Effects of arc current and electrode size on electrode erosion in ac plasma torches

Abstract

Heavy erosion of the electrode is a problem in using ac plasma torches. In a series of our studies, the effects of electrode stem size (diameter and length) and tip shape on the electrode erosion were clarified at a low current of 200 A. In this paper, on the basis of the 200-A data, in order to reduce the electrode erosion with the current increased, the effects of current and electrode size on electrode erosion were clarified experimentally and analytically between 100 and 550 A. The electrode used was of solid type and the material was tungsten containing 2 wt% of lanthanum oxide. The plasma gas was argon. Main results are as follows. (1) As the current increased, the electrode erosion increased linearly. In the case of thin electrodes (8 mm ø), between 450 and 550 A, the electrode tip was molten partially and the electrode erosion increased rapidly over 5 mg/h. However, for thick electrodes (13 mm ø), the electrode erosion did not increase rapidly. (2) The temperature distribution along the electrode axis was calculated. As a result, a good correlation was obtained between the calculated temperature of electrode tip and the measured electrode erosion. As the tip temperature decreased to 3000 K (cf. tungsten melting point: 3660 K), the electrode erosion decreased below 5 mg/h of the linearly increasing range.