Patent GB-524444-A (GB)
Title: Improvements in or relating to electrical measuring arrangements
IPC: G01R27/26
Filed: 1939-01-26
Abstract: 524,444. Power-factor and capacitance measurements. FIDES GES. FUR DIE VERWALTUNGr UND VERWERTUNG VON GEWERBLICHEN SCHUTZRECHTEN. Jan. 26, 1939. Nos. 2764, 2765, [Convention dates, Jan. 26, 1938], and 2766, [Convention date, Dec. 19, 1938]. [Class 37] A bridge circuit for measuring capacitance and loss angle by means of a complex compensator has in the arm containing the standard condenser a part, capable of being tapped, of an ohmic resistance which is connected in parallel with the complex compensator. In the bridge shown in Fig. 1, one arm contains the article to be measured, shown as a condenser CX with a loss resistance of conductance g x in parallel. The adjacent arm contains a standard condenser shown as a loss-free condenser CN with a loss resistance having a conductance gN in parallel. The other two arms contain resistances RX, RN. The resistance RN is a tapped portion of a resistance the other part RO of which is in the circuit of the complex compensator, which is in parallel with the total resistance RN+RO. The complex compensator consists of two slide wires 2, 3 connected to the secondary windings of an ironcored transformer 4 and an air-cored transformer 5, the primary windings of which are connected in series with a resistance rK, having a parallel-connected condenser cK, and one winding 8 of a choking-coil. The second winding 10 of the choking-coil has a tapping 9 connected to the primary winding of a transformer 11 which is connected to the diagonal point 14. For varying the range of measurement of the compensator, the secondary winding of the transformer 11 has a tapping contact 12 connected through a zero galvanometer 13 to the tapping contact of the slide wire 3. The displacements of the tapping contacts on the slide wires 2, 3 when the galvanometer 13 is brought to zero are a measure of the deviations of the capacitance Cx and the angle of loss gX of the article to be measured from those of the standard condenser.. The resistances of the primary windings 4, 5 are so high that the current iN is a small fraction of the total current IN through the standard condenser CN. The resistance rK and capacitance cK are so adjuste,d that the compensating current iN is in phase with that in the resistance RN. In order to eliminate the effect of the earth capacitance of the transformer 1 supplying the bridge, a variable condenser co and resistance ro are connected in series with the resistance RN, and a variable choke La is connected in parallel with the resistance RX. In a modification, Fig. 2, there is arranged in parallel with the resistance RN a resistance 2 and the primary winding 3 of a transformer having two secondary windings 4, 5 connected to a complex compensator 6, the sliders of which are adjusted by knobs 7, 8. The lead from the diagonal point 12 to the article Cx is provided with a screen 13 in the form of a conductive covering. The lead from the other diagonal point 14 to the standard condenser CN has concentric screening covers 15, 16. Screens 17, 18 are provided between the windings of the transformers, and - further screens 19, 20 are connected to the primary windings 3, 11. The screens 13, 16, 17, 18 are earthed, the intermediate screen 15 being connected to the diagonal point 12. The capacitative wattless component due to the screen capacities in parallel with Rx is compensated for by a choking-coil 22 and a series resistance 23. A condenser C6 of such capacitance that C6+C4#C1+C2+C3 is connected as shown. Either of the points 12, 21 may then be earthed without affecting the measurement. The sliding contact on the resistance RN+ RO, Fig. 1, may move only over a small part of the total resistance, for fine adjustment, coarse adjustment being effected by a second slider movable over studs connected to tapping on the resistance RN. In order that the potential applied to the complex compensator may be independent of the position of the sliders, the part of the resistance moved over by the fine adjustment slider is shunted by a second resistance tapped similarly to the coarse adjustment resistance and mechanically connected thereto.