The operation of permanent
magnet moving coil type Instruments are based on the principle that when a
current carrying coil is placed in a magnetic field , torque acts on the coil.
PMMC Instruments construction
Visualise the figure showing various parts of a PMMC instrument.
Magnetic field system
:- As suggested by its name , the instrument consists of a powerful horse-shoe
shaped (U-shaped) permanent magnet made of Alnico. Between the
poles is a rectangular shaped multi-turn coil wound on a light aluminium frame
inside which is a cylindrical soft iron core.
The soft iron core makes the
magnetic field radial & uniform and at the same time decreases the
reluctance of air path between the poles and hence increases the magnetic flux.
Damping system :- The aluminium frame provides damping by the eddy currents induced in it when the
coil moves.
Controlling system
:-
Two spiral shaped Phosphor-bronze springs are connected to the two ends of
the moving coil , one above and one below. The springs provide not only controlling
torque but additionally serve the purpose of lending current in and out of the
coil and
∴ should have low resistance.
The two springs are spiralled in opposite sense to
compensate for the temperature and aging effects on reading .
PMMC Instruments Working
As mentioned earlier , PMMC type Instruments work on the principle that when a current
carrying coil is placed in a magnetic field , a torque acts on the coil.
The top-view explains the working of PMMC instruments
as shown.
Let
, B=magnetic flux density in wb/m2
N=number of turns in the
coil
L=length (vertical arm )
of the coil in meter
b=breadth of the coil in
meter
When current I passes through the coil , force acts on
its vertical arms which constitutes the necessary deflecting torque.
Deflecting torque ,
Td=BINA = BINLb
This same , maximum torque , acts on the coil at each
of its positions as the magnetic field is radial.
This torque move the coil and hence the pointer. The
coil movement will produce controlling torque in the springs.
Controlling torque ,
Tc =K θ ... let K be the spring
constant
At equilibrium,
Td = Tc
i.e K
θ=
BINLb
Let K1=
( BNLb/K ) be a new constant
Then,
θ ∝ I
Showing that PMMC instruments have linear scale.
PMMC instruments Advantages
:-
- High torque to weight ratio.
- Uses very effective and efficient eddy-current damping.
- Since operating field is very strong therefore, they are not much affected by stray magnetic field.
- Have linear scales therefore, readings can be taken easily.
- Since there are no magnetic reversals so no hysteresis loss.
PMMC instruments Disadvantages
:-
- Cannot be used on ac as the average torque over an ac cycle would be zero and pointer always shows zero value.
- These are costlier compared to moving iron type instruments.
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