Investigation of parameters affecting the ignition of arc discharges and the development of a high frequency ignition supply

Non-contact ignition of TIG welding arcs has been studied. The variation of dc voltage with dc current of combined acdc discharges indicated that an ac-dominated discharge, a dc-dominated discharge and a transition region exist during the initial current rise after breakdown from cold. These measurements enabled the conditions for reliable ignition of dc arcs using a continuous sinusoidal hf source to be predicted. The minimum current to sustain a cold arc and the time taken to reach the steady-state were investigated using a novel capacitor discharge supply. The results showed that to initiate a3 mm TIG welding arc from cold supplied by a power supply with an open circuit voltage of 80 V, a minimum current of about 0.9 A may be required and the time taken for the arc to reach the steady-state may take several hundred milliseconds. The results of investigations on combined ac-dc discharges, minimum current to sustain a cold arc and the time taken to reach the steady-state indicated that for safe, interference-free and reliable non-contact arc ignition, a continuous sinusoidal hf supply was the best method. A high voltage (about 3 kV) and high current (about 1 A) were required simultaneously to initiate a3 mm TIG arc from cold. A single continuous sinusoidal hf supply required an ignition power of the order of 1.35 kW which was not feasible. An arc ignition method using two continuous sinusoidal hf supplies has been devised which provides safe, interference-free and reliable arc ignition, and which requires less than 75% of the output power of a single continuous sinusoidal hf system. A solid-state hf ignition system based on the new method was designed and constructed.