Fig. 3 — Large panel pushed all the way into the throat of the welding machine. line voltage. Therefore, lower voltage going into the welding machine will result in proportionally lower secondary voltage and lower weld current. This leaves two choices. First, you can adjust your welding program settings for each shift. That is not a great fix but can help. The second is to use welding controls that have automatic voltage compensation systems. These systems, called AVC or LVC, will electronically increase or decrease the voltage on the secondary of the welding transformer to produce a reasonably constant voltage for each of your heat settings. All quality modern resistance welding controls should have this feature available for your use. They should be able to hold the RMS (effective) voltage going into the welding transformer to no worse than ± 1% variation with a line voltage variation of ± 10%. On a 460-V line, welding machine output voltage should be within this 1% window for line voltages as low as 414 V and as high as 506 V. 2. Metal into welding machine throat. As you push the metal panel back onto the welding machine, the steel increases the impedance (AC resistance) of the machine secondary. This means that for the same voltage at the machine’s secondary, a lower current is produced. This lower current reduces the weld strength considerably. I worked on a welding machine that had a 40-in. throat. The parts being produced were, at times, pushed pretty much the full depth of the arms (Fig. 3) and at other times only 4 in. into the area between the arms (Fig. 4). With no change to the welding program, the current produced at when the part was pulled almost out of the throat was 12,640 A. When the part was pushed fully into the throat, the amperage dropped to 10,690 A. The welds produced at these two locations were very different. One solution is to rotate the part so that it does not go as deep into the throat. This means handling large heavy sheets and requires a really good support system and a strong operator. This also reduces production output by adding a lot of wasted material handling. The best solution is to use a weld- 24 WELDING JOURNAL / JULY 2016 Fig. 4 — Large panel pulled all the way out of the throat of the welding machine. For info, go to aws.org/adindex
Welding Journal | July 2016
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