GB953658A - Improvements in or relating to control-devices for use in resistance welding systems - Google Patents

Abstract

953,658. Control systems for resistance welding. PHILIPS ELECTRICAL INDUSTRIES Ltd. Jan. 24, 1961 [Jan. 27, I960], No. 2773/61. Heading H2H. To delay initial conduction of the main valves in a resistance welding system comprising opposed parallel connected main discharge valves feeding a welding transformer and which are controlled by auxiliary discharge valves B1, B2 themselves controlled by main pulses 7 superimposed upon a direct voltage 3 defining the welding period X further pulses 8 which lag the control pulses 7 are uttlized to control the auxiliary discharge valves during an initial period in which a third auxiliary valve B3 shorts the main control pulses 7. Pulsatory voltages 7 from a saturated transformer T1 supplied through a phase shifting bridge 1 control the gas discharge valves B1, B2 which themselves, through the outputs from transformers T3, T4, control the ignition of the main discharge valves, not shown. A negative bias for valves B1 and B2 is obtained from a capacitor C2 and is applied in series with the positive voltage 3 which allows the pulses 7 to initiate welding for a " Mark " X (Fig. 2) whilst suppressing it for a " Space " Y. Valves B1, B2 respectively control welding on odd and even half cycles of the welding supply. Secondary pulses 8 lagging the pulses 7 by a variable amount are generated by a saturated transformer T2 fed from an adjustable phase shifting bridge. A second square wave voltage 5 having a positive front which coincides with that of the square wave 3 is applied in series opposition with a smaller positive bias voltage, obtained at capacitor C1, to a delay circuit R; C3 in the control circuit of valve B3, which short circuits the positive pulse 7 which would otherwise ignite the discharge valve B1 as soon as the square wave voltage 3 occurs, the valve then being conductive at this instant due to the positive bias across the capacitor C1 Valve B1 thus ignites under the control of transformer T2. Square wave 5 becomes effective after a time lag determined by the time constant R.C3, to cut off auxiliary valve B3, so that subsequent ignitions of valves B1 and B2 during the welding period are determined by the voltage pulses from the secondary winding of the transformer T1. The time constant R.C3 is chosen so that the auxiliary valve B3 is out off within half or one cycle of the supply. At the end of a welding period X the square wave 5 again becomes zero so that the bias voltage at capacitor C1 becomes operative with a time lag due to R.C3 as shown by the line 10. In order that valve B3 may again be sufficiently conductive within the period of one cycle, the resistor R may be shunted by a rectifier 11. In the above description, it is assumed that valve B1 is always ignited first, but if this is undesirable, a second secondary winding of transformer T2 may be connected between the right-hand secondary winding of T1 and the grid of the valve B2. A second valve B3 controlled from the same circuit may then be connected in parallel with the right-hand secondary winding of T1, to short circuit the pulses 7 and valve B3 must then be blocked within half a cycle.