July 24, 2003
It's easy to look at such a long-established technology as welding and believe that little or no technology development is taking place. In truth, however, the capabilities of welding power supplies are changing constantly and rapidly, especially in the area of inverter technology. These power supplies are suitable for welding aluminum alloys, including thin aluminum alloys.
All welding power supplies transform relatively high-voltage, low-current incoming power to lower-voltage, high-current welding output using a transformer. In the past the transformer operated directly from 50- or 60-hertz incoming alternating current (AC). At these frequencies, a lot of heat is generated in the transformer, so it must be relatively large and heavy. Additionally, if 60 Hz is used, control signals are limited to being issued at no more than 120 per second.
Inverters were introduced into welding power supplies first to generate direct current (DC) and later to generate AC. In these power supplies, incoming 50- or 60-Hz AC power first is rectified to DC and filtered and then is fed into the inverter section of the power supply, where solid-state controls switch it on and off at frequencies as high as 20,000 Hz, effectively converting it back to high-frequency AC.
This pulsed, high-voltage, high-frequency AC then is fed to the main power transformer, where it is transformed into low-voltage, 20,000-Hz AC suitable for welding. Finally it is put through a filtering and rectifying circuit to obtain DC welding current. Output is controlled by solid-state controls that modulate the switching rate of the switching transistors.
Because the power transformer runs at 20 kilohertz, it is much more efficient than one that runs at 60 Hz. This means the transformer can be much smaller and lighter, so the power supply itself can be lightweight. Inverter-based DC gas tungsten arc welding (GTAW) power supplies typically weigh 30 to 50 pounds. With some of these power supplies, the current draw at 205 amps is 29 amps on 230-V, single-phase power. While the resulting cost savings of an inverter power supply often are overstated, annual power supply savings typically are 10 percent of the power supply purchase price.
Inverter power supplies also "chop up" the incoming AC very finely, resulting in a steady DC without the typical 60-Hz ripple and a stable welding arc.
For many years inverter power supplies were capable of supplying DC only. Inverters that output AC simply did not exist. This limited the use of inverters for welding aluminum, which normally is GTA welded using AC. Then someone had the idea of packaging two inverters inside one case. Running them at different polarities and alternately switching them on and off generated a pseudo-AC output. Some inverters still generate AC in this manner.
The ability to generate AC is what makes inverter power supplies suitable for welding aluminum using GTAW. Because the arc voltage never truly goes through zero, the AC arc is made more stable. Most inverter-based GTAW power supplies do not need the high frequency to be on continuously for stability, which reduces the amount of radio frequency interference (RFI) generated by the power supply.
Because control signals can be sent at any frequency up to twice the inverter frequency (40 kHz), the frequency of the AC welding output can be varied. Some machines can output AC at rates from 20 to 150 Hz. As the frequency is raised, the arc cone and the weld become narrower, resulting in deeper weld penetration.
In GTAW, weld penetration comes from the electrode-negative part of the AC cycle. During the electrode-positive part of the cycle, weld penetration is reduced, and more heat goes into the tungsten electrode, but the arc actually removes oxides from the surface of the aluminum, making welding easier. Thus, while most other materials are GTA welded using DC, aluminum usually is welded using AC.
Early GTAW power supplies supplied a simple sine-wave AC output with equal amounts of electrode positive and electrode negative generated. However, that much electrode positive was unnecessary for adequate cleaning. Later power supplies allowed the proportion of electrode negative to electrode positive to be varied, typically with about 65 percent electrode negative and 35 percent electrode positive.
Inverter power supplies provide adequate arc cleaning with as little as 15 percent electrode positive. Reducing the amount of electrode positive helps increase weld penetration and decrease the amount of heat going into the tungsten electrode. This allows smaller-diameter, pointed electrodes to be used, which concentrates and narrows the weld.
Blunt-ended pure or zirconianted tungsten electrodes are normally recommended for AC welding with conventional power supplies. The situation changes when inverters are used. Most inverters are optimized to ignite the arc and weld best using sharpened 2 percent thoriated tungsten electodes. Many users also report good results using sharpened ceriated or lanthanated tungsten electrodes.
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