PMMC | PERMANENT MAGNET MOVING COIL TYPE INSTRUMENTS

Hello friends , in this article we gonna study about "PERMANENT MAGNET MOVING COIL TYPE INSTRUMENT".

CONSTRUCTION OF PERMANENT MAGNET MOVING COIL TYPE INSTRUMENT

As it 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 figure , it is surrounded by 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. Aluminium frame also provides Eddy current damping.
Two springs are spiralled in opposite direction in order to neutralise the effects of temperature changes.

Fig. Permanent Magnet Moving

Coil Type Instrument

WORKING OF PERMANENT MAGNET MOVING COIL TYPE INSTRUMENT.

When current is passed through the coil, a force acts upon on its both sides which produces a deflecting torque .

 Let ,B = flux density.

L = length/depth of coil in metre.

b = breadth of coil in metre.

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 side is equal to
"B*I*L"

For N turns , the force on its 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 eqn.1 that "torque is directly proportional to the current passing through the coil"

Damping is provided through Eddy current.

ADVANTAGES

1. They have low power consumption.

2. Such instruments have uniform calibrated scale.

3. They have high torque/weight ratio.

4. They have no hysterisis loss.

5. They have very effective and efficient Eddy current damping.

6. They are not much affected by stray magnetic field.

DISADVANTAGES.

1. Due to delicates 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 aging of control springs and permanent magnets.

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