RESISTANCE: THE UNKNOWN WELDING VARIABLE

A welding arc is part of an electrical circuit, and every electrical circuit has at least three components: electromotive force (voltage), current (amperage) and resistance (ohms). As welders, we?re often concerned with voltage and amperage, but we rarely think about resistance. That can be a mistake, since introducing unintended resistance into the welding circuit can lead to equipment failure, poor weld quality, porosity, the inability to weld to specification and the frustration of trying to diagnose an intermittent problem.

To keep it simple, let?s look at a garden hose. We can look at voltage as the water pressure, amperage as the gallons per minute of water flowing past the outlet and resistance as the kink in the hose restricting how much water flows out of the hose. That kink increases the pressure before the kink and lowers it after the kink.

SOURCES OF RESISTANCE

Anything that impedes the flow of current increases resistance and adds a voltage drop to the circuit. If resistance is added to the circuit, the output at the arc can be significantly lower than what is shown on the front panel of the welding machine, which reads output at the terminals. To get the arc to ?feel? right, the operator must turn up the ?juice? to overpower the resistance in the circuit.

The conditions of your cable, work clamp, and connections contribute significantly to resistance in the welding circuit. How much they affect your welding arc depends on the quality of the connection, cable condition, tightness of the connection, and the list goes on. Simply stated, check all of the cable conditions everywhere along the weld circuit from the lugs on the front panel to the contact tip on the MIG gun.

Check the strength of the spring on the work clamp. If this spring is weakened (which will happen over time), it will not clamp the work piece tight enough, causing excessive resistance in the weld circuit. The tighter the grip, the less the resistance.

Additionally, a loose contact tip in your MIG gun or a loose lug on your welder will restrict how much current is transferred to your welding wire and cause problems in your arc.

Cable size plays an important part in resistance, too. Consider the mobile welder, who may have 150 ft of welding cable on the work clamp end and another 150 ft on the electrode end. That is a total of 300 ft of cable in the weld circuit. On the job, welding at 120 amps 150 ft away from the machine, everything reaches, the cables are extended, and everything works fine. On the next job, where the repair is only 20 ft away, he uses the same machine setting and same electrode, but only uncoils 20 ft of cable. This time the arc magically runs ?colder? and the operator needs to turn up the machine. What happened?

That extra cable length needs to go somewhere. If you coil it, you will be introducing inductance to the circuit. Inductance opposes change in the flow of current and is similar to resistance.

Similarly, as the diameter of a wire decreases, its resistance increases. Use too small of a cable and the extra resistance will take up some of the voltage and turn the current into heat. If the resistance were too high, the cable would quickly heat up and burn through. Don?t try to save money by buying cable that is too small.

Even if the correct size cable is purchased, however, its resistance can change over time. If you drive a forklift over your cable and break some of the copper strands, you?ve essentially decreased the size of your cable, added resistance to the circuit and need to increase voltage to compensate. Additionally, the heat caused by a break in a MIG gun from bending the gun cable back and forth repeatedly can burn through the gas hose of a MIG gun and allow the shielding gas to escape, which will lead to porosity in the weld.

While introducing unintended resistance into the welding circuit can lead to some difficult-to-diagnose problems, preventative maintenance can eliminate them before they show up in your weld beads. Inspect your cables regularly, especially at the connectors, and repair or replace as necessary. Keep cables where they can?t be run over by forklifts or repeatedly stepped on and follow the manufacturer?s recommendations for cable size.

Finally, remember that the work piece is also part of the circuit, so make sure the work clamp has a solid electrical connection with the work and that it is as close to the weld as possible.

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Nick Peterson is a welding engineer for Miller Electric Mfg. Co., 1635 West Spencer Street, P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, www.millerwelds.com.