On a simple method for wattmeter calibration

Abstract

A simple method for wattmeter calibration, good also in the case of quadrature between current and voltage, is described. Essentially a mutual inductor is loaded with a non-inductive resistor. First the value of the power loss inside the resistance which loads the secondary of the mutual inductor is measured, and then, by calculation, the value which should be read on the wattmeter is obtained. In a practical case, using instruments with an accuracy of about 0.1% to measure the circuit parameters, in a range of frequencies from 1 to 400 Hz and phase angles between current and voltage to 88 degrees , the accuracy obtained for the wattmeter calibration is about +or-0.5%. The calibration accuracy essentially depends on the accuracy evaluation of the ratio between the value of the mutual inductance and the self inductance of the secondary winding. Another advantage of this method is that it can be used to calibrate the whole wattmeter, including the input attenuators, but making use of a suitably dimensioned mutual inductor.