Transformer core.

[ozzie34231]

I'm hoping someone can give some answers to this electronically challenged tinkerer.
I'd like to wind a transformer according to some directions and plans that I have to build a 240V MIG welder. The plans use a stripped stator from a large motor as a core. I don't have one of those, but I have a core that has three legs, kind of a squared figure 8 laying on its side?
The plans use the stator by winding at two places 180 degrees apart, putting both primary and secondaries at each spot.
Can I use the two outer legs of my core to accomplish the same thing, or does the center leg mess with something,(waveform? or what?), that shouldn't be messed with?
Ozzie

[Al_The_Man]

Maybe post a photo of what you have.
Al.

[ozzie34231]

Hi Al,
Here's a picture. What I'm suggesting is that I strip the present windings and wind on the two outer legs.
The plans show the primaries on each side connected in series.
The secondaries are also connected and that connection used as "Ground".
The other two secondary leads feed through diodes to a heat sink, "positive"

Ozzie

[Al_The_Man]

That is a standard EI core transformer, the primary is usually wound on first and any secondaries on top.
You could strip the secondaries and wind on what you need, if you can power it up, you can firstly wind on 10 turns of any gauge insulated wire and measure the voltage, this will give you the turns/volt you need to take off or wind on.
It can be a pain to wind these type, if many turns are needed and the core does not have a weld streak across it, I have actually taken one apart down the winding bobin and wound on the secondary on a lathe.
But this requires much time and patience as you have to put all the EI laminations back one at a time.
Al.

[ozzie34231]

The transformer came from a power supply that output 76 volts at 15 amps max. input 115 volts.
I need/want to create 240volt input, 30 volts out at 180 amps plus. This requires some heavy wire and I doubt that I can get it all onto a single post. That's why I'm asking about winding on the two outer posts only. That would give twice the space.
The reason I'm not sure it would work is that I have a very old booklet about building a stick welder, and in it they wind on two posts and then tune it by putting core stack material between the windings. I realize this is a totally different sort of transformer.
Another idea I had is to cut the center post away. I could take the stack apart or maybe cut it in place with a manual hack saw. However I'm under the impression that the pieces of a stack should be isolated from each other. Is that true.
In the old booklet, they make their own stacked core without coating the plates.
So, I'm kind of back to my original questions plus.
1. Can I wind on the outer posts without removing the center core?
2. What are the problems associated with cutting the center post away?
Thanks,
Ozzie

[Al_The_Man]

Originally Posted by ozzie34231 The transformer came from a power supply that output 76 volts at 15 amps max. input 115 volts. I need/want to create 240volt input, 30 volts out at 180 amps plus. This requires some heavy wire and I doubt that I can get it all onto a single post. That's why I'm asking about winding on the two outer posts only. That would give twice the space. The reason I'm not sure it would work is that I have a very old booklet about building a stick welder, and in it they wind on two posts and then tune it by putting core stack material between the windings. I realize this is a totally different sort of transformer. Another idea I had is to cut the center post away. I could take the stack apart or maybe cut it in place with a manual hack saw. However I'm under the impression that the pieces of a stack should be isolated from each other. Is that true. In the old booklet, they make their own stacked core without coating the plates. So, I'm kind of back to my original questions plus. 1. Can I wind on the outer posts without removing the center core? 2. What are the problems associated with cutting the center post away? Thanks, Ozzie

The are a few obstacles and a few misunderstandings.
Firstly, if your transformer was rated for 76v at 15a max, this would indicate a little over a 1kva transformer.
You want 30amps at 180 volts, this is 5.4kva!.
You may run into core saturation if you were to run at this current on this TXFR.
Also, the closest coupling you can get is either one winding over the top of another, or side by side.
A stick welder TFMR has VERY different characteristics over a MIG.
You might want to look out for surplus 3 phase transformers with 240v pri and cut one down for 1 phase this may save rewinding the primary.
Al.

[ozzie34231]

Thanks Al,
Unfortunately I live in a small town, not close to any industrial area, or even a good junk yard. With shipping as high as it is the transformer you suggest seems unlikely.
The transformer I was contemplating using might be of greater capacity than you calculated. In its original case it was fused at 20 amps on the primary and apparently had two outputs of 76V 15 amps. two separate diode pairs and two large capacitors, two voltmeters and two ammeters. So maybe we're talking 2.25 kva. And to say that I want 5.4 kva is to imply 100% duty?

Anyway, besides a waste of time and money, what would the consequences of saturating the core? Heat? Complete failure? Just won't work?

Is there a way to calculate the needed core dimensions?

Ozzie

[Al_The_Man]

30v is around right for MIG, but 180amps?
I don't remember a MIG running this high.
A rough guess, I would think that the TXFR 2kva max.
To rewind the primary as well as the secondary is an undertaking, either way.
If you are bent on it, I would be tempted to strip it down as I mentioned and wind the bobbin on a home made jig, and restack the laminations.
Al.

[Zig]

What I am goig to suggest will sound a bit daft. but here goes.

If we assume the core is capable of 2kva operation then if all the windings are connected in series they would form a primary windaing suitable for operation on something that operates on say 250V. Close enought to be deemed 240V.

Now Al has indicated a little trick of winding only a say 10 turn secondary to determine the transformer ( new composite?) primary volts per teurn design point.
Generally for larger transfomrers it will be typically two volts per turn.
So having put on say 10 turns you should measure the open circuit secondary voltage.

Note the volts per turn on secondary .

Calculate turns for the volts you need.
select wire gauge for the current you need.

wind the new secondary on top of old primary and old secondaries ( the new primary is formed from old primary and old secondaries connected in series and PHASED CORRECTLY.

A bit of kludge but might work out for You. Main problem is insulation rating on old secondary.

[Al_The_Man]

One hic-up there is to wind on 60 turns capable of 180amps ( I still think that is way high), on a transformer that appears already full to capacity, winding wise.
Al.

[Zig]

Yes All,

Looking at the photo it would seem the copper window is full.

God bless the clever designers.