Converting a six phase rectifier to three phase rectifier

Thanks for yesterday' suggestions regarding 3 phase transformers.

I have a hobart cybertig welder. It has a "six phase rectifier" shown in this schematic:

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I received a three phase SCR control/firing system made by PCTI, that seems to be extremely easy to hook up.

By the way, PCTI loves children and is highly recommended, they are a wonderful company. The best of the best.

I want to convert this Cyber Tig to both TIG/Stick (CC) as well as MIG (CV) mode. This way I could both tig and mig with it.

Here's its wiring diagram:

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My issue is that the PCTI schematic refers to a "three phase bridge" and my welder has a "six phase" rectifier where all SCR's "point" to one DC bus, and where another side is connected to an interphase transformer. Very different schematics.

So, I have maybe one and maybe two possibilities:

1) Rewire the transformer. It is made of six secondary windings, two independent ones on each phase. Two leads from two transformers on every phase are tied into the interphase transformer.

The ones that are tied together I could disconnect them from interphase transformer, parallel each pair, and then connect them so that I have three leads instead of six. (I am not 100% sure how to connect them, but I think that it is easy. I have an idea).

Then simply put together a new rectifier bridge wired as PCTI suggests, using new SCR half bridges which I already have in my pile.

2) Maybe there is a possibility of wiring in the PCTI board so that it works just fine controlling the six SCRs that make up the current "six phase" rectifier.

I spoke to their engineer yesterday (did not give him the schematics) and he was kind of vaguely positive about it, I asked if ony K2, K4 and K6 were used for synchronization and he said yes.

What I am thinking here is that I would wire all six existing Hobart SCRs to the PCTI board, it would monitor voltage on K2, K4, K6, and then adjust phase angles of firings of all six SCRs based on desired current/voltage vs. actual. And that it would not matter that the SCRs

1, 3, and 5 are wired the opposite of the PCTI sample schematic.

Note that the SCRs that I have now are pointing with their gates to the DC+ bus, which makes it impossible for K2, K4, and K6 to meaningfully monitor voltage/phase. So, what I think I can do here is turn all SCRs UPSIDE DOWN, so that K2, K4, and K6 could be connected to 3 phase voltage.

The side effect of this is that the negatve bus will become positive, and vice versa, but that is no big deal as I could simply switch contacts leading to commutator.

Is this making any sense? Is approach #2 possible?

thanks

i
Reply to
Ignoramus12493
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the "6 phase" rectifier is essentially 3 center tapped transformers wth

6 diodes. This design optimises the number of diodes at the expense of extra trnasformer windings. This would be an older design when coper and iron were cheap, tube type diodes were very expensive.

Your other > Thanks for yesterday' suggestions regarding 3 phase transformers.

Reply to
RoyJ

This is absolutely correct. A great description.

And my question is, can I "flip" the SCRs upside down, as that would possibly seem to let me use the PCTI control board. (I understand that it will also switch positive and negative).

I think that I would end up with the same number of diodes. Both the six phase rectifier and the three phase rectifier have six diodes.

I would, however, like to avoid major rewiring, for two reasons:

1) it saves time/space/is backwards compatible 2) It allows me to later use double voltage as a plasma power supply

If I cannot avoid rewiring , that's not the end of the world, but I would prefer to be able to get by just flipping SCRs.

i

Reply to
Ignoramus12493

In a 6 pulse bridge the diodes are in series and so you allways need to fire them in pairs. The gate pulses consist of 2 pulses 60deg apart. In your original rectifier they are fired sequentialy 1 at a time, so the fireing pulse trains are not compatable. It is possible to seperate them with some logic but that would mean making an extra board.

Reply to
cbarn24050

This is bullshit. The 6 phase approach has lower losses when it comes to high current. There is only one semiconductor loss in the centertapped recifier and not two as are in the bridge approach. A semiconductor loss means in the order of 2V for an SCR. An alternative approach would use FETs, as they allow even lower losses.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

While we're not on the topic, how many people here call rectifiers 'rectum fryers'?

:)

Reply to
Simon Scott

Thanks... Sounds like you are fully right.

i
Reply to
Ignoramus8092

Perhaps the OP meant "optimizees the number of dioes in the path of current". There is only one voltage drop in a six phase rectifier.

I am not too worried, I can easily make 3 phase rectifier, I have all the parts already.

i
Reply to
Ignoramus8092

You mention six secondary windings, but I only see 3 connections to the transformer on your schematic. The schematic looks like one of these:

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..but the description (six secondaries, interphase transformer) sounds more like one of these:
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I am not sure whether this describes your Cyber Tig, , but I have often seen see welder transformers that consist of a 3-phase wye secondary (which may or may not bring the center tap out) and a eperate 3-phase delta secondary, each driving 3 diodes or triggered SCRs. The the delta and wye are 60 degrees phase shifted from each other. The advantage of this is that the rectifiers/SCRs send twice as many pulses per cycle to the capacitor, lowering the ripple amplitude, increasing the ripple frequency, and improving the power factor -- which is the main reason for doing it in a welder.

--
Guy Macon
Reply to
Guy Macon

That's similar the schematic for the three phase rectifier firing system that I have and am considering installing into my CyberTIG:

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Yes, that is what I have right now.

I do not think so, no, what describes my CyberTIG right now is

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Yes... You are correct...

It seems as though this PCTI firing system cannot be adapted to the current 6 phase wiring. Rewiring is not a huge deal and can always be reversed.

i
Reply to
Ignoramus8092

It seems to me that you are on the right track. I am currently designing systems that are in the 50KW to 1MW range; I am doing the embedded processing and precision analog bits, but one of my coworkers is among the top names in power electronics. I will be happy to run things past him if your current effort runs into a snag. I don't think it will, though.

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Guy Macon
Reply to
Guy Macon

Thank you Guy. My plan is as follows: wire the transformer as three phase Wye, and use three SCR half bridges in a scheme similar to that by PCTI. These three half bridges will be added without disturbing current SCRs.

I have six secondaries, two on each leg, I will parallel each pair.

Does it make sense?

i
Reply to
Ignoramus8092

That appears to be correct, subject to the usual limitations of talking about a circuit rather than having it in front of me. Normally I bring up any such circuit slowly with a three phase variac or, in a pinch, with three single-phase variacs and a lot of moving from knob to knob. You might also consider using temporary fuses to do the paralleling at first as an extra precaution. All of the jumpers we use in our lab have inline fuseholders; putting a regular clip lead across a 440V 2KA supply by mistake makes quite a spectacular flash.

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Guy Macon
Reply to
Guy Macon

No. Wiring the transformer as (a single) Wye requires having access to all 6 secondary leads on each transformer half (12 secondary leads available vice the 6 I think you actually have) so you can reverse the phase on one half (switch the neutral connection with the 3 line connections on one side) so they can be paralleled. This will totally screw your existing 6-phase rectifier of course. I suggest you re-read the description by Lawrence of how the 6-phase system works (previously posted to ABSE).

You now have 2 separate 3-phase Y secondaries of opposite phase, if you wish to add a separate SCR rectifier of standard 3-phase design without screwing your existing rectifier you will need two of them, they cannot be paralleled. The DC output of two separate rectifiers can of course be paralleled.

Reply to
Glen Walpert

Glen, I have physical access to all 12 leads: I have three legs, two secondaries on each, and two leads on each secondary, all plainly visible and physically accessible.

Some secondaries are connected in parallel to form the existing "6 phase rectifier", but I could disconnect them.

Here's the schematic of my welding xfmr:

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Here's its actual picture:

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Right now, the following points are connected:

A3, B3, C3;

A2, B2, C2;

These connected pieces are connected to the interphase transformer.

I want to rewire it by disconnecting A3, B3, and C3 from one another, and by connecting

(A3, A1); (A2, A4) (B3, B1); (B2, B4) (C3, C1); (C2, C4)

Then I will have a common point of (A2, A4, B2, B4, C2, C4); and the wye leads (A1, A3), (B1, B3), (C1, C3).

Is this wrong?

i
Reply to
Ignoramus17838

That is the correct way to rewire the transformer secondary as single

3-phase Y (and not what I thought you were planning), but I thought you said you wanted to leave your existing 6-phase rectifier connected and operational, and rewiring your transformer as 3-phase will preclude that. This also gets rid of the current balancing action of the interphase transformer, which is of course completely disconnected.

Why not use the installed rectifier as wired, and build a new controller for it, adaptable for either CC or CV? If you have problems you can just reconnect the original controller.

If you are willing to abandon the existing rectifier entirely then transformer rewiring and a standard 3-phase SCR rectifier module should work. But it seems like the hard way to do the job to me.

Reply to
Glen Walpert

Glen, just to make sure that you are aware, I was given a three phase firing system by PCTI. As a sign of appreciation for certain activity of mine (algebra.com). Wiring it into a existing 3 phase supply is next to trivial. All I have to do is supply SCRs and potentiometers and switches. I already have it sitting on my desk.

Here it is:

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The question that we are discussing is how to make my power supply compatible with this PCTI system.

So, yes, I was hoping that I could simply use my "6 phase rectifier" to be controlled by this PCTI system. As of now, it does not look like I could do it, because it is really a 6 phase system with different phase angles that necessitate different firing angles.

Well, I think that reconnecting the xfmr (which is easily reversed) is quite easy, will take a couple of evenings. And I already have a well working SCR controller, made for rough industrial conditions, by company that is.

It is hard if we assume that I need to make a SCR controller in either case. But it is not my situation, my situation is that I have a super nice controller for a 3 phase bridge.

Also, Glen, if I wire this transformer as a Wye vs. Delta, is it true that I would get different voltages phase to phase? I have a suspicion that the side effect of my Wye wiring would be increase of voltage by sqrt( 3 ). Is that true?

Thanks, I always listen very closely to what you say. i

Reply to
Ignoramus17838

I got around to looking at the picture; not the easiest connections to rewire with those brazed neutral bussbars!

Your 6-phase rectifier is two 3-phase rectifiers in parallel, with 30 degrees phase difference between them, so you could use a pair of the PCTI controllers for your pair of 3-phase rectifiers.

The transformer winding connections you have shown above are still Wye connected (you still have a neutral), but if you are planning to connect your new rectifier in Delta (and not use the neutral) then the line-to-line voltage will be sqrt(3) times the line to neutral voltage seen by the original rectifier. You could reduce the line-to-line voltage to the same as your present line-to-neutral voltage by rewiring the transformer secondary as Delta, with no neutral. (The winding voltages will stay the same.) But since you are using a motor based rotary converter with inherent voltage imbalances a Delta connection could result in excessive circulating current - you would want to measure this with a current shunt in the loop before using a Delta winding connection.

How about starting out by using 1/2 of your transformer secondary windings only; either one of the two Wye connected windings provides

3-phase power as is. If it works and you decide you need full power instead of half power, get another control module or rewire the transformer.

How about temporarily disconnecting half of your transformer and rectifiers, so that you have only a 3-phase rectifier, and you can use your existing SCRs and wiring too?

Fine, just don't always assume I know what I am talking about :-).

Reply to
Glen Walpert

Not the easiest, but, definitely, not terribly hard. A little bit of cutting and maybe a little bit of drilling will "get me there".

Well, I have only one controller set.

Makes sense.

That (about the phase converter) is a great point indeed.

It is a f*&^ing brilliant idea. I will do exactly that! I will try starting tonight.

Well, my current SCRs are wired with gate towards the common DC+ bus, so there is no way for the PCTI controller to sense voltage.

I already have enough isolated dual SCRs, adding them to the octagonal DC+ bar is very easy. Note that the DC+ bar will no longer be energized, it will only be used for cooling.

If you do not understand what I mean in the above paragraph, it is my fault, my main message here is that I have all parts for using half windings.

I have to admit to it! I think that you know what you are talking about.

My plan is as follows:

1) use one half of the secondaries as a Wye connected 3 phase source 2) add new dual SCR modules to the octagonal heatsink (that used to double as DC+ bar) 3) wire everything together with the PCTI modules for some test runs.

If everything works and produces voltage and current, then I could work on doing a few things such as:

1) reconnecting second half of secondaries 2) making a proper welding control panel.

Glen, do you know anything about plasma cutting? The reason for this question is that if I put secondaries in series, I would get a power source that can supply 190V OCV and about 135V continuous voltage.

I could make this transformer switchable from paralleled secondaries to secondaries in series, with three contactors (not too expensive). Would it be able to do any plasma cutting?

It would be good to decide on whether to do it, sooner rather than later. But, first things first, I will definitely try your great idea about using only hald of the windings "as is".

i
Reply to
Ignoramus21673

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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