| 摘要 |
A first stop valve (4) is connected in parallel with a serially connected second stop valve (7) and a gas flow regulating means (6) between a supply of working gas and a plasma torch (1). In response to a start signal ST, the second stop valve (7) is opened so as to supply the working gas at a small flow rate QP to the plasma torch (1) via the gas flow regulating means (6). After a pilot arc is started, the first stop valve (4) is gradually opened so as to gradually increase the flow rate of the working gas from the small flow rate QP up to a normal flow rate QM, and at the same time, the pilot current is gradually increased from an initial pilot current level IS to a pilot current level IP, corresponding to the gradual increase in the flow rate of the working gas up to the normal flow rate QM. In response to a stop signal SP, the arc current is gradually reduced from the cutting current level IM to a lower level ID at which the main arc (13) is extinguished. A resistor (12a,12b) is connected in a current path between a constant current source (8) and the plasma nozzle (1a), with the current source (8) having a plurality of switchable current levels and the resistor (12a,12b) having a corresponding plurality of resistance levels, whereby the nozzle-workpiece voltage can be maintained substantially constant.
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