Patent GB-725004-A (GB)
Title: Improvements in or relating to apparatus for controlling the supply of current from a polyphase source of alternating current to a load such as electric-resistance welding apparatus
IPC: H05B7/02
Filed: 1953-03-23
Abstract: 725,004. Converting. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. March 23. 1953 [April 23, 1952], No. 7885/53. Class 38 (2). The single primary winding of a welding transformer W is fed from a polyphase source L1, L2, L3 through two groups of main electric discharge devices such as ignitrons I1-I6, I7-I12 which are fired by a group of valves FT1-FT6 equal in number to that of each group of main discharge devices, each valve controlling one main discharge device from each group, the valves being connected to the members first of one group and then of the other group by an automatically controlled reversing switch. This arrangement has the advantage over that described in Specification 730,064 where there is only one group of ignitrons with a reversing switch or switches between them and the welding transformer, in that the switch contacts have only to carry firing current, as against load current, so that the switches may be of lighter construction and speedier operation, e.g. 1/ 60  as against <SP>1</SP>/ 5  second. It also has the advantage over circuits with two groups of ignitrons each having their own firing valves, in that changes in firing valve characteristics with age do not introduce a difference between the values of current passed through the welding transformer in opposite directions. In the arrangement of Fig. 2, each firing valve FT1-FT6 controls a pair of ignitrons connected in parallel opposition between one phase of the supply and one terminal of the welding transformer primary winding. The reversing switches in this case have 24 contacts 821-867, twelve contacts being open while twelve are closed. In the preferred arrangement of Fig. 1A, the paired ignitrons are those which have their anodes or their cathodes connected to the same one of the supply phases L1, L2, L3, and their other main electrodes connected to opposite ends of the welding transformer primary winding. In this way the reversing switches need only fourteen contacts, 601-627, two of them, 613, 615, being necessary to maintain the connections between valves FT4-FT6 and the anodes of those ignitrons whose cathodes are connected to the supply. Not only are the number of contacts reduced over the arrangement of Fig. 2, but the firing valves operate in the same order during output half-cycles of opposite polarity. The firing valves and reversing switches are controlled by circuits similar to those described in Specification 470/53 in such a way that the welding current comprises a train of pulses alternating in direction and separated by a gap of length determined by the time of operation of the reversing switches which train continues while the start switch, Fig. 1C (not shown), is closed. Each pulse comprises a main heat period during which the current builds up while all the ignitrons of one group fire in succession, one or more times, followed by a post heat period in which the same ignitrons fire but with a lowered current setting. The firing valves are energized upon the short-circuiting of resistances in the grid circuits by current from transformers energized from heat-control networks, Figs. 1D-1F (not shown). The operation is controlled by a sequence timer, Fig. 1B (not shown), similar to that in the earlier Specification, comprising main thyratrons which initiate the squeeze, main heat and post heat periods, switching thyratrons and associated timing circuits. The reversing switches are controlled through a light current relay and a stepby-step ratchet-operated relay which rotates one step each time the welding transformer is energized, Fig. 1C (not shown). An interlock prevents the control circuits from rendering the firing tubes conductive, if the reversing switches have not operated fully.