TWI Industrial Member Report Summary 140/1981
By G B Melton
The buildup of current from zero, on applying a short circuit via a solid state switch, is examined for commercially available equipment for direct current (dc) gas tungsten arc (GTA) welding. The form of the current buildup is described for different types of control circuit - transductor, thyristor converter and switched mode inverter - over periods up to 40 milliseconds. In addition the effects of current setting and instant of start with respect to open circuit voltage are also noted, particularly for single-phase welding sets.
The short-term current characteristics are determined over the first 400 microseconds, and explained in terms of basic circuit components (resistance ,capacitance and inductance). Difficulties in arc initiation due to the form of current. buildup are discussed and an example given of how a welding set with a generally poor arc starting capability was improved by modifying the short-term current characteristics.