[oil pan 4]

To adress "power factor is always a moving target in a welding machine", umm its not.
Basically the excessive reactive power is a fixed amount. Then when you strike an arc true power and apperant power increase in unison, raising the power factor from say 0.22 or 0.33 to 0.6 or 0.7 at full load while welding depending on the machine. There is still roughly 400 more volt amps than watts, at no load versus full load but the ratio of watts versus volt amps is a lot better when under load.
The machines draws the same power factor reguardless of the output amps setting at no load.

I power factor corrected my mig welding machine with great results. I figured I needed at least 30uf of capacitance to get my machine into greater than 0.9 power factor territory.
So in installed a terminal board and put 30uf of 200v metalized film capactors in parallel with the welding transformer. At no load it brings my mig welders power factor from 0.33 to 0.55, while welding it increased the power factor from 0.7 to 0.92. I have some 2.2uf 200v metalized film caps in the mail, I am going to fine tune the pf up to .96 at full power.
And best of all its putting out 10 to 15 more amps than ever before, its drawing more true power and apparent power compared to before with the same settings as before and putting out more welding amps. Thats so not not a problem for a little 110v mig, I just turn down power or move faster.