GB1067678A - Improvements in apparatus for measuring electrical current - Google Patents

Abstract

1,067,678. Measuring D.C. current. UNIVERSITY OF SOUTHAMPTON and BRITISH PETROLEUM CO. Ltd. Oct. 9, 1963 [Nov. 1, 1962], No. 41360/62. Heading G1U. [Also in Division H3] Apparatus for measuring direct current flowing in a bar form conductor 15 such as a pipe comprises a magnetic transducer including a closed loop magnetic flux circuit, 11 which is placed around the conductor, an input circuit consisting of windings 12a and 12b to which A. C. from a source 13 is applied to saturate the magnetic circuit and an output circuit 14 which provides a signal indicative of the current in the conductor 15. An arrangement for eliminating any error signal in the output circuit is provided externally to the transducer and includes squaring, amplifying and phase and amplitude control circuits 19, 20, 21, to derive from the A. C. supply source 13 a balancing signal of the same form and amplitude as the error signal. The balancing signal is subtractively combined in the differential amplifier 18 with the transducer output, so that the output from the amplifier 18 accurately indicates the current in the conductor 15. The magnetic transducer consists of two magnetic cores 11a and 11b upon which symmetrical input windings 12a and 12b are provided, these windings being arranged to provide opposing flux in the cores. If perfect symmetry is achieved no signal will appear on the output winding 14 in the absence of a current on conductor 15 when the input windings are supplied from the generator 13. Any signal which does appear is fed to a filter 16 which blocks signals of the same frequency as the supply 13, the remainder being passed to the amplifier 17 where the second harmonic of the supply is selectively amplified and passed to the difference amplifier 18. The control circuit 21 in the path of the signal which is fed directly via the circuits 19, 20, 21, is adjusted so that in the absence of current in the conductor 15, zero output is indicated on the meter 22 in the output circuit of the amplifier 18. When a direct current is passed through the conductor 15, the flux patterns in the cores 11a and 11b are affected so that an output appears on the winding 14. This output is passed through the stages 16, 17, 18 previously described and is measured on the meter 22.