Patent GB-699783-A (GB)
Title: A device for measuring direct currents
IPC: G01R19/20
Filed: 1951-11-07
Abstract: 699,783. Current measurement; measuring bridges. ALLMANNA SVENSKA ELEKTRISKA AKTIEBOLAGET. Nov. 7, 1951 [Nov. 13, 1950], No. 26046/51. Class 37. In apparatus for measuring direct currents by means of a transductor provided with at least two separate magnetic cores having windings, an alternating current source feeds the transductor windings, rectifier elements associated with the alternating current fed windings produce unidirectional ampere turns therein to magnetize the transductor in dependence on the alternating current, and the transductor is magnetized by the direct current to be measured in opposition to the unidirectional ampere turns; further rectifier elements being combined with the alternating current fed windings to admit at each instant to a current measuring instrument a direct current equal to the numerically largest of the currents in the alternating current windings. Fig. 1 shows two transductor elements traversed by the field of a busbar 3 (or measurement winding) carrying a D.C. Is to be measured; whose A.C. windings 1, 2 are connected in series with and between the half-wave rectifiers 4, 6 and 5, 7 of a fullwave rectifier bridge energized by an A.C. supply 12 across the junctions of rectifiers 4, 5 and 6, 7. A D.C. instrument 13 is connected between the junction of rectifier 6 with winding 1 and the junction of rectifier 7 with winding 2, and it is shown that if unidirectional rectified currents i, i 2  traverse windings 1, 2 respectively in a particular half cycle, i 2  is greater or less than i 1  according to the direction of the busbar current and the net effect of the current unbalance is to unidirectionally and degeneratively magnetize the transductor cores in opposition to the magnetization by the busbar current, whereby accuracy is increased and the consumption of reactive power is decreased, while the larger of the two unidirectional currents in the windings is a measure of busbar current and deflects the measuring instrument accordingly. Fig. 5 shows two transductor elements traversed by the busbar field which have each two opposed windings 20, 21 and 22, 23, each of which windings is connected in series with corresponding half-wave rectifiers 24, 25, 26, 27 and an A.C. source 28. Windings 20, 22 are connected in series with a fullwave rectifier bridge 29 and windings 21, 23 in series with a similar bridge 30; the D.C. terminals of the bridges being connected in series with a measuring instrument 13, which indicates the larger of the unidirectional currents flowing in windings 20, 22 and 21, 23 as a measure of the busbar current Is, while the field of the unbalance currents degeneratively opposes the busbar magnetization. In a modification, Fig. 6 (not shown), the transductor elements are double wound and in each transductor one winding is connected in series with two bridge rectifiers in a corresponding branch across the A.C. supply. The first bridge rectifier output of each branch energizes the second winding of each transductor to oppose the busbar magnetization, while the outputs of the second bridge rectifiers are connected in series through a D.C. instrument indicating the busbar current, as measured by the instantaneously larger of the transductor currents. Fig. 7 shows three single-wound transductors 43, 44, 45 magnetized by the field of busbar 3, whose individual windings are respectively connected in series with halfwave rectifiers 50, 51, 52 and the secondaries of 3-phase transformer elements 47, 48, 49; each series combination being shunted by a respective half-wave rectifier 53, 54, 55 connected in series with each other and a D.C. instrument 13, energized by the instantaneously largest current in the transductor windings to measure the busbar current Is and the unbalance current in the transductors degeneratively opposing the busbar magnetization. Fig. 8 shows a modification of Fig. 1 for heavy busbar currents wherein further windings 56, 57 on each transductor element are traversed by the unidirectional current through the meter to further oppose the busbar field. To ensure that the device operates over the most linear branch of the response curve, the transductors may be premagnetized by a unidirectional current rectified from an A.C. source which energizes two additional series-connected windings of the transductors, and also traverses the measuring instrument to cancel out the D.C. derived from the transductors due to their premagnetization, Fig. 9 (not shown). In Fig. 10 a premagnetizing current in transductor windings 58, 59 derived from an A.C. energized bridge rectifier 60 is adjusted by a variable resistance 68, and the unidirectional measuring current derived from the transductors energizes the excitation winding 62 of an auxiliary transductor 63, having another excitation winding 65 energized from the rectifier 60 through a variable resistance 69 to cancel out the effect on the auxiliary transductor of the D.C. derived from the main transductors due to their premagnetization. The auxiliary transductor output is coupled through a current transformer to a bridge rectifier 67 supplying the D.C. instrument 13, which indicates the value of the current in busbar 3.