©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II 2.4.4
Power
Source -
A direct current, constant voltage power source is recommended
for gas metal arc welding. It
may be a transformer-rectifier or a rotary type unit. The lower
open circuit voltage and self-correcting
arc length feature, as described in Lesson I, makes it most
suitable. Constant voltage power sources used for spray transfer welding
and for flux cored
electrode welding (to be covered later) are the same. However, if the unit
is to be used for
short-circuiting arc welding,
it must have "slope"
or slope control. Slope
control is a
means of limiting the
high short-circuit current
that is characteristic of
this type welder. Figure
13 shows the effect of
slope on the short-
circuiting current.
2.4.4.1 If
we were short-arc
welding at approximately
150 amperes and
18 volts, as shown in Figure 13, and
had no slope components in the power source, the current at short-circuit or when
the wire touches
the work, would be over 1400 amperes. At this high current, a good length
of the wire would
literally explode off the end, cause much spatter, and the arc would be erratic.
With the slope
components in the circuit, the short-circuiting current is in the neighborhood
of 400 amperes,
and the molten ball is sort of pinched off the end of the wire more gently. For
those with an
electrical background, it might be added that in some machines, slope is achieved
by adding a reactor
in the AC secondary of the power source. In others, a slope resistor is
added in the DC
output portion of the circuit. Slope may be adjustable for varying wire
diameters or it may
be fixed, giving a good average value for .035" and .045" diameter wires, the
two most popular
sizes. 2.4.4.2
Another factor influencing the arc
in short-circuiting welding is the rate that the amperage
reaches the short-circuiting current level. Using the example in Figure
13, we know that
the current goes from 150 amperes to 400 amperes during each shorting period.
If we were
to plot the current rise on a graph, as in Figure 14, we would see that the current
rise if very rapid,
as shown by the broken line. FIGURE
13 25
20 15
10 5
200 400
600
800 1000
1200
1400 OPERATING
POINT CONSTANT
VOLTAGE V/A CURVE SHORT
CIRCUITING CURRENT
NO SLOPE SHORT
CIRCUITING CURRENT WITH
SLOPE EFFECT OF
SLOPE ON SHORT CIRCUITING CURRENT V
O L
T S
