Weston Frequency Meter

Weston Frequency Meter Points : Weston Frequency Meter This is the moving iron instrument which operates on the principle of variation of impedance in an inductive circuit with the variation in supply frequency. The construction and internal connections of the instrument are shown.

It consists of two pairs of fixed coil set with their magnetic axis at right angles to each other, and a long and thin soft iron needle at the centre of the coils, as shown. The spindle carries a pointer moving over a scale, damping vanes but no control device. One pair of fixed coils AA is connected in series with a non-inductive resistance across a highly inductive coil L8, as shown. The non- inductive resistance RA and the inductive coil LB are connected in series with an other inductive coil L across the supply circuit, whose frequency is to be measured. The inductive Coil L damps out the harmonics in the waveform of the supply through the instrument and SO eliminates errors on account of such harmonics. The two sets of coils A and RB set up two magnetic fields at right angle to each other, whose magnitude depends upon the magnitude of the current flowing through the respective set of coils. Both of these fields act upon the pivoted iron needle, so the position taken by the soft iron needle depends upon the magnitude of two fields or of the currents in coils AA and BB. The values of various resistances and inductances, are so chosen that for normal frequency of supply, the voltage drop across non-inductive resistor RA and that across inductance LB so that the needle takes such a position that the pointer is at the centre, as shown. When the frequency decreases, opposite action takes place and the pointer indicates decreased frequency. Range The general range is from 47 to 53 Hz.