High Current Arc Discharge Phenomena with Particular Reference to the Plasma Torch

Abstract

The purpose of this investigation was to study high current arc discharge phenomena with particular reference to the plasma torch, The principle object was to provide basic information where required to enable the factors affecting the operation of a plasma torch used for industrial process heating to be evaluated. This has been achieved. An extensive literature survey of ( i) Arc discharge processes relevant to the operation of a plasma torch, - (ii) Existing arc devices used for heating gases has been made. A plasma torch operated from both direct current and alternating current has been used to establish the areas where further research was required. The preliminary investigations indicated that further fundamental information on the behaviour of arcs rotated at high velocities between co-axial electrodes in an axial magnetic field was necessary. In particular the conditions required for the apparently diffuse nature of the arc that had been observed at high rotational velocities and the variation of the erosion rate of the arc electrodes with arc velocity were required, These were investigated experimentally and the results employed in an a.c. plasma torch operating at 410V 50 Hz at power inputs up to 60 kW and with conversion efficiencies greater than 60%, measured in terms of electrical power input to the arc and the useful heat output. The results from the literature survey, basic investigations and the final version of the plasma torch have been used to establish the design criteria for a d.c,. or a.c, plasma torch suitable for industrial process heating. In addition results of a wider application have been obtairad including the development of a synchronous switch suitable for supplying accurately controlled high current pulses, improved methods of arc ignition and a new definition of arc stability that enables quantitative measurements of arc stability to be made.