The AM Forum

I have several chokes and transformers that were stored in a shed that I would like to use in a few projects. These are not military style sealed units. Size runs from receiver to KW+.

What is the consesus on time and temperature in an oven to dry them out?

Carl
KM1H

It would depend on many factors. size to some degree, time for the entire mass to heat, tightness of the windings and if they are sealed up with something, (some have a goop), the temperature any goop will run at, the brittleness of any paper.

Some gas ovens reach peak temperatures much higher than the set average temp, especially leaky older ovens. Just gut feelings and a little experience having done this says to be gentle and use maybe 140-150 degrees F for 3-4 days to make sure, but don't take just my word for it, I'm not an expert on this.

Make sure to use the wife's new electric oven so as to avoid those temperature peaks.

Another method for drying transformers is to put a short on the secondary and use a variac to increase the current to the rated level so as to heat the transformer itself. A variac connected to a 24V high current transformer will be easier on the variac since the device under treatment won't need 120V but rather only a few volts to draw the maximum current. Similar methods have been used to dry generators and other large electromagnetic devices that don't fit in an oven.

Lastly, there may end up being some mildew etc you will never get rid of, but if it is dry, should not cause a problem, just don't let it carbonize from heat.

The oven part sounds better. Energizing (even with a VARIAC) and shorting the secondary would seriously stress the xfrmr.

Fred

Phred,
         Bake the transformer at somewhere around 170 -190 deg for at least 6-8 (or more) hours. bake it until the core itself is good and hot. After baking it, drop it into a bucket of oil based varnish untill it cools to room temperature. (this will suck the varnish into the windings as it cools). After it cools to room temperature, pull it out and let it drip dry. A day or too later, dunk it back into the varnish and let it dry again. (Do it at room temp this time) (dont rebake it) Repeat the dunk and drip dry
a few more times untill you get a nice glaze on it. Let it dry for a week or 2 and let her rip. this will add years of new life for an old transformer if it was good when you started. however it wont bring a dead one back to life.

                                                   The Slab Bacon 

Hadn't thought of the varnish step, Frank - good idea. I usually just bake them for a few hours around 150 deg. (my oven is old and varies upward a bit in temp), maybe for the afternoon, 4-8 hours depending on size.

BTW Frank, dug the parts rig out of storage last week, will have your un-drilled panel soon.

somewhere in the late 1930's QST's Kenyon had a running series of ads showing their manufacturing process from start to finish. one of the things they used was a vacuum varnish tank capable of drawing down to -30 pounds vacuum and they did exactly what slab suggests, except they also drew a vacuum down on it to suck the varnish into any space or air bubble. I wonder which would work better, hot varnish or a hot xfmr or both? Any thermo dynamic engs in the house? I B curious now.  :)

Place I used to work for had a vac/pressure chamber big enough to hold several KW sized at a time. It was pumped down to - 45. (they had upgraded the tank) Engineer strolled by and asked my boss if there was anything in it as he wanted to use it. Boss said "No, not even air".  ::)

-30 lbs of vacuum, wow. I only do my vacuum dips at -15 inches of vacuum. First I preheat at 150 degrees then put it in my home made vacuum tank that is full of baking varnish. Pop the lid on, turn on the vacuum pump. Let this run for about 2 to 3 minutes, depending on size of transformer. Remove from vacuum dip tank, reheat again, another dip, "Hang Em High" and let er cool.
Regards,
Gary...WZ1M

That BC reactor you did for me shines so much, you could skate on it.   ;D

don't pull 30 inches on it. I have made some potting materials foam out.
not sure if varnish will break down. 15 to 20 inches better.
Mil epoxy is best if you can find it, but hope it never has to come apart.

the varnish dip "glaze" also does wonders for preserving and reviving crumbling fish paper and paper outer wrappings on them as well. It makes them look amazingly good, just about like new again. Once it has been baked and glazed it is prety tough for moisture to get back into them. I have done quite a few old transfomas like this and never had one fail.

                                                       The Slab Bacon

they look like kinda open frame military jobs. That slick glaze over everything reminds me of the WW2 mold and mildew coating you still see on some military gear.

I should do that with the Gonset transformers....matter of fact I will while I still have easy access to both of them.  ;D 8) bet they will look awesome. They have the 'fish' paper outside the windings. and some of it is a bit crumbly on the corners. 2 transformers and 2 chokes.  :D

Kenyon made great iron but I hate the $#@&*^%& connection terminals on them. The little posts are a PITA to deal with, and if you put too much heat on one it will break the inside joint.

I think everyone else feels the same because (when I was looking ) it was almost always Kenyon iron that went for songs at festers. Some really desirable iron like 500W mod transformers and such for 5 or 10 bucks.

I have one (plate RF power amp plate xfmr in my HF-300 rig).  The ceramic terminal block is cracked. I have never been able to figure out how to get the damn thing apart to replace it.

But it has worked for decades with the cracked ceramic, so I don't worry too much about it.

Thanks for the ideas, its time to try a few and also make a vacuum chamber, Ive already got a pump taking up space on a shelf in the garage.

Carl
KM1H

the "vacuum chamber" goes by another name... a big pressure cooker from the flea market.

               _-_-bear

FWIW, I tried another method recently to "burn in" an old plate transformer.

It was a 1200v job with (I'm guessing because I don't have the spec) a 300 MA current output rating.

To burn it in, I put 12 VAC from a high current transformer on the primary. That gave me 120 VAC out the HV winding.

I fed that into a 40 watt light bulb and left that on and running for a day.

I figured that drew about the rated current thru both the primary and secondary, but at lowered voltage. So if there was any moisture in the windings I hoped it would bake out and the lower voltage would prevent breakdowns.

Didn't think of baking it in the oven. I don't think my wife would approve of that operation, plus with 105 degree weather outside I hate running the oven.

Dave

 Didn't think of baking it in the oven. I don't think my wife would approve of that operation, plus with 105 degree weather outside I hate running the oven.
Dave
[/quote]

If it is a fairly small transfoma, use a toaster oven and set it out on the porch. I do it that way a lot of times.

                                                                     the Slab Bacon

I heard talk of 107 next week. If it gets that hot, maybe I'll just set the transformer out in the sun and I won't even need an oven.  :)

Fwiw, if ur gonna bother with the varnish, you might as well buy some purpose made varnish, intended for electrical applications. It has a higher temp rating and better voltage breakdown than varnish for wood. Glyptal was the old standby, I suspect that they have more modern, better flowing stuff for the purpose today.

The biggest motor building shop (into the hundreds of horsepower) out this way has a big tank that they dunk the windings in - no vacuum! The stuff looks red and thin. They say they do a couple of dunks between drying time, which apparently is fairly fast.

          _-_-bear

Be careful of what type of motor varnish you use. There are the kind that is just "AIR DRY" and then the other that needs to be baked for it to cure.
Gary...WZ1M

Vacuum is much better to get the varnish in deep. Also less chance of hum

If you have access to high pressure steam lines thru work or a friends work, you can use them to bake the items for a few days or a week. I did this with a slightly leaky choke, put it directly on the uninsulated part of the pipe at a valve and then put a can over top as an oven. Did megger out clear after that. The potting did stink a bit, not good for the home.

A quick trip to the local landfill often yields medium to large size Toaster ovens.  Free and perfect for the average Iron-baking requirement.  Let it run outside on the back porch or something to keep the XYL happy...