GB1367900A - Apparatus for measuring two parameters of impedances - Google Patents

Abstract

1367900 Measuring impedances INSTITUT ELEKTRODINAMIKI AKADEMII NAUK UKRAINSKOI SSR 30 June 1971 8130/71 Heading G1U In apparatus for measuring the inductance and loss angle tangent of a coil 9, a balancing voltage U x is applied to the coil 9, a reference voltage U o is applied to a standard inductance 13, and a comparator 15 detects the difference I x -I o between the currents in the coil and inductance respectively and then reduces this difference to zero by adjusting two balance controls to vary the magnitude and phase of the balancing voltage, the balance controls being brought to their respective balance points independently of each other. The balancing voltage U x is produced by adding together the voltage U<SP>0</SP> x from an adjustably-tapped winding 4 and the voltage U y provided at the output of a 90 degree phase shifter 5 which receives (7) an adjustable proportion of the voltage U<SP>0</SP> x . To reduce the out-of-balance current I x -I o (phasor CD) to zero, a selectable first trial current, from a source 8, of either #I L <O degrees or #I L <180 degrees, is superimposed on the current I x . The phasor representing this trial current is inclined relative to U<SP>0</SP> x at an angle equal to the phase angle of the unknown impedance. A control device 14 senses which of the two selectable phases of the trial current causes a decrease in the outof-balance current, and adjusts the tapping on winding 4 in the corresponding direction until the condition is reached where the addition of the trial current in either phase increases the out-ofbalance current. This will occur when the tip of the I x phasor reaches d 1 , where Cd 1 is perpendicular to the phasor #I L , the locus of the I x phasor as the tapping is adjusted being along OD since the magnitude of the balancing voltage U x is changed but not its phase. When this minimum has been reached, the source 8 is rendered inoperative and a further such source 18 rendered operative to inject second selectable trial current #I tg # <O degrees or <180 degrees into the comparator 15 via a resistor 19, the phasor representing this trial current being parallel to U<SP>0</SP> x , and the control device 14 then adjusts the tapping on winding 7 in a similar manner. Since only the voltage U y is being varied the locus of the tip of I x phasor is now along a line perpendicular to I<SP>0</SP> x , and so I x will be brought to coincide with the current I 0 through the standard inductance 13. Balance is thus achieved in only two steps because the locus of the I x phasor when the second adjustment is being made is perpendicular to the trial current phasor during the first adjustment. Winding 7 may be the first one which is adjusted in which case the I x phasor moves to C via the path D-d 2 -C.