What's in a welder?

[Maker]

I've taken the case off my Kemppi to blow the spiders out before I test/sell it but there's a few interesting things inside:

First off:

There's nine pieces of round, about 1/4" diameter, connecting the two haves, one side is welded to the heatsinks with the SCRs and the other side is bolted to a 95 square mm cable. Some kind of weird fuse?


Here's an SCR sandwiched between two heatsinks, there's 4 in total. I was going to ask what they were but I googled the part number and found a datasheet.


That's the bulk of the weight, the big transformer towards the back is where the mains input goes, haven't figured out what the secondary is wired to yet. What I'm more curious about is the smaller (Though by no means small) one at the front. It appears to have a 1:1 turns ratio, no extra taps on either coil and the coils are shorted together? A giant inductor? What does it do?

Some more random pic of its innards if anyone's interested:

The control panel and associated circuitry all slide out the front, two screws and a big 20 pin connector. Test sticker on the back of that circuit board:

Probably due a service.

Mains supply (Through a switch on the front) goes to the middle two posts, then swapping the brass bars around lets you run it from 220, 380, 415 or 500 volts mains. Each post is connected to different taps on the big transformer.

Heat sinks and drive circuits (?) for the SCRs. They're wired side to side on top (See the wires?) and front to back on the bottom.


There's some more interesting stuff behind the big transformer but I can't get the back cover off so I can't really see it to take pictures.

Richard; Its only got a 4 pin amphenol on the front so sadly I can't test your mysterious remote.

Snowcat; I found a thread from you a little more than 10 years ago where you said you had one of these, don't suppose you have any information about it? A manual? There's very little about it online.

 

[Jim Davey]

The device in the first picture is called a shunt. Voltage passes through it and it is also bridged by the machines amp meter. It is carefully fettled so that at a given amperage (say 500 amp) it will allow a certain amount of milliamps across the little meter wires also coming off it, the more metal you remove from the legs of the shunt the more voltage will bypass it via the meter wires. If it’s a 50 millivolt @ 500 amp shunt it will let 10 millivolts through the shunt wires for every 100 amps of current that passes through it.
Hope that makes sense?

 

[Maker]

The device in the first picture is called a shunt. Voltage passes through it and it is also bridged by the machines amp meter. It is carefully fettled so that at a given amperage (say 500 amp) it will allow a certain amount of milliamps across the little meter wires also coming off it, the more metal you remove from the legs of the shunt the more voltage will bypass it via the meter wires. If it’s a 50 millivolt @ 500 amp shunt it will let 10 millivolts through the shunt wires for every 100 amps of current that passes through it.
Hope that makes sense?

So it's basically a known (very low) resistance so it can measure current?

 

[Jim Davey]

Exactly, they come in different amperage and millivolt ratings to suit various applications and machine sizes. 50 and 60 millivolts are common moving coil and digital volt-meter input ranges.

 

[monkeh]

 

[Jim Davey]

Most invisible angry pixies go through the big metal thing and some angry pixies go off down the little wires across it but not so many as through the big metal thing.
If you take away some thickness in the big metal thing more angry pixies have to flow through the little wires and trough your amp meter. More pixies = more digits on the meter.

 

[bourbon]

and I understand it now! LOL

 

[bigegg]

I'm guessing, from my brief foray into turning my Oxford into a DC supply, is that the 1:1 "transformer" is an inductor/choke ?
Something to do with current stability?

I don't know much about electrickery.

 

[Jim Davey]

It’s a choke yes, it smooths the output characteristic of the set by adding inductance to the welding circuit. This has an effect on how quickly the short circuit current rises when the wire electrode touches the workpiece and stabilises the arc. Bigger migs have larger inductors with multiple tapping points, A to handle the extra current flowing through them and B to allow you to fine tune the inductance required to get a smooth arc across the whole voltage range in dip to spray on various wire diameters. On most small sets the inductance is fixed.

 

[Maker]

Really wanted to know what was behind the transformer so I took it out, all 95Kg of it.

Interesting design, the primary(s) are wrapped around both sides of the core and the secondary is copper "wires" wrapped over the top.

That' what's behind it:

At the bottom there's a secondary transformer, it takes a 220v input from one of the taps on the big transformer and provides an (Isolated) 24v to the control circuitry and 220v to the two sockets on the back to power a watercooler and something else, maybe a cold wire feeder? Maybe one of the many add-ons I'm told Kemppi used to produce?

The thing above it I assume is some kind of HF transformer? Hopefully someone can explain how it works. The wires coming off it connect to the dinse sockets on the front (+ and AC torch side), the big SCRs and these capacitors:

One of the wires goes to that blue 1.2mF () cap at the back, another one connects to a small SCR on that plate in the middle and then to the 60uF capacitor "bank" at the front.

In order to get it back together I've taken the top off so I can lift the transformer straight back on rather than try to shimmy it back in the side. I've cut off the wheels/crap trolley too, that way it'll fit much more tightly against the wall and will fit on a euro pallet when I come to sell it.

 

[Kent]

Magic n voodoo best not fiddle

 

[Maker]

Magic n voodoo best not fiddle

Does not compute.

 

[Maker]

Does anyone know why there should be 0.22uF capacitors shorting the outputs to the case?

Two have cracked, one has caught fire and one is missing.

 

[Kent]

Does anyone know why there should be 0.22uF capacitors shorting the outputs to the case?
View attachment 137105
Two have cracked, one has caught fire and one is missing.

Told you not to fiddle, now the evil spirit has escaped

 

[Seadog]

Does anyone know why there should be 0.22uF capacitors shorting the outputs to the case?

Probably RF suppression so you don't annoy the neighbours.