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Resistance-Welding Machines Are Thirsty...And What to Do About It

By: Tom Snow

Wednesday, August 1, 2018
 


These photos show an overall view and a closeup of water-cooling circuits on a standard press-type RW machine, including water-in and water-out manifolds, plus visual-flow indicators and a water-flow switch.
Long known as water hogs, resistance-welding (RW) machines, including spot, projection, seam, butt and flash welders, generate heat due to the high secondary-welding amperages used.

Therefore, with an RW machine running at high speed, an adequate flow of cooling water is one of the most important variables of the resistance-welding process. The typical machine requires 1 to 3 gal./min. per cooling circuit, and sometimes more.

The standard AC resistance welders used for decades by metalformers can tolerate some overheating, but the newer medium-frequency direct-current (MFDC) inverter-power supplies, more commonly used today, are prone to failure due to inadequate water flow.

Often, the required total water flow required is substantial due to several parallel water circuits per machine. For example, the water-cooling needs of specially designed automated multi-gun resistance welders with multiple transformers and tips easily can total water flows of 10 to 20 gal./min.

In the past, manufacturers often connected RW machines to their incoming municipal water supply without giving it another thought. However, the days of cheap and plentiful city water are long gone, and additional sewer charges can be substantial.

Starting with the large welding transformer typically located inside the machine frame, the RW process employs numerous heat generators. As the transformer converts incoming line voltage and current into the high secondary amperage necessary for resistance welding, heat generates internally.

When projection welding large fasteners or spot welding aluminum, some RW applications require 50,000 to 100,000 secondary A to generate enough localized heat.

Additional heat, produced in the large copper conductors, comprises the machine’s external secondary loop and final heat source in the RW process. This can be the hardest to cool with heat coming from the electrodes, which make contact and conduct the welding current.

Small, easily clogged water-cooling copper tubes built into the welder’s internal secondary circuit cool most resistance-welding transformers. Other water circuits cool the high-voltage contactor section of the control.


This illustration shows proper use of a water-cooling tube inside of a spot-welding electrode holder and tip. With one end cut on a 45-deg. angle, the tube forces water flow as close as possible to the end of the internal water-cooling cavity in a typical spot-welding tip.
Importantly, water-cooling tubes supplied with electrode holders are there for a reason and should remain in place. With one end cut on a 45-deg. angle, the tubes force water flow as close as possible to the end of the internal water-cooling cavity in a typical spot-welding tip.

Common Water-Cooling Problems

Large roof-mounted water towers long represented the most common method to supply cooling water to spot welders, but they present a unique set of problems, including inconsistent process water temperatures due to varying ambient air temperatures throughout the year.

In addition, dirt and other contaminates in the tower can cause problems. I remember visiting a Japanese-owned automotive-parts manufacturer with cherry trees planted around its building. Although beautiful, their blossoms clogged the system each spring.

Additional problems resulted from this plant’s use of black-iron water lines throughout the facility, which corroded internally after a couple of years and introduced flakes of rust into the system, clogging the small-diameter internal water-cooling circuits in their transgun-type portable spot welders.

Another reason for not recommending a cooling tower for use with spot welders: A plant’s water-flow capacity often doesn’t increase upon installation of additional water-cooled machinery.

Sometimes, plants cool RW machines with well water. Often viewed as a free cooling method, the minerals present eventually clog a machine’s water-cooling circuits. In addition, well water may be too cold and cause condensation.

Some plants use large recirculating reservoir tanks cooled by truck radiators. I’ve also seen welders cooled by water pumped from 55-gal. drums into which bags of ice are poured. Due to inconsistent water temperatures, neither option is recommended.

The Big Chill

Finally, small radiator-type water recirculators for use with arc-welding machines are usually inadequate for cooling a production RW machine.

Steve Derrick, manufacturing engineering manager at Innovative Hearth Products in Russellville, AL, recalls inheriting a group of 30 RW machines cooled by those little recirculators. Dramatic improvements resulted from replacing those units with self-contained refrigeration-type water chillers properly sized for his welders.

“It was obvious that the little radiator-type recirculators were not doing the job because our spot-welding tips were too hot to handle,” says Derrick. “Sometimes we had to stop production to let the welders cool down. In addition, our weld quality varied throughout the day.

“A water chiller solved that problem, and I will be buying more in the future,” he adds. “And we order our chillers with optional castors so that we can move them easily from welder to welder.”

Water, Water, Everywhere

Although a recirculating chiller can be filled with city water, it’s better to use a mixture of distilled water and glycol, especially for outdoor chillers. Never acceptable: the use of automotive antifreeze in a chiller.

In addition, many of our customers assume that using deionized water is fine, but doing so will quickly destroy a chiller due to leaching of minerals from the chiller’s internal components.

Condensation Kills Resistance Welders

Although common sense would indicate that water to cool RW machines should be as cold as possible, condensation quickly becomes a problem during the summer, when the prevailing dew point (the temperature at which water vapor condenses), surpasses the process-water temperature.

Since dew-point temperatures greater than 70 F are common during the muggy summer months, it’s best to set chillers at temperatures exceeding the dew point to avoid internal sweating of the machine’s transformer and silicon-controlled rectifier (SCR), which could cause an electrical short.

Also, avoid puddles of water inside of the control cabinet and condensation dripping off of the electrodes and holders. A water-saver relay and solenoid valve used to turn off water to the welding transformer about 1 min. after the last welding cycle will prevent condensation damage to the welding transformer from condensation.

In addition, some chillers now include a highly recommended feature, automatic dew-point compensation, which removes the need to adjust manually the chiller’s water-temperature setpoint during humid weather.

Caution: If your RW machine has a direct water-cooled SCR assembly, never leave the machine sitting idle with the power on and the water turned off. Doing so can cause the stagnant water in the hose to generate heat and steam, causing the hose to burst and creating the potential for a catastrophic failure the next time the water is turned on.

Go With the Flow

Research has proven that water chilled to 55 to 60 F and circulated through the spot-welding tips will improve electrode life. Therefore, in the ideal world, and if one doesn’t mind the potential for condensation dripping off of electrodes, two chillers per machine would be nice, with one set to a low temperature and connected to the machine’s secondary circuit and a second chiller set to a temperature exceeding the dew point, cooling the transformer and control.

However, if using just one chiller to remove heat generated by the machine, compromises are necessary. That’s when water flow should be as high as possible. For this reason, select a chiller with a properly sized pump, and plumb the system to minimize head pressure caused by running overhead water lines.

Installing petcocks for each circuit, along with inline water flow meters or visual indicators, makes it easier to balance the water flow.

Chiller Sizing

The ideal setup: Connect to just one new machine, or a group of similar machines located close together, and run water lines on the floor.

Selecting the proper refrigeration capacity of a resistance-welder chiller is not always an exact science, since the welder or group of welders to be cooled may not run continuously. However, a good rule of thumb for standard single-phase AC resistance welders is to specify 1 ton of refrigeration for each 100 KVA of welding capacity.

MFDC (inverter type) resistance welders require additional heat-removal ability to cool the secondary diodes. A good rule of thumb here: 3 tons of refrigeration for every 150 KVA of welding capacity.

Reservoir tanks typically built into a chiller serve as a thermal flywheel, helping to prevent short cycling of the refrigeration compressor.

When choosing a large chiller to serve several welders or even an entire plant, consider a multicompressor system. This allows the system to ramp up and down automatically to meet the required cooling capacity.

With continuing droughts worldwide and the possibility of future wars over water rather than oil, buying self-contained chillers for use with your resistance welders could be one small way to help save the world. At the very least, it can improve your resistance-welding process and save money. MF

Snow acknowledges and appreciates input for this article from Raschell Hickmott of Dimplex Thermal Systems, Roger Hirsch of Unitrol Electronics and Don DeCorte of RoMan Manufacturing.

 

See also: T. J. Snow Company

Related Enterprise Zones: Welding


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