Welcome to webEnotes. In this article, we gonna study “Permanent Magnet Moving Coil Instruments”.
Construction Of Permanent Magnet Moving Coil Type Instrument
As its name indicates, this instrument consists of a permanent magnet and a rectangular coil of many turns wounded on a light aluminium or copper former.
As shown in the figure, it is surrounded by a U-shaped permanent magnet made up of ALNICO.
Between the magnetic poles is fixed a soft iron cylinder whose function is to increase the magnetic flux. Surrounding the core is a rectangular coil of many turns wounded on a light aluminium frame which is supported by delicate bearings and is attached with a light pointer. The aluminium frame also provides Eddy current damping.
Two springs are spiralled in opposite directions to neutralise the effects of temperature changes.
Permanent Magnet Moving Coil Instruments
Moving Coil – The coil is the current-carrying part of the instruments which is freely moved between the stationary field of the permanent magnet. The current passes through the coil and deflects it due to which the magnitude of the current or voltage is determined. The coil is mounted on the rectangular former which is made up of aluminium. The former increases the radial and uniform magnetic field between the air gap of the poles. The coil is wound with the silk cover copper wire between the poles of a magnet.
The coil is mounted on the rectangular former which is made up of aluminium. The former increases the radial and uniform magnetic field between the air gap of the poles. The coil is wound with the silk cover copper wire between the poles of a magnet.
Magnet System – The PMMC instrument uses a permanent magnet for creating stationary magnets. The Alcomax and Alnico materials are used for creating the permanent magnet because this magnet has a high coercive force (The coercive force changes the magnetisation property of the magnet).  Also, the magnet has high field intensities.
Control – In the PMMC instrument the controlling torque is because of the springs. The springs are made up of phosphorous bronze and placed between the two jewel bearings. The spring also provides the path for the lead current to flow in and out of the moving coil. The controlling torque is mainly because of the suspension of the ribbon.
Damping –  The damping torque is used for keeping the movement of the coil at rest. This damping torque is induced because of the movement of the aluminium core which is moving between the poles of the permanent magnet.
Pointer & Scale – The pointer is linked with the moving coil. The pointer notices the deflection of the coil, and the magnitude of their deviation is shown on the scale. The pointer is made of lightweight material, and hence it is easily deflected with the movement of the coil. Sometimes the parallax error occurs in the instrument which is easily reduced by correctly aligning the blade of the pointer.
Working Of Permanent Magnet Moving Coil Type Instrument.
When current is passed through the coil, a force acts upon both sides which produces a deflecting torque.
 Let, B = flux density.
L = length/depth of coil in metres.
b = breadth of the coil in metres.
N = number of turns of coil.
If ‘I’ is the current passing through the coil then the magnitude of the force experienced by each of its sides is equal to
“B*I*L”
For N turns, the force on each side of the coil is N*B*I*L
Deflecting torque = Force * perpendicular distance.
T = N*B*L*I*b
T = N*B*I*(L*b)
T = N*B*I*A……………….1
where A = face area of the coil.
It is clear from eq.1 that “torque is directly proportional to the current passing through the coil”
Damping is provided through Eddy Current.
Torque Equation for PMMC Instrument
The deflecting torque is induced because of the movement of the coil. The deflecting torque is expressed by the equation shown below.
Where, N – Number of turns of the coil
B – flux density in the air gap
L, d – the vertical and horizontal length of the side.
I – current through the coil.
The spring provides the restoring torque to the moving coil which is expressed as
Where K = Spring constant.
For final deflection, By substituting the value of equations (1) and (3) we get,
The above equation shows that the deflection torque is directly proportional to the current passing through the coil.
Error in PMMC Instruments
In PMMC instruments the error occurs because of the ageing and the temperature effects of the instruments. The magnet, spring and the moving coil are the main parts of the instruments which cause the error. The different types of errors of the instrument are explained below in details.
1. Magnet – The heat and vibration reduce the lifespan of the permanent magnet. This treatment also reduced the magnetism of the magnet. The magnetism is the property of the attraction or repulsion of the magnet. The weakness of the magnet decreases the deflection of the coil.
2. Springs – The weakness of the spring increases the deflection of the moving coil between the permanent magnet. So, even for the small value of current, the coil shows a large deflection. The spring gets weakened because of the effect of the temperature. One degree rise in temperature reduces the 0.004 per cent life of the spring.
3. Moving Coil – The error exists in the coil when its range is extended from the given limit by the use of the shunt. The error occurs because of the change of the coil resistance on the shunt resistance. This happens because the coil is made up of copper wire which has high shunt resistance and the shunt wire made up of Magnin has low resistance.
To overcome this error, the swamping resistance is placed in series with the moving coil. The resistor which has a low-temperature coefficient is known as the swamping resistance. The swamping resistance reduces the effect of temperature on the moving coil.
Advantages Of Pmmc Instrument
1. They have low power consumption.
2. Such instruments have uniform calibrated scales.
3. They have a high torque/weight ratio.
4. They have no hysteresis loss.
5. They have very effective and efficient Eddy current damping.
6. They are not much affected by stray magnetic fields.
Disadvantages Of Pmmc Instrument
1. Due to delicate construction, necessary accurate machining and assembly of various parts, such instruments are somewhat costlier as compared to moving iron instruments.
2. Some errors are set in due to the ageing of control springs and permanent magnets.
