Patent GB-676971-A (GB)
Title: Improvements in a synchronous control device for electrical welding machines
IPC: B23K11/24
Filed: 1949-03-15
Abstract: 676,971. Control systems for resistance welding. EVANS, O. (Schlatter, Ltd., H. A.). March 15, 1949, No. 7065/49. [A Specification was laid open to inspection under Sect. 91 of the Acts, Sept. 16, 1949.] Class 38 (iv). A spot or seam welding transformer 1 is connected to an A.C. supply R, S through back-to-back ignitrons 2, 3 having associated firing tubes 4, 5 which are normally maintained non-conducting by negative grid bias provided by a transformer 6 and rectifiers 7, 8 but which may be rendered conducting by closure of a welding switch 11 which completes a circuit for applying pulses of positive bias to the grids of the firing tubes, this circuit comprising a D.C. source u, v, a system of rotary distributers driven synchronously with the supply to the welding transformer by a motor 12, and a system of switches 22 by which the welding period or pattern is determined. The rotor brushes of distributers 14-16 each make one revolution per cycle of the A.C. wave and the speed of the brushes of distributers 20, 21, which are driven through reduction gearing 17, 18, is such that brush 20a of distributer 20 dwells on each of its co-operating stator segments for a period of half a cycle. Distributer 16 connects, alternately, the two cathodes of the firing tubes to the negative line v of the D.C. source, whilst when the ganged switches 30, 31 are in the position for seam (continuous " spot ") welding, viz. opposite to that shown, the distributer 15 intermittently and alternately connects the grids of the firing tubes to the positive line u through the distributer 20, switches 22 and a normally-closed switch 41. Each closed switch 22 causes the flow of welding current for one half-cycle; the welding current may, therefore, flow continuously, or may comprise pulses of any number of half-cycles of current separated by currentless periods of any number of half-cycles, according as to whether all, or only certain, of the switches are closed. The switch 41 is a safety switch which opens upon failure of the cooling system for the ignitrons. Switches 22 may be in the form of a drum controller. For spot welding, switches 30, 31 are set to the position shown and a number of the switches 22 closed, corresponding to the desired period of the weld. Switch 11 is then closed, whereupon relay coils 23, 25 become energized from the source u, v over contacts 28 of a further relay 24 when the brushes of distributers 21, 14 make contact with their fixed segments. Response of relay 23 causes energization of relay 24 over contacts 26 of the first relay, so that 24 picks up and holds in over its own contacts 27. Response of relay 25 connects the common pole of switches 22 to the positive line u over contacts 29, thereby initiating welding. The energizing circuits of relays 23, 25 are interrupted at contacts 28 when relay 24 picks up ; relay 23 drops out in consequence, but the contacts 29 of relay 25 are maintained closed by a holding coil 35 fed from a circuit including the closed switches 22 and the rotor of distributer 20. This circuit is interrupted and thus terminates the weld when the rotor reaches a segment connected to the first open switch 22. The stators of distributers 14, 15 may be angularlyadjustable to permit control of the points in the A.C. cycle at which the ignitrons fire.