Welding Power Supply Types: How to Choose a Power Supply

In spite of the name, welding power supplies are not typically the source of electrical power for arc welding. Instead, they are power inverters and transformers that work by converting the electrical power from a building’s power or a generator into a current type that is suitable for the welding application being performed. Inverters using an insulated-gate bipolar transistor (IGBT) technology are more common in newer welding power supply types, while transformers are usually found in older power supplies. In welding processes like shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) for welding pipe, welding power supply types commonly combine power generation, inverters, and a few extra controls needed to modulate the current.

The choice of welding power supply is what makes the difference between a slaggy lump of melted metal and a quality weld. For this reason, it’s critical when choosing a welding power supply type that their nuances and the elements they control are understood.

The Basics of Supplying Power for Arc Welding

An arc is essentially a short in between the positive and negative sides of a circuit. Usually, it involves creating an air gap between conductive materials that is small enough for the voltage in the circuit to jump across. It is possible to create an arc in nearly any electrical circuit, such as when you reach for a doorknob and get shocked on a dry day. Arc welding simply involves sustaining this arc. A welding power supply is what enables the controlled conditions to create smooth, continuous beads of structurally sound metals.

At a minimum, a welding power supply type will allow the welder control over the following elements of current flow:

• Voltage: A measure of electrical pressure needed to overcome resistance and start the flow of current. This is roughly synonymous with arc length, as the bigger the gap between the electrode and the workpiece, the more voltage will be needed to make the leap. 
• Amperage: The measure of the frequency of electrons flowing past a point. The higher the amperage, the more electrical energy is flowing into the point and the more heat it has. 
• Polarity: This refers to the direction that electrons are flowing. In most electrical systems, the negative is the ground that electrons flow toward and the positive represents the source of electrons. This is called straight polarity, and the majority of welding power supply types will allow polarity to be reversed either through a switch or by manually swapping the ground and electrode leads. It should be noted that in some arc welding processes, positive grounds are the norm and represent straight polarity.

Welding power supply types are defined by how they modulate electrical currents, and what arc welding process is best supported by this modulation:

• Direct Current (DC): Direct current is a flow of electrons in a single direction through a circuit. In welding, it creates a steadier arc and smoother output. It can be used to weld with a negative ground, or the flow of electrons can be reversed to flow toward a positive ground in a reverse polarity.
• Alternating Current (AC): Alternating current is a bidirectional flow of electrons in which the polarity shifts a hundred or more times per second from a negative to a positive ground. Arcs tend to be less stable and welding harder to control. However, AC welding can break apart oxide formation and allow for purer welding in some processes.
• Pulsed Current: Pulsed current is a form of DC welding in which the current goes from high peak current to a lower background current at a frequency determined by the operator. This narrows the arc, allowing greater penetration while reducing the effect on surrounding materials. As a result, pulsed current welding is an excellent choice for welding thin metal or performing deep welds on thicker materials. 
• Pulsed Voltage and Heat: Newer GMAW power supplies focus on controlling pulsed voltage and heat applied to the consumable electrode. Controlling the pulsed voltage (heat), and wire feed speed allows greater control over how the wire melts and the rate of deposition. Adaptive pulse GMAW carefully monitors feedback and automatically compensates to keep the arc consistent despite variation by the welder and differences in height and joint location.

There are many general-purpose welding machines that offer the choice between welding using AC or DC current. DC is a better welding option for steel and other ferrous metals, while AC current is almost a requirement for welding aluminum. Alternating current breaks up the oxides that form on aluminum welds and allows the metal to be welded at lower amperages than would be possible with DC. This is a consideration in other metals and alloys that share a similar oxidation profile with aluminum, and that also need the oxide layer broken up before welding can commence. AC welding is also used to speed up material deposition and welding overall in more typical steel and steel alloys.

In GTAW processes, however, DC pulsed current welding power supply types are used almost exclusively (AC welding power supplies are occasionally used for GTAW welding aluminum). This is because GTAW is typically used for very high-specification welds and pulsed current welding results in a much purer weld, with increased penetration and far less heat distortion than with non-pulsed welding. 

Choosing the Best Welding Power Supply Types for GTAW

Welding aluminum with GTAW requires an AC power supply to break up the oxides that form on the surface when the metal is heated. However, in nearly all other GTAW welding applications, DC with pulsed current is superior. It allows greater control of bead formation and heat input into the weld. Given the sort of high-spec welding that GTAW is used for, a DC pulse-type welding power supply is generally preferable. 

Stainless steel is notorious for how difficult it is to weld and how easily it warps during welding. GTAW already has a low heat input compared to other types of arc welding, and DC pulsed current lowers this heat input still further while simultaneously increasing the heat focused on the weld. This deepens penetration while narrowing heat-affected zones. As a result, DC pulse welding power supplies are preferred over other welding power supply types for challenging stainless steel welding like welding thin-walled stainless steel tube. The ability to narrowly focus heat also makes DC pulse welding the best way to weld Inconel® and other high-specification austenitic metals. For challenging pipe and tube welds, the best approach is orbital GTAW backed by a DC pulse welding power supply. 

Welding Power Supplies for Orbital GTAW

Pipe and tube welds are challenging due to the geometry of working around a cylinder. Orbital welding offloads the movement of the electrode around the workpiece to an automated system, removing the challenge for the welder of having to position themselves as they work around the circumference of the piece. 

The best welding power supply types for orbital GTAW welding of steel, stainless steel, exotic alloys like Inconel or Monel, or even titanium, are DC pulse power supplies. Within this category, the choice of power supply will depend on the thickness of the materials being welded. Fusion welding power supplies are made for thin-walled materials and are available in both bench-mounted and more portable forms. Higher amperage welding power supply types are made for welding pipe and other thick-walled materials and support welding accessories such as wire feed weld heads. These work in tandem with power supplies designed for hot wire welding that speed up deposition.

The choice of a welding power supply type should be made to support the material and arc welding process being used. In particular, any welding power supply type used for high-specification welding like GTAW orbital welding should come from a manufacturer with extensive experience in the design of orbital welding power supplies and familiarity with the ins and outs of the orbital welding process.