<P> A transformer - style welding power supply converts the moderate voltage and moderate current electricity from the utility mains (typically 230 or 115 VAC) into a high current and low voltage supply, typically between 17 and 45 (open - circuit) volts and 55 to 590 amperes . A rectifier converts the AC into DC on more expensive machines . </P> <P> This design typically allows the welder to select the output current by variously moving a primary winding closer or farther from a secondary winding, moving a magnetic shunt in and out of the core of the transformer, using a series saturating reactor with a variable saturating technique in series with the secondary current output, or by simply permitting the welder to select the output voltage from a set of taps on the transformer's secondary winding . These transformer style machines are typically the least expensive . </P> <P> The trade off for the reduced expense is that pure transformer designs are often bulky and massive because they operate at the utility mains frequency of 50 or 60 Hz . Such low frequency transformers must have a high magnetizing inductance to avoid wasteful shunt currents . The transformer may also have significant leakage inductance for short circuit protection in the event of a welding rod becoming stuck to the workpiece . The leakage inductance may be variable so the operator can set the output current . </P> <P> Welding power supplies may also use generators or alternators to convert mechanical energy into electrical energy . Modern designs are usually driven by an internal combustion engine but older machines may use an electric motor to drive an alternator or generator . In this configuration the utility power is converted first into mechanical energy then back into electrical energy to achieve the step - down effect similar to a transformer . Because the output of the generator can be direct current, or even a higher frequency AC, these older machines can produce DC from AC without any need for rectifiers of any type, or can also be used for implementing formerly - used variations on so - called heliarc (most often now called TIG) welders, where the need for a higher frequency add - on module box is avoided by the alternator simply producing higher frequency ac current directly . </P>

Who decides how welding power sources are rated