A Weston frequency meter is a moving iron type instrument. The action of this instrument depends on the variation of the current distribution in the parallel inductive and resistive circuits with a variation of the supply frequency.
That means there are two parallel paths in this instrument. One is inductive and the other is a resistive path. So, these parallel circuits divide the current through them according to the reactance of the inductive circuit. When frequency changes the reactance also changes. Therefore, the current distribution between inductive and resistive path also changes.
Construction of Weston Frequency Meter
There are two fixed coils in a Weston frequency meter. Both of these coils have two equal halves. We attach these coils in such a way that the magnetic axis of one fixed coil is perpendicular to that of the other fixed coil. Then we pivot a lightweight thin and long iron piece at the center of the space surrounded by the two fixed coils. Also, we fit the pointer and the damping vanes with the spindle of the soft iron piece. Although, the Weston frequency meter does not require any controlling device.
We connect an inductor LA in series with the coil named as A. Then we connect the series combination of coil A, and the inductor LA across a resistance RA. On the other hand, the next fixed coil is connected with a resistance RB. This series combination of coil B and resistor RB is connected across an inductor LB. Then we connect one inductor in inductance L in series with the Weston frequency meter. The purpose of this inductor is to eliminate the harmonies in the waveform of the current. Although we have not shown this inductor in our figure.
Working Principal of Weston Frequency Meter
When we connect the Weston frequency meter with supply, the current starts flowing through both of the fixed coils. The direction of the magnetic field produced by these two fixed coils is perpendicular to each other. But the magnitude or strength of the magnetic fields depends on the magnitude of the current through the respective coils. Both of these magnetic fields act on the pivoted iron piece. The position of the pointer attached with the spindle of the iron piece depends on the strength of the two magnetic fields. The value of the resistance and inductance connected with the coils are so chosen that at the normal frequency the pointer of the Weston frequency meter remains at the middle position of the dial.
Indication of Increased Frequency
When the supply frequency increases the reactance of inductor LA increases. So, the current of coil A decreases. Therefore the magnetic field strength of coil A decreases. Also, the increasing frequency increases the reactance of inductor LB. Therefore, the voltage drop across the inductor LB increases. Again, this voltage appears across coil B. As a result, the current of coil B increases. So the magnetic field strength of coil B increases. Due to this difference of magnetic field strength of coil A and B the pointer of the Weston frequency meter deflects. Because the soft iron piece attached to the pointer tail will try to align more along the axis of the stronger magnetic field. Now it is the field of coil B.
Indication of Decreased Frequency
Again when the supply frequency decreases, the reactance of inductor, LA decreases. Therefore, more current flows through Coil A. Consequently the field strength of coil A increases. At the same time, the reactance of inductor LB decreases due to decreased frequency. Therefore, the voltage drop across this inductor decreases. As a result, the voltage appearing across coil B decreases. Therefore, the current through coil B decreases. Hence, the magnetic field of coil B becomes weaker. Now the pointer will try to rotate in the opposite direction. This is because at that time the soft iron piece will try to align more along the axis or direction of the magnetic field coil A.
So, we have seen that due to change in frequency, the position of the pointer tip on the dial scale also changes. So the different position of the pointer on the dial can be calibrated with supply frequency to be measured.