GB1285346A - Connectionless wattmeter - Google Patents

Abstract

1285346 Wattmeter L N LIEBERMANN and S H LAI 4 Nov 1969 [7 Nov 1968] 53924/69 Heading G1U A wattmeter for measuring the A.C. power at a known voltage taken by a load through a conductor 2 comprises an openable transformer core 1 for clamping on the conductor and a secondary winding 3 on the core coupled via an F. E. T. switch 4 to a D.C. meter 5; the core also developing by electrostatic induction a voltage, in phase with the load voltage, for controlling the switch 4. The average current through the meter is proportional to the line current times the cosine of the phase angle between the line current and line voltage and since the line voltage is constant the meter can be calibrated to indicate power. Any necessary phase adjustment can be obtained by capacitor 9 and a quasi ground connection 6 is obtained via the instrument casing and the operator. In a second arrangement, Fig. 2 (not shown), the F. E. T. switch is replaced by a balanced modulator, and the electrostatically induced voltage is amplified by a circuit which obtains its power from a second winding on the transformer core.