A current comparator bridge for power measurement

Abstract

A current comparator technique for the measurement of sinusoidal ac power at 60 Hz is described. The reference power is that generated by applying the voltage across a 10-kΩ resistor. The voltage itself is measured by comparing it directly to a dc reference with an ac/dc rms voltage comparator. The current is compared with the current comparator to the current in the resistor for the in-phase or active power component, and to the current in a capacitor for the quadrature or reactive power component. Six-digit resolution is provided for both components. The bridge can be operated over a voltage range from 50 to 150 V and a current range from 0.01 to 1 A at any power factor. An auxiliary current transformer provides extension of the current range to 10 A. Comparison measurements with a Drysdale wattmeter used as an ac/dc power transfer standard indicates agreement at unity power factor to within 25 ppm.