ROOTING OUT RESISTANCE: ELIMINATING THE HIDDEN VARIABLE

Resistance is frequently characterized by random and intermittent welding issues that are difficult to repeat. But even though resistance-related problems can have many symptoms, their root cause is always the same – not enough current is flowing in the electrical circuit. Here are some tips on how to pinpoint and root out this frustrating problem.

Regardless of your experience level, diagnosing and troubleshooting welding problems caused by resistance can be difficult and frustrating. Too often, problems caused by excessive resistance in the welding circuit are misdiagnosed as problems with the welding power source or the wire feeder.

Bryan Erpelding of Erp’s Mobile Maintenance LLC (Glendale, AZ), a welding repair technician and facilities maintenance expert, says that resistance is often the last factor welders usually consider when diagnosing their welding symptoms. “There are so many variables that go into the welding operation, and yet most people immediately look to their power source when something goes wrong,” says Erpelding. “In fact, the machines are almost always doing what they should and it’s something outside of the machine, such as a poor work lead or dirty work piece, that is causing the problem. Sometimes the machine just isn’t wired to the primary power correctly.”

Resistance-related problems can have many symptoms, but their root cause is always the same – not enough current is flowing in the electrical circuit. Frequently, resistance is characterized by random and intermittent welding issues that are difficult to repeat. One minute your welds are smooth and flawless, and the next minute they’re lumpy and porous and you can barely sustain the arc. Fortunately, excessive electrical resistance is fairly easy to diagnose and troubleshoot if the welder is aware of its common symptoms.

RESISTANCE BASICS
To understand how and why excessive resistance can lead to welding problems, it first helps to understand what it is and the role it plays in the welding circuit.

Resistance, in the context of the welding circuit, is in opposition to the flow of electrical current in a conductor. This opposition to current flow changes electrical energy into heat energy and is measured in ohms (Ohm’s Law: V=IR [Voltage (V) = Amperage (I) X Resistance (R)]. “In every welding circuit there is voltage and amperage, but there is also resistance, and many people don’t realize that resistance is not a constant,” explains Erpelding. “Resistance can change from one minute to the next, critically impacting your weld quality and consistency.”

Eliminating excessive resistance is particularly important for applications that use advanced welding technologies that constantly change the arc conditions. These advanced MIG processes rely on accurate information from the arc in order to adjust the welding current to the optimum levels. They also adjust the welding current levels from high to low values within a fraction of a second. If there is resistance in the work clamp or work lead, the correct current levels cannot be reached as precisely and as fast as they need to and weld quality may suffer.

Poor arc starts, increased weld porosity and inconsistent weld quality are common symptoms associated with excessive resistance in the welding circuit. These problems can also have other sources, but checking the common signs of resistance – unusually hot cables, loose couplings and poor work clamp connections – are easy to conduct and should be one of the first steps in the troubleshooting process.

Another factor that plays into the impact that resistance has on the welding circuit is heat. Heat is both a cause and a result of resistance, creating a cycle whereby resistance causes heat in the circuit and that heat causes even more resistance. Leaving a resistance problem unresolved can exponentially hasten the failure of the welding components!

“Many welders will respond to reduced welding output by simply increasing their current and voltage at the power source,” says Erpelding. “Doing this might work temporarily, but it ends up exponentially increasing the problem until a total failure occurs. It also makes your machine work harder than it needs to, causing other problems like using more power than you need to, or a failure in the machine.”

Troubleshooting these problems requires attention to many different possible sources of resistance in the circuit, including the work lead and work clamp, as well as the contact tip, liner, gas tube, connectors and lugs.

TROUBLESHOOTING: WORK CLAMP
Whether it’s insufficient spring tension, using the wrong materials or a dirty work surface, the work clamp is one of the most common sources of resistance in the welding circuit. “On one of the calls I went on, a guy actually took the time to make a steel work clamp with a spring rather than just purchasing one,” recalls Erpelding. “Steel is not a very good conductor, and resistance built up from the current trying to flow from the copper cable. His weld quality was very poor, and he didn’t understand why.”

“Work clamps made of aluminum, copper or bronze,” notes Erpelding, “are superior due to their higher conductivity. Some work clamps have rounded ends on both jaws to accommodate different material sizes.” Not having at least one flat end to create good clamp-material contact minimizes the clamp ends’ surface area and thereby affects conductivity. Excessive weld spatter on the clamp, or paint or mill scale on the work piece also will interfere with good contact with the work clamp and will increase resistance.