Welding power supply

A welding power supply is a device that provides an electric current to perform welding. Welding usually requires high current (over 80 amperes) and it can need above 12,000 amperes in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good example. A welding power supply can be as simple as a car battery and as sophisticated as a modern machine based on silicon controlled rectifier technology with additional logic to assist in the welding process.

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.

Power supply designs

Most welding power supplies are of the following designs:

Transformer

A transformer style welding power supply converts the high voltage and low current electricity from the utility mains into a high current and low voltage, typically between 17 to 45 volts and 55 to 590 amps. A rectifier converts the AC into DC on more expensive machines.

This design typically allows the welder to select the output current by either moving a magnetic shunt in and out of the core of the transformer or allows the welder to select the output voltage from a set of taps on the transformer. These machines are typically the least expensive.

Transformer designs are often bulky because they operate at the utiltiy 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. The leakage inductance may be variable so the operator can set the output current.

Generator and alternator

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 some older machines 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 insulated gate bipolar transistor (IGBT), it is now possible to build a switched-mode power supply capable of coping with the high loads of arc welding. These designs are known as inverter welding units. They generally first rectify the utility AC power to DC; then they switch (invert) the DC power into a stepdown transformer to produce the desired welding voltage or current. The switching frequency is typically 10 kHz or higher. Although the high switching frequency requires sophisticated components and circuits, it can drastically reduce the bulk of the step down transformer. The circuitry can also provide features such as power control and overload protection. 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.

References

• http://www.millerwelds.com/education/articles/articles31.html - Miller Electric news release on IGBT technology for welding inverters 8 April 2003
• http://www.lincolnelectric.com/en-us/support/process-and-theory/Pages/inverter-based-power-detail.aspx -Inverter Based Welding Power Supplies for Welding Aluminum By Frank G. Armao, The Lincoln Electric Company ( Lincoln Electric tutorial on inverter-based welding machines )