| 摘要 |
In a method for facilitating the striking of a pilot arc and main arc in plasma arc cutting torches in plasma gases that are difficult to ionise, when cutting workpieces covered with an electrically insulating surface layer, without establishing electrical contact between nozzle and workpiece, a direct current voltage source is connected on the one hand to the electrode E of the plasma arc cutting torch, and on the other in series with a resistance R to the cutter nozzle D, and an ignition element, which by way of a transformer T generates an ignition pulse between electrode and nozzle. Plasma gas is fed to the gas duct between electrode and nozzle. The ignition pulse energy necessary for ionisation of the plasma gas is kept constant. The ignition pulse is given a duration and polarity in relation to the direct current voltage source such that the pilot arc is established without the involvement of an auxiliary starter circuit. An ignition pulse is produced at a point in time essentially equal to 0.3T, where T is the time constant of the gas system. The time constant T is selected having regard to the area in the inlet and the nozzle duct and the gas flow volume between valve device and nozzle, so that the gas flow at the point 0.3T is essentially equal to 0.3φ max, where φ max is the maximum plasma gas flow, at which the pilot arc is struck by the ignition pulse at the point 0.3T and at the gas flow 0.3φ max. The current-voltage characteristic of the direct current voltage source and the series resistance R in relation to the maximum gas flow φ max are selected in such a way that the pilot arc goes out at point T, and the pilot arc during its burning time T-0.3T melts the surface layer on the workpiece, so that a main arc is established. <IMAGE>
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