Patent GB-669306-A (GB)
Title: Improvements in or relating to electric supply stabilising apparatus
Inventor: PATCHETT GERALD NORMAN
IPC: G05F1/52
Filed: 1948-03-16
Abstract: 669,306. Automatic voltage, current and power control systems. TINSLEY & CO., Ltd., H. March 16, 1949 [March 16, 1948; June 22, 1948], Nos. 7887/48 and 16773/48. Class 38 (iv). In an automatic control system for voltage, current, or power, fluctuations in the output quantity are detected by a bridge which includes a non-linear element, the voltage/current characteristic of which has a negative slope over at least a part of its range. As applied to an A.C. voltage regulator, Fig. 2, the secondary of a controlled booster transformer 15 is in series between the input 10, 11 and the output 12, 13. A potential transformer 21 connected across the regulated output, suplies, via a temperature compensating network 25, 26 a measuring bridge 28 including as one arm a thermistor 29 which is shunted by a power factor correcting condenser 30. The output of the bridge is passed by a low-pass filter to amplifier V1, V2, V3, a phase splitter V4 and push-pull connected valves V5, V6, which, by supplying a current of appropriate phase to the booster transformer 15 maintain the output voltage constant. A transformer 18 and rectifier 19 provide heater and anode circuit supplies for the valve circuits. When the regulator is first put into operation, a relay 31 connects the thermistor 29 in series with a resistor across the transformer 21 so as to heat the thermistor into its working range. A pushbutton 50 is then closed and relay coils 47, 48, change over relay 31 to connect thermistor 29 into the bridge circuit, and open contacts 51 which have been shortcircuiting the regulating transformer 15. Range limiting is effected by applying the output of the bridge, amplified by valves V1, V2, to the grids of a pair of valves V7, V8, the anodes of which are fed in phase opposition through relay coils 52, 53, from tappings T7, T8 on the transformer 18. Normally, relay coils 52, 53 are energized to move contacts 39, 36, to the position shown and to close the hold-in contact 49. If the supply voltage rises beyond the controllable range of the regulator, valve V8 ceases to conduct, and by de-energizing relay coil 53, changes contact 36 over to apply a preset A.C. voltage from potentiometer 42 to the grid of valve V3 so that the control effect remains constant at the fullload value. Conversely, if the supply voltage falls below the controllable range, valve V7 ceases to conduct and contact 39 changes over to earth the grid of V3, contact 49 is opened, contact 51 closes, and contact 31 changes back. The regulator remains inoperative until the input voltage is increased and push-button 50 again operated. In a modification, Fig. 7 (not shown) shutting down at low inputs is avoided by providing an auxiliary transformer, normally short-circuited, which injects into the bridge input circuit a voltage to maintain the stabilizer at maximum output, if the input voltage falls below the lower limit. The starting button and regulator transformer shorting contacts are omitted, and other minor circuit re-arrangements are made. In a further modification, Fig. 8 (not shown), a wider operating range is obtained by supplying the regulator from a tapped transformer, tap changing being effected by an electric motor, relay controlled by the upper and lower limit control valves V7, V8. The above arrangements may be used to control current instead of voltage. To control power, for tertiary A.C. or D.C. watt-hour meters, a bridge 28, Fig. 3, including an indirectly-heated thermistor 29, is connected across a resistor R in series with the current coil I of the meter. A second bridge 54, which may include a thermistor in one arm, is connected across the potential coil P of the meter, the output of this bridge feeding the heater of thermistor 29 so that the output of bridge 28 controls a device S to maintain the product of voltage and current constant. Alternative temperature compensating arrangements are described and also alternative circuits for correcting the power factor of the bridge thermistor.