Weston (M.I) Type Frequency Meter
Principle: It is a moving iron instrument which operates
on the principle of variation of impedance in an inductive circuit with
variation of frequency.
Weston Frequency Meter
Construction: It consists
of two coils A and B mounted perpendicular to each other. Each coil is divided
into two sections. The connections are as shown in the above fig. The branch of
coil has a resistance R1 connected in the series with it while coil
B
has a reactance coil L2 in series with it. Circuit of coil A is in
parallel with a reactance coil L1 while that coil B is in parallel
with a resistance R2. The moving element is a soft iron needle. The needle
is pivoted on a spindle which is also carries a pointer and damping vanes. There
is no controlling torque. The series reactance L0 acts to suppress
higher harmonics in the current of instrument and therefore tends to minimize
the waveform errors in its indication.
Working: When the
meter is connected across the supply, the two coils A & B carries currents
and set up two magnetic fields which are at right angle to each other. The magnitude
of the field depends upon the value of current flowing in the coil. Both these
fields act upon the soft iron needle and the needle takes up a position which
depends upon the relative magnitudes of the two fields and hence of the
currents.
The meter is so designed that the
values of carious resistances and inductances are such that for normal
frequency of supply the value of voltage drops across reactance L1
and resistance R2 send equal currents through coils A and B.
Therefore, the needle takes up a position which is at 45o to both
the coils and the pointer is at the centre of the scale as shown in the fig.
Now if the frequency increases above
its normal value, reactance of L1 and L2 increases while
resistances R1 and R2 remain same. This means that with
an increase in frequency, the voltage impressed upon coil A increases as
compared with that across coil B. hence the current in the coil A increases
while it decreases in coil B. the tendency of the needle is to deflect towards
the stronger field and therefore, it tends to set itself in the line with axis
of coil A. Thus the pointer deflects to the left.
When the frequency decreases, an
opposite action takes place and the pointer deflects to the right.
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