[Johnson] Transformer "cooking"

[Dennis Deaton]

On 01/06/2014 12:59 PM, Roy Morgan wrote:
> On Jan 6, 2014, at 3:14 PM, Carl <km1h at jeremy.mv.com> wrote:
>
>> I dont know if Measures used the method I described decades ago but there are methods that work.
>> ...
>> Short the secondary with an ampmeter and fuse in series. Place a Variac on the primary with another ampmeter and fuse. ...
>> Bring up the Variac very slowly while monitoring both meters. ...
>> It might take days or even weeks until it is running at even half its CCS KVA rating;
> I assume you mean the CURRENTS that would be seen if it were running normally under full load.
>
>> consider the electric bill vs saving money on a new transformer.
>>
>> The last one I recovered was ... a 275# monster ...
>>
>> It took a week of cooking out in the garage, at up to around 75% of calculated KVA, ... followed by a dunking in a vat of transformer varnish after a hi pot at 13.5KV which was the limit of my tester.
>   Again, I assume you mean the primary and/or secondarily CURRENTS that would be expected in normal operation.
>
> I’ll check my garage for a vat of transformer varnish. heheh
>
> I’m pretty sure I don’t have a variac the right size for a 275 pound transformer, but:  How about using a lower voltage transformer such as a monster filament transformer, to feed the patient being warmed?  I wonder what sort of voltage the primary of your monster took during the warming process.
>
> Thanks much for your post.  It gets added to my transformer notes file.
>
> Roy
>
> Roy Morgan
> RoyMorgan at alum.mit.edu
> K1LKY Since 1958 - Keep 'em Glowing!
>
>
>
>
> ______________________________________________________________
> Johnson mailing list
> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Johnson at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
Hi guys,

I've been following this thread for a while. As a retired engineer I 
cringe at the thought of applying any voltages to a wet transformer 
(especially one that has a high turns-ratio like a plate transformer) 
with even a variac. Somewhere in that stack of windings the voltage will 
exceed the insulating qualities of the wet paper insulation and cause an 
arc to the next set of windings or the core.

Probably the best way of handling this situation is to take the 
transformer to a motor / transformer rewinding shop and have them bake 
it in their curing oven. Most or all of the reputable shops have them 
and they will do this, probably, for a nominal fee. After that, you can 
have them dis-assemble the core from the windings and dip it in 
insulating varnish and re-bake it. I know that sounds like a lot. But 
you'll end up with a better-than-original transformer in the end.

Dennis Deaton, Redlands, CA
WA6ACC