Inversion is the opposite of rectification (Fig. 5.1). Rectification is the conversion of alternating current into direct current and is achieved using rectifiers or diodes (described on pp. 199–203): four diodes in bridge connection for single phase and six diodes for three-phase input (shown in Fig. 4.44b, c). Inversion converts direct current into alternating current. An inverter is not simply a device like a diode, but a network of many components designed to achieve this conversion (Fig. 5.2).

The great majority of arc welding today uses direct current, and since the mains are invariably alternating current, some method is required for changing from a.c. to d.c. Inversion is only part of what happens within the inverter power source. Many other electronic and electrical conversions and transformations are required to convert the mains supply into one suitable for welding. Figs. 5.3 a, b and c illustrate a typical power source, transformer and inductor of the inverter type, together with one of similar output but of non-inverter design. There is a great reduction in size and weight (and hence portability) resulting from the inverter-based design. There are other advantages discussed later but size and weight are the most obvious.

Transformer and inductor

In a transformer there are two separate coils (primary and secondary), or windings, wound on a laminated iron core, the laminations being coated with insulating varnish; the bolts or rivets fastening the laminations together are mounted on insulating sleeves to decrease the circulating eddy currents further.