Advanced Welding Processes: Technologies and Process Control

Power
source
Advanced gas tungsten arc welding 77
however, encountered with high-voltage leakage and the system has not
been exploited commercially.
Arc stabilization
Resistor
Electronic
control
A B C D
8CR
Electrode
Current
Voltage
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
6.1 Programmed touch striking system for GTAW. Top: simplified
circuit diagram. Bottom: current and voltage during programmed
touch striking.
Stage II arc initiation or stabilization is largely dependent on the rate at
which the power source can supply current to the embryo arc after initial
breakdown. Conventional GTAW power sources have been shown to have a
current response of around 10 4 A s –1 at the optimum settings and with a
favourable phase relationship, [84] although much lower rates of rise could
be experienced, particularly on single-phase units, when the arc initiation
takes place out of phase with the mains supply. It would be expected that this
rate of rise of current could lead to striking difficulties especially at low set
currents and when long, inductive welding cables are used. In these
circumstances effective arc initiation often depends on the use of a capacitor
in parallel with the output which can discharge into the arc.
Electronically controlled power sources, such as the series regulator and
inverter designs described in Chapter 3, are capable of giving much higher
rates of initial current rise (typically 5 ¥ 10 4 A s –1 ) and are less sensitive to
phase relationship problems.