DETECTING CIRCUIT FOR MEASURING CURRENT

摘要

For the measurement of large currents, d.c. as well as a.c., a so called zero-flux current transformer may be used. It comprises two identically wound toroidal cores (11,12), in which the magnetic flux induced by a main or primary current (I1) is neutralized by a counter-flux induced by a compensation current (I2), which by means of an amplifier circuit (40) is passed through a compensation winding (14) common for both cores (11,12) and having many turns. The compensation current (I2) is adjusted by means of the amplifier (40) to such a value, that the mean flux in the cores (11,12) is zero. The intensity of the compensation current (I2) is measured by the voltage drop across a measuring resistor (15). Any unbalances between the primary and secondary ampere-turns are sensed by a detecting circuit comprising a sensing winding (21,22) wound on each core (11,12) and divided into two subwindings which carries a magnetizing current (I3) from a current source (23) in such a way, that the odd-harmonic components contained in the magnetizing current are outbalanced, whilst the even-harmonic components contained therein are added and conducted to a synchronous rectifier (32) to provide a control signal to the amplifier (40) for the adjusting of the compensation current (I2). …Hereby is obtained a largely improved signal-to-noise ratio for the signal applied to the synchronous rectifier (32), and the need for a complex and expensive active 2nd-harmonic filter is eliminated.