The design and performance of high-precision audio-frequency current transformers

Current transformers of existing design are found to have large high-frequency errors. The causes of these errors are examined and it is shown that they are due to the high values of leakage inductance, self-capacitance and concentrated inter-winding capacitance that result from the conventional multi-layer form of construction. Greatly improved performance can be obtained if the windings are arranged in uniformly-distributed single layers with a suitable thickness of insulation between them. Approximate formulae for calculating the various constants and errors of a transformer having single-layer windings are given, together with a design procedure for any specified limit of error over a wide band of frequencies. The practical aspects of the design are also discussed. Multi-ratio transformers in which all the ratios have virtually identical errors over a band of frequencies can be obtained by sub-dividing single-layer primary windings into sections, all of which are used by means of series-parallel connections for each ratio. Details are given of the design and performance of precision single-layer multi-ratio transformers rated from 5/5 to 400/5 amp. Their errors do not exceed 5 parts in 105 in ratio and 0.3¿ in phase over the frequency range 400c/s-10kc/s and are less than 2 parts in 104 and 1¿ over the range 50c/s-30kc/s.