Arc Applications, Inc.

In The Weld Lab: Variable Polarity GTAW, Tungstens, and Waveforms

VP Tungsten Waveform Collage

Variable Polarity Experiments

Every once in a while we need to strip everything away from a welding process until we’re left with the bare essentials. We did this recently with a variable polarity GTAW (VP-GTAW) power supply to investigate its output characteristics, and along the way we made some other interesting observations that we’re sharing here. To do this, we recruited the services of our handy water cooled copper block, AAI fabricated current sensing coil, and data acquisition system.

With the system configured, we ran a series of tests with various current settings. We varied electrode negative (EP) and electrode positive (EP) current, wave balance, frequency, and pulse settings. What we observed was rather interesting.

In the waveform in the upper right, notice the intermittent amplitude of the negative cycle of the waveform. This represents the EP cycle of the VP arc and a saturation of the current going to the tungsten. This was only observed with lanthanated tungsten, and during operation presented a very unstable arc. Pure, thoriated and ceriated tungsten did not behave this way.

In the waveform in the lower right, notice the step in the middle of the waveform on the positive side. This is the change from the background pulse to the primary pulse. This indicates that the pulsing of this machine operates entirely independent of the VP portion. While not particularly significant from a practical standpoint, it’s interesting from an academic standpoint.

In the large picture in the upper left of the image, notice the area directly under the tungsten. For those who have welded with AC before, you’ll recognize this as the area cleaned by the EP cycle of the arc during welding. Waveform balance is said to change the size of the cleaning zone, but testing is more fun than taking someone’s word for it. At a wave balance of 75%, the arc cleaned the copper block all the way out to the black soot ring. At a wave balance of 90%, the arc cleaned the copper directly under the tungsten as indicated by the distinct gold ring concentric with the black ring.

These tests allowed us to quickly examine the power supply’s current characteristics, and along the way, observe some of the behaviors that are fundamental to VP welding.
Until next time.
-Mike, in the lab.