METHOD OF CONTROLLING THE OPERATING CONDITIONS OF AN APPARA TUS FOR PLASMA-ARC TREATMENT OF ARTICLES AND AP APPARATUS FOR PLASMA-ARC TREATMENT

摘要

1331139 Welding by fusion I S SHAPIRO L M PERSITS M V TKACHEV and L E LINETSKY 29 July 1971 35613/71 Heading B3R [Also in Division H2] In a method of controlling a plasma-arc device 1 for cutting or welding, the voltage between the cathode 2 and the gas supply nozzle 3 is sensed to enable the increase of the arc current to be discontinued when an abrupt voltage increase is detected. The voltage is sensed by a circuit 8 comprising a filter 24 and a threshold circuit 25, the output of which is connected to a current supply source 7 for the plasma-arc device. The source 7 comprises a d.c. generator 11, a circuit 12 for increasing the supply current, a circuit 14 for decreasing the supply current and a counter 16. When the arc current exceeds a certain value the circuit 8 supplies a pulse to a circuit 13 to switch off the circuit 12 and to the counter 16 which enables the circuit 14 to reduce the arc current. Other pulses may subsequently be received by the counter, for example if the nozzle contacts the article 4 or if the gas supply to the nozzle is reduced, each pulse causing a successive decrease in the arc current. When the counter is full it operates a unit 15 to disconnect the generator 11. The sensing of the abrupt voltage increase which occurs when the arc shorts across to the nozzle enables this shorting to be stopped by reducing the arc current before damage to the nozzle occurs. The cathode is made of lanthanized tungsten and the copper nozzle is water-cooled. In a modification, the source 7 includes a D.C. and a pulsed-current generator connected in parallel and a two-position switch which selectively enables either one of the generators to provide the bulk of the arc current.