[Johnson] Transformer "cooking"

Mon Jan 6 15:14:28 EST 2014

I dont know if Measures used the method I described decades ago but there 
are methods that work.

First, you have to realize that using an oven only works for small 
transformers that can actually be brought up to a sufficient core 
temperature to drive moisture out. Trying that method with the wrong 
transformer will only drive moisture out plus in deeper.

My first experience with "heavy iron" was in the 60's with a 150# oldie 
6000VCT that was rescued from a flooded basement (not mine) following advice 
from a ham decades my senior.

Short the secondary with an ampmeter and fuse in series. Place a Variac on 
the primary with another ampmeter and fuse. Dont even bother hi potting if 
it is a flood recovery.

Bring up the Variac very slowly while monitoring both meters. The primary 
will draw more power due to the magnitizing current plus I2R and core losses 
but it should be a linear increase. Stop if there is a sudden increase in 
current, back off and let it cook. A portable heater with a fan will help 
dissipate moisture escaping.

It might take days or even weeks until it is running at even half its CCS 
KVA rating; consider the electric bill vs saving money on a new transformer.

The last one I recovered was close to 5 years ago and Ive recounted this on 
several forums over the years. It is a 275# monster built in 1946 with a 
6000/7500V CT secondary. This beast lived in an old mill building with 
broken windows and then an old garage with no door for about 40 years after 
removed from service in a shoe factory.

It took a week of cooking out in the garage, at up to around 75% of 
calculated KVA, during a nice summer dry spell followed by a dunking in a 
vat of transformer varnish after a hi pot at 13.5KV which was the limit of 
my tester.

Then towed on its dolly to the basement bulkhead with a lawn tractor and 
lowered down with my engine hoist and chain fall. It continues to power a 
"big" AM amp and modulator which currently runs a pair of 6C21's with PP 
QB5/1750 modulators.

Carl
KM1H

----- Original Message ----- 
From: "David C. Hallam" <dhallam at knology.net>
To: <johnson at mailman.qth.net>
Sent: Monday, January 06, 2014 8:17 AM
Subject: Re: [Johnson] Transformer "cooking"

> Rich Measures put out some information on how he dries out old power 
> transformers by applying AC with a variac.  It's probably in the 
> ham_amplifiers archives some where or maybe on his web site.
>
> David
> KW4DH
>
> On 1/5/2014 11:28 PM, Richard Knoppow wrote:
>>
>> ----- Original Message ----- From: "Brian Burns" 
>> <brian at lessonsinlutherie.com>
>> To: <johnson at mailman.qth.net>
>> Sent: Sunday, January 05, 2014 11:55 AM
>> Subject: [Johnson] Transformer "cooking"
>>
>>
>>> Hello Richard et al,
>>>
>>> I heard on the air a few months ago a suggestion for drying out a 
>>> modulation
>>> transformer, and was wondering if it is just an "Olde Ham's Tale". The 
>>> ham
>>> that was suggesting it shall remain call-less.
>>>
>>> The suggestion: Short the secondary, and apply a few metered volts to 
>>> the
>>> primary with a Variac.  Raise the voltage in small steps, with extended
>>> intervals of time for temperature stabilization. When the transformer 
>>> gets
>>> warm to the touch, let it sit for a long period of time (days?) to dry 
>>> it
>>> out thoroughly.
>>>
>>> Whaddaya think?
>>>
>>> Cheers,
>>
>>     Would scare me to do.  If there is suspicion that there is moisture 
>> in a transformer I would remove it and bake it out. The ideal is an 
>> electric oven with thermostat control but it can be done with a makeshift 
>> using lamps to supply heat. The heat should not be too high,  you don't 
>> want to melt out any insulation. Say around 150F for a few days.
>>     While pure water is not conductive even a small amount of any salt 
>> will make it quite conductive.  I suspect most transformer failures that 
>> are not from plain old overloading or arc overs are from old insulation 
>> crumbling. It would be interesting to hear from someone who has done a 
>> lot of re-winding on what he found.
>>
>>
>> -- 
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> dickburk at ix.netcom.com
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>
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