MFDC Dominates

Though thousands of single-phase RW machines will continue to have their place, over the past 25 years automotive manufacturers have led the way in converting to inverter-type medium-frequency direct-current (MFDC) machines, designed to be connected to all three legs of the incoming electrical service.

In some cases, especially when dealing with an expensive single-phase special 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.

This comparison of the different methods of controlling and regulating the energy shows how the more advanced methods, MFDC and TJSWC AC inverter, deliver more efficient energy by reducing the off time between welding pulses.

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 rectifiers 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 resistance-welding power supplies, reports that MFDC technology dominates the global market.

“Today, MFDC has replaced about 85 percent of AC transformers purchased for most general process use in resistance welding around the world,” says DeCorte. “As power constraints become more common, especially in heavily industrialized or fringe areas where the power grid is challenged, 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, MFDC power supplies have a distinct advantage over AC technology because the power supplies are dramatically smaller and lighter. This allows spot-welding guns to be designed with integral power supplies instead of remotely mounted transformers.

Previously, heavy AC transformers supplied welding current to robotic-welding guns with long and inefficient water-cooled cables. Another benefit of lighter MFDC spot-welding guns is that robots can now be smaller and faster.

Shown here, an example of typical part geometry and tooling for a part being welded using capacitor-discharge welding. Notice the ring projection.

Further, installation of MFDC welding machines enable a plant’s copper wiring and feed breakers to be smaller and less expensive. “An important savings with MFDC over AC is in the reduced cost of plant wiring,” says Mike Morrow, electrical engineer at T. J. Snow Co. “As an example, an AC RW machine requiring 400A primary service would only need 250A service, if converted to MFDC.

“The same heat energy would still be put into the weld.” adds Morrow. “However, the peak currents would be lower with MFDC, resulting in lower demand charges from the power company. In addition, MFDC presents a balanced load to the line, which greatly improves the machine’s power factor.”

Another benefit: MFDC technology, which typically operates at 1000 Hz, produces more controllable and often superior welding quality than 60 Hz technology. This allows some new MFDC controls to have feedback circuits that monitor and adjust each weld on the fly.

Jeff Morgan, a T.J. Snow welding engineer, adds that “because the output of the welding power supply is DC, inductive loss in the throat of an MFDC machine is virtually eliminated, thus requiring less energy. This is especially evident when welding ferrous materials with pedestal machines and with robotic guns with large throat areas.

“In theory, since the waveform seen at the weld is very much like a square wave,” he continues, “the energy and heat signature delivered to the part is virtually continuous during the weld, rather than cyclic, as seen with AC machines. This is particularly evident in projection welding, where we can produce excellent results using the equivalent of one cycle of AC weld time (16 msec.). In the past, this type of weld would have typically required six to seven cycles of AC output.”

Capacitor Discharge Coming on Strong

The waveform shows how two of the more efficient forms of energy delivery, medium-frequency DC and the AC inverter, compare with the conventional single-phase AC process.

Capacitor-discharge (CD) technology represents the ultimate greening of the resistance-welding process. The machines operate on stored energy and recharging the built-in capacitors can be slowed and low amperage demanded from the power lines.

The popularity of capacitor-discharge technology continues to rise, though the machines cost more than other types, and are physically larger than comparable AC and MFDC machines.

“Since CD is a stored-energy resistance-welding process, it uses the lowest of all instantaneous primary amperage draws, with the energy output not dependent on the stability of the primary voltage,” says Morgan. “In addition, a CD RW machine does not affect surrounding welders.”

If time permits, a large-output capacitor-discharge machine can be recharged slowly, with only 60A primary service. However, since more welds/min. are often needed to reach desired production rates, recent advances in electronics allow faster recharging of CD resistance welders.

Although CD machines operating at higher speeds require a larger primary service, the savings in the cost of primary wiring and power supplied to the machine can be greater than a comparable MFDC unit.

The technology is especially suitable for projection welding fasteners (nuts and studs) to high-strength steel. Moreover, since secondary welding currents as high as one million amps have been obtained, other ideal welding applications for CD technology include products with large ring projections, such as threaded water-heater connections and automatic-transmission clutch baskets, where CD technology has replaced laser welding in some cases.

Other advantages: Due to the short duration of capacitor-discharge welding current flow, the heat-effected zone is small and the part can be handled immediately after completing the weld; and the short weld time increases the life of copper electrodes. MF

 

See also: T. J. Snow Company

Technologies: Welding and Joining

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