What the kVA Number on the Nameplate Does Not Say

Two quotations arrive for the same bracket. Both machines are described as 160 kVA. One puts 14 kA into the part; the other, on the same schedule, puts in about 11.5. Neither supplier is lying, and neither figure is wrong. The number on the plate is simply measured somewhere other than where your weld happens.

For anyone comparing machines across suppliers, that gap is the single most expensive thing not to understand, because the plate looks like the one number you can compare without knowing anything.

latest company news about What the kVA Number on the Nameplate Does Not Say 0

A platform machine with a long throat, built for a panel that could not be lifted over a short arm. The two horizontal arms are the secondary circuit, and their length is the price of reach.

The Plate Describes a Transformer

On a resistance welder, the kVA figure is the apparent power the transformer can deliver at its own secondary terminals, at a stated duty cycle, normally with the secondary shorted. It answers a question about the transformer: how much current can this winding push when nothing is in the way. It does not answer the question a buyer is actually asking, which is how much current arrives at the tip when the tip is clamped on 2 mm galvanised sheet.

Between those two points sits the whole secondary circuit: the shunt, the moving arm, the holder, the electrode, the workpiece, and the same list back again on the return side. Every bolted joint in that path is resistance. The loop they form is inductance. A machine can have an excellent transformer and a mediocre secondary, and the plate will still say 160 kVA.

The Arms Are an Inductor, Not a Bracket

Current leaves the transformer, travels out along one arm, crosses the workpiece and comes back along the other. That loop encloses an area, and an area enclosing current is an inductor. Its reactance rises with both the loop area and the frequency, and on a mains-frequency machine the reactance term is not a detail: it is the dominant part of the impedance at the electrodes.

This is why throat depth is an electrical decision and not a handling convenience. Take the same transformer from a 600 mm throat to a 1,000 mm throat and the loop grows, the reactance grows with it, and the current at the tips falls. The machine has not become weaker; the circuit has become longer.

It is also the honest explanation for why inverter machines changed long-throat welding. Reactance scales with frequency, so raising the welding frequency from the mains to about a kilohertz takes the reactance term down by an order of magnitude. The same transformer then reaches much further down the same arms. That is a property of the frequency, not of the marketing.

Where the Current Actually Leaks Away

Resistance in the arms grows with length and falls with section, and the arms carry the entire secondary current, so a long throat needs more copper, not the same copper stretched. The shunt is the other half of the story and the part most often neglected. It is a stack of thin copper laminations, flexible because the moving arm has to move; its job is to carry the full current across a gap that changes length all day.

A shunt with too few laminations, corroded ends or the wrong length turns into a heater. We have opened machines two years into service and found the ends black and the insulation crisped. Nothing had failed. The welds had simply become harder to set, and the operator had compensated with time, which is why nobody noticed until the cycle time was 30% over the quotation.

latest company news about What the kVA Number on the Nameplate Does Not Say 1

The copper loop behind the gun head is the shunt: flexible copper laminate carrying the whole secondary current from the transformer to the moving arm.

A Quotation We Got Wrong

A customer sent a stainless sink bracket with a deep return flange. On the drawing the only thing that changed was the reach: the weld was 900 mm inside the part, so we quoted a platform machine with a 1,000 mm throat on the transformer we used for our 600 mm machines. On paper it was the same machine, longer.

It was not. At run-off the tips gave us about 11.5 kA where the schedule needed 13, and the stainless would not fuse without a time so long that the visible face discoloured. We had to step up to the next transformer size, which also meant a larger cabinet and a longer lead time, and we absorbed the difference. The lesson was not that long throats are difficult. It was that we had sized the transformer from a table and not from the throat.

We now write the throat depth on the order sheet before the transformer is chosen, and for anything past 800 mm we measure the secondary current on a coupon of the customer's material before the machine is released.

The Return Path Is Part of the Weld

There is a second loop nobody draws on the quotation, and it is the one that causes arguments. The secondary current has to come back to the transformer through the arms. On a correctly built machine the frame is earthed for safety and carries no welding current. On a machine where somebody has bonded the work table to the frame, or where the return arm has a poor joint, part of the current finds its way home through the frame, the guards or the conveyor. The weld gets whatever is left.

The symptoms are confusing precisely because they are intermittent: welds that are fine on one fixture and weak on another, arcing marks on a clamp, a bearing in the fixture that pits. We have chased one of these for a fortnight, changing schedules on a machine that was never at fault, before finding a corroded joint on the return arm.

A buyer can test for it cheaply. Clamp a meter around the return arm during a weld and compare with the current measured at the electrode; and check the short-circuit current between the tips against the figure the supplier quoted. A small shortfall against the plate is normal. A large one is not a schedule problem, and no amount of turning the current up will put the metal back.

What to Ask Instead of the Figure

None of this means the kVA number is useless. It means it is a component rating being used as a system rating, and five questions will get you past it.

What is the throat depth, and what is the cross-section of the arms at that depth?
Is the kVA figure a short-circuit rating or a working rating, and at what duty cycle?
Is the machine mains frequency or inverter, because the two do not compare at equal kVA?
What secondary current was measured at the tips, on a stack like mine, and with what schedule?
What is the shunt made of, how many laminations, and what is the inspection interval?

On the plate	What it really describes	What it leaves out
160 kVA	The transformer at its own terminals	Arms, shunt, holder, electrode, workpiece
50% duty cycle	A thermal limit at a stated ambient	Your ambient, your part mix
Max secondary current	A short-circuit figure	The current under a real stack
Throat depth	Mechanical reach	That reach costs current as well as money
Electrode force	Clamping capability	Whether the frame deflects at that force

A supplier who can answer the fourth question has measured a machine. A supplier who answers only the first has read a plate.

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

What The KVA Number On The Nameplate Does Not Say