Classification
Welding machines are usually classified as constant current (CC) or constant voltage (CV); a constant current machine varies its output voltage to maintain a steady current while a constant voltage machine will fluctuate its output current to maintain a set voltage. Shielded metal arc welding will use a constant current source and gas metal arc welding and flux-cored arc welding typically use constant voltage sources but constant current is also possible with a voltage sensing wire feeder.
The nature of the CV machine is required by gas metal arc welding and flux-cored arc welding because the welder is not able to control the arc length manually. If a welder attempted to use a CV machine to weld with shielded metal arc welding the small fluctuations in the arc distance would cause wide fluctuations in the machine's output. With a CC machine the welder can count on a fixed number of amps reaching the material to be welded regardless of the arc distance but too much distance will cause poor welding.
Machine construction
Most welding machines are of the following designs:
Transformer
A transformer style welding machine converts the high voltage and low current electricity from the utility into a high current and low voltage, typically between 17 to 45 volts and 190 to 590 amps. This type of machine typically allows the welder to select the output current by either moving the core of the transformer in and out of the magnetic field or by allowing the welder to select from a set of taps on the transformer. These machines are typically the least expensive to purchase for hobbyist use.
Generator and alternator
Welding machines may also use generators or alternators to convert mechanical energy into electrical energy. Modern machines of this type are usually driven by an internal combustion engine but some older machines may also use an electric motor to drive the 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, these older machines can produce DC from AC without any need for rectifiers of any type.
Inverter
Since the advent of high-power semiconductors such as the IGBT, it is now possible to build a switching power supply capable of coping with the high loads of arc welding. These are known as inverter welding units. These supplies generally convert utility power to high voltage and store this energy in a capacitor bank; a microprocessor controller then switches this energy into a second transformer as needed to produce the desired welding current. The switching frequency is very high - typically 10,000 Hz or higher. The high frequency inverter-based welding machines can be more efficient and have better control than non-inverter welding machines.
The IGBTs in an inverter based machine are controlled by a microcontroller, so the electrical characteristics of the welding power can be changed by software in real time updates. Typically the controller software will implement features such as pulsing the welding current, variable ratios and current densities through a welding cycle, variable frequencies, and automatic spot-welding; all of which would be prohibitively expensive in a transformer-based machine but require only program space in software-controlled inverter machine.
power distribution transformers
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