Patent GB-515032-A (GB)
Title: Improvements in or relating to alternating current measuring arrangements
IPC: G01R1/28
Filed: 1938-05-21
Abstract: 515,032. Current, capacitance, inductance, and phase difference measurements. SIEMENS & HALSKE AKT.-GES. May 21, 1938, No. 15245. Convention date, June 4, 1937. [Class 37] The magnitude and phase of an alternating current is measured by directly superimposing in the circuit of a zero instrument the current to be measured and two auxiliary currents of the same frequency but phase displaced 90‹ with respect to one another, and varying the auxiliary currents until no current flows through the zero instrument. The invention is applicable to the measurement of the losses in condensers and transformers. Preferably one of the auxiliary currents is in phase with a reference voltage or current, e.g., the voltage supplying the current to be measured. Fig. 1 illustrates a known method of measuring the current flowing through an unknown impedance M in which the voltage drop across a non-inductive and non-capacitative resistance Rv in series with the impedance M is balanced by means of a zero instrument VG against a regulatable voltage produced by a compensator K. This comprises potentiometer wires R<1>, R", connected at their mid-points and supplying, respectively, in-phase and quadrature voltage components, which, connected in series balance out the voltage across the resistance Rv. The potentiometer or slide wires R<1>, R<11>, are fed respectively through a resistance R and capacitance C from independent secondaries of a transformer T connected to the mains supplying the current to be measured. The positions of the sliders on the slide wires R<1>, R", are thus a measure, respectively, of the in-phase and quadrature components of the voltage across Rv from which the current required and the impedance M can be calculated. To avoid these calculations, according to the invention, the zero instrument VG, Fig. 3, is connected in series with the unknown impedance M and the current therethrough balanced out by regulatable currents obtained from potentiometer slide wires R<1>, R", connected directly across the independent secondaries of transformer T. Slide wire R<1> supplies a current in phase with the supply voltage U through a resistance R and slide wire R" supplies the quadrature current through capacitance C. The positions of the sliders on the slide wires R<1>, R", are thus a measure, respectively, of the power and wattless components of the current flowing through the impedance M the value of which is thus directly given, since the voltage drop across the zero instrument VG is zero. For comparing a condenser Cx with a standard condenser. Cn, Fig. 6, the two condensers are connected differentially across a high potential secondary W of transformer T, in known manner, the differential current (Ix-In) being balanced out in the zero instrument VG as previously described by in-phase and quadrature current components taken, respectively, from slide wires R<1>, R<11>, the respective sliders of which serve to indicate the loss factor and capacity error of condenser Cx. A current transformer Tx, for example, may be compared with a standard transformer Tn, Fig. 8, by connecting the two primaries in series, the secondaries being connected differentially with a load B and the primary of a further transformer Th. The differential current Im = Ix - In or a portion thereof as determined by resistances R1, R2, and selector switch W, is balanced out in the zero'instrument VG as previously described by power and wattless current components derived respectively from potentiometer wires R<1>, R<11>, connected across independent secondaries of the transformer Th, the positions of the sliders of the potentiometers R<1>, R<11>, being measures of the transformation ratio and angle errors, respectively. The balancing means, i.e., the sliders of the potentiometer wires R<1>, R<11>, may be operated automatically as described in Specification 479,771, for directly indicating or recording the measurements.