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Vacuum pump for high voltage transformer potting
TimFox:
On your step 5/: was "1 mm Hg" the absolute pressure or the gauge pressure? I assume it was the absolute pressure, where normal atmosphere is 760 mm Hg, but vacuum measurements are tricky.
(Back in 1975, we had an ionization gauge that could read "10-10 mm Hg", which was amusing since an Hg atom is much larger than that.)
xzswq21:
--- Quote from: jonpaul on August 12, 2021, 04:25:00 pm ---Bonjour a tous: Just now reading the OP post by xzswq21
in 1980s and 1990 we made many potted HVPS with transformers for 12 KV aviation PSU. ~ 10,000 pcs were made over the years.
1/ The potting compound is critical esp for fill and cure chemistry as well as creation of internal mechanical stress during cure.
2/ Generally a two part heat cure epoxy/silica/filler is used.
3/ The device used is a vacuum oven, which are available from industrial equipment suppliers in all sizes. Check epay for used ones.
4/ Homebuilt vacuum is possible but somewhat risky from safety and yield. Consult a few vacuum technique textbooks, many are available.
5/ We pulled ~ 1mm Hg and kept the pumps running for the entire cure cycle, many hours or overnight.
6/ After cure, an annealing cycle is used to relieve internal stresses.
7/ Stress can crack the ferrite and damage insulation and other parts. Poor Yield kills profit and results in useless bricked units. We had 50% initially and 95% after a year of process and compund improvement.
8/ Stress is proportional to potting shell dimensions, our parts were ~ 1x1x3". Larger shells may have worse stress.
I hope this info is useful to the OP and others.
Just the ramblings of an old retired EE.....
Bon Chance!
Jon
--- End quote ---
I found an epoxy with the below specification:
Dielectric coefficient= 4.6
Viscosity= 1080 Centipoise
Density= 1.47 gr/cm3
Pressure resistance= 742 kgf/cm2
Volume resistivity= 5e14 Ohm.cm3
Surface resistivity= 9.8e13 Ohm
Heat Deflection Temperature= 50 'C
Practical thermal resistance= 80'C
Gel Time= 40 min
Curing time= 50 min
minimum curing temperature : 10'C
maximum curing temperature : 40'C
according to the curing temperature I think I don't need to have a vacuum oven.
Do you see any weakness in the epoxy I found? a local producer offered me the epoxy! (I'm concerned about the Heat deflection Temperature!) could you please offer me a suitable epoxy?
jonpaul:
Rebonjour,
To TimFox: This was decades ago, I am sure this was with the guage that comes with common vacuum pumps (piston/rotary) and I am not certain of the 1 mm number.
It was not a "high vacuum". Due to epoxy outgassing during cure the pump must handle corrosive vapors.
To the OP xzswq21:
Impossible to evaluate the epoxy suitability from the specs you mention.
If I had photos and specs of the potting shell, transformer to be potted and exact epoxy mfg/number I can be more helpful.
Without a manufacturer and Epoxy number/spec sheet I have no idea. I can say that a 10 deg C cure means room temp, generally not suitable.
In 1980s..1990s, bought the epoxy and insulation from disty like EIS Electrical Insulation Specialty.
The potting compound was a heat cure 2 component epoxy with 100% solids, perhaps silica or quartz.
It took a YEAR of trial error and R&D to develop a reliable system.
The exact mix was proprietary.
If you have a serious business requirement, I recommend to contact an experienced HV transformer/PSU manufacturer.
Finally notice that many DIY, vintage equipment aficionados used beeswax or tar potting.
Bon Chance
Jon
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