Power Supplies.

Welding power supplies take alternating or direct current from a power line or portable generator and convert it to current usable at the welding arc. Check National Electrical Manufacturers Association (NEMA) Standards Publication EW1, Electric Arc Welding Power Supplies, and manufacturers' literature for construction and performance requirements of individual machines.

Among types of power supplies, alternating-current (AC) transformers are least costly-most use 230 V single-phase input power. Direct-current (DC) transformer-rectifiers convert AC to DC, using single-or three-phase power to produce constant voltage (CV) or variable voltage (VV). A constant current (CC) power source has a volt-ampere relationship that yields a small current change from a large arc voltage change. AC-DC transformer-rectifiers do both AC and DC welding.

Transformer-rectifiers have moveable parts subject to wear and tear. However, these machines can produce some of the best arc characteristics.

A silicon-coated-rectifier (SCR) power source, electronically voltage-controlled, has no moving parts other than the primary current contactor and cooling fan--the only parts that can wear out. Some recent designs even eliminate the mechanical contactor to imporve reliability. The SCR machine is the most popular power source today. However, if a fabricator requires pulsed welding, the scr power source starts to show its limitations and inverters become the power supply of choice.

Taking over the market are AC-DC inverters, which convert ac input frequency to very high frequencies--up to 1,000 times that of input frequency--allowing transformers and other components to be small in size and the power supply to be relatively lightweight. This high operating frequency enables the inverter to precisely control output power and to deliver smooth arc initiation with minimal or no spatter. Current ripple is low for a very stable arc, even at low amperages. And the improved arc control can, in some cases, minimize electrode overheating and fume emissions--one fabricator reduced its fume emissions by 50 percent by switching over to inverter power supplies. Finally, the small transformer allows for more efficient use of primary power compared to conventional power supplies--energy savings can be substantial. For example, one inverter on the market operating at 30 V and 400 A draws 23 kW; a typical rectifier operating under the same conditions draws 33-34 kW.

Multiple-operator power supplies use a high-amperage high-voltage power source to feed power to more than one welding station. Where line power may be unavailable, as in the field, contractors operate engine-driven generators powered by natural gas, propane, or diesel fuel. Units rate 150 to 650 A at up to 100-percent-duty cycle. Some come rated as high as 10,000 W, enough power to drive any power tool plus extra for lighting, cooling, or other field requirements.

For welding in confined spaces, damp, wet, or awkward locations, AC power supplies pose more of an electric-shock hazard than do DC power supplies. Use AC power supplies in these cases when possible; also try to weld using the gmaw process rather than SMAW, due to the lower open-circuit voltages associated with GMAW. When using an ac power supply, outfit it with a low open-circuit-voltage attachment to limit open-circuit voltage to 35 V maximum during nonwelding times.