Taking the mystery out of air plasma power supply specifications – part 2
Guest post by Jim Colt, technology applications engineer
Last week, I posted about specification sheets with a goal of helping you interpret all of the numbers found on one. I explained rated open circuit voltage, rated output current, and rated output voltage. Today I want to cover duty cycle.
Duty Cycle at 40C: Many, but not all, plasma and welding manufacturers use IEC (International Electrotechnical Commission) standards when establishing duty cycle. These standards are based on a known ambient temperature of 40 degrees Celsius (104 degrees Fahrenheit.) Here’s what the test looks like:
- The power supply is brought up to 40 degrees Celsius
- Cutting is done at maximum current at a fixed arc voltage
- When the components inside the power supply (normally transformers, switching devices, etc.) reach the manufacturers maximum operating temperature, you have reached duty cycle
Temperature sensors added by most manufacturers to most systems then kick in and automatically shut down the system before damage occurs. Duty cycle may not be much of a concern for the occasional hobbyist, however anytime you put a plasma system on a CNC machine you can expect higher arc-on times than when cutting by hand. Now, determining duty cycle is tricky, because unfortunately, there is no agreed upon standard among plasma (and welding for that matter) manufacturers. Though many use IEC standards, there is no law requiring it. Two different manufacturers can come up with different duty cycle ratings for the exact same system! So how do you know if a particular system will meet your needs? You can start by looking at (1) the ambient temperature that the duty cycle is based on and (2) the load voltage of the system. Let’s start with ambient temperature and why this matters.
Let’s say manufacturer A uses an ambient temperature of 70 degrees Fahrenheit while manufacturer B uses the IEC suggested temperature of 104 degrees. If this case, manufacturer A will come up with a higher duty cycle because its system didn’t have to work as hard as the system from manufacturer B. It’s like our bodies; we can go a lot further on a 70 degree day than we can on a 114 degree day.
Load voltage is also important. If manufacturer A determines duty cycle when the system is cutting gauge material, while manufacturer B is running the system at maximum capacity, which manufacturer do you think will come up with a better duty cycle rating? Again, like our bodies, we can carry 1 pound a lot further than 50 pounds!
Even though a manufacturer may state a 60 percent duty cycle, when cutting thicker material on a hot day, the duty cycle can actually drop to10 percent. I have experienced this!
I've inserted the specification sheet used in our last post below so you can see what it says about duty cycle. In this case, the specification sheet contains three different duty cycle calculations. The first is the advertised 60 percent duty cycle rating for cutting the advertised production thickness of ½ inch thick steel. The second number provides a duty cycle for that same ½ inch thick steel cut at a slower speed. And finally, the third number tells you how to achieve 100 percent duty cycle. So now you know how to read and interpret duty cycle.
The next question is what duty cycle do you really need? A 50 percent duty cycle is usually more than adequate for those cutting by hand. For mechanized cutting you should consider purchasing a system with a higher duty cycle. Does this mean you should dismiss any system with a 50 percent duty cycle rating? No. It means you may have to buy a larger system and run it a lower amperage level to get a higher duty cycle.