Selecting a GTAW power source

One of the benefits of gas tungsten arc welding (GTAW) is its ability to control amperage, or heat input, with a high degree of precision, especially when using it in mechanized applications.

This article focuses on manual GTAW applications, although many of the factors discussed also apply to mechanized operations.

A key element of GTAW is the power source, which generates the arc. Choosing the right power source can be a confusing decision. Consider several factors before purchasing a GTAW power source:

The application

Material(s) to be welded

Power requirements

Usage

Time constraints

Additional capacity

Basics of the Process

GTAW uses a nonconsumable tungsten-alloy electrode. It's held in a torch assembly, which initiates and maintains the arc. In manual GTAW, the heat of the arc commonly is controlled one of two ways: through the use of a foot pedal or as a static setting on the power source. The foot pedal provides variable control of the welding amperage, or heat.

The torch assembly is either water- or air-cooled. A solenoid on the power source activates the flow of shielding gas to the torch. Some power sources provide an additional solenoid, which switches the water flow to the torch, while others don't have a solenoid for the torch coolant so the cooler can circulate coolant constantly through the torch, providing cooling all the time.

High frequency (HF) may be required to start the arc for non-touch-start applications. Most GTAW power sources provide HF capability, but not all do.

A GTAW power source is a constant-current machine. Unlike constant-voltage power sources, it will do its best to maintain a constant current (i.e., heat) despite changes in the voltage (i.e., arc length). A constant-voltage power source does just the opposite, which is required for gas metal arc welding (GMAW).

One Size Does Not Fit All

The most important consideration when deciding to purchase a power source is the intended application of the process. One size does not fit all, and if you purchase the wrong machine for the job, then nothing but problems and frustration will result.

When choosing a GTAW power source, consider first output current and duty cycle. The output current of the power source is one of the primary factors, because it dictates the thickness of the materials that can be welded. This is a complicated measurement that requires you to understand your application in terms of material type, thermal conductivity, and part size. These are all contributing factors that affect amperage requirements.

Duty cycle is a rating of the amount of power a machine can produce in a given time. The duty cycle rating is broken down on a 10-minute scale. For example, a machine with a duty cycle rating of 150 amps at 60 percent duty cycle means that the machine can produce 150 amps for six minutes and then must cool for four minutes. A machine with a rating of 300 amps at 100 percent duty cycle means that the machine can produce 300 amps of current continuously.

The combination of output current and duty cycle is crucial. If you choose a power source with a lot of output current and a poor duty cycle, for instance, you'll be able to weld at higher amperages, but for short periods of time. For welding at home this may not be a problem, but when time is critical, waiting for a machine to cool isn't cool.

For productivity and safety, regulating input and output power is essential. This often is overlooked until the machine is hooked up and running. Serious electrical problems can occur in a home-based application if the machine is not installed properly. If you don't have a comprehensive understanding of electricity, consult a professional before attempting to wire or install power to a welding machine.

However, the type of input power also should be a deciding factor when choosing a GTAW power source. Some machines can operate at multiple-line input voltages, for example, 208 V, 220 V, and 440 V. As a rule, the higher the input voltage, the less current (amperage) the machine will draw through your electrical system. In a shop where several power sources operate, this is a critical factor.

Newer Power Sources

Newer inverter-based machines offer additional flexibility. This is especially apparent in the use of AC for welding nonferrous materials. Some manufacturers offer power sources with a maximum control over the arc waveform. Some of these newer machines also are considered to be multiprocess power sources, which differ from conventional power sources because they are not limited to either a constant-current or constant-voltage mode.

Inverter-based machines can offer efficiency in terms of power input versus output, which translates into more work with less energy, thus reducing cost and space. These machines are powerful for their size, but also cost more.

While some newer machines offer technology and flexibility, achieving high-quality welds with a less advanced, less expensive power source is possible. Technology can't beat what a skilled welder can provide. Many machines are affordable and can provide years of service.

A plug-and-chug equation doesn't exist for buying a power source. However, a comprehensive understanding of the process and the materials to be welded is imperative and can lead to a satisfying purchase.

How Knowledge Affects Choice

Once you define your application and power requirements, choosing a power source becomes easier. When you know the output current, duty cycle, and special requirements, matching a power source to your specifications becomes a matter of manufacturer preference.

Manufacturers also offer many different equipment packages, which can be customized based on need. Also consider used equipment. If a used power source was not abused and was properly maintained, it can perform well for less money.

Dealers are another source of information. They can provide details about the equipment and its limitations.

Matt Hoppes is president of ProFab Industries L.L.C., 519 W. Lone Cactus Drive, Suite 105, Phoenix, AZ 85027-2921, phone 623-879-8182, fax 623-879-0391, e-mail matt@profabllc.com, Web site www.profabllc.com. ProFab Industries provides precision welding services to the commercial, automotive, and aerospace industries.