Advanced Welding Processes: Technologies and Process Control

Voltage (V)
50
40
30
20
10
Gas metal arc welding 119
0
0 100 200 300
Current (A)
400 500 600
a
Low voltage set
b
Normal voltage
c
High voltage set
7.22 Switch output power supply control.
example during dip transfer. With a CV characteristic power source this
level may be very high and result in explosive rupture of the short circuit
with high levels of spatter. In order to limit this current, it is usual to incorporate
inductance in the output of the power source, this reduces the rate of rise of
current and limits the maximum value reached before the short circuit ruptures.
If the inductance is too high, the current will not reach a sufficiently high
level to cause detachment and irregular operation will result. In the dip
transfer mode, the voltage and inductance should therefore be adjusted to
optimize the short circuit current and obtain stable spatter-free detachment.
7.5.5 Alternative control techniques
Arc characteristic
Power source output
If a constant-current output power source is used, the heating effect (current)
of the arc does not vary with small changes in stand-off and voltage.
Furthermore, an inherent self-adjustment mechanism operates with resistive
wires since, if current and wire feed speed are fixed, a unique value of ‘l’, the
electrical stick-out, is defined by equation (7.7). If l decreases momentarily
due to some process disturbance, the second term in the equation decreases
and the melting rate decreases thereby restoring the original extension length.
Unfortunately, this self-regulation does not occur with high conductivity
materials such as aluminium.
Constant-current power sources may be used for GMAW of high
conductivity materials by using a variable-speed wire feed unit which responds
to arc length changes by adjusting the wire feed speed. Electronic AVC (arc
voltage control) systems may be used with more modern power sources to
cope with this situation. Some of the developments in this area are described
in Section 7.7.