In GTAW AC and Aluminum, Increased frequency (up to 220 Hz.) has its greatest effect in directional control of the arc in fillet welds on thinner material <less than 0.090>. Greater control allows pinpoint arc placement at the root of the fillet and greater travel speeds which together can produce smaller fillet profiles.

Penetration is controlled for the most part with AC Balence (max clean/Max pen) The greater dwell time on the Straight polarity (DCEN) the greater the penetration.

Travel speed (especially on groove welds) has the next greatest effect on penetration.... the longer you stay in one place the deeper the melt and the more filler required.

Third is electrode preperation. A pointed electrode will produce a more directional arc. A hemispherical tip prep is slightly less controlable and a ball that exceeds the diameter of the tungsten is least easy to control and thus produces a more erratic arc and wider bead profile. If you have a large balled electrode or you have dipped it just once and don't stop to clean it up completely, forget about the benefits of that high tech powersource.

Now on heavy sections <greater than 200 amps> a reduction of frequency, lets say 20-40 Hz. will provide greater heat input and will sometimes allow you to weld a massive part that otherwise would not be weldable or require a helium mix. It can also allow you to get in one more pass when your preheat is beginning to fade.