Moving Iron Frequency Meter
Moving iron frequency meters are the meters in which the two coils
are fixed and a moving iron is attached to the spindle. This moving iron
frequency meters depend on the following working principle.
Working Principle
This meter depends on the variations in an electric current
drawn by inductive and non-inductive circuits connected in
parallel. The current flows from these circuits when the frequency changes its
value.
Construction
This meter consists of two fixed coils A and B that their
magnetic axes are perpendicular or each other. A long and soft iron needle in pivoted
at their centers. This circuit remains balanced at the supply frequency
to be measured. Coil A consists of a series resistance Ra and
a reactance La in parallel and the coil B consists of a
series reisitance Rb and a reactance Lb in parallel. The
series inductance helps to suppress higher harmonics in the current waveform
which helps to minimize the waveform errors in the indication of the
instruments.
Working of Moving Iron Frequency Meter
When the supply is connected to the meter, the current pass
through the coils A and B and these two coils produce opposing torques.
When the supply frequency increases then the current of the coil A increases
and decreases in the coil B. The iron needle lies more nearly to the magnetic
axis of the coil A. For low frequencies, the current of coil B increases and
the current of the coil A decreases.
Range
These instruments are designed to measure large amount of
frequency and also for measuring very small amounts.