux waveform. And in case of voltmeters, the effects of harmonics upon the inductor.

Advantages of moving iron instruments

  1. The instruments can be used both in ac as well as dc circuits.
  2. These instruments are simple in construction of moving part.
  3. The stationary part of the instruments is very Simple.
  4. They are less costly.
  5. These instruments possess high operating torque.
  6. These instruments can withstand overload momentarily.

Disadvantages of moving iron instruments

  1. Scale of these instruments are not linear.
  2. Power consumption is higher for low voltage range.
  3. If the temperature increases then the stiffness of the spring decreases.
  4. The errors that are caused by hysteresis in the iron of the operating system and due to stray magnetic field.
  5. Change in frequency and in waveform causes serious errors in ac measurements.

Also read : introduction to measuring instruments
types of measuring instruments