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Chapter 5 SMAW: Equipment and Supplies
dc welding machines. Combination machines have a
greater flexibility in their use. However, they are more
expensive than ac or dc machines.
Machine designs can be different types. The most
common are:
• Transformer—used for ac.
• Transformer-rectifier—used for ac and dc.
• Inverter power supply—provides ac and dc
options.
• Motor generator—used to create dc power.
• Motor alternator—used to create ac power; with
addition of a rectifier, can produce dc power. See
Figure 5-9.
The function of a power supply is to take the high-
voltage, low-current supplied power and change it
into the low-voltage, high-current power needed for
welding. Welding transformers are called step-down
transformers because the voltage is reduced.
Alternating current is supplied to one side of a
transformer. This is called the primary current. The cur-
rent coming out of the transformer, which is used for
welding, is called the secondary current. Alternating
current is used as the input to the transformer. The
output of the transformer is also alternating current,
but at a higher amperage and lower voltage.
To obtain direct current, the alternating current
is passed through a set of diodes or silicon-controlled
rectifiers (SCRs). Diodes and silicon-controlled rectifiers
are electrical devices that allow current to flow in only
one direction. (Remember, alternating current flows in
two directions.) A bridge rectifier is a group of diodes or
SCRs used to transform ac into dc. Figure 5-10 shows a
bridge rectifier circuit.
Another type of power supply is an inverter.
Inverter machines are very small and lightweight.
Current range
selector
Current adjustment
dial
Figure 5-8. An ac shielded metal arc welding machine has two
current ranges. The desired current is set by selecting a current
range and setting the desired current using the adjustment
dial. (Miller Electric Mfg. Co.)
Generator
engine
AC/DC
polarity
switch
Figure 5-9. A portable engine-driven ac/dc welding power
source has a switch used to select ac or either dc polarity.
(Lincoln Electric Company)
Transformer
Inductor
SCR SCR
SCR SCR
Controls
+
–
Figure 5-10. This is a bridge rectifier that uses SCRs to
change ac current into dc current. SCRs conduct current in
only one direction.