Patent GB-674614-A (GB)
Title: Device for testing the equality of phase, amplitude, or phase and amplitude of a.c. voltages of equal frequency
Inventor: OBERMAN ROELOF MAARTEN MARIE
IPC: G01R29/16
Filed: 1950-11-07
Abstract: 674,614. Voltage and phase difference measurements. STAATSBEDRIJF DER POSTERIJEN TELEGRAFIE EN TELEFONIE. Nov. 7, 1950 [Nov. 11, 1949], No. 27238/50. Class 37. For determining if the amplitude of an A.C. voltage falls below a certain minimum, or for observing phase opposition between a comparison phase and an unknown phase, a transformer is employed for producing a polyphase from a single phase voltage. The apparatus shown in Fig. 1 compares two A.C. voltages which can, e.g. be taken out of a twelve-phase system with equal amplitudes of the different phases. The comparison voltage 'is applied to one of the unconnected terminals of a transformer. BT. The phase to be tested, in the example shown, is scanned by the control wiper of a selector EK to be position, which is connected to the testing apparatus through a group selector GK and cord finder KZ. The control wiper circuit is earthed through a resistance R1, the primary of a transformer PT, and half the primary of the transformer BT, and also through a resistance R2 and the other half of the primary of the transformer BT. This arrangement causes the sum of the two A.C. voltages to be compared to appear in the primary of the transformer PT, while no reaction coupling can occur between the A.C. voltages to be compared. The secondary of the transformer PT is connected to a condenser C1 and resistance R3, by which the single phase A.C. voltage is converted to three phase. This is applied to a rectifier S1 ... S6 and resistance R4 to provide a rectified voltage which disappears immediately when the control wiper scans the correct phase and thereby reduces the single phase primary voltage to zero. Two valves B1, B2, which may be in a common envelope, have anode resistances R8, R9 and a common cathode resistance R10. In each anode circuit is a polarized testing relay WP. When the two A.C. voltages to be compared are unequal the control grid of the valve B1 is negative relative to earth. The control voltage is supplied to the control grid of the valve B2 by a rectifier S7, S8 in the secondary circuit of the transformer BT, a smoothing circuit R11, C2 and charge resistance R7 being included in the circuit. When there is a phase difference between the voltages to be compared the valve B1 is non-conductive and the valve B2 conductive. When phase equality occurs the valves B1, B2 change conductivity and the relay WP operates and interrupts the energizing circuit of the driving magnet KM of the selector. By decoupling the transformers BT, PT and applying the A.C. voltage to be tested to the primary of the transformer PT, the apparatus may be used in combination with a normal zero switch to test for A.C. voltage as regards amplitude, irrespective of phase. In a modification, Fig. 2, a cold cathode tube SV1 is employed having three controlling electrodes, each of which may produce an arc with respect to the two others. The tube is extinguished in the main path if no voltage is applied to the primary of the transformer PT, which has the same function as in Fig. 1. A tube SV2 immediately ignites. Resistances R7, R8, R9 form an artificial star point to which one pole of the anode supply of the tube SV1 is connected, the other pole being connected to a resistance R10.