Taking the mystery out of air plasma power supply specifications

By Hypertherm
Posted on 09/16/2014 in SPARK the blog, Plasma cutting

Guest post by Jim Colt, technology applications engineer

Air plasma systems, ranging from 20 amps on the low end on up to 200 amps, are made by companies all over the world. These systems are available at varying price points with a wide range of cut thickness and speed ratings. Reading a shiny, multi color brochure is one way to get up to speed on the various offerings, but brochures don’t always tell the whole picture. After all, when was the last time you read a brochure that said “In all honesty, this system isn’t as good as system x.”

This might be why people often ask me to recommend a system for them. After 35 (close to 36!) years in the plasma cutting business, I still try to consider actual needs. If all you ever cut is 10 gauge steel, then you certainly don’t need a 125 amp, 100 percent duty cycle system. If you are a hobbyist that only uses a plasma cutter to make a cut now and then, then maybe a lightly used or low cost import plasma is all you need. If you are a job shop needing to cut a variety of thicknesses both by hand and on a table, and rely on tools like this for your income, you’ll likely make a different choice.

One of things I always recommend looking at before choosing a system is the specification sheet. Admittedly, specification sheets aren’t all that exciting, but it’s important to read one before deciding on a system because these show the design output capabilities of a typical system.

Over the next week or so, I’m going to write about some of the specs you might find on a specification sheet and also explain how to interpret each one. I’m going to start with three specs today: (1) rated open circuit voltage, (2) rated output current, and (3) rated output voltage.

Rated open circuit voltage is a safety spec present under certain conditions, usually if there are exposed torch wires or a damaged torch handle. This is a case where a higher number is not better. In fact, the only thing a higher open circuit voltage gets you is a greater chance of electric shock. Generally older power supplies and torches have higher open circuit voltages to make arc starting easier and more reliable.

Rated output current is the designed operating range of a plasma system. I will bring up the importance of having a range of amperage in a later post when talking about consumables and cut quality.

Rated output voltage is one of the things duty cycle is based on. Arc voltage and arc length are proportional. When cutting thicker material, the arc gets longer, and the voltage gets higher. The spec sheet at the end of this post shows the voltage required to cut the advertised production thickness. Be careful when looking at this as many companies (even major manufacturers) will use a lower arc voltage to publish a higher duty cycle. For example, they might cut a ¼ inch thick piece of steel rather than the ½ inch piece the system is rated for.

Another thing to know is that current or amperage and voltage are two distinctly different measures of cutting capability. Amperage is a fairly good indicator of a plasma systems power, and can be set with a knob by the operator. Voltage is entirely dependent on the length of the cutting arc. If you multiply the maximum amperage by the rated voltage you will get the rated wattage of the plasma system. (Read our post on amps for more on this.) Wattage is a true measure of the amount of power a plasma power supply can produce though it’s probably a good idea to also consider duty cycle and the ambient temperature rating. All plasma power supplies can produce a higher voltage than the rated voltage and a higher wattage output. However, the power supply has to work harder. This causes it to produce more internal heat, which will shorten the effective duty cycle.