Patent GB-727779-A (GB)
Title: Improvements in or relating to systems for controlling the supply of current from analternating current source to a load
IPC: G05F1/34
Filed: 1953-06-23
Abstract: 727,779. Rectifying; supply systems for welding arcs. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. June 23, 1953 [July 26, 1952], No. 17342/53. Classes 38 (2) and 38 (4). [Also in Group XXXVII] In a rectifying or other supply circuit which is periodically interrupted by a contact switching device in series with which is a saturable choke for producing current steps at the opening and closing instants, the positive terminals of two asymmetrical conducting devices having their negative terminals connected together are connected respectively to the two terminals of the switching device, it being so arranged that a current slightly greater in value than that forming the beginning of the steps flows through that asymmetrically conducting device connected to the current input contactor terminal. Fig. 1 shows a rectifying circuit in which D.C. supply leads L1, L2 are fed from a three-phase supply 1 through six contactors 11-16 connected in pairs to the star-connected secondary windings of transformer 2, each contactor having in series a saturable reactor 17 which may carry a biasing winding 17<SP>11</SP>. Each contactor, e.g. 11, has associated with it a low voltage drop gas-filled valve, e.g. T1, having two anodes connected respectively through leads A1, A1<SP>1</SP> to the output and input contactor terminals. The latter anode is energized through rectifiers 20 from two phases of the star-connected secondary winding 23 of an auxiliary transformer 21, the star point of which is connected to a common cathode of valve T1 over a period including the time during which the main contactor 11 can conduct. Closing contactor 11 is initiated when switch 37 is closed and the grid of valve T1 receives a suitable voltage from secondary winding 36 of transformer 31 and battery B at a time controlled by phase shifter 35. Current then flows through the anode-cathode circuit A1-C1, the valve T1 is ionized and an arc strikes in the anode-cathode circuit A1<SP>1</SP>-C1, and current flows through reactor 17 which being unsaturated maintains it at the low, current-step level. The A1<SP>1</SP> anode being at a higher potential than the A1 anode, an anode-anode current flows and passes through the operating winding 19 of the contactor which then closes. The contactor being effectively short-circuited by the low-voltage drop valve T1 the contactor closes onto a very low voltage, and once closed no further current flows through anode circuit A1<SP>1</SP>. The anode circuit A1 continues to carry a current limited by resistor Ra to a value only slightly greater than that of current at the beginning of the current step, so that the valve T1 remains ionized, and when the contactor is opened by its return spring when the main current has fallen to its low current step value at the end of the conducting period, the residue current is transferred to the shunt path provided by the valve T1 where it is finally extinguished. The magnetically-operated contactor (see Group XXXVII) may be replaced by rotating contact devices driven by a synchronous motor, Fig. 2 (not shown), and the double anode valves such as T1 may be replaced by two single anode valves, Fig. 4 (not shown). If the load current is heavier than can be carried by one contact, several contacts may be operated in parallel by a single magnetic structure bearing control windings equal in number to the contacts, each winding being controlled by a separate valve, with anodes connected to the common contactor input terminals, and with heavy grids, in place of second anodes, connected to the separate output terminals and controlling windings, Fig. 5 (not shown). The connections to the valve anode and grid may be reversed, Fig. 6 (not shown). Fig. 3 shows a circuit in which a welding transformer 70 is energized from a single phase supply through a contactor 11 which is closed at a controllable instant in each half-cycle to control the output voltage. Two double anode valves T7, T8 are reversely connected to the contactor, respectively operating when the current flows through the contactor, two directions during consecutive half-waves. The currents through the first fired anodes are supplied from transformers 23<SP>11</SP>, 2411 and the timing of the grid energization from transformer 31<SP>1</SP> is controlled by phase-shifter 35.