The Development and Performance of Current Comparators for Audio Frequencies

The design and construction of five different multiratio current comparators developed for use at audio frequencies is presented. The effectiveness of the various shields on the reduction of the current null detector error over the operating frequency range is shown. Equations are developed which illustrate the effect of several different arrangements of windings and grounded shields on the ratio winding error. All of the comparators are of the compensated type and each is rated at a secondary current of five amperes. Four have fixed primary windings which can be series-paralleled to obtain various ratios from 1 to 1 to 12 to 1. The fifth comparator is a feed-through type with a 100 turn secondary winding which provides ratios up to 100 to 1. Circuits are described which enable the calibration of these comparators to be performed, and the results of calibration at frequencies up to 16 kHz are given. A method of estimating the error at different ratios to within a part in a million, based on experimentally determined parameters and assumed uniform distribution, is also described. The main conclusion of the study is that for current transformer calibration the best design includes a grounded copper shield between the primary and secondary windings.