Level of vacuum to purge transformer coils

[abstruse1]

After winding flyback secondaries, I immerse them in mineral oil and put that assembly into my vacuum chamber for several hours at ~28" Hg vacuum.

Does anyone have any idea about the relationship between vacuum level and flashover tendency? I don't want to spend more money on a vac pump if the gain would me small. About 29" seems to be the best my pump will do.

[rikkitikkitavi]

Increase temperature a bit to compensate for less vaccum. And TUV, Time Under Vacuum.
Give those small water molecules time to fly away...

Diffusion through oil will be much slower than the free molecule to be going away into the vacuum connection. So 28 or 29 " vacuum wont make a big difference, at bes 28/29  ~3 % .

By increasing time 3% you will have the same effect, and temperature about the same, but it is easy to to increase both with significantly more.

How much , I cant say. It is relatively speaking so to say from a certain setup.

Br
/Rickard

[petespaco]

Look up "Steam Tables" to see what partial pressure it takes to evaporate water, at various temperatures.
  We used to take certain potted assemblies weighing about 2 pounds down to about 25 microns (pressure)  for at least an hour or two.  Those circuits ran at 20,750 VDC,  24/7/365.
We had lots of mechanical (vacuum) roughing pumps around that backed up oil vapor diffusion pumps.  They all "blanked off" at that 25 micron point.

Here's one simple reference that I found:

"around 0.03atm
Water evaporates when the vapor pressure is greater than partial pressure of water in the atmosphere. At room temperature, the vapor pressure of water is around 0.03atm. So 0.1atm is insufficient to cause boiling.  Aug 22, 2018"
---
Anyway, you need well over 29 inches Hg to do the job at room temperatures.  A simple 0-30 in Hg bourdon tube vacuum gauge isn't going to be accurate enough for you to see the difference anyway.

!! DO check out the steam tables to see if you can heat your stuff hot enough to get the water to "boil" at a pressure that you CAN obtain.

Pete Stanaitis
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[Patrick]

I get to 400 micron with my simple setup. But my epoxy has volatiles in it so I temporarily paused on that project but I do have pictures.
Getting any amount of oil or epoxy in there is better than nothing, but even with non ideal conditions I found that common Amazon transformers could be reasonably potted.

[davekni]

I get to 400 micron with my simple setup. But my epoxy has volatiles in it so I temporarily paused on that project but I do have pictures.
Getting any amount of oil or epoxy in there is better than nothing, but even with non ideal conditions I found that common Amazon transformers could be reasonably potted.

Looks like you are getting reasonable results in spite of epoxy outgassing.  Thank you for the sectioning results.

I found the same issue with volatiles in urethane epoxy when potting small transformers.  Found that centrifuging worked better than vacuum.  Best results I had were with a short vacuum exposure to get some air out followed by centrifuging.  Simultaneous vacuum and centrifuging would likely be ideal, but more complex and expensive than I was willing to construct.

[klugesmith]

Pete talked about assemblies being left under 25 micron vacuum pressure (3 Pa, 0.03 mbar) for an hour or two.   

I bet they were dry (not under oil or any potting resin) until long after the chamber was evacuated.   
What's the point of pulling vacuum lower than a few torr (a few mbar) over oil or resin, when most of the liquid is under higher pressure because of its own weight?

Many years ago I got a gallon of transformer oil from a fellow 4hv forum reader.  He said that at work, they would evacuate HV transformer tanks and then admit oil into the vacuum through a spray nozzle near the top.   Degassing and drying the oil is not a thing you can do efficiently with a transformer already immersed.  And as oil progressively floods a fine-wire winding under vacuum, there is no trapped air needing to escape by tilting, vibration, and limited enlargement of bubbles.

[Patrick]

He said that at work, they would evacuate HV transformer tanks and then admit oil into the vacuum through a spray nozzle near the top.   Degassing and drying the oil is not a thing you can do efficiently with a transformer already immersed.  And as oil progressively floods a fine-wire winding under vacuum, there is no trapped air needing to escape by tilting, vibration, and limited enlargement of bubbles.

I meant to say this as well, I read the same in formal papers and tried this myself and found better results than "dipping and pulling" vacuum. "Raschig rings" were all mentioned as well,
"https://en.wikipedia.org/wiki/Raschig_ring"

And my YouTube Vid: