©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II 3. Distance
from Work
- If the distance from the work to the power source is great,
AC is the best choice since the voltage
drop through the cables is lower than with DC.
Even though welding cables are made of copper or aluminum (both good
conductors), the resistance in the
cables becomes greater as the cable length increases.
In other words, a voltage reading taken between the electrode and the
work will be somewhat lower than a
reading taken at the output terminals of the power
source. This is known as voltage drop. 4. Welding
Position (See Appendix A - Glossary
of Terms) - Because DC may be
operated at lower welding currents,
it is more suitable for overhead and vertical welding
than AC. AC can successfully be used for out-of-position work if proper
electrodes are selected.
5. Arc
Blow - When welding with
DC, magnetic fields are set up throughout the weldment.
In weldments that have varying thickness and protrusions, this magnetic
field can affect the arc by making
it stray or fluctuate in direction. This condition is
especially troublesome when welding
in corners. AC seldom causes this problem because
of the rapidly reversing magnetic field produced. 2.2.2.2
Combination power sources that produce
both AC and DC are available and provide
the versatility necessary to select the proper welding current for the application.
2.2.2.3 When
using a DC power source, the question of whether to use electrode negative
or positive polarity arises. Some
electrodes operate on both DC straight and reverse polarity,
and others on DC negative or DC positive
polarity only. Direct current flows in one direction in
an electrical circuit and the direction
of current flow and the composition of the electrode coating
will have a definite effect on the welding arc and weld bead. Figure 3 shows
the connections
and effects of straight and reverse polarity. 2.2.2.4
Electrode negative (-) produces welds
with shallow penetration; however, the electrode
melt-off rate is high. The weld bead is rather wide and shallow as shown
at "A" in
Figure 3. Electrode
positive (+) produces
welds with deep
penetration and
a narrower weld bead
as shown at "B"
in Figure 3. FIGURE
3 DC
POWER
SOURCE ELECTRODE
DC POWER
SOURCE ELECTRODE
A HIGHER
BURN-OFF RATE, LESS PENETRATION
DEEP PENETRATION,
LOW BURN-OFF RATE
WORK PIECE B
STRAIGHT POLARITY
REVERSE POLARITY
WORK PIECE
