In the past, welding power supplies have been based on transformers. The power supply took in 60 Hertz 230, 460 or 575 volt power. A metallic transformer changed it from the relatively high input voltage to 60 Hertz current at a lower voltage. This low voltage current was then rectified by some sort of rectifier bridge to get direct current (DC) welding output. Control of this output was usually performed by some sort of relatively slow magnetic amplifiers.
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.
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.
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 other significant advantage of inverter power supplies is that, by "chopping up" 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.
By packaging two inverters inside one case inverters can supply AC. 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.
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.
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.
Those are not my words but Frank G. Armao from the Lincoln Electric Company. It's clear to anyone that coil based technology is dying just like push rod technology in cars.
