Inverter Controls and Seam Welding

by Robert K. Cohen

adaptive control used on heat exchanger
To produce a gas-tight seam weld, the WeldComputer® adaptive control
is used on a Tranter, Inc., heat exchanger. (Photo courtesy of T. J. Snow.)

To overcome limitations imposed by SCR control technology, manufacturers that perform high-speed seam welding are switching to inverter technology. The expectation is that the newer inverter control technology will deliver superior weld current regulation, improve weld quality, and increase production throughput.

Manufacturers seeking expert advice are often informed to take advantage of the newer inverter technology, it will be necessary to throw away the existing AC welding transformer and replace it with a newer technology MFDC welding transformer.

In case studies of seam welding manufacturers that made the conversion from single-phase AC to MFDC, they reported that instead of increasing production throughput and improving weld quality, decreased production throughput, reduced weld quality, and increased maintenance occurred instead. These issues worsened when the manufacturers programed a shorter weld impulse time and shorter cool time between each impulse in an attempt to try meeting or exceeding the 120 weld per second impulse rate realized with the older technology control.

Instrumentation of these welding operations reveals two causes, listed below, for the degraded welding performance.

The inverter control selected, when programmed to produce short duration impulses, delivers inaccurate and/or unstable current regulation that results in greater weld impulse current variability than what was previously achieved with the older SCR- based control.

During the programmed cool time between each impulse, the current decays slowly, and often doesn’t decay to zero before the next welding impulse begins. This high residual current during each cool interval, which is caused by the introduction of the MFDC transformer, degrades the effectiveness of the cool time function — Fig. 1.

Current trace of a MFDC control
Fig. 1 – The current trace of a MFDC control documents that current has not stabilized at the programmed value prior to completion of an 8-ms duration weld, has big current fluctuations occurring twice per ms, and excessive current decay time.

This causes the seam wheels to operate at a higher temperature to make the same size welds than what previously occurred when the current was able to be brought to zero during the majority of the programmed cool interval. The elevated wheel temperature caused from switching to a MFDC transformer creates secondary issues, including faster material pickup on the wheel surfaces.

WeldComputer has solutions to address these issues and deliver the increased production throughput and improved weld quality originally sought by these manufacturers.

Learn more about WeldComputer and our solutions for resistance welding applications.

Rita Cox