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Inverter Based Welding Power Supplies for Welding Aluminum
What's New? In inverter controlled power supplies, the same incoming 60 Hertz power is used. However, instead of being fed directly into a transformer, it is first rectified to 60 Hertz DC. Then it is fed into the inverter section of the power supply where it is switched on and off by solid state switches at frequencies as high as 20,000 Hertz. This pulsed, high voltage , high frequency DC is then fed to the main power transformer, where it is transformed into low voltage 20,000 Hertz DC suitable for welding. Finally it is put through a filtering and rectifying circuit . Output control is performed by solid state controls which modulate the switching rate of the switching transistors.
The other significant advantage of inverter power supplies is that, by "choppingup" the incoming AC so finely, we end up with a very steady DC, without the typical 60 Hertz ripple. This results in a much smoother, more stable DC welding arc. So far, we've only discussed inverters that supply direct current. For quite a few years, this was all that was available. Inverters that supplied AC output simply did not exist. Then, someone had the idea of packaging two inverters inside one case. By having them run at different polarities and alternately switching them on and off, a pseudo AC output was generated. Some inverters still generate AC in this manner. There are also more sophisticated methods of generating AC today, but for the purposes of this article, it's easier to think of generating the AC from two inverters at opposite polarities.
Second, the fact that we can send control signals at 20 kilohertz means that we can vary the frequency of the AC welding output. Older machines were 60 Hertz AC output only. The V205 can put out AC at anywhere 20 and 150 Hertz. Higher frequencies can be beneficial in welding thin materials. As the frequency is raised, the arc cone, and the weld, become narrower, resulting in deeper penetration. It was realized many years ago that in GTAW, weld penetration comes from the electrode negative part of the AC cycle. During the part of the cycle when the electrode is positive, weld penetration is reduced and more heat goes into the tungsten electrode. However, during the electrode positive part of the cycle, the arc actually acts to remove the oxides from the surface of the aluminum, making welding easier. It is for this reason that, although most other materials are GTA welded using direct current, aluminum is usually welded using AC. Very early GTAW power supplies supplied a simple sine wave output where equal amounts of electrode positive and electrode negative were generated. However, this was inefficient. We didn't need that much electrode positive to get adequate cleaning. Later power supplies allowed us to vary the proportion of electrode negative to electrode positive. It was found that approximately 65% electrode negative and 35% electrode positive gave adequate arc cleaning and good penetration. However, a lot of the arc energy was still going to heat the tungsten electrode., so that large diameter tungsten electrodes were required. The inverter power supplies provide adequate arc cleaning with as little as 15% electrode positive. Reducing the amount of electrode positive makes the process more efficient, increases weld penetration, and reduces the amount of heat going into the tungsten electrode, which means smaller diameter, pointed electrodes can be used. This further concentrates and narrows the weld.
The future is here.
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©1999-2008 The Lincoln Electric Company. |
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The
world is changing. That's no surprise to anyone who is even remotely conscious
of their surroundings. However, it's tempting to look at long- established technologies,
such as welding, and believe that there is little or no technology development
taking place at this late date. However, the person who took that view would
be wrong.. In fact, the design and capabilities of welding power supplies has
changed , and is continuing to change, rapidly. One of the technologies driving
this change is the development and popularization of power supplies based on
inverter technology. This technology is particularly well suited to welding
aluminum alloys, especially thin aluminum alloys.
Transformers
are relatively inefficient operating at 50 or 60 Hertz. A lot of heat is generated
in the transformer, and the transformer must be relatively large and heavy.
A significant part of the power cost goes into heating the transformer and the
surrounding air. Most such welding power supplies weigh around 400 pounds and
have a shape something like a 32 inch cube. Additionally, if 60 Hertz is used,
control signals are limited to being issued at no more than 120 per second,
so it's impossible to pulse the welding current any faster than this.
What
advantages does this new inverter controlled design offer? First, the main power
transformer, which operates at 20,000Hertz is vastly more efficient than 60Hertz
transformers, which means it can be much smaller. Remember, transformer - based
machines typically weigh 400 pounds plus and are a 32 inch cube. The accompanying
photo shows the Lincoln line of inverter - based gas tungsten arc welding (GTAW)
power supplies. The machine in the center, the V205, weighs 33 pounds and is
9 inches wide, 19 inches deep and 15 inches high. The other two machines are
DC only inverters and are even lighter and smaller. So there is a huge advantage
in weight and portability in favor of the inverter - based machines.
There
is another advantage of the inverter power supplies - power cost. The inverter
equipment is much more efficient than transformer equipment. For instance, the
current draw at 205 amperes for the Lincoln V205 is 29 amperes on 230Volt single
phase power. The current draw of an older transformer welder is typically 50
to 60 amperes on 230 Volt single phase power when welding at similar currents.
While the cost savings in switching to inverters is often overstated, under
normal circumstances, it is safe to say that annual power savings are approximately
10% of the power supply purchase price.
The
ability to generate AC is what really makes the inverter shine for welding aluminum
using GTAW. The fact that the arc voltage never truly goes through zero means
that the AC arc is much more stable than previously. Most inverter - based GTAW
power supplies do not need the high frequency to be on continuously for stability.
In fact, the Lincoln V205 has no provision for using continuous high frequency.
It will automatically be extinguished as soon as the arc starts.. The elimination
of continuous high frequency drastically reduces the amount of RFI generated
by the power supply.
Finally,
the newer inverter power supplies are software programmable. This makes it much
easier to change power supply characteristics. The accompanying photo shows
another Lincoln power supply, the 