Here’s How an Inverter Welder Works and Why You Need One

A MIG welder is an almost universal tool for resto work. But in recent years, inverter power technology has been incorporated into newer units, and with that advancement came many improvements. Old-school welders with transformer power-delivery have been around for more than 100 years because they are very reliable, but they’re heavy and take more skill to operate, especially with delicate work. Inverter welders are much lighter and are easier to learn and control, although they do cost more.

How a MIG Welder Works

The main challenge with MIG welding is controlling the arc. Heat levels vary depending on the type and thickness of the steel, so a stable and controllable arc is crucial for consistency in welds.The traditional way of doing that is with an electrical transformer that converts the high-voltage (120 or 240 volts) and low-amp (15 to 20 amps) line power to low voltage (15 to 20 volts) and high-amp (more than 200 amps) welding current.

How an Inverter Welder Works

What is an inverter welder? How is it different from a transformer type? An inverter welder takes standard 60Hz AC power and runs it through an initial rectifier, turning it into high-voltage DC power. A microprocessor using extremely high-speed transistor switching turns the DC power off and on again at a very high rate (in Miller welders, for example, 60,000 times a second), which then simulates the waveform of AC power, but at 1,000 times the frequency. That high-frequency AC power then passes through a transformer to another rectifier that converts the power back to DC on its way to the tip of the welding wire.

Much as an engine’s computer can sense the air/fuel ratio in the exhaust and make the adjustments needed to keep the engine running correctly, the welder’s microprocessor can sense the welding arc conditions and adjust voltage, amperage, or both to maintain the specific arc required for the application.

The Right Welder for the Job

If you are welding ¼-inch steel plate, a traditional transformer-type welder will work beautifully. On the other hand, butt-welding much thinner 20-gauge sheetmetal found on classic musclecars will be significantly easier with a welder that has a greater level of precision and control over the arc. The range of acceptable heat generated is much narrower.

Another major benefit of inverter welders is their ability to weld more types of material. For example, the Millermatic 211 has programming built in for welding mild steel, aluminum, and stainless steel. It also allows for the use of either 25- or 100-percent CO2 shielding gas, or even flux-cored, self-shielding wire. This is possible because of the programming inherent in the inverter system that drives the secondary transformer. Switching the welder from stainless to mild steel to aluminum alters the programming to provide precisely the arc needed for each application. This Millermatic even has an Auto-Set feature that allows the user to dial in the metal thickness and wire diameter, then the welder sets the power and wire speed levels automatically.

This programming is a wonderful aid for new users. Auto-Set mode allows the welder to establish very accurate settings right from the start so that a beginner can concentrate on technique while trying to learn. Of course, the welder can be run in manual mode if desired once a greater level of expertise is gained.

Muscle Car Restorations in Chippewa Falls, Wisconsin, has been performing precision welding for more than three decades, experiencing success with the older-style welders. But when the techs in the welding shop got to try out a new inverter-type MIG welder, there was instant competition over who got to use it. The old welders were quickly pushed aside in favor of the new inverter type. We thank them for providing us a demo for this article.

Tack Welds and Spot-Stitch Welds

A series of overlapping spot-stitch welds were made between the tacks to fill in the gaps. It’s similar to pulse-welding, but actually more accurate to call it spot-stitching. An air quench followed each set of spot stitch welds to minimize the chance of heat warp.

Stitching is done until the seam is completely welded. The cosmetic look of welds is irrelevant, as they will be ground flush with the panel until the seam becomes invisible. It’s important that welds have the correct penetration.

The true test to determine whether welds were done correctly is to check the back side of the weld to verify penetration. This photo is from the inside of the quarter-panel.

Anatomy of a Transformer Welder

This is the inside of a Miller transformer welder. Note the huge transformer in the center. The rectifier and other electronics can be seen above it. This is considered a state-of-the-art transformer-type welder, and it weighs about 80 pounds.

There is nothing terribly complicated about the basic design of a traditional transformer welder, but control of the arc is somewhat limited, and done by manual input.

The inside of the Millermatic 211 inverter welder is mostly microprocessors on a circuit board. A much smaller transformer is hidden within. This unit weighs about 30 pounds, making it very portable.

As you can see, there is a bit more going on inside an inverter welder. The key difference is the increase in input current to 60,000 Hz, which dramatically improves the system’s response time. There are also sensing and control circuits for Auto-Set, and to ensure presets are maintained when the welder is in manual mode.

Pros and Cons of a Transformer Welder

An issue with traditional transformer welders is that they tend to produce a ball at the end of the wire during repeated spot-stitch welds. That ball increases the energy needed to start the arc, which can lead to false starts and incomplete welds. Experienced welders are used to having wire cutters nearby to trim the wire.

Inverter welders practically eliminate the formation of a ball at the end of the wire, allowing for more consistent welds.

Another benefit of inverter welders is that they maintain a correct and stable arc even when the conditions change. Experienced welders know the importance of maintaining the wire at a constant distance from the work, usually ¼ to ⅜ inch. The inverter’s microprocessor can monitor resistance at the work surface and adjust the arc accordingly to minimize the effects of distance variations.

How To Use an Inverter Welder

Using this inverter welder couldn’t be simpler. The small knob in the center selects the type of weld, in this case mild steel with 25-percent CO2 shielding gas. The right hand knob is for wire size, and material thickness is on the left.

These spot-stitch welds were produced with the welder set at position 2 for 20- to 22-gauge thickness. They looked great from the top side.

Flipping the material over showed the welds to be much smaller with barely adequate penetration, however. What was going on? Isn’t this how the welder should be set for this thickness?

Setting the welder at level 4 for 16-gauge metal still yields excellent results on the top side.

Here is the backside of these welds. The welds are similar in size to the top side with full penetration. So, what’s going on? Why is the higher setting needed?

Testing Different Bead Types

This series of welds was made with the Millermatic 211 set to number 2 for 20- to 22-gauge material. The first weld was made with a continuous bead. The second was a spot-stitch weld. The last was another spot-stitch with an air quench after. The size of the heat ring got smaller as it went from bead to spot-stitch to spot-stitch with quench. The smaller the heat ring, the less chance of heat-warping the metal.

This is the backside of the same welds. The continuous bead had full penetration. The first spot stitch was OK, but the third was not achieving proper penetration. The reason was that Auto-Set programming is designed for running continuous-bead welds, so it works better in that context. However, running bead-welds creates heat-warping issues when joining metal this thin. Starting the bead takes a bit more energy than maintaining it, and since tack welds and spot-stitching are nothing but starting and stopping, a higher setting is needed to achieve a good weld.

This is not a bead-weld but rather a series of spot-stitch welds. As each weld is laid down, it cools before the next is applied. The welds are not actually next to one another but rather each new weld starts almost on top of the previous one. Doing only four or five at a time followed by an air quench is what keeps the heat under control and prevents panel-warping. The generous overlap gives the effect of a continuous bead with no gaps or pinholes.

Of course, not all resto work involves welding 20-gauge panels. Often, parts of varying thickness need to be joined. Inverter welders excel at this as well. This piece of 20-gauge was joined to ⅛-inch plate. In fact, this Millermatic can comfortably weld up to ⅜-inch mild steel. So, rollbars, frame connectors, motor mounts, or any other heavy parts can be welded using just this one machine.

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