Converting a six phase rectifier to three phase rectifier

Mabey, I am not sure it would actually work out to be a problem, just something to check.

Sounds like a good plan to me, I trust you'll let us know how it goes.

Alas, I know next to nothing about plasma cutting. Never even tried one. They weren't all that common when I was welding a lot in the early 70's. I think it is a constant current process with the arc confined to the torch and not transferred to the workpiece as in plasma welding, but that is just a foggy memory of something I may have read 20 years ago. Perhaps you can pick a particular torch and find out what it needs.

Reply to
Glen Walpert
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OK, I will keep this in mind. My phase converter is 17.5 HP idler capacity. When I loaded it with full 200 amps of welder output (shorted leads), the third leg sagged to about 190 volts.

I already started, the first thing that I am working on is mounting the SCR modules.

I see. Well... If I connect my secondaries with each pair in series, wye, it should give me about 208 volts AC to play with. I think that it would result in about 300 volts OCV and 200 volts welding power.

But first things first, I want to make it work as a welder first to get the proof of concept.

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Reply to
Ignoramus22091

Don't claim to know much about plasma cutters, but my one time neighbors unit seemed to require an inert gas supply like GMAW and GTAW welders.

Reply to
joseph2k

Thanks... I know nothing about plasma cutters... Actually right now my both kids are asleep and this evening i will try to get this to work in the most basic configuration..

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Reply to
Ignoramus12938

Glen, I put together the most basic configuration, and voila, the controller did produce the voltage that I expected. No current tests yet, but that the voltage was correct, makes me think that everything is alright. Now that I know that it is working, I will actually wire it up properly, that will take quite a while to get all the copper wiring, contactors etc, right.

I am now going to ask about making a plasma power supply.

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Reply to
Ignoramus19383

Sounds good so far. Do you have a MIG torch for testing under load yet?

An alternitive to transformer reconfiguration with a bunch of large contactors would be a patch panel with the transformer windings and load connections brought out to studs on an insulating panel, with copper bussbar links and a few jumper cables able to switch to any of the 5 configurations (6-phase, and 3-phase wye and delta both with series or parallel winding connections). Compare the time difference in assembly with the likely time spent changing configurations, and of course parts on hand ...

When connecting transformer windings in parallel it is always a good idea to connect one end first, power up and check the voltages between the unconnected ends, which will not only let you know if you have a winding backwards without letting any smoke out, it will show any winding imbalance with the correct connection, which should be very small. Do the same thing with the delta connection, first connecting the 3 windings in a U configuration and checking voltages between the open ends before making the final delta connection. If there is a significant voltage present (more than a few percent of phase voltage) then check for circulating current at no-load with an amprobe or shunt when you make the connection. Since it is not an always-on power dist transformer you could live with some circulating current; eg 10% of rated current circulating still leaves 90% available for welding and my hunch is it will not be that high, what there is will tend to equalize phase voltages, and it is just part of the cost of using unbalanced power from your rotary converter.

(BTW I think it is considered poor netiquette to stick a persons name in a message subject line, even though it is done all the time. Glen or GW would be better than the full name in caps if you want my attention, but I tend to keep an eye out for your project so you could put that in the body and I would still probably notice.)

Reply to
Glen Walpert

A little update, I took out the old rectifier from my Cyber Tig, which freed up a lot of space, for a nice installation of isolated case dual SCRs.

I will hopefully install the heatsink and SCRs in a couple of days. It will need some brackets to fit into the space available.

No, all I have is TIG equipment...

Well, I already bought five 50a contactors for $9.99 each. I drew some schematics, they should be enough to switch between parallel delta, vs. series wye (for plasma).

I am going to start with wiring for just welding (parallel delta), and make the welder part work fully. I want to move in smaller steps.

After that, I will add contactors to switch between parallel delta, vs. series wye.

Great point, I will do just that.

This is a very nice idea, I was quite afraid of this delta connection, but this is a safe way to go.

Yes. Also, I want to finally make the transformer switch on when I need to actually weld. It should not be on all the time, like it used to be, it is a waste of electrons.

Sorry about that Glen, I changed the Subject of this post.

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Reply to
Ignoramus5749

So I assume you are scanning ebay for a deal?

50 A is in the ballpark but seems a bit light for your transformer; assuming a nominal rating of 200 A, 100 A per 3-phase section, 100 / sqrt(3) = 58 A per winding. Probably close enough.

Your original rectifier did not make very effective use of the transformer secondary windings, as each winding was used in one direction only. With your full wave bridge each winding is used in both directions, reducing copper losses by 1/sqrt(2) IIRC, so you could use about 82 A per phase from one of the two windings for 82 * sqrt(3) = 140 A RMS total welding current. (If you did that for both secondary windings at the same time you would probably overload the transformer primary, which was designed for the half wave rectifier configuration). So you could do reasonable load testing with your existing single winding configuration. Also you get the same VA with either delta or wye, so the delta config of one winding would be good for 140 * sqrt(3) = 245 A. With both windings paralleled the delta connection will provide 200 * sqrt(3) = 346 A, limited by the transformer priimary.

Thanks!

Reply to
Glen Walpert

Yes. I would like, I think, to buy some old industrial wirefeed unit.

Yes.

First, the contactors will not open or close under load.

Second, I believe that for most instances, none of the contactors would carry full "two winding" load, they would carry one winding load.

For the high voltage wye configuration (plasma), they would carry full load in the wye point, but that would be much less than welding current.

(Glen, also note that this Cyber TIG seems to be the same model as the

300A 60% duty cycle unit, but with a controller only capable of 200A, with the benefit of 100% duty cycle rating).

This is great. I will wire it up so that all wiring and semiconductors would be capable of the ultimate 300A rating. If it works in 200A configuration (using a 100 mV, 200A current shunt) I may switch to a

300 A shunt.

I am working right now on mounting a forced air cooled heatsink (with a nice heatsink fan) in my welder. This heatsink will carry SCRs. I hope to finish this today and install the heatsink. It will be mounted on the main fan frame assembly. I had to make some mounting brackets.

The heatsink will have its own fan, and will be mounted next to the big 0.11 HP fan also. The reason why the heatsink will have its own fan is that the fins would be perpendicular to the flow of air inside the welder, there is no other way to mount it. It's a nice heatsink with big fins glued in and a shroud around the fan to direct ait flow.

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Reply to
Ignoramus32263

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