The Design and Performance of Multirange Current Transformer Standards for Audio Frequencies

Abstract

The design, construction, and performance of two nearly identical, multirange current transformers for operation 400 Hz to 10 kHz and with errors of only a few ppm are discussed. A general discussion of the parameters that influence the performance of single-stage transformers and their consideration in effecting minimum errors throughout a wide frequency band is included. Construction of the two standards having consecutive ratios 1 to 1 to 6 to 1 and operating at a rated secondary current of 5 amperes is described. Circuit diagrams are used to describe 1) the self-calibration circuit for measuring the errors of the 1-to-1 ratios and 2) the (N+1) circuit together with an auxiliary circuit for measuring the errors of ratios> 1. Balance equations are derived in the Appendix. Measured values are presented; included are the results up to 16 kHz obtained at the National Research Council, Canada, during an international comparison and described in a companion paper [6]. The effects of polarization at low frequencies are emphasized by utilizing the 400 Hz data together with information obtained in supplementary experiments; also the role of winding and interwinding capacitances and the necessity for a more explicit definition of current ratio at the higher frequencies is highlighted.