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Vacuum pump for high voltage transformer potting
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xzswq21:
Hello
I want to make a few hundred Flyback transforms but no manufacturers replied me so I decided to make the transformers in our workshop. I will develop my small business soon. I could find many simple vacuum chambers in our country or Aliexpress, but most of vacuum pumps are Chinese and I'm skeptical to use Chinese pumps.
I think the vacuum pressure should be around 10^-2 miliBar for transformer potting process (I'm not sure).
Do you know any small size and reliable vacuum pumps and vacuum chambers?
10 Liters (2.6 Gallon) Vacuum Chamber is good for my application bcoz the transformer boxes are only 3x3x3 cm (1.2"x1.2"x1.2"), I will put several transformers in the vacuum chamber simultaneously .
Thanks
T3sl4co1l:
Any HVAC or roughing pump should do.  Over here anyways, they're available at some hardware stores, or equipment wholesalers.

I suppose the thing to watch out for with an Ali Special is, maybe not so much whether it works at all (but, if they send utter junk, that's certainly not beyond the realm of possibility), but if the seals, or fitment of the pump mechanism, or vacuum oil, are incorrect, it won't go to nearly as low pressure as desired.

A vacuum gauge might be a good idea, and, I don't think you need anything exotic to measure pressures in that range?

Tim
Kleinstein:
There is no need for a really good vacuum for the potting. The main point is to remove bubbles of trapped air. The liquid itself does provide some pressure inside anyway. 1 mbar corresponds to 10 mm of water column and the epoxy is not much different.  With a very low pressure some of the epoxy will evaporate - so one usually does not want too low a pressure to limit the evaporation (more gas slows down the speed the vapor is moved away).

So I would consider some 10-50 mbar sufficient. I would use a simple mechanical vacuum gauge. So not really high demand. The idea is that bubbles will expand some 100 times and will than rise to the surface. So the maximum bubble size will be reduced about 100 times in the process. Very small bubbles are additionally compressed by the surface tension and the gas may dissolve.
It can help to do the mixing of the potting compound in a way that does not produce many small bubbles to start with. Expanding and compressing the bubbles is a kind of extra stirring that helps the bubbles to move up.

There is still a chance there will be quite some contamination of the pump from evaporated potting compound. So a relatively cheap pump that allows for an easy oil change may be a good idea. A membrane pump may be sufficient too. For very frequent use one may have a special cold trap / absorption filter to catch much of the contamination.

It is some time ago I did vacuum treatment on epoxy glue. IFAIR it is not about applying the vacuum for a long time, but a few cycles of vacuum and normal pressure worked well. It helps of the chamber is relatively small, not much more than needed. I helps to have visual control. It also helps to have separate valves to the pump and adding air in. Normally with vacuum one want's rather thick hoses, but here a limited pump speed should be sufficient, so one has some control over the pressure and stop pumping if the pressure is OK.

I would consider to send the pump outlet side to the outside and not the room. One may still need some oil mist filtering.

I had used a plastic vessel made for drying stuff in vacuum - not really large, maybe some 5 L or so.
TimFox:
The traditional vacuum chamber for these purposes is a transparent bell jar sitting on a gasket above a solid plate that has the plumbing fixtures for connecting to the pump, up-to-air valve, and gauge (if needed).  This allows you to watch the bubbles from the liquid to monitor the process.  You won't be able to lift the bell jar until the interior has come back to atmospheric pressure, hence the need for the valves.
T3sl4co1l:
Also common is a sturdy bucket with rubber rim, and acrylic cover.  Valves may be in the bucket or cover.

Tim
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