Moving iron instrument - Engineering notes

Moving iron instrument:

Moving iron instruments are of two types:

1. Attraction type Moving iron instrument

2. Repulsion type Moving iron instrument

Attraction type Moving iron instrument:

Principle of Attraction type Moving iron instrument:

These instruments are based on the principle that when an unmagnetised soft iron piece is placed in the magnetic field of a coil, then the piece is attracted towards the coil. The moving system of the instrument is attached to the soft iron piece and the operating current is passed through a coil placed near it. The operating current sets up magnetic field which attracts the iron piece and thus moves the pointer over the scale.

Construction of Attraction type Moving iron instrument:

It consists of a hollow cylindrical coil or solenoid which is kept fixed as shown in the fig. An oval shaped soft iron pieces is attached to the spindle in such a way that it can move in or out of the coil.The pointer is attached to the spindle so that it is deflected with the motion of the soft iron piece. The controlling torque on the moving system is provided by spring control method while damping is provided by air friction.

Attraction type Moving iron instrument

Working of Attraction type Moving iron instrument:

When the instrument is connected in the circuit, the operating current flows through the coil. The current sets up magnetic field in the coil. In other words, the coil behaves like a magnet and, therefore, it attracts the soft iron piece towards it. The result is that the pointer attached to the moving system moves from zero position.

If current in the coil is reversed, the direction of magnetic field also reverses and so does the magnetism produced in soft iron piece. Therefore, the direction of deflecting torque remains unchanged. It follows, therefore, that such instruments can be used for both D.C as well as A.C work.

Deflecting torque of Attraction type Moving iron instrument:

Field strength h produced by the coil.

Pole strength m developed by the piece

F  proportional   mH

F  proportional   H^2

Td  proportional  F  proportional  H^2

If permeability of iron is assumed constant then

H  proportional   I

Td  proportional  I^2

If the controlling torque is provided by the springs,

Tc  proportional   deflection

In the steady state position of deflection,

Td  =  Tc

Deflection   proportional   I^2

Deflection   proportional   Irms

Scale of such instruments is non-uniform, being crowded in the beginning. In order to make the scale of such instruments uniform, suitably shaped iron piece is used.

Repulsion type moving iron instruments:

Principle of Repulsion type moving iron instruments:

These instruments are based on the principle of repulsion between the two iron pieces similarly magnetized.

Construction of Repulsion type moving iron instruments:

It consists of a fixed cylindrical hallow coil which carries operating current. Inside the coil, there are two soft iron pieces or vanes, one of which is fixed and other is movable. The fixed iron piece is attached to the coil whereas the movable iron piece is attached to the pointer shaft. Under the action of deflecting torque, the pointer attached to the moving system moves over the scale. The controlling torque is produced by spring  control method and damping torque by air friction damping.

Working of Repulsion type moving iron instruments:

When the instrument is connected in the circuit, current flows through the coil. This current sets up magnetic field in the coil. The magnetic field magnetizes both iron pieces in the same direction i.e. both pieces become similar magnets and hence they repel each other. Due to this force of repulsion only movable iron piece moves as the other piece is fixed and cannot move. The result is that the pointer attached to the moving system moves from zero position.

Repulsion type moving iron instruments:

Deflecting torque of Repulsion type moving iron instruments:

The deflecting torque results due to the repulsion between the similarly magnetized iron pieces. If two pieces develop pole strengths m1 and m2 respectively, then,

Instantaneous deflecting torque  proportional to   repulsive force

proportional  m1.m2

Since pole strengths developed are proportional to H, therefore

Instantaneous deflecting torque, Td  proportional   H^2

Assuming constant permeability, h  proportional  current i through the coil

Td  proportional   i^2

Controlling torque provided by springs,  Tc  proportional  deflection

In the steady position of deflection,

Td  =  Tc

Deflection  proportional  i^2

proportional  I^2

Proportional  Irms

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

Advantages of moving iron instruments:

1. These are cheap, robust and simple in construction.

2. The instruments can be used for both A.C as well as D.C circuits.

3. These instruments have high operating torque.

4. These instruments are reasonable accurate.

Disadvantages of moving iron instruments:

1. Such instruments have non-uniform scale.

2. These instruments are not very sensitive.

3. Errors are introduced due to changes in frequency in case of a.c measurements.

4. Higher power consumption.