Advanced Welding Processes: Technologies and Process Control

36
Advanced welding processes
Mains input
Primary
rectifier
The welding output is smoothed and stabilized and, although it is not
possible to achieve the same response rates as those obtained with the series
regulator, it is possible to produce the output characteristics required for
recent process control developments. This type of circuit was initially used
for MMA power sources, but it is now being employed for GTAW and
pulsed GMAW units. It has particularly good electrical efficiency and a
comparison of inverter and conventional power sources at current settings of
250 A has shown that idle power consumption is only one-tenth of that of a
conventional machine and, during welding, the efficiency is around 86%
compared with 52% for a conventional unit. [31, 35]
3.4.5 Hybrid designs
Inverter
3.12 Primary rectifier–inverter.
Transformer
Control electronics
Output
control
Secondary
rectifier
Output
It is possible to combine the electronic control techniques outlined above to
improve the performance and cost effectiveness of the power source. For
example, the use of a secondary chopper to pre-regulate the supply followed
by a small air-cooled transistor series regulator for final control of the output
has been described, [32] the circuit is shown schematically in Fig. 3.13 and
the advantages of this approach are summarized in Table 3.1. Hybrid designs
may also be adopted to produce a square-wave AC output by adding a
secondary inverter to the output of a DC phase-controlled unit. SCR phasecontrolled
power sources may be used in conjunction with an SCR inverter
[33] or, alternatively, the system may be based on an integrated primary
rectifier–inverter design. Sophisticated experimental hybrid units have been
developed, for example for high-frequency AC plasma welding, [34] in order
to investigate potential improvements in process control.
Alternative power devices such as asymmetrical SCRs (ASCRs) [35] or
metal oxide–silicon field-effect transistors (MOSFETS) may also be used to
improve the efficiency of conventional electronic and hybrid systems.