[Boatanchors] Drying power transformer

[Ron]

Hi Brian.

What you say makes no sense to me.
If one were to design a transformer to perform inside a closed case 
where the ambient temperature could reach 90 Deg F on a really hot day, 
then what you are saying is that you leave 25 Deg F for a temperature 
rise due to eddy and I**2R loss and including the safety margin.
General stuff running in a closed case gets to 110 F . Stuff sitting in 
the sun will reach 130 F.

Wax melts at 200 Deg C and solidifies below that temperature without 
damage .  Asphalt (pitch)) melts at about 60 Deg C and is the same when 
it cools ....otherwise they wouldn't be able to inject it into the 
windings as a solid.
Polyurethane will not melt but will char and so it is coated on the wire 
itself before the it is wound onto the transformer bobbin.
I can take the writing paper on my desk and put it in a dryer at 100 deg 
C and it won't even char.

---
Ron


Brian Clarke wrote:

>Whoa there, Dobbin.
>
>Polyurethane, wax and pitch all start to fail irreversibly long before 100 C. So 
>does the paper used in the inter-winding layer of insulation. Wire often had 
>polyurethane coating in the later transformers, too. So, see previous 
>sentence. Irreversible damage starts about 45 C, say, 115 F for many 
>carbon-based protectives.
>
>Vacuum is a good idea because the boiling point of water and many other 
>volatiles drops with atmospheric pressure. So, to accelerate drying, you 
>don't need to stress things thermally quite so heavily.
>
>How do you know when the transformer is dry? If you are using a vacuum 
>dryer, put a couple of fine-pointed needles in the outlet stream that have 
>say 10 to 15 kV between them. When the crackling stops, the exit air is 
>dry. Or, use the Hi-Pot test. In Australia, we expect a frame to winding, 
>and winding to winding withstand of 4,500 V for telecomms applications.
>That may have changed with the new CISPR regulations coming in.
>
>73 de Brian, VK2GCE.
>
>  Ron reminisced:
>  Put it in a dryer and if you happen to have a vacuum chamber , use that 
>  as well.
>  If you can get a heated vacuum dessicator ...all the better.
>  Go to 100 C as the transformer should easily handle that.
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