Cleaning rust from transformer laminations?

I think the thin paper is called "fish paper" in the rewinding trade. I've no idea where the name came from. Bob Swinney

Jim, go back and read axolotol's statement about 4 posts above. Insulated lams are not required, and probably not desireable. There can be no shorted turn effect because current in the windings cannot be diverted to the lams -- unless, of course, there is a short between the windings and the lams. Actually it would take 2 shorts in 2 different places on the winding to cause appreciable current to flow in the lams. Read that, other than eddy currents, which the lams are designed to suppress.

Bob Swinney

It would be a very hot lousy transformer!

It isn't about current connected from windings to lams. It's about a closed conductive path enclosing the time-varying flux in the core. Any closed conductive path enclosing the flux linking the winding is a shorted turn.

Individual lams are still little shorted turns, but they each individually enclose relatively little flux so eddy currents induced in them are minimized.

A weld along one side of the stack doesn't create a closed path so it doesn't hurt anything. If you placed another weld on the other side of the stack, it would be a very different situation.

Yep. Try that sometime, put a turn of coat-hanger wire fully through a core sometime, with the ends connected.

Back when transformers were new, they had all kinds of tricks.

Atwater Kent made them by winding the bobbin on a form, and then slipping it over a bundle of steel wires. Then bend the wires over to overlap to complete the flux circuit. Pot the whole thing in tar and there it is, laminated core without the need to punch the E and I pieces.

Jim

Have to respectfully dissagree here, guys. Your "average" Microwave oven transformer is welded in at least 4 or 5 places. It is an "E" core with a straight closer welded on at both ends. The closer is also welded across at the middle, and the "E" core is welded at both bottom corners to the bracket, as well as being welded across the middle. On the one on my bench (modified to be an isolation transformer) there are SIX welds, well distributed around the core - and it wuns perfectly cool.

I assume that the welds were all on the outside. This can be OK, so long as there is no complete circuit in the iron more or less paralleling the path the copper in the coils takes.

Joe Gwinn

Right. Welds in the plane of the lams don't count either because the lams are already conductive in that direction.

According to Don Foreman :

But only because of magnetic eddy currents, not shorted turns..

Enjoy, DoN.

And the difference is?

Hint: current is current. "Eddy" just connotes current circulating in a small region. What constrains the region size?

I suppose one might argue that there is no "turn" unless there is a physical entity, separate from the core metal, that looks like a shorted turn. OK, absent that it's an eddy current -- one honker of an eddy current!

If we have many such axially-adjacent regions, are they in parallel? (Ducking........)

The laminations will be, by definition, at right angles to the windings. ANd yes, the welds are on the outside. Would be quite a trick to weld the inside with the coils installed.

It is the "shorted turn" concept I have a problem with. A powdered iron toroid core will show a low DC resistance. It will, however support a respectable flux density due to the distributed air gaps (or, if you prefer, multiple domains) in it.

Kevin Gallimore

Sorry Kevin, I've got to disagree with you; powdered iron cores have high electrical resistivity since they consist of iron powder and an insulating binder. The whole reason for using them is that they don't support eddy currents which is what makes them good for high frequency use. Martin

step-lap cored transfomers are interesting too. They wind the core out of strip steel, then cut it apart, and reassemble it around the coils. They're really efficient and used in distribution transformers.

Martin,

You are absolutely right. I really do know better. I must plead a virus and too many drugs. After some sleep the only thing I can figure out I was trying to say was that the iron was an energy storage medium and could not be considered a simple conductor. What makes this worse is that long ago, in a land far away, I used to have to measure the bulk resistivity of ferrites. I will now slink off and change my drug mix.

Kevin Gallimore

Now put a single weld (or bead of solder) on the central leg of the E and try it again.

Aren't disposable scrap microwave parts fun ! 8-)

You can weld all you like on the "outside" of the lamination stack, but this doesn't create a shorted turn, it just creates a long C-shaped conductor, bent into a doughnut (and then joined to a friend on the other side to make the completed figure 8). So long as there isn't also a conductive path on the "inside" of the E, then it's OK. You can connect up laminations all you like, so long as you only connect them along a plane that's at more or less constant B (or maybe H - I could never remember).

As the coil has relatively little flux outside the coil, as soon as you get a small distance away, then it's easy to draw a roughly equipotential surface. You can even approximate this and follow the straight-line outside of the E laminations instead. The field varies, but not by much. _Inside_ the core though, in that central leg, then unless the core is very long and thin then there will also be a field variation lengthways on the core. Trying to put the welds here instead of on the outside would probably cause enough losses to be significant.

I think Andy is right here, although I can't quite visualise all the C-shapes contorted into doughnuts and figures of 8. Let's suppose that you only weld where you would in order to fix the laminations together, i.e., any welds are perpendicular to the plane of the laminations. If you put welds on the outer surfaces of both the outer legs of the E, that's fine, because the loop you create encloses two equal and opposite quantities of flux: the flux in the central leg is equal and opposite to the flux in the two outer legs, so there will be no voltage induced in the loop. But if you made two welds on one of the outer legs of the E (one on the outer surface and one on the inner), or alternatively one on the outer surface of each outer leg, and two on both surfaces of the inner leg, you'd have a problem as the net flux enclosed isn't zero. I think you get the idea.

Sorry I've taken a while to respond to this thread. My own computer (faithful Sun Ultra 2) has broken down. The clock stopped working, so I tried to replace the NVRAM chip and now I can't get the machine to boot. If there are any Sun hardware experts here let me know - I'd be very glad of the help. I asked the question at comp.unix.solaris but so far haven't received much advice.

Best wishes,

Chris

You have remembered to tell it where to find /vmunix haven't you?

Mark Rand RTFM