Page 40 - MetalForming May 2019
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  Fabrication: Welding Well
 Shown here, an example of typical part geometry and tooling for a part being welded using capacitor-discharge welding. Notice the ring projection.
motive manufacturers have led the way in converting to inverter-type medi- um-frequency direct-current (MFDC) machines, designed to be connected to all three legs of the incoming elec- trical service.
In some cases, especially when deal- ing with an expensive single-phase spe- cial design AC production machine welding a specific part, it may make sense to convert the machine to MFDC by retrofitting new components in place of the original AC transformer and control.
However, there are trade-offs because MFDC power supplies don’t withstand abuse as well as single- phase resistance-welding transformers, and the water-cooled secondary rec- tifiers require about three times more water cooling (heat removal) than a comparable single-phase machine.
MFDC machines also have other water-cooled components and to remain green, a water-cooled MFDC machine (and most other RW machines) should be connected to a refrigeration- type water recirculator/chiller with adequate flow.
Although MFDC transformers and controls are more expensive than AC, Don DeCorte, vice president, RoMan Manufacturing, a Wyoming, MI-based manufacturer of AC and MFDC resist- ance-welding power supplies, reports that MFDC technology dominates the global market.
“Today, MFDC has replaced about 85 percent of AC transformers pur- chased for most general process use in resistance welding around the world,” says DeCorte. “As power con- straints become more common, espe- cially in heavily industrialized or fringe areas where the power grid is chal- lenged, MFDC technology helps plants expand and add additional machines without severely overloading plant or local power grids.
MFDC Benefits
The benefits of MFDC are more than a reduced primary amperage draw and a balanced three-phase load.
When used with spot-welding robots,
AC Inverter
In single-phase RW applications, where primary current requirements present an excessive load on incoming power lines, AC inverter wave-synthesis technology offers a practical way to retrofit an existing machine while still retaining its robust original single- phase transformer. AC inverter technol- ogy reduces primary-current demand by distributing the load over all three phases.
In addition, the three-phase AC inverter control system decreases the weld current and time required to pro- duce the same weld strength as the original machine operating on single- phase AC.
Two benefits of converting to AC inverter technology come from Bob Cohen, CEO of WeldComputer, a Troy, NY-based manufacturer of RW controls. He reports that a newly installed resist- ance flash/butt welding machine in Southern California was putting 1500A spikes on the power grid when it fired. Since the local power company report-
ed that the machine produced voltage flicker experienced by customers con- nected to the 12kV power system, it demanded that the manufacturer stop operating the machine.
The power company suggested that the manufacturer install a new power feed at a cost of $250,000. Instead, Cohen received permission to install an AC inverter control on the existing machine. With no other changes to the system, the current draw on the power line dropped to 107A and the power grid flicker fell to within allow- able limits.
Similarly, a manufacturer of wire- mesh welding machines in South Africa reports that installing an AC inverter control on an existing multigun RW machine resulted in a 45-percent improvement in electrical efficiency, while still producing quality welds.
MFDC Dominates
Though thousands of single-phase RW machines will continue to have their place, over the past 25 years auto-
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