For resistance spot welding, resistance projection welding, and resistance seam welding, the RMS current needed to weld a specific material of a specific thickness will be similar for MFDC as for AC. If all things are considered equal, where the only variable is the weld transformer type, the same KVA MFDC or AC transformer is needed for resistance welding the same job.
It can be expected, though, that an MFDC transformer will have a lower life expectancy than the same KVA AC transformer. So, it is advisable to select a larger KVA MFDC transformer than needed, because operating it below the KVA rating will allow the diodes in the transformer to last longer before they fail. The AC transformer has no diodes and can take a lot more abuse without failing.
Whichever transformer type is selected, it should be sized to meet the electrical requirement of the biggest weld job that will be produced on the machine.
When resistance seam welding materials such as Inconel, current impulses with well-defined cool times between each impulse result in more robust welding performance. This is achieved with an AC welding transformer because the current quickly decays to zero after each impulse. MFDC impairs the ability to make well-defined cool times between each impulse. This is because it takes more time for the current to decay to zero after each impulse. The result is less robust welding performance. This also causes the seam wheels with the DC transformer to degrade faster and run hotter than when making the same welds with an AC transformer.
The Peltier Effect, resulting from welding with a DC current, causes the wheel connected to the positive polarity lug of the transformer to run hotter than the wheel connected to the negative polarity lug of the transformer. This shifts the position of the nugget formed between the workpieces being welded. The positive polarity wheel degrades faster than the negative polarity wheel, and accelerated pickup of material alloying with the positive polarity electrode surface occurs. These phenomena do not occur with AC current.
The DC welding current also magnetizes the part and the machine. Magnetic filings attract to the magnetized machine, which over time, work their way into the machine moving linkages. This degrades the smoothness of the ram and its motion dynamics, resulting in poorer machine mechanical repeatability and more frequent maintenance. AC welding current naturally degausses the part and machine, which keeps the machine from turning into a magnet that attracts filings.
AC welding current is also more effective than DC welding current at burning through surface contaminants such as oil and oxides.
WeldComputer® inverter systems can drive an MFDC transformer with greater accuracy than every other inverter system we have tested. However, using WeldComputer® proprietary wave synthesis technology, more adjustments per millisecond and better granularity of control is achieved driving an AC transformer than is theoretically possible driving an MFDC transformer.