Patent GB-994531-A (GB)
Title: High voltage alternating current bridge
IPC: G01R15/14
Filed: 1963-03-28
Abstract: 994,531. Measuring bridges. CANADIAN PATENTS & DEVELOPMENT Ltd. March 28, 1963 [April 2, 1962], No. 12322/63. Heading G1U. [Also in Division H1] An A.C. bridge comprises a current comparator having a toroidal magnetic core, with primary and secondary windings and a detection winding surrounding the core, and two capacitors; one capacitor being connected in a first series circuit with one such primary or secondary winding, other such capacitor in a second series circuit with other such primary or secondary winding, a detector means indicating zero output of the detection winding, and first and second alternating voltages referred to a predetermined reference voltage connected to corresponding ends of the first and second series circuits, the remaining ends of the series circuits being returned to the reference voltage and to an impedance element whose impedance is small compared with that of the capacitor in the second series circuit and which element is returned to the reference voltage through a further source of alternating voltage in such means that currents flow opposedly in the series circuits to generate a difference flux in the core; while there is provided means modifying the output of the alternating voltage source in magnitude and phase to adjust the junction of the second series circuit and the impedance element to the reference potential level; means responsive to the voltage across the impedance element to develop a control voltage referred to the reference voltage whose magnitude and phase is related to the current in the second series circuit, and means, for generating therefrom a control current which is applied to the comparator device to generate a flux in the core thereof equal and opposite to the difference flux, so that the output of the detection winding is zeroised. A capacitance measurement bridge (Fig. 2) comprises windings 15, 16 the former of which is tapped; respectively connected in series with unknown capacitor C x  and known capacitor C s  returned to earth over secondary of energization transformer T; winding 16 being connected to earth and winding 15 connected over capacitance C 1  (of a small impedance compared with C s ) to point X from which an autotransformer T 3  with an earthed centre tap is connected over variable resistor R 2  to the junction of C s  and 15. Point X is connected to a tap on energization transformer T 1  through mutual inductor M introducing an out of phase component from energization transformer T 2 . An A.C. detector D 1  is connected between the junction 15 and C 1  and earth, while a detector winding 11 linked to 15, 16 energizes A.C. detector D. Winding 11 surrounds toroidal core 10 and is covered by split electrostatic shield 12 overlain by split magnetic core M 1 . A deviation winding 14 (which may be omitted) is covered by split magnetic core M 2  overlain by windings 15, 16, (Fig. 1). The bridge ratio is set by the tapping on 15 and a voltage is developed across C 1  which is proportional to the energization voltage by the ratio of C 1  to C s  while a similar voltage opposed in phase is applied to the upper half of T 3 , developing an equal voltage again phase reversed across the lower half thereof. The voltage tappings of T 1  and T 2  are adjusted until D 1  reads zero and point P is electrically equivalent to earth, after which R 2  and if necessary the tap on 15 are adjusted until null is indicated on indicator D. R 2  may be calibrated in terms of the losses of C x  while the capacitance thereof is related to C s  by the ratio of arms 15, 16. In a modification (Fig. 3) windings 15, 16 are both tapped, and the input of an amplifier A is connected between point P and earth, whose output excites transformer T 3 , the ends of whose secondaries are selectively connectible over a switch and variable resistance R to the junction of 15 and C s . Input point P is automatically held at an approximation to earth potential, and the balance at detector D is achieved by adjusting the taps on 15, 16, the tap on an additional deviation winding 14 energized from T 3  over capacitance C 2  for fine in phase adjustment, and variable resistor R for fine quadrature adjustment. At null it is shown that where n p , n s , n d  are the numbers of turns tapped on windings 16, 15, 14 respectively; and that the loss factor of C x  is given by  # 1/RC 1  so that R may be directly calibrated. The quadrature current through resistor R may be introduced into a separate deviation winding 14a (Fig. 5 not shown) whose variable tapping is gauged with that of winding 15. Voltages may be compared by replacing capacitors C x , C s  by known capacitors Ca, C b  returned to earth through windings of a voltage transformer delivering voltages V p  and V s  (Fig. 6). Having primarily balanced the bridge for equal voltages applied to the capacitance circuits, a second balance of the bridge connected as in Fig. 6 establishes the magnitude and phase relationship between the voltages.