Electrolytic Capacitor Series Connection

[joeavaerage]

Hi,
its not so much as 'enough' but rather 'more is bette'.

The 'crack' that you are talking about sounds to me like you don't have enough output inductance.
You may recall in one of my first posts in this thread I indicated that you need some inductance in the output between the capacitors
and the weld arc. If you don't, or have to little, the capacitors discharge in a short circuit manner and tend to stick the rod to the job and produce a crack.
With enough inductance you will get a 'constant current' which is the appropriate mode for stick welding.

The capacitors are to make the most DC voltage from your transformer/rectifier combination but the its the output inductance turns that into constant
current required for MMA welding.

Craig

[bmn]

Hi Craig,

Yes, I did exactly as the schematic you posted, I do have two big air core inductors wired in series.

is it possible that the two inductors are cancelling each other if the winding directions are opposite ? (I id my best back then to match the winding directions to act as a bigger single inductor but I am not sure I have it right)

Maybe the moisture is shorting/making the inductors act as a simple conductor ? (35mm² "braided" copper with varnish insulation bought'em used)

After the test the bridge rectifier heat sink(no fan) was very warm to the touch and the inductors were warm to the touch, it was about 15°C ambient during the test.

Thanks very much !

bmn

Just came across this: https://www.electronics-tutorials.ws...inductors.html

[joeavaerage]

Hi,
more likely because they have no core they are very low inductance. All the output inductors I've seen in MMA welders
have thumping great cores.

Craig

[CitizenOfDreams]

I do have two big air core inductors wired in series.

is it possible that the two inductors are cancelling each other if the winding directions are opposite ?

If the inductors are physically separated from each other (no magnetic coupling), then the winding direction does not matter.

[bmn]

Hi Craig,

Well I have no inductance meter but, they are about 90mm air core diameter and wound in two layers, maybe 100mm long. Any figure about the required inductance ?

Can I use the rotor from an AC compressor as a ferrite core (loooose fit ~5mm gap around)? (Hermetic Rotary compressor)
Does it have to be tightly wound around the core ?

Thanks very much !

bmn

[bmn]

If the inductors are physically separated from each other (no magnetic coupling), then the winding direction does not matter.

Thanks for pointing that out !

Yes they are physcially separated from each other and at about 90° angle(cores' axes) because of short leads / cramped connection.

BTW, I did check and they are wound/wired in the same direction, not in opposition, but, the current goes through the internal layer to the external layer of the first inductor then the same for the second inductor(internal->external->internal->external).

Thanks very much !

bmn

[joeavaerage]

Hi,
I would expect those air-core coils to be adequate for a MIG, but not MMA.

MIG is essentially a constant voltage process however due to the capacitors ability to short out and dump their charge in very short order its
necessary to have some inductance in the output. 5-10 mH is probably enough, and I would guess that's what those air-cores will provide.

MMA is a constant current process and requires a great deal more inductance, 500mH to maybe 1H.

May I suggest getting a big lump of iron and shoving it inside the air core? If the core has a complete circuit, that is to say a toroidal shape or closed
rectangular shape then you would be concerned that the core will saturate and thereby get hot. A six inch length of two inch solid steel won't saturate (much).
Its a cheap way to experiment and determine whether you are on the correct path.

Craig

[bmn]

Hi Craig,

Its working ! Thanks very much !

It's burning 3.25mm 7018DCEP beautifully. 4mm is hard to start/ sticks on cold metal but welds ok/(colder side of ok) on hot metal.

Measured the inductors, they are ~82mm I.D and ~82mm long (square...) too small for a motor rotor I had so, I removed/unscrewed the Bakelite clamps holding the wire together and slid the inductors over a lathe spindle blank (80mm to 75mm O.D) I had laying around, it's night & day.

On the plus side, the mod was easy/no rewinding & I burnt a few 3.25mm rods like in the movies with the slag cracking on its own but, the down side is that I now have to return to the scrap yard to find/get a fat and heavy piece of metal to make the heavy welder even heavier

Thanks very much !

bmn

[joeavaerage]

Hi,
sounds like increasing the inductance is the right way to go.

If you get scrap to fill the void of the air-core it will almost certainly be steel. Steel does not have great magnetic properties.

The core should be made of laminated transformer steel. Transformer steel is a high silicon steel designed to lower hysteresis and increase
saturation level.

You would be advised to approach electrical companies that specialize in welder repair and by a genuine inductor. Such inductors are usually
wound on rectangular laminated cores. They will almost always have a small non-magnetic gap in the core, maybe as small as 1/16 inch, usually
plastic or fibreglass, but up to 3/8 inch in behemoth inductors for Deltaweld's and DC600's.

A single solid slug (non-laminated and not a closed path) cored inductor will have low inductance, probably 100mH, and you really want more, much more.
Closed loop cores are much better, within the proviso that they need a small non-magnetic gap to prevent saturation.

Craig

[bmn]

Hi Craig,

The local repair shops(very few) around are into $$$ electronic TIG/MIG welders I only found a very small air core inductor and the ESAB technician never saw anything bigger than that.

About two months ago there were big power transformer frames(Those in the big die cast aluminium finned boxes) at the scrap yard, I was looking at the "I's" which were like 50mm to 80mm wide by about a meter long, didn't have the courage to mess around the oily stuff(PCB's?) thinking about the work involved in cleaning trimming...etc. I guess microwave transformers are too small ? maybe another Chinese welder
transformer core ?

Looking back at all the mods I did over the years, sure I learned a lot but, for the same budget I could buy a brand new Chinese inverter welder (portable but can't easily fix it myself), I didn't break the bank nor did I loose money as it is still under $200 everything included

What have I got ? :

500A welder which makes barely 200A
+ solid state PWM control circuit ("a la" Dan Hartman's diy welder plans)
+ Hot/easy start feature
+ DC feature*New!*(Thank you!)
+ cables+clamps...

I still have to make a box for housing the DC guts + buy quick release cable terminals this is one of those projects that are not worth anything as soon as the learning comes to an end...

What do you think ?

Thanks very much !

bmn

[bmn]

Distribution transformer frames is what I had found...

[bmn]

Took a closer look at the test weld beads, and I can say that it's the first time I got no undercuts on the sides, its easy to look down on mods which took way longer to accomplish than expected but, the results are beautiful welds with no visible porosity using stronger rods...etc. it's easy to look down on details but the devil hides in the details