Cleaning rust from transformer laminations?

Hi all,
I did more work on my phase convertor project this evening (I'll post some pictures soon). I stripped a 230 V -> 110 V transformer in
preparation for winding a new secondary to give me a 415 V output. It's amazing how many individual pieces of iron make up a transformer core - in this case it must be close to 1000.
Anyway, the laminations show a little surface rust. It's minimal but still makes a mess and it would be good to clean it off in order to do a really nice rebuild job. There are two sizes of lamination: approximately 3" x 1.5" and 9" x 1.5" (and a third size in the centre, which I can't yet measure). Both are about 1/64" thick.
Any ideas on how I might clean off the surface rust? They're too delicate for tumbling in a concrete mixer, and I don't have a blast cabinet. I've thought about electrolytic de-rusting, but haven't tried it before. Does it work okay with a whole bunch of little components resting in a metal basket? Cleaning 1000 laminations with steel wool seems like an endless task!
Any thoughts would be appreciated.
Best wishes,
Chris
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Chris,
Why not just leave them alone? Conductivity between laminations is not much of an issue in transformer and motor design. IMO, you'd do well to put them back together again and make sure you have adequate protection where the new wires will be in contact with the bare laminations.
Bob Swinney

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Robert Swinney wrote:

Bob,
I might well do this. I just thought it would be nice to clean off the rust as the dust makes quite a mess. But if I clean them, I want to do it properly and then give them a fresh coat of insulating varnish. The windings sit on a rubber/plastic former so that they aren't actually in contact with the core.
Best wishes,
Chris
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much
them
new
Chris, don't clean them! The purpose of laminations is to prevent (or at least reduce) eddy currents flowing through the thickness of the core, the laminations are effectively insulated from each other (I think they oxidise the surfaces of the laminations); cleaning them may well increase the magnetic losses in the core by allowing high currents to flow between the laminations. Martin
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On Fri, 13 Jan 2006 00:15:34 GMT, "Martin Whybrow"

Yes they are coated to prevent Eddy currents. I believe we used a magnesium oxide but too much time has elapsed to recall details.

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"Hmmm"!
I may be misinformed, but over my many years of working on and around ship and docks, I would hear a loud humming sounds coming from "some" very large transformers. The electricians would refer to this a "lamination hum" and was often blamed on rust between the laminations which would increase the gap between them.
It never seem to be a detriment to the performance of the transformer other than an annoying noise.
I often wondered if it didn't, to some small extent, effect the efficiency of the transformer.
Just my experience, FWIW.
Steve

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On Fri, 13 Jan 2006 00:14:16 +0000 (UTC), Christopher Tidy

Hey Chris,
Leave the laminations alone. No need to do anything. And note the "Insulating Varnish" is not applied to make it look nice or to not to let it rust, but in fact to assist in what another poster said, and that is, while in use, to keep the windings from moving and rubbing together, and rubbing the sharp edges on the laminations too. If you feel you need this to look nice after you get laminations back together, just wire brush the flat sides, and not the part where you can see the ends of the laminations. You don't want anything to "short them out". Best way to apply the varnish after re-winding is to dip the whole thing in a pail of varnish and let it sit for a few hours, then bake it in the oven, ******Yeah right!! I'd be dead in my house!!!****** So, while it's in the dip, make up a cardboard carton with a nice big light bulb (150 watt?) and figure out a way to suspend the transformer in with it, and then let it sit there for a day to bake dry.
If you can't do that, then use air-drying Glyptol instead of varnish. Easiest way, of course, is to take your efforts into a motor shop and let them do the dip & bake properly. Then megger it.
Take care.
Brian Lawson, Bothwell, Ontario.
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On Fri, 13 Jan 2006 00:14:16 +0000 (UTC), Christopher Tidy

A soak in salt & vinegar or dilute muriatic acid will probably take care of the rust -- but you will definitely then need to coat the lams with insulating varnish before reassembly. That could increase the height of the stack enough so not all of the lams will fit inside the coil former. If you have fewer lams, then the voltage capability of a given winding will be reduced: less core sectional area supports and produces fewer volts per turn at given max flux density.
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Conductivity is an issue - the magnetic currents induce currents. The plates are normally isolated with lacquer or 'dope'. Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Robert Swinney wrote:

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    I would advise not doing it.
    The laminations are designed to be electrically separate to minimize eddy currents. There is an insulator coating between laminations, and your cleaning would remove this. It also keeps the magnetic fields oriented along the laminations, not across them.
    I'm not sure what process is used to generate the insulating layer, but at least the rust will not conduct electricity like the cleaned laminations would.
    Good Luck,         DoN.
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DoN. Nichols wrote:

The insulating coating on transformer and motor laminations is generally a thin oxide layer that is put onto the steel, with some sort of a steam process. It is a finish not entirely unlike some types of gun blueing. Usually the transformer is varnished after the laminated core is assembled onto the coils. They are immersed into a varnish tank, and often the tank is closed and a vacuum drawn in the tank to help draw any trapped air out from the windings and the laminations.
You don't want eddy currents in the core, and you want to minimize any gaps in the magnetic circuit to minimize core losses and idle currents. Coating the individual laminations with varnish will introduce lots of gaps in the core, which is not good. that is why they are steam finished, it forms a very thin insulating layer.
Don is correct. You would probably be best off to leave them alone, stack your core and varnish.
A number of years ago, I stacked hundreds and hundreds of these things by hand. Used to work for a custom transformer house.
Hope that helps.
Al A.
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Al A. wrote:

Alright, it looks like the consensus is that I should leave them alone. I thought they were varnished to prevent eddy currents, but now you mention it there is a noticeable green layer on the plates. I assume that this is the oxide layer formed by the steam. I had to dismantle the core to rewind, so that isn't wasted effort. The reason for cleaning off the rust was to make reassembly a bit cleaner. The rust is thin and dusty and gets everywhere. Perhaps I should just wipe them with a rag soaked in white spirit as I reassemble?
Thanks for the suggestions.
Chris
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Christopher Tidy wrote:

Just wiping the loose dusty stuff off as you describe sounds about right to me. You are correct, that green is the oxide layer. On some laminations the oxide layer makes some really cool multi-colored patterns.
have fun, and let us know how it comes out.
-AL A.
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Jm
After you get the whole thing back together take it to a motor shop and have them dipped in an epoxy type of material. They will evacuate the chamber to remove all the voids and then bake and dry as needed.
Bob AZ
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Thanks for all the suggestions. There is nowhere near enough rust to make reassembly a problem. There's just a bit around the edges and some dust between the plates. I would try electrolytic de-rusting, except that I'm concerned that it may remove the intentional green oxide layer (it is an oxide layer and not varnish in this case). I'll give the laminations a wipe with white spirit before I reassemble. Before that I need to build some kind of jig so that I can wind neat coils. It didn't occur to me at first that using thick (2.5 mm) wire would make winding more difficult, but it looks like I need to avoid twisting the wire as it is transferred from the reel to the transformer bobbin. I'll post some pictures when it's done (probably a week or two).
Chris
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    [ ... ]

    That will probably do well enough.

    Hmm ... I've seen transformers with heavy wire (around this size) wound with *square* wire -- to maximize the conductor's cross-sectional area. Otherwise, there are air gaps between wires which are just waste space.
    Obviously, this would require even more care in the winding, assuming that you can find the square wire in the first place.
    Older transformers that I've disassembled have had a very thin waxed insulating paper between layers of the wire -- to keep the wire from bunching up, and especially to minimize the voltage between closely adjacent wires, which could otherwise be a problem if an upper layer dropped into a much deeper layer.
    obviously, if you are going to be using 2.5mm wire, round or square, you won't be developing enough voltage to have to worry about this unless the transformer is a *lot* larger than I have been thinking. :-)
    Good Luck,         DoN.
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| Voice (all times): (703) 938-4564

I think the thin paper is called "fish paper" in the rewinding trade. I've no idea where the name came from. Bob Swinney
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On Fri, 13 Jan 2006 01:08:41 +0000, snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote:

You could sandblast the laminations with low pressure 80 grit, then put a coat of Manganese Phosphate (parkerize) or Zinc Phosphate on them. Soak them in a thinned varnish or drying oil (tung or linseed) and assemble. The manganese or Zinc phosphate is a poor conductor, and particularly when oiled keeps the silicon steel from rusting.When the oil "dries" it forms a varnish type resin that keeps the laminations from vibrating - then you just need to keep the windings and cores from singing. "Parkerizing" is a relatively simple process.
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Nichols says...

It's varnish.
If the varnish is stripped off there will be one giant conductive shorted turn. The transformer will smoke.
JIm
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jim rozen wrote:

In this you are mistaken. The steel is a magnetic material. The insulated laminations reduce the eddy current losses. It is not unusual for laminated cores to be welded to reduce acoustic noise. A solid lump of iron would still function as a (lousy) transformer core.
Kevin Gallimore
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