jimcolt
Gold Member
I thought I would put together a few bits of information regarding Plasma Cutter design and performance....with no particular models brought up.....except to make points about power, etc. I am a long term Hypertherm guy, however my responsibility is not with sales, rather my title is " Applications Technology Manager", and that means I am supposed to help any and all potential or existing users ensure that they choose the correct metal cutting system that best fits their needs. That includes all brands and also incudes waterjet, laser and oxy-fuel cutting applications.
If these two postings seem (the other I posted earlier is about Plasmacutting angularity) to be of value on this forum...let me know and I will continue adding to this series. I certainly am happy to respond and clarify anything posted here, or expand on it as well. New topic suggestions (in my area of expertise) are welcome as well! Jim Colt Hypertherm
Important facts.
Think of the maximum amperage rating of a plasma cutter as its general power output, however when pushed near its factory rated limits, the load voltage and duty cycle start to get very important when comparing one plasma to another. Most 30 amp air plasma systems can cut 1/4" steel with reasonable cut speeds. I expect a 40 amp plasma system to be a little faster, and perhaps have the ability to slowly sever material a slight amount thicker than the 30 amp model. These thoughts are true for the majority of systems. One of the other important components of producing a powerful plasma cutting arc is load voltage. Load voltage is the actual voltage reading of a plasma arc between its (negatively charged) electrode and the (positive) workpiece or plate being cut. If your are cutting thicker material, the plasma arc needs to naturally be longer in order to penetrate all the way through?and a longer arc will measure as higher voltage.
So, if you are cutting a piece of 1/8 steel at 30 amps with a plasma cutter the power supply is producing 30 amps of DC cutting energy, and an arc voltage (I'm going to provide an experienced guess here) of approximately 110 volts. Next, I decide to cut some 3/8 steel with the same plasma cutter, still set at 30 amps. Since I am know cutting 3 times the thickness I am quite sure the arc will be 3 times the length, so I expect that the output arc voltage will be higher and with the plasma systems that I am familiar with the voltage for cutting 3/8 will be in the 124 volt range. So on the 1/8", 30 amps at 110 volts, we can simply calibrate the amount of cutting power that the plasma power supply has to produce by multiplying its output amperage by the output voltage. 30 amps x 110 volts = 3,300 watts, or 3.3 killowatts. To cut the 3/8 requires more power, 30 x 124= 3,720 watts (3.72 kW).
Most of the air plasma cutters on the market today are high tech inverter based power supplies. The electronics and the technology involved in inverters are continuously improving, with higher amounts of power now able to be produced in smaller power supplies. When I started in the plasma cutting business in 1978?t took a power supply that ran on 3 phase power and weighed around 800 lbs to cut 1/2" steel, today we can cut 1/2" steel with a 22 lb. power supply that runs off a household 120 volt outlet. The limitations within these inverter power supplies are with internal heat that builds up. More heat is produced if more wattage is required for the cutting process, so even though we are cutting 1/8 steel and 3/8 steel with the same Amperage (30 in this case), the 3/8 uses more power in Watts, therefore the power supply worked harder and produced more internal heat. This clearly proves that amperage is not the best indicator of the amount of power your plasma cutter can produce!
OK?now we have developed a bit of an understanding of Amperage, Voltage, and how these two factors (along with material thickness) determine the true power requirements for cutting steel?.now lets throw duty cycle into the mix. The air plasma cutting market is very competitive. Most air plasma cutters (well over 90%) are used strictly for hand cutting, and with hand cutting the human component generally does not run at 100% duty cycle?meaning that the torch operator does not cut for 8 hours continuously on his shift. In reality, the numbers for hand cutting are closer to a 20% to 30% hand cutting duty cycle even in a very busy shop. Next fact: It cost more to produce a plasma cutting power supply that can produce high wattage output?.as it is producing more heat?and too much heat on electronic components can shorten their life, or worse can cause catastrophic failure. Again a competitive market. If a power supply from manufacturer A costs more than one from Manufacturer B, yet both are advertised to cut the same thickness, most buyers will buy the lower priced plasma. So, to keep costs and physical size of an air plasma system as low and small as possible, engineers design them with duty cycles that will closely match the way these plasma cutters will get used in the field.
What does this have to do with cutting power? Duty cycle in the welding and plasma cutting industry are rated on a 10 minute cycle. Engineers laboratory test their designs after the power supply has warmed up to an ambient temperature, then they operate the system at its design rating while measuring the temperature of internal critical components such as transformers, IGBT switching devices, inductors, etc. From this testing, which must be done at a known output wattage, in a temperature controlled environment the amount of on time vs off time of the power supply can then be determined, based on a ten minute cycle. The major plasma system manufacturers state in their system literature the ambient temperature that the system duty cycle was tested at, and they determine the wattage that it was tested at, Then a percentage such as 35% is assigned as the duty cycle rating. The Hypertherm ambient (room temperature) temp used in all of its duty cycle ratings is 40 degrees C. (104 degrees F). So this means that on a 104 degree (F) day you can cut at the rated wattage (or kW) output for at least 3.5 minutes out of every 10 minutes.
Think about this. If the day is only 55 degrees (instead of the 104 that the duty cycle was rated at), expect the 35% will be considerably higher. Also, if you are cutting on thinner plate?eaning your kW usage was lower than the rated kW?expect also that the rating will be higher than 35%. Look in any Hypertherm operators manual and you will find a duty cycle rating based on 104 degrees ambient, and the rating will have a percentage of on vs off time, and it will have a kW output that it was tested at under those conditions. Can you cut thicker? Yes, any plasma system will produce higher output voltages than they are rated to produce? So if you cut thicker, it will produce a longer arc (higher voltage , higher kW) and more power supply heat so the duty cycle likely will become less than it was rated for.
The reason I always talk about duty cycle is because most manufacturers of plasma cutting systems only provide a duty cycle number. No wattage or voltage rating, no ambient temperature that the system was rated at. All you will find on the power supply specifications is a duty cycle percentage with no facts to back it up. Beware that plasma system you just bought that produced 40 amps and has an advertised 50% duty cycle may more realistically have a 10 or 20 percent duty cycle when cutting the same thickness as comparable unit. This can seriously shorten the life of a system, and will prove to be a lesser performing system from a cut speed and thickness point of view.
Bottom line: while more amperage generally will cut thicker, you need amperage, load voltage (to make higher wattage) and a properly rated duty cycle in order to produce long term reliability and performance in a plasma system.
If these two postings seem (the other I posted earlier is about Plasmacutting angularity) to be of value on this forum...let me know and I will continue adding to this series. I certainly am happy to respond and clarify anything posted here, or expand on it as well. New topic suggestions (in my area of expertise) are welcome as well! Jim Colt Hypertherm
Important facts.
Think of the maximum amperage rating of a plasma cutter as its general power output, however when pushed near its factory rated limits, the load voltage and duty cycle start to get very important when comparing one plasma to another. Most 30 amp air plasma systems can cut 1/4" steel with reasonable cut speeds. I expect a 40 amp plasma system to be a little faster, and perhaps have the ability to slowly sever material a slight amount thicker than the 30 amp model. These thoughts are true for the majority of systems. One of the other important components of producing a powerful plasma cutting arc is load voltage. Load voltage is the actual voltage reading of a plasma arc between its (negatively charged) electrode and the (positive) workpiece or plate being cut. If your are cutting thicker material, the plasma arc needs to naturally be longer in order to penetrate all the way through?and a longer arc will measure as higher voltage.
So, if you are cutting a piece of 1/8 steel at 30 amps with a plasma cutter the power supply is producing 30 amps of DC cutting energy, and an arc voltage (I'm going to provide an experienced guess here) of approximately 110 volts. Next, I decide to cut some 3/8 steel with the same plasma cutter, still set at 30 amps. Since I am know cutting 3 times the thickness I am quite sure the arc will be 3 times the length, so I expect that the output arc voltage will be higher and with the plasma systems that I am familiar with the voltage for cutting 3/8 will be in the 124 volt range. So on the 1/8", 30 amps at 110 volts, we can simply calibrate the amount of cutting power that the plasma power supply has to produce by multiplying its output amperage by the output voltage. 30 amps x 110 volts = 3,300 watts, or 3.3 killowatts. To cut the 3/8 requires more power, 30 x 124= 3,720 watts (3.72 kW).
Most of the air plasma cutters on the market today are high tech inverter based power supplies. The electronics and the technology involved in inverters are continuously improving, with higher amounts of power now able to be produced in smaller power supplies. When I started in the plasma cutting business in 1978?t took a power supply that ran on 3 phase power and weighed around 800 lbs to cut 1/2" steel, today we can cut 1/2" steel with a 22 lb. power supply that runs off a household 120 volt outlet. The limitations within these inverter power supplies are with internal heat that builds up. More heat is produced if more wattage is required for the cutting process, so even though we are cutting 1/8 steel and 3/8 steel with the same Amperage (30 in this case), the 3/8 uses more power in Watts, therefore the power supply worked harder and produced more internal heat. This clearly proves that amperage is not the best indicator of the amount of power your plasma cutter can produce!
OK?now we have developed a bit of an understanding of Amperage, Voltage, and how these two factors (along with material thickness) determine the true power requirements for cutting steel?.now lets throw duty cycle into the mix. The air plasma cutting market is very competitive. Most air plasma cutters (well over 90%) are used strictly for hand cutting, and with hand cutting the human component generally does not run at 100% duty cycle?meaning that the torch operator does not cut for 8 hours continuously on his shift. In reality, the numbers for hand cutting are closer to a 20% to 30% hand cutting duty cycle even in a very busy shop. Next fact: It cost more to produce a plasma cutting power supply that can produce high wattage output?.as it is producing more heat?and too much heat on electronic components can shorten their life, or worse can cause catastrophic failure. Again a competitive market. If a power supply from manufacturer A costs more than one from Manufacturer B, yet both are advertised to cut the same thickness, most buyers will buy the lower priced plasma. So, to keep costs and physical size of an air plasma system as low and small as possible, engineers design them with duty cycles that will closely match the way these plasma cutters will get used in the field.
What does this have to do with cutting power? Duty cycle in the welding and plasma cutting industry are rated on a 10 minute cycle. Engineers laboratory test their designs after the power supply has warmed up to an ambient temperature, then they operate the system at its design rating while measuring the temperature of internal critical components such as transformers, IGBT switching devices, inductors, etc. From this testing, which must be done at a known output wattage, in a temperature controlled environment the amount of on time vs off time of the power supply can then be determined, based on a ten minute cycle. The major plasma system manufacturers state in their system literature the ambient temperature that the system duty cycle was tested at, and they determine the wattage that it was tested at, Then a percentage such as 35% is assigned as the duty cycle rating. The Hypertherm ambient (room temperature) temp used in all of its duty cycle ratings is 40 degrees C. (104 degrees F). So this means that on a 104 degree (F) day you can cut at the rated wattage (or kW) output for at least 3.5 minutes out of every 10 minutes.
Think about this. If the day is only 55 degrees (instead of the 104 that the duty cycle was rated at), expect the 35% will be considerably higher. Also, if you are cutting on thinner plate?eaning your kW usage was lower than the rated kW?expect also that the rating will be higher than 35%. Look in any Hypertherm operators manual and you will find a duty cycle rating based on 104 degrees ambient, and the rating will have a percentage of on vs off time, and it will have a kW output that it was tested at under those conditions. Can you cut thicker? Yes, any plasma system will produce higher output voltages than they are rated to produce? So if you cut thicker, it will produce a longer arc (higher voltage , higher kW) and more power supply heat so the duty cycle likely will become less than it was rated for.
The reason I always talk about duty cycle is because most manufacturers of plasma cutting systems only provide a duty cycle number. No wattage or voltage rating, no ambient temperature that the system was rated at. All you will find on the power supply specifications is a duty cycle percentage with no facts to back it up. Beware that plasma system you just bought that produced 40 amps and has an advertised 50% duty cycle may more realistically have a 10 or 20 percent duty cycle when cutting the same thickness as comparable unit. This can seriously shorten the life of a system, and will prove to be a lesser performing system from a cut speed and thickness point of view.
Bottom line: while more amperage generally will cut thicker, you need amperage, load voltage (to make higher wattage) and a properly rated duty cycle in order to produce long term reliability and performance in a plasma system.
