6
INDUCTANCE Power
sources do not respond instantly to load changes. The current takes a finite time
to attain a new level.
Inductance in the circuit is responsible for this time lag. The effect of inductance
can be illustrated by analyzing
the curve appearing in Figure 3-5. Curve A shows a typical current-time curve
with inductance pre- sent
as the current rises from zero to a final value. Curve B shows the path which
the current would have taken
if there were no inductance in the circuit. The maximum amount of current attainable
during a short is determined
by the slope of the power source. Inductance controls the rate of rise of short
circuit current. The rate
can be slowed so that the short may clear with minimum spatter. The inductance
also stores energy. It supplies
this energy to the arc after the short has cleared and causes a longer arc.
In ”short arc” welding, an
increase in inductance increases the ”arc on” time. This, in turn, makes
the puddle more
fluid, resulting in a flatter, smoother weld bead. The opposite is true when the
inductance is decreased.
Figure 3-6 shows the influence of inductance on the appearance of ”short-arc”
welds made both with
an argon-oxygen gas mixture and with a helium-argon-carbon dioxide mixture. Weld
No. 1, made with a mixture
of 98% argon and 2% oxygen shielding gas and no added inductance, is rolled or
peaked as seen in the
top cross-section. Midway along the sample, inductance of about 500 micro-henries
was added. Freeze lines
are not as prominent, and the bead remains convex. Continued
on next page... Figure
3-5 - Change in Current Rise Due to Inductance
