Invert the GTAW equation

WWW.THEFABRICATOR.COM FEBRUARY 2013

February 5, 2013

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Wondering if it’s time to transition from a transformer-style gas tungsten arc welding (GTAW) power source to an inverter-based machine? What are the differences between the two, and what can you expect if you switch?

Modern GTAW inverter-based power source

Modern GTAW inverter-based power sources are considerably smaller and lighter than their transformer-based counterparts, making them easier to fit in confined spaces.

Classic transformer-style gas tungsten arc welding (GTAW) power sources continue to be used extensively throughout the U.S. and the world because they’re bastions of reliability. It’s not uncommon to see a 20- or 30-year-old power source still putting in a full day’s work on the shop floor. These classic machines have no problem producing code-quality welds, but consider this analogy: Will a rotary phone still make a call and connect you with the person on the other end? Sure. Can you do a lot more with a smartphone? Absolutely. The same applies to modern GTAW inverter technology.

Quality = Productivity

Quality and productivity are synonymous in many shops—welding faster is good only if you are maintaining or exceeding current quality standards. But ensuring that a part is fabricated right the first time in itself is a productivity improvement, because it helps eliminate waste and rework—one step forward, no steps back. Modern inverter-based technology helps achieve this through a number of factors, including faster travel speeds, better arc control, reduced heat input and distortion, and reduced pre- and postweld cleanup. It also establishes the weld puddle faster and makes it easier to manage.

So what are the differences between older transformer-based technology and inverters?

Arc Starts. Conventional GTAW power sources generate a burst of current during starts that helps initiate the arc, but that burst also can damage the base metal or create undesired weld characteristics, especially on thin materials where minimizing heat input and preventing burn-through are important.

GTAW inverters allow welders to regulate that output down to a microsecond and to the exact amount of starting amperage (as low as 1 amp) needed to light the arc without damaging the base material. This gives welders more time to establish the puddle and control heat input and puddle characteristics.

It also means welders can start welding immediately. There’s no preheat. The welder strikes the arc and he’s ready to go.

AC Balance. AC balance controls arc cleaning action. Old transformers typically allowed only the electrode-negative (EN) portion of the cycle to be dialed up to 70 percent. Today’s inverters allow operators to set it up to 99 percent. This extended balance range allows for the arc to be fine-tuned according to base metal conditions in each application.

Making the EN portion last longer provides faster travel speeds, narrows the weld bead, and may permit the use of a smaller-diameter tungsten to more precisely direct heat or further narrow the weld bead. Reducing the EN portion produces greater cleaning action and widens the bead profile, but shortens tungsten electrode life and increases balling action.

AC Frequency. Transformer-based GTAW units typically are locked in at the frequency of the incoming electrical line: 60 hertz in North America and 50 Hz elsewhere around the world. New inverters give operators the ability to dial that in between 20 and 400 Hz. Higher frequencies limit the time that the arc expands on each half-cycle, which creates a narrow, focused arc with enhanced directional control.

This is helpful when welding in tight corners. The welder can turn the frequency up, and he can push that puddle right into the corner very sharply without it trying to bounce off and ricochet on other gussets or anything else in the area. Also, it really focuses that arc down to the puddle and keeps that tungsten very pointed and very clean.

Waveform Options.Traditional transformer-based GTAW power sources typically are limited to one AC waveform option: soft square wave. Inverter-based power sources give welders four options to further tailor the bead to meet their needs: soft square wave, advanced square wave, sine wave, and triangulated wave.

  • Advanced square wave gives fast transitions for a responsive, dynamic, and focused arc for better directional control.
  • Soft square wave provides a smoother, softer arc with a more fluid puddle than the square wave.
  • Sine wave gives the soft-arc feel of a conventional power source, while using square transitions to eliminate the need for continuous HF.
  • Triangular wave combines the effect of peak amperage while reducing overall heat input. This leads to quick puddle formation and, because of lowered heat input, reduced weld distortion, especially on thin material.

The advanced square wave of older inverters would leave a bead that you could almost see after machining. The wave selection capability gives a welder the ability to go back to a sine wave, so that in the machining operation, the bead is transparent. Also, with the triangulated option, thin materials are extremely easy to weld.

Independent Amplitude Control.This ability allows for the electrode-positive (EP) and EN amperages to be set independently to precisely control heat input to the work and electrode. More current in EN than EP provides deeper penetration and faster travel speeds, while more current in EP than EN provides a shallower penetration. 

For instance, if a welder needs a small bead for cosmetic reasons, he might increase negative amperage. That narrows the arc and makes the accuracy of the weld better.

Pulsed GTAW Pulsing Frequency.The pulse frequency of conventional welding transformers tops out at about 20 Hz. New inverters offer pulse frequencies ranging upwards of 100 Hz, with some machines having capabilities up to 5,000 Hz. This high-speed pulsing focuses and constricts the arc, which reduces average amperage levels, narrows the heat-affected zone, and lowers heat input.

Energy Efficiency and Space Savings

A 350-amp conventional machine draws 128 amps of input power under a rated load on 230-volt, single-phase power. Under the same circumstances, a comparable GTAW inverter draws only 32 amps. This energy savings can have an effect on utility bills and is obviously good for the environment, but the greatest benefit comes in the flexibility to potentially add more machines and workstations on existing power without having to expand or build in more power.

As many companies are landlocked by the buildings they inhabit, doing more in the same amount of space is a good way to expand. This is further enhanced by space savings afforded by modern inverters, which are considerably lighter in weight and smaller in both height and width, making them easier to fit into weld cells and move around.

Total Efficiencies Drive ROI

All of these features might sound great, but how can they be quantified into real numbers that justify updating your equipment? You can work with your local welding distributor to audit your welding operation and determine the realistic gains that you might achieve. You also can use online calculators that rely on basic operational information to determine ROI, but keep in mind that they are not a guarantee of good results. (An example can be found here.) Inputting fairly common statistics into the calculator can project potential payback numbers and future savings.

If you’ve been looking for new ways to expand within your existing footprint, are looking to produce more, or are just looking to improve your total quality, now might be the time to retire that old transformer and work with your local welding distributor to examine replacements that can help you do more and save more.



Andrew Pfaller

Product Manager and Weld Engineer
TIG Solutions Group
Miller Electric Mfg. Co.
1635 W. Spencer St.
Appleton, WI 54912-1079
USA
Phone: 920-734-9821

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