GB729745A - Improvements in electric arc welding systems - Google Patents

Abstract

729,745. Supply systems for welding arcs. FOSTER TRANSFORMERS, Ltd., [formerly FOSTER TRANSFORMERS & SWITCHGEAR, Ltd.], and JENNINGS, R. E. Jan. 16, 1953 [Jan. 22, 1952], No. 1732/52. Class 38 (4). In an A.C. arc welding system in which the stability of the arc is improved by injecting into the welding circuit pulses of high voltage high frequency current obtained from a spark-gap oscillator, the spark-gap is arranged to flash over once only per half cycle of the main supply so as to produce a single non-oscillatory pulse of high-frequency energy timed to occur at or immediately after the instant when extinction of the welding are would occur. In the system shown in Fig. 1; the welding arc W is supplied from a power transformer T through a current regulator Z and the secondary winding of a high-frequency transformer 10, the primary winding of which is in the discharge circuit of a condenser 9. The last, together with a triggered spark gap 14-16 and a charging resistor 8, form a relaxation oscillator energized over a transformer 7 and phase shifter PS, the condenser slowly charging over part of each half cycle of the mains voltage and then being quickly discharged by the spark-gap when triggered by a transformer 17, triggering occurring when the welding circuit voltage passes through zero. A condenser 20 and resistor 19, connected as shown, or across the reactor Z, or fed from a separate transformer, may provide further arc-maintaining current assisting the high-frequency pulse. A relay, controlled by a current transformer in the welding circuit, may have contacts in the primary circuit of transformer 17, the arrangement being such that during normal welding the transformer triggers the spark-gap as described above, but in the absence of flow of welding current the primary circuit of transformer 17 is open at the relay contacts. In consequence the spark-gap now breaks down when the A.C. voltage is at or near its peak value and the high-voltage pulse is thus applied to the welding circuit at the most favourable instant in the cycle to initiate an arc. In a modified arrangement, Fig. 6, the transformer 7 supplies a simple spark gap 26 and is energized over an integrating circuit 23, 24. A change-over relay 22 controlled by current transformer CT provides that under non-welding conditions the transformer 7 receives only the voltage across condenser 24, so that the spark gap breaks down at a voltage peak of the A.C. cycle, whilst, during welding, the voltage across the welding circuit is added to that across condenser 24-the gap then breaking down at a point in the cycle when the welding current passes through zero.