GB674700A - Improvements in and relating to electric welding control systems - Google Patents

Abstract

674,700. Control systems for resistance welding. BRITISH THOMSON-HOUSTON CO., Ltd. July 6, 1950 [July 21, 1949], No. 16924/50. Class 38 (iv). Back-to-back resistance welding ignitrons 5, 6 are controlled, so as to pass increasing current as the weld proceeds, by so controlling the phase of the voltage applied to a peaking transformer 24 for the firing valves 7, 8 that the firing-point is progressively advanced, phase control being effected by a condenser 10 which controls the conductivity of hard valve rectifiers 36, 37 arranged, when conducting, to form a variable shunt (through transformers 35) on a " heat control " rheostat 31 which forms one element of a phase-shift bridge through which the transformer 24 is supplied, the other elements of the bridge comprising capacitors 28, 29, inductor 27b and resistor 27. A capacitor 38 charged over a rectifier 39 normally holds the rectifiers 36, 37 non-conducting, but upon closure of a weld-initiating switch 51 (and the accompanying actuation of a contactor 49 and conduction of an initiating valve 46) condenser 10 commences to charge and applies a progressively increasing " turn-on " voltage to the grids of rectifiers 36, 37. Timing of the weld is effected by normally non-conducting valves 11, 12, the former being held off by D.C. bias obtained from a potentiometer 18 and the latter (which is arranged to trail valve 11) by A.C. bias obtained from transformer 25. Valve 11 conducts, upon actuation of the initiating contactor 49, as a result of depression of cathode potential consequent upon change-over of'contacts 49b, 49c, the cathode potential subsequently rising as condenser 10 charges and eventually terminating conduction. During the conducting period, a transformer 13 fed through valves 11, 12 generates a voltage in the grid circuits of the firing valves 7, 8 which nullifies a hold-off voltage obtained from a transformer 26 so as to allow the peaked voltage from transformer 24 to be effective. In a modified arrangement the charging voltage for condenser 38 is obtained through a transformer shunting the " heat control " rheostat 31, a change-over contactor actuated in response to conduction of the timing valves providing for reversal of charge.