Duty Cycle, Ambient Heat, and Why the Claim Fails in Summer

A machine is sold as 50% duty cycle. The customer installs it in a workshop that reaches 42 °C in July, runs it for eight minutes out of every ten, and reports that it "loses power" after two hours. Nothing has failed, and nobody has been misled: the 50% was established at an ambient temperature that the workshop does not have.

Duty cycle is the most quoted and least examined number on a welding specification, and for any buyer in a hot climate it is worth more attention than the kVA figure beside it.

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A seam welding machine on our floor, with its two copper electrode wheels. A machine like this is bought for continuous running, which is exactly when the duty cycle claim starts to matter.

What the Number Is Measuring

Duty cycle is a thermal statement, not an electrical one. It says that at a stated current, for a stated proportion of a ten-minute period, the hottest part of the winding stays below the temperature its insulation is rated for. Fifty per cent means five minutes of welding and five minutes of cooling, averaged, not five minutes whenever you feel like it.

The measure is deliberately conservative because the consequence is not a bad weld but a burnt transformer. What it is not is a statement about the electrodes, and on most production lines the electrodes give up long before the transformer does.

The Electrode Is Usually the Real Limit

A copper electrode that has been run too hot softens, and softened copper is no longer an electrode: the tip mushrooms under force, the contact face grows, current density falls, and the weld drifts thinner and thinner while every gauge and meter on the machine still reads normal. The transformer is comfortable. The weld is not.

This is why a machine can be inside its duty cycle and still lose the weld on a long shift, and why the useful question is not "what is the duty cycle" but "at what tip temperature, measured how, and how often do we dress".

The arithmetic of mushrooming is worth knowing because it is not linear. Current density falls with the square of the face diameter, so a tip that has grown by a fifth has lost roughly a third of its density. No meter on the machine reports that. A caliper on the tip once a shift does.

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Electrode holders on a platform machine, with the coolant tubing that keeps the tips below the temperature at which copper stops being an electrode and starts being a soft metal.

Ambient Is Half of the Rating

The rating assumes an ambient temperature. The winding has a permitted temperature, and the cooling available to it is the difference between that permitted temperature and the air around it. Raise the air by fifteen degrees and you have taken fifteen degrees off the cooling, which is a large fraction of the margin the machine was designed around. The permitted duty falls with it.

Manufacturers publish a derating curve for exactly this, and it is worth asking for it in writing rather than reasoning about it in a meeting. In our export markets the relevant condition is rarely the average; it is a 43 °C afternoon, in a shed with no air movement, with the transformer sitting at the top of the frame where the heat collects.

Where the Heat Actually Goes

Cooling water is the part most easily got wrong, because a chiller is bought by tonnage and installed by whoever is on site. What matters at the tip is flow and temperature, not capacity: a chiller with headroom and a partly blocked tip channel is worse than a smaller chiller with clean passages, and the symptom in both cases is a tip that discolours.

Water quality is the other half. Hard water scales the narrow channel inside the tip, scale insulates, and an insulated tip becomes a fuse: it heats from the inside, softens, mushrooms, and the weld drifts. We now ask for the water analysis at the enquiry rather than at commissioning, because the answer changes the chiller and sometimes the tip design.

Some of the heat never reaches the cooling circuit at all. On 0.7 mm coated sheet most of it goes into the panel, which is why a machine can run cool while the part walks out of shape. Duty cycle has nothing to say about that, and no number on the plate will warn you.

Where Duty Cycle Meets a Weak Supply

There is a third mechanism that looks like a cooling fault and is not. On a weak supply the voltage at the machine sags during the weld pulse, the controller sees the current falling short and compensates by extending the time to put the same energy in. The weld itself may still be acceptable. What changes is that every weld now takes longer, the machine spends a larger fraction of the cycle actually welding, and the thermal margin quietly disappears.

We have seen this on sites where the welder shares a transformer with a large compressor. The customer reports overheating; the cooling is fine; the schedule has been stretched by the controller to about 30% more than it should be. Measuring the supply voltage during the pulse, not between pulses, is the way to find it, and it takes ten minutes with a meter that records.

A Line We Under-Cooled

A customer in a hot climate ordered a seam-welded tank line to run continuously, two shifts. We sized the chiller against the nominal duty at a comfortable ambient, which is how the quotation had always been done. In the first summer the return water came back at 38 °C, the wheels softened, the seam width drifted, and the tanks failed leak tests at the rate of one in six.

The fix was a larger chiller, a tip temperature check every two hours, and a dressing interval in the work instruction rather than in somebody's head. We air-freighted the chiller and lost about a week of their production. The mistake was not the equipment; it was sizing against an average instead of against the worst week of the year.

What to Put in the Enquiry

Four numbers turn duty cycle from a claim into a specification.

The real duty: how many seconds of welding in every ten minutes, on your part mix, not on the best case.
The maximum ambient in the building, in the month it happens, and whether air moves there.
The incoming water temperature and its hardness, or the fact that there is no water.
Who checks the tips, how, and how often, because that interval is part of the duty cycle whether it is written down or not.

Claim or assumption	What it depends on	What goes wrong when it is wrong
50% duty cycle	Ambient at which it was rated	Power fades after two hours
Water-cooled electrodes	Flow, temperature, water hardness	Tips soften and mushroom
Rated current	Tip face area, kept by dressing	Weld drifts with no meter moving
Continuous seam welding	Wheel cooling between passes	Seam width wanders, leaks appear
Shop-floor ambient	The worst week, not the average	Everything above, at once

A machine that is honestly sized for the worst week costs a little more and stops being a conversation in July.

PDKJ continues to provide customized laser welding and resistance welding solutions based on different materials, workpiece structures, welding requirements, and production applications.

Duty Cycle, Ambient Heat, And Why The Claim Fails In Summer