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View Full Version : How to figure out Duty Cycle of your Welding machine

Joker11
07-10-2011, 09:15 PM
This should be a sticky. Most welders are sold with a 20% or 40% duty cycle rating. But what if you want to know the duty cycle at other amperages? The owner manual doesn't tell me that at all. Nor does it tell me how to figure it out.

So here you go guys. An equation to post on your shop wall. You may never use it or may use it just once. I just enjoy knowing how to do it. Plus it was handy for using my new Precision Tig 275!!

69957

A= Adjusted duty cycle at desired welding current. (percent)

S= Specified current at rated duty cycle. (amps)

I= Desired (or actual) welding current (amps).

R= Rated duty cycle (i.e. at current S). (percent)

DSW
07-10-2011, 09:32 PM
Curious where you got this formula and if it applies to all types of machines, transformer, engine drive, inverter...

Joker11
07-10-2011, 09:44 PM
I asked my buddy, who teaches math and science at the local high school to help me. We both tried ratio formulas and that didn't work at all.
I did take my owner manual and it gave 40% and 100% duty cycle ratings for different modes, as in stick or tig, and did the math and the numbers matched. But that's the only numbers Lincoln gives, 40% and 100%. well, laaaa di daaaaa. I wanted to know amperage at all percentages.

My Lincoln engine driven Ranger 9 was 100% duty cycle, so I couldn't compare.

I don't know if it applies to inverters or not. Let's see how many people tell me I am wrong. LOL.

Unstaybull
07-20-2011, 05:08 PM
The rule of thumb that i use for that is as follows......40 percent duty cycle means to me I can run that straight for 4 minutes out of ten....60 is 6 minutes out of ten....Thats what I was taught about that...lol Right or Wrong?

Peace

Joker11
07-22-2011, 02:38 PM
Unstaybull,

Yes, if your rated output is 40%, you can run 4 minutes out of 10. But when can you run it nonstop? This formula will tell you that if I only run at 200amps, I can run 100% duty cycle.

But if I want to run 340amps, do you know that if you run 4minutes, out of 10, you will exceed the limits of this machine?

That was the purpose of my post, to show a relationship between duty cycle and output amps. Because 40% does not cover the WHOLE range of output, only one amperage.

Rbeckett
07-22-2011, 05:09 PM
I like it. Being an empisicist this answers a lot of vaguaries and gets me close enough to be comfortable. Thanks.
Bob

Joker11
07-22-2011, 06:31 PM
what is an empisicist?

Bluewelders
07-22-2011, 07:43 PM
empiricist

lotechman
07-24-2011, 04:47 PM
Some operating manuals actually provide a graph with the duty cycle and amperage settings. I doubt that you could "fit" the lines to the odd curves I have seen.
In any case ambient temperature will have an effect and there are many other influences.

Silicon-based
08-22-2011, 10:14 PM
Some Thermal Arc manuals have this formula, stated in terms of amps at desired duty cycle. It is not absolutely accurate but usually close enough.

John

Rick V
08-22-2011, 10:55 PM
I've found it darn useful the last few days for comparing welders. Thanks!

Welderskelter
10-09-2011, 10:33 PM
Never paid any attention to duty cycle. I started welding for a company in Protection Kansas. Filsons Manufacturing. They had about 40 of these little AC buzz boxes and we ran them as hard as we could. Cant remember anybody being man enough to burn one up. They are tough. Harold

Rick V
10-10-2011, 12:57 PM
Often for a given welder, the owner's manual will give the 230 volt input amps required to output so many amps (say 225) at some specified (say 20%) duty cycle. However, what we often want to know is how many welding amps are available at another duty cycle - usually 100%. Note too that duty cycle as affects the input amperage requirment!

Joker11 equation can be restated as:
Amps at Desired Duty Cycle = Amps at Rated Duty Cycle x Square root (Rated Duty Cycle / Desired Duty Cycle)

Using that relationship, here's some tables I made up for several widely used stick welders and a typical 230 volt MIG welder that outputs 130 amps DC.

Old & New Lincoln Idealarc 250 welders.
73796

An assortment of other welders.
73797

Hopefully, someone may find these tables useful.

Warpspeed
12-01-2011, 02:51 AM
Mathematically the formula is correct, as it indicates the heating effect produced by different load currents.
But that is not the entire story.......

At 100% duty cycle rated current, the internal heating produced can be safely dissipated basically forever, and nothing inside your welding power source will rise to a high enough temperature to case any reliability problems.
The original designers very carefully determine the 100% rating after running the welder at that continuous load for maybe an hour or more.

Now just about all of the internal heating is created by electrical resistance of the various parts, and the heating power produced is proportional to current squared for most things. Hence the above formula.

BUT there is another factor, "time" which comes into this as well.

You obviously cannot run any welder at 50% duty cycle thinking that means eight hours continuous welding and eight hours to cool down !!

Most people just assume the rating is taken over a ten minute period, maybe five minutes welding and five minutes cool down for 50% duty cycle.
That may be how some manufacturers rate their machine, and it may have a lot of historical precedent, and is a good practical working rule of thumb.

But realize that many of the latest modern very light weight portable inverter based welders now have very small highly stressed internal parts, with small thermal mass, compared to a traditional transformer welder.

A 200 Lb welding transformer made of pretty solid iron and copper, is going to heat up and cool down a lot slower than a high frequency inverter welder with a mini 10 Lb transformer.

My own thoughts are that an older style transformer welder can be pushed a lot more safely as far as duty cycle goes than a low cost hardware store "made in China" inverter welder.

So use the above formula with extreme caution when trying to push a light weight portable welder out to very low duty cycles. It will probably do it, but the internals are going to get very hot very quickly, and you may need to think more terms of tens of seconds of operation rather than tens of minutes, at absolute maximum current.

assassin_works
12-01-2011, 10:12 PM
kool thanks i might ask questions later

Welding Fool
12-27-2012, 11:14 PM
Great formula, Thanks! It is still almost impossible to cross check Lincolns technical specifications because they list duty cycle currents at 3 different voltages. What do they have against being honest about the machines capabilities?:angry:

01-06-2013, 10:12 PM
Rick V.

Very interesting table:)

It appears you took the ratio of AC input to weld output from the manufacturers original duty cyle rating and used it to determine the AC input at other output settings. Is that correct?

Dave J.

joshuabardwell
01-06-2013, 10:27 PM
But realize that many of the latest modern very light weight portable inverter based welders now have very small highly stressed internal parts, with small thermal mass, compared to a traditional transformer welder.

Don't most of those newer inverter welders have a built-in over-heat shutdown anyway? I don't have one, but I got the impression that with those the duty cycle was just a guideline, and if you wanted to, you could just weld until it shut down and then wait until it said "good to go" again.

joshuabardwell
01-06-2013, 10:43 PM
I just crunched the numbers for the duty cycle rating on the faceplate of my welder, and they don't quite add up. My welder is rated at 100% duty cycle up to 80 amps DC, and then 20% duty cycle from 80 to 140 amps.

If I put in 140 amps/20% and solve backwards to 80 amps, I get a duty cycle of 60% when it should be 100%.

If I put in 80 amps/100% and solve forwards to 140 amps, I get a duty cycle of 35% when it should be 20%.

I guess reality is somewhere in the middle?

SquirmyPug
01-06-2013, 10:49 PM
Don't most of those newer inverter welders have a built-in over-heat shutdown anyway? I don't have one, but I got the impression that with those the duty cycle was just a guideline, and if you wanted to, you could just weld until it shut down and then wait until it said "good to go" again.

I just checked my Maxstar, it says.

Duty Cycle is percentage of 10-minutes that unit can weld at rated load without overheating.

If unit overheats, output stops, the Overtemperature Light comes on, and the cooling fan runs. Wait fifteen minutes for unit to cool. Reduce amperage or duty cycle before starting to weld again.

NOTICE - Exceeding duty cycle can damage unit and void warranty.

Mondo
01-09-2013, 02:03 PM
Seeing how Duty Cycle is all about heat dissapation I have a solution for all these duty-cycle issues:

Build a double-walled box with 2" of fiberglass insulation between the walls. The box should have a tight-fitting lid with a single 1" ventilation hole through it. Make the box at least twice as tall as the welder with one wall cut-out for the welder face (control panel) to be exposed. Install a shelf perforated with many 1" holes over the welder. Put a block of dry ice on this shelf and close the lid. The block of dry ice could be 50 lbs for a small machine, and maybe 100 lbs for a larger machine like a Tombstone. Now you can weld as long as as you like providing you can still see CO2 vapor pouring from the vent hole!! When the CO2 "smoke" cloud thins and stops flowing stop welding. You are out of dry ice! :laugh:

Now if you want to get really scientific about this you could build in deflectors and auxillary fans to channel the airflow add some thermometers to permit monitoring the temperature inside the welding machine, the lower compartment space around it, and the upper compartment space around the block of dry ice! A really nerdy type could use electronic sensors connected to a computer that would provide real-time measurements and statistical information in graphical format on a video display screen. :D

:jester:

Does this sound far fetched? Data networking gear installed in the wiring closets and computer "machine rooms" of schools, municipalities, and companies large and small have all of this but the dry ice. A network operator or manager can monitor the temperature of not only the inside of each cabinet in the closet but the temperatures inside each chassis in the cabinet and the temperature of individual circuit boards inside each chassis from his network management work station in the next room or 12 thousand miles away. He can tell when a fan fails and for cabinets and chassis that have more than one fan he can tell which fan it is that failed. Our gear can even send him an email, a page, or a Tweet to tell him he has an over-temp condition in a device in his company's data network the moment the condition is first detected! This is the sort of equipment I support doing my day job every day.

So how about welding machine makers adding a temperature monitoring display on the control panel so the operator can see what the temperature inside the cabinet really is and provide him the opportunity to monitor how long it takes to heat up and how long it takes to cool down, when he should stop and when it is safe to start welding again?

What a weldor has now isn't any more than guess work! Weld for a period of time then stop and look at your watch and say "I guess I should stop for a while". Several minutes pass and you say "I guess the machine has had some time to cool off, I suppose it is OK to do some more welding for a while!" But do you really know what the temp is inside the machine or how much duty cycle you are using? NO! Pretty low-tech in a high-tech world, I think.

- Mondo

TheJackal
02-05-2013, 02:29 AM
I am no rocket scientist, but I am a realist. To me, you guys are on the right track, however, A realist would view a duty cycle as being a cold machine, and at x amps, How long can it run before it goes into thermal overload, and how long does it take to cool down to reciprocate that same process, As a starting point, however I agree with most that has been said and have not written out an equation. I understand that duty cycle is more of a maintained rating, but you need to also address the starting point to focus on what your machine will do, also, you are not allowing ambient temperature into the equation, not sure if it makes a difference because heat into the work and how hard it is on the machine @ 80* might not be linear to same scenario @ 0* but I guess amps out and duty cycle would still hold true.. Just kicking the "hornets nest" :)

jtcnj
02-11-2013, 02:36 PM
I've had concerns about this as well. My Lincoln AC/DC 225/125 says 20% at all taps, except Lincoln has acknowledged the 75A AC setting (with the circle) is rated at 100% as intended for long-period use during pipe-thawing operation. (member Super Arc, I think, got hold of someone at Lincoln, and they still insist 20% duty cycle for everything else).

Others here with repair experience have stated the lower amp secondary circuits and wiring to the controls are of a smaller diameter wire, so maybe the concern at the lower amp taps would not be overheating and damaging the primary windings on the transformer, but other components. Makes sense from that perspective, but not much sense from a product design standpoint in negating the benefit of increased duty cycle at lower amps.

I think its CYA, so if I blow it up with the following Duty Cycle Operational Protocol, the answer is "See, we told you so!".

At 65 DC amps and below, I really don't pay much attention to it. When I'm running 3/32" 6010 on usually 1/8" or tig on 11 or 16ga.,typical time in between beads allows enough to cover it.

When I go to 3/32" 7018, from 85-105A, I make sure to allow more time in between. It's still just ballpark, but I am consciously keeping it in mind.

Above that, I'll run maybe 2 or three rods or tig passes, then clock out a full 8 minutes.

02-11-2013, 03:19 PM
On my first weld/fab/mechanic job I used a standard Lincoln 225 AC welder. I never paid any attention to the duty cycle using 1/8" rods and under.:)

Naturally, I do think about it now...sometimes...:D

Dave J.

Crash5291
02-13-2013, 10:46 AM
. . . . . Pretty low-tech in a high-tech world, I think.

- Mondo

I Agree Mondo, and with all these digital controlled welders one would think its and easy adaption for the manufacture some may even have temp monitoring to control the safety's.

hmmm

Joe