Frequently Asked Questions
The primary functions of the power source are to produce sufficient heat to melt the joint and to generate a stable arc and metal transfer. As the welding processes require high current (50-300A) at relatively low voltage (10-50V),the high voltage mains supply (230 or 400V) must be reduced by a transformer. To produce DC, the output from the transformer must be further rectified (Fig 1).
Five types of power source exist: AC transformer; DC rectifier; AC/DC transformer rectifier, DC generator and inverter.
The type of control, e.g. primary tapped, saturable reactor, thyristor and inverter is an important factor in the choice of power source. A simple primary tapped machine may be the ideal and robust choice for many MIG (GMA) welding jobs but it has its limitations. If there are insufficient steps, it may be impossible to tune the optimum condition and supply fluctuations will affect the output. Thyristor control allows continuously variable adjustment of the output, is independent of supply voltage variations and can be controlled remotely. Thyristor power sources may be used for most welding processes, i.e. can have either a flat (MIG [GMA]) or drooping (MMA [SMA] and TIG [GTA]) output characteristic.
Inverter power sources offer all the advantages of thyristor control, but with additional performance, weight savings and efficiency. Transistors are used to convert mains AC (50Hz) to high frequency AC (>500Hz) before transforming down to a suitable voltage for welding and then rectifying to DC. Thus, the inverter is essentially a power block which may be controlled, often by software, to give the static and dynamic characteristics required for the selected welding process. Hence, most inverters offer multi process capability. Also, the response of modern inverters opens up the possibilities of high frequency pulsing as required for pulsed MIG (GMA) and dynamic feedback to control metal transfer as in dip transfer MIG.