The AM Forum

I had to banish a lot of my large transformer to the garage, no room in the shack. Unfortunately several have devolved rust on the laminations over the winter.  I don’t think it will kill them but it’s ugly. I was just going to sand them and use brush Rustolem on them.  Next winter they will be in the basement. Anybody have any suggestions?
73 N8QPC

You might just as well have set the transformers outside in the weather.  At least they would have dried out a  little during dry, sunny days.  The problem with leaving them in an unheated garage or barn is that  during cold snaps the iron drops to freezing temperature.  Then when the weather changes, and warm humid air moves in, the thermal inertia of the iron causes it to remain colder than the outside air for hours if not days, and it sweats like an ice-cold glass of beer on a hot summer day. Not only does the exterior of the  transformer sweat, there is also condensation at the interior, where the insulation contacts the core, and even inside the windings, where the metal wire contacts the insulation. In other words, the entire transformer is impregnated with moisture, unless it is hermetically sealed, a rarity with vintage radio transformers. Not only does the moisture deteriorate the insulation, it causes the wire and the iron core to corrode, further damaging the transformer.

I would bring those transformers ASAP into a warm, dry place and keep them there for several weeks or months before applying any high voltage. A friend of mine found an old KW size modulation transformer that had been stored in an unheated shed for several decades. He brought it home and checked the resistance between windings  and between core and windings with a hi-potter before putting it in a rig, and it showed severe leakage, and probably would not have withstood the high voltage in a transmitter.  He figured it was hopelessly crapped out and sat it in his dry, heated basement for several months while trying to decide whether to  just toss it out or save the frame and core for rewinding. Then, before finally disposing of it he ran another hi-pot test, and the insulation showed close to normally expected leakage! He put it in his 833-A's modulated by another pair rig, and it was still working fine when he sold the transmitter.

Small transformers, that can be lifted in one hand (up to roughly 50#), can be baked in an oven to drive moisture out. Dont rush it and keep the temperature below the insulation ratings.

For big iron I short the secondary and Variac up the primary slowly, and with an Amp meter, until it is pulling around half its rating. This may take days or a week.

Mod iron can be done the same way. Chokes just get the direct variac; Ive been told this can get tricky with real high inductance and its better to use DC.

As always a hi-potter is your best friend, check progress daily.

Carl

I guess it all depends on your climate. Here in Central California I keep extra transformers in the garage. It never gets below about 45 degrees. And there is no condensation ever. I've never had a rust problem -- except of course on transformers that were rusted before I got them.

To dry them I put them out in the garden shed during the summer. It probably gets up around 140 degrees in there every day. And we have zero rain in the summer so not much humidity.

I was born in Tucson -- Moving there would also be a good idea for people with rusty transformers.  :)

After my last episode with plate iron that was crapping out for no apparent reason.
(mostly old stuff) I have become extreemly leary of rusty iron. I will no longer even consider using (or buying) it. But if I must use it, I will first Hi-pot it and if it looks reasonably good on my Hi-Potter I will then remove the end bells and bake it for an entire day at somewhere around 160-180 degrees and then drop it into a bucket of oil based varnish to cool.      (and hopefully suck some of the varnish into the windings)

After it cools down to room temperature, I will them pull it out and let it "drip dry."
Then give it a few more dunk and dry cycles until it forms a nice glaze on everything. Then put it somewhere warm to dry thoroughly. After a few weeks of dry time, I will HI-Pot the living hell out of it before reinstalling the end bells and putting it into service. This procedure will give new life to old transformers.

Insofar as rust is an indication of moisture exposure it may be cause for some concern. A light rust on the lamination edges is not by and of itself an indicator of certain doom however. So long as the transformer workings have been completely dried before operation. The pointers given by KM1H are spot on  regarding drying and testing .
  My transformer collection resides right next to the heater in my shack. No signs of moisture ingress whatsoever. I have about 25 years use on a lot of older gear with mucho rusto on the edges of the laminations, I merely removed the surface rust and varnished the edges. No problems at all. (After a good drying cycle )

If you have access to steam lines... as in the boiler plant at work or a friend that has access to some. Let it bake there for a week on an exposed valve. Instead of using your gas or electric and stinking up your house.

I just keep those things in an old shed.  Runs about 130 deg inside during the summer here (about 7 months long). 8)

Humidity stays in single digits most of the year, rainfall is around 4-5" annually.

Creak pop snap.  Never mind, that was our skin...  ;)

73DG

Last year I moved all the iron that was in the garage into the basement where I run a dehumidifier and keep it at 40%.  I hate humidity.  Summers in the South are a horror show.  I have been to New Orleans in July for trade shows and seen the sidewalks wet like it just rained but it didn't--it was water rolling off air conditioned building windows.  :P