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

Reply to
Christopher Tidy
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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.

Reply to
Robert Swinney

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

Reply to
Christopher Tidy

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

Reply to
Martin Whybrow

"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

Reply to
Steve

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.

Reply to
Brian Lawson

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.

Reply to
Don Foreman

According to Christopher Tidy :

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.

Reply to
DoN. Nichols

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.

Reply to
Al A.

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

Reply to
Christopher Tidy

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.

Reply to
Al A.

You _might_ need to de-rust the laminations, just to get the stack height small enough to fit back in the coil former. If they're really rusty, then they're also likely to hum more than usual.

You should re-varnish too, if they're particularly scabby. Missing varnish increases losses in the core, leading to wasted heat.

Yes - a rotating basket would be ideal, but just turn them over every few hours. You need a big shallow basket, and to have the anode on the lamination side of the basket. Generally a plastic basket works best, if you can arrange good contact to the workpiece. A conducting basket tends to steal too much current flow for itself.

Electrolysis may also leave the varnish intact, compared to abrasives. Or it might strip it - you don't know for sure until you've tried that particular varnish.

Reply to
Andy Dingley

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.

Reply to
clare at snyder.on.ca

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

Reply to
Bob AZ

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

Reply to
Unknown

Did you think about using a spray on rust converter? I use it for anything from engines to rectifier assy's.

Reply to
Modat22

It's varnish.

If the varnish is stripped off there will be one giant conductive shorted turn. The transformer will smoke.

JIm

Reply to
jim rozen

The laminations MUST be electrically insulated from each other when you are done or there will be one large shorted turn. This is bad.

If you strip them down to clean metal you will have to re-varnish them before putting it back together.

Jim

Reply to
jim rozen

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

Reply to
Christopher Tidy

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

Reply to
axolotl

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