Chapter 29 Shielded Metal Arc Welding 781
Copyright Goodheart-Willcox Co., Inc.
Duty Cycle
Welding machines are classifi ed by their maximum amperage output and
their duty cycle. A duty cycle is a rating that indicates how long a welding
machine can be used at a given output current without damaging it. Duty
cycle is based on a 10-minute time period while the machine is switched on.
A welding machine with a 50% duty cycle can produce a welding current at
its maximum rated output current for 5 out of every 10 minutes. It must be
left running to cool down for the remaining fi ve minutes. A welding machine
is likely to overheat if the duty cycle is exceeded. Some machines have an
automatic cutout, or switching device, to prevent damage to the power unit.
At lower current settings, the duty cycle typically increases and the power
source may be used for longer periods of time.
When selecting a welding power source, choose a unit that can meet
most welding needs at the middle of its output range. For example, if most
of your welding will be done at the 100 A to 120 A level, a machine rated at
200 A to 250 A maximum output will supply adequate power at a reasonable
duty cycle.
Welding Leads
The cables that connect the welding power source
to the work are known as welding leads. Welding
leads must be able to carry large amounts of current
while remaining fl exible enough to be manipulated
by the welder. Since electrons travel on the surfaces
of a conductor, a multiple-strand conductor can carry
more current than a single-strand conductor of the
same size. Welding leads consist of a twisted copper
wire core composed of many tiny strands bound into
multiple bundles and covered by a heavy rubber or
neoprene insulating sheath. See Figure 29-5. The
leads are connected to the power source directly
with crimp-on lugs or quick connectors, Figure 29-6.
Goodheart-Willcox Publisher
Figure 29-5. Welding cable is composed of many
small strands of wire to carry large amounts of
current and to remain flexible.
B
Goodheart-Willcox Publisher
Figure 29-6. Welding lead connectors are available in assorted styles. A—Assorted quick
connectors. B—Lug connector.
A
Goodheart-Willcox Publisher