The Power Source
Direct current, constant potential
(voltage) power sources are used for most mig welding. This contrasts with
tig and stick electrode welding which
use constant current power sources. A mig power source
provides a relatively
constant voltage to the arc during welding. This voltage determines the arc length.
When there is a sudden
change in wire-feed speed, or a momentary change in arc length, the power source
or decreases the current (and thereby the wire burnoff rate) depending on the
arc length change. The
burnoff rate of wire changes automatically to restore the original arc length.
As a result, permanent changes
in arc length are made by adjusting the output voltage of the power source. The
wire-feed speed, which
the operator selects prior to welding, determines the arc current (see Fig.
3-1). It can be changed over a
considerable range before the arc length changes enough to cause stubbing to the
workpiece or burning back
to the guide tube. Power
Source Variables The
self-correcting arc length feature of the constant voltage welding system is very
important in producing stable
welding conditions. Specific electrical characteristics are needed to control
the arc heat, spatter, etc. These
include voltage, slope, and inductance.
Figure 3-1 - Affect of Wire Feed Speed