• Here the situation is different, the voltage setting of the power-source and not the welder controls the arc length.

• In GMA/GTA Welding the feed wire diameter is usually very small and the burn-off rates are far higher than in SMA or TIG Welding, and they vary much more with current. A small variation in current causes significant change in burn-off rate. Some typical burn-off curves for low-carbon-steel wires with carbon-di-oxide shielding are shown in Fig. 3.5. Change in burn-off rates with change in current are also shown. We find that the electrode burn off rate changes rapidly with change in current. Thus we should have a power source which can accomodate these large changes in the

burn-off rates. For a small change in voltage, there should be a large change in current. Special power-sources have been designed for this purpose.

• Some welding power sources are designed to give a flat volt-ampere characteristics as shown in Fig. 3.6 with a voltage falling by 2 V for each 100 A fall in current. This type of characteristics is also known as constant potential characteristics.

increases and the burnoff matches with wire feed rate. The system returns to equilibrium.

• Conversely, if the arc-length shortens, the voltage falls, the current rises, burn-off rate increases, wire melts faster than it is being fed into the area, arc length thus increases continuously till it reaches the preset value. This is called self-adjustment of the arc.

• With electrode wires 0.8-1 .6 mm diameter, this requirement for rapid self-adjust­ment is readily met. For example, with 1 .2 mm wire using carbon dioxide shielding, a change in 20 A causes a change in burn of rate of 0.5 m/min. Thus a change of 1 mm in arc length will be adjusted in (60/500) seconds = 0.12 seconds. Proceeding in the same way we find that in manual metal-arc (MMA) welding a change in arc length of 1 mm

will require 3 seconds to self-adjust itself. This is too long as compared to the time taken by the operator to adjust it manually. Thus, for MMA Welding better results will be obtained if the current is kept constant by the use of drooping characteristics power supply.