Patent GB-1192077-A (GB)
Title: A.C. Regulators
Inventor: KING KENNETH GORDON
IPC: G05F1/44
Filed: 1967-12-05
Abstract: 1,192,077. Automatic voltage and current control. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. Nov. 29, 1968 [Dec. 5, 1967], No. 55282/67. Heading G3R. The A. C. voltage across a load 7 is maintained at a value set by a control circuit 13 by the regulating arrangement shown, in which the circuit 13 varies the firing angle of controllable semi-conductor rectifiers 9, 10 to vary the voltage across a capacitor 8 connected across the D. C. terminals of a rectifier bridge 3---6 in series between the A. C. supply 1, 2, 11 and the load 7. The firing angle is varied between 90‹ and 180‹ in relation to the A.C. supply voltage at terminals 1, 2, the effect being to vary the amplitude of the square wave alternating voltage appearing across the A. C. terminals of the rectifier, which voltage is opposed to the voltage at terminals 1, 2 and so produces at the load 7 a resultant clipped A. C. voltage, Fig. 2 (a) (not shown). The auto transformer 11 provides for voltage stabilization at a load voltage equal to the input voltage at terminals 1, 2, the output waveform being substantially undistorted by virtue of the inclusion of a harmonic filter comprising members 63, 64, 65 of which members 64, 65 form a shunt tuned circuit tuned to the fundamental frequency. Further controllable semiconductor rectifiers 61, 62 are anti-parallelconnected across the A. C. terminals of the bridge 3---6 and, during normal operation, are triggered by pulses from a control unit 66 at an instant 120‹' after the reversal of the load current, thus producing a stepped squarewave voltage across the D.C. terminals of the bridge, Fig. 5 (not shown). The unit 66 also senses the voltage across the bridge 3---6 and the current through it (as sensed by the voltage across the inductance 63) and, should either of these parameters exceed a predetermined value, the unit 66 fires the rectifiers 61, 62 to short circuit the bridge and thus protect the voltage regulating members. Normal triggering of the rectifiers 61, 62 reduces the harmonic content of the unfiltered output waveform and eliminates the third harmonic. Instead of being coupled to the input side of the circuit, the D. C. terminals of the rectifier may be coupled via a converter to the output side of the circuit, which is arranged to regulate in a manner similar to that described. The current through the load 7 may be stabilized by making the unit 13 responsive to load current instead of voltage. A discussion is given of the operation of another embodiment excluding members 61---66, Fig. 1 (not shown) for resistive and reactive loads, Figs. 2 (a) and 2 (b) (not shown). This basic embodiment may also be applied to a three phase system, wherein each rectifier bridge is transformer -coupled to its phase and the bridges are connected to a common capacitor, choke, converter arrangement, the converter being directly connected to the input of the three phase system, Fig. 3 (not shown). In a further three phase embodiment, Fig. 6 (not shown), a star-delta transformer isolates the input and output sides and the star primary has further star-connected windings, these two sets of starconnected windings respectively supplying the rectifiers and the converter which take the form of two six-element bridges. Instead of the two controllable rectifiers 61, 62, a single controllable rectifier may be connected across the D.C. terminals of the bridge, in which case a diode is connected in series with the capacitor 8.