An inverter power supply uses electrical circuits to
change the input frequency to the transformer. See
Figure 5-11.
First, an inverter machine takes single-phase or
three-phase power and passes it through a bridge rec-
tifier. This creates dc current. Next, this dc current is
chopped into very high-frequency ac current. The fre-
quency can be between 1,000 and 50,000 cycles per
second or 1kHz to 50kHz (the k means 1000). This
high-frequency current is passed through a small, effi-
cient transformer. Low-voltage, high-current power
useful for welding is produced. This power still has a
very high frequency. Another bridge rectifier and an
inductor are used to create smooth dc output. Inverter
power supplies can be used for all arc welding
processes and for plasma arc cutting.
Some welding machines taken to job sites where
no electricity is available have a gasoline or diesel
engine as the power supply. The engine turns a gen-
erator or alternator. These create dc and ac power,
respectively.
After selecting an ac, dc, or ac/dc machine, the
following variables must also be considered:
• Input power requirements.
• Rated output current rating.
• Duty cycle.
• Open circuit voltage.
Input power requirements. Welding machines
used in school shops, trade schools, and industry are
connected to commercially available electric power.
Power requirements for a welding machine must be
specified to the electrician when the machine is wired
into a building. The input power of a welding
machine must correspond with the type of power that
is available. It is fairly expensive to rewire existing
120V power to 240V or 440V.
The input voltage to a welding machine may be
120V, 240V, 440V or higher. These high voltages are
reduced by a transformer within the welding machine
to the required welding voltage. Welding voltage
ranges from 5V to 30V.
Engine-driven welding machines are used in the
field for pipelines, construction, and other welding
operations where electric power is not available.
Engines are connected to the welding machine to turn
an alternator for ac welding current or a generator for
dc welding current.
Rated output current. A nameplate on each weld-
ing machine shows the rated output current. See
Figure 5-12. A welding machine must be able to sup-
ply the current required for the welds being made.
The rated output current depends on the duty cycle.
Duty cycle. The duty cycle is a rating that indi-
cates how long a welding machine can be used at its
maximum output current without damaging it. Duty
cycle is based on a ten-minute time period. A welding
machine with a 60% duty cycle can be used at its max-
imum rated output current for six out of every ten
minutes. The welding machine may overheat if the
duty cycle is exceeded. At lower current settings, the
duty cycle may be increased and the power source
used for a longer period of time.
A duty cycle chart should be provided with a new
welding machine. In Figure 5-13, the duty cycle is
20% at the maximum rated output current of 200
amperes (A). The duty cycle is 100% at a rated output
current of 100A.
Welding Technology Fundamentals 56
Input bridge
rectifier
1- or 3-phase
primary
Output bridge
rectifier
Inverter Inductor
+
-
Transformer
Inverter
control
circuit
Figure 5-11. An inverter power supply takes a number of steps to change the supplied power into power used for welding. The
different steps and the resulting waveform are illustrated.