GB1294135A - A process and apparatus for the direct-current arc welding of workpieces - Google Patents

Abstract

1294135 Welding by fusion UNITED KING- DOM ATOMIC ENERGY AUTHORITY 28 April 1970 [6 Feb 1969] 6554/69 Heading B3R Arc welding apparatus includes means for determining whether or not a stable arc has been established, and means for switching off the welding current if an unstable arc is found to exist after an initial period usually sufficient for the establishment of a stable arc. As shown, a variable potentiometer 7 (Fig. 3) is set to a reference voltage, say 10 V., corresponding to a stable arc. The potentiometer is connected in circuit with a reference resistor 9 and relay contact DR-1; terminals A, B of this circuit are connected across the main current conductors of the welding circuit (Fig. 1). D.C. welding current is supplied to the torch 2 from rectifier 1; the open circuit voltage may be 100 V. After striking of the arc, the voltage across a relay SR/3 between terminals A, B drops from the open circuit voltage to the arc voltage. If the arc is stable, relay SR/3 experiences a voltage drop of say 10 V. which is insufficient to maintain it energized and, as a result, a weld sequence timer is energized to thereafter control the weld cycle as is usual. If, however, the arc is unstable, relay SR/3 experiences a voltage which may fluctuate in the range 25-40 volts which maintains it energized and prevents the timer being energized. The arc voltage is supplied to terminals A, B so that where the applied voltage exceeds the reference voltage on potentiometer 7, a voltage difference across reference resistor 9 causes a relay FR/1 to be energized, opening its contacts (FR-1). (Fig. 2, not shown), to shut off the welding current. Comparison of the arc and reference voltages is delayed by a delay relay DR/1 whose contact DR-1 retains the reference circuit open for a period usually sufficient to allow a stable arc to be established. This period may be about 10 milliseconds. Thereafter contact DR-1 closes. In the event of an unstable arc being determined, the welding sequence is terminated in a few milliseconds from attempting to strike the arc, before damaging heating of the work can occur. Additionally, the commencement of welding is prevented by harmful impurities in the arc shielding gas; these impurities cause the arc voltage to be higher than under stable arc conditions. The apparatus includes a variable resistor 4 for welding current control, and a spark igniter 5. A Zener diode 12 in parallel with reference resistor 9 prevents a too large voltage input to relay control amplifier 11. Arc ignition can be assisted by providing a large capacitor across the electrode/workpiece gap.