After watching his video 4 more times, I think you are correct. It does appear that he re-routed the feed motor wires to the secondary of the second transformer.
Since he apparently spliced into the trigger relay on the motherboard and connected that lead to one of the primary leads on the second transformer, it looks like he spliced into the the blue fan wire (its blue in my HF) on hot side of the switch and connected that to the other lead on the primary side of the second transformer. So assuming the trigger relay is the AC neutral and the fan wire splice is the AC hot side, then that all makes sense now.
Have I got it right?
I believe there is a newer version and an older version of the welder in the different videos. I never changed anything to do with the small transformer. I only added the rectifier, the choke, and the resisters to the transformer outputs to change them from ac to dc.
I would be tempted to put the motor on a separate circuit using the second transformer. The trick would be finding a way to tap the trigger relay input signal with a second wire to another relay that controls the new circuit. It might be to tricky to do and it might have been his original intent.
Brian,great job on the welder will do mine also now, by the way the bridge rectifier (MDS150A 3-Phase Diode Bridge Rectifier 150A Amp 1600V) is in fact a full wave rectifier.To get half wave rectification you use only one diode from ac, this is not the case here,where two diodes are used for each phase.
Harbor Freight seems a good brand for welding machines.
At risk of starting an argument, You can do better. One of the good things about Harbor freight, they are one of the OK Cheep brands. You might be able to get some parts for some models…
Check into used Lincoln, Miller, Hobart, ESAB machines. Check less expensive but well supported machine brands like Lotos, Eastwood, Everlast, USAWeld (HTP) and some others turning up the heat on the main-stream brands. Always check the internet for reviews before buying, wisdom comes with many opinions averaged out!
With the mods, you are essentially upgrading it. I got an old well-used mig century model, for what I could have gotten a HF model for, but I looked for about a year for it and it leaks some gas, but I don’t use it often enough to justify buying a new gun/hose kit for it.