Converting Miller GoldStar 300SS to single phase???

I recently sold a Miller GoldStar 300SS welder. It is a very basic three phase stick welder, SCR controlled. Old, big, heavy, simple and
bulletproof.
I showed it to the buyer, we did some sample welds from my home three phase, and asked how is he going to use it in his garage.
He said something strange: that he has a friend who is a Union electrician and who said that he can modify it to run from single phase. The buyer said that he is "not an electrician" but he said that his friend sais something about adding a capacitor.
I can imagine, possibly, how he would use just one leg of the three phase transformer, use rectified single phase, and then adding capacitance would somewhat smooth the voltage. But would that not somehow degrade the "constant current" feature of the welder? Maybe producing giant sparks when starting an arc?
The manual of the welder, with schematic, is here:
http://millerwelds.com/om/o221aa_mil.pdf
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Ignoramus23772 wrote:

My guess is his fried would hook up a capacitor like a motor phace converter. This will work on the REALLY old, like WW-II vintage motor-generator welders. I doubt the transformer-rectifier welder will work well at all on single phase, capacitor or not. Maybe he didn't catch that his friend also mentioned "motor" in his conversion scheme. A capacitor plus a motor would be adding a traditional rotary phase converter, and would make sense. How well that would work depends on the motor and the welder, I guess. But, it worked at your place.
Jon
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Ignoramus23772 wrote:

To quote just about everyone in Starwars: "I've got a bad feeling about this."
technomaNge
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I feel the same. my feedback to him said "Be careful converting to single phase. Ask some forums first".
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I see it's got a 220V jumper setting so that looks good for trying to run it off a 220V single phase house circuit.
The basic power design looks like it wouldn't care too much whether it was run off of one phase or 3. At worse, the current wouldn't be as well rectified but it should work for welding (you can weld on AC with no rectification so welding doesn't need a smooth DC signal to work).
The part that looks questionable is the control board (which they don't include the schematic to). It receives three phase input and uses some type of circuit to regulate when the 6 SCRs are turned on based on the phase of the three inputs. Whether it can work when fed with a single phase is hard to guess without knowing more about how the control card works. It might. But he's not going to get the same power levels out of it - capacitor or no capacitor.
I don't get the point of adding a capacitor. If you add a capacitor to the other phase it will cause some phase shift but not enough to make it act as true 3 phase power.
I also see the fan is driven off one of the three phases. That better be the phase he wires up or else the fan either won't work, or not work well enough.

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I was thinking similar. The power input looks like it can be properly jumpered, the 3rd phase input to the circuit board looks problematic. If the machine is old and the circuit is unsophisticated, a zero voltage input to phase 3 may mean that it just doesn't fire the phase 3 SCR pair. If it is more exotic, it might (will) have some protection circuits to shut things down.
The SCR circuit looks quite similar to some early 80's DC motor controls that I have been playing with. Same problem there: hard to fake the feedback signals.
If it were my project, I'd take a quick look at what pins 19, 20, and 21 feed to on the circuit board. Faking out signal level voltages and currents is a LOT easier than main power.
Curt Welch wrote:

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I think that there are two possibilities:
1) The SCR driver board does not care about missing 3rd leg. In which case you may end up with a working, but slightly less good, welder that does not quite make good "DC", more like rectified single phase AC.
2) The SCR driver board would fault when it sees missing 3rd leg, and then, he's be looking at spending $500 for a new SCR driver board if he wants to continue.
Personally, I am happy with my phase converter. I am very glad that it is so big.
i

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I was thinking that they considered putting a cap on the output, to smooth out voltage.

yep, that is the easy part
i

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Some years ago, 4 or 5 IIRC, A major contributor to this lot and metalworking worked on a static design and had it working.
That design - IIRC - is in the http://www.metalworking.com/ pages.
The design is working in a number of shops. I went rotary or would have gone our friends(Mitch) way.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Ignoramus23772 wrote:

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