Vacuum pump for high voltage transformer potting - Page 1
Forum section: electronics / general

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 11, 2021, 07:05:24 pm
content: HelloI 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 07:23:23 pm
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2021, 07:46:23 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: August 11, 2021, 08:11:11 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 08:20:03 pm
content: Also common is a sturdy bucket with rubber rim, and acrylic cover. Valves may be in the bucket or cover.Tim

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2021, 09:06:12 pm
content: You don't need (or want) a really good vacuum for potting, all you're trying to do is coax the air bubbles out and too good of a vacuum will just cause the resin to outgas like crazy. Even one of those handheld vacuum pumps for bleeding brake systems and testing vacuum actuators in cars will do the job, but a diaphragm type vacuum pump or cheap single state HVAC service pump will be less work. Compressors salvaged from old refrigerators and AC units also make reasonably good vacuum pumps.Be careful about pulling a vacuum in various vessels, there is actually very little difference in force on the vessel walls between a lousy single stage mechanical pump and the deep vacuum that can be achieved with a diffusion or turbomolecular pump. Atmospheric pressure is almost 15 pounds per square inch of surface area and that adds up quickly. Vacuum chambers can fail violently if they are not up to the task.

author: langwadt
author_rank: Super Contributor
timestamp: August 11, 2021, 09:59:56 pm
content: will a shopvac do? mind the cooling if you do it for long

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 10:55:14 pm
content: Nah, that's a few inches of water. A few cascaded can do a good job for vacuum clamping:Or vacuum forming or the like (which doesn't really need much and one is enough).If you have compressed air, a venturi pump is alright, but won't give a great expansion ratio so you need to do it a lot more, and it still won't be as thorough. Also takes a shitton of air, probably not something you'll get by with a small some-CFM compressor running flat out(?).Tim

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2021, 10:55:24 pm
content: Maybe, but that's probably going to be marginal at best. A shopvac is optimized for high volume, the actual static vacuum you can pull with one is not very good at all.

author: langwadt
author_rank: Super Contributor
timestamp: August 11, 2021, 11:08:00 pm
content: Quote from: T3sl4co1l on August 11, 2021, 10:55:14 pmif you have plenty of water (or a water pump) a water aspirator?

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2021, 11:30:32 pm
content: Quote from: xzswq21 on August 11, 2021, 07:05:24 pmSorry but I have no idea what country you are even in.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 12, 2021, 05:21:24 am
content: Quote from: langwadt on August 11, 2021, 11:08:00 pmThe only thing venturi type vacuum pumps are good for is applications where they're likely to suck contaminants that would damage most pumps. It's not at all difficult to find a mechanical pump that can pull lower.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 12, 2021, 06:53:10 am
content: What do you think about the Leybold TRIVAC series (but used not a new)? For example Trivac D 2.5 E? Thanks

author: geggi1
author_rank: Frequent Contributor
timestamp: August 12, 2021, 12:46:50 pm
content: Check on youtube.You will find several videos on how to make a vacuum chamber with a fridge compressor and a pressure cooker.Mos of the time these builds are used for potting and is probably sufficient for your use.

author: jonpaul
author_rank: Super Contributor
timestamp: August 12, 2021, 04:25:00 pm
content: Bonjour a tous: Just now reading the OP post by xzswq21in 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

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2021, 05:08:16 pm
content: 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 "10-10mm Hg", which was amusing since an Hg atom is much larger than that.)

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 12, 2021, 05:56:21 pm
content: Quote from: jonpaul on August 12, 2021, 04:25:00 pmI found an epoxy with the below specification:Dielectric coefficient= 4.6Viscosity= 1080 CentipoiseDensity= 1.47 gr/cm3Pressure resistance= 742 kgf/cm2Volume resistivity= 5e14 Ohm.cm3Surface resistivity= 9.8e13 OhmHeat Deflection Temperature= 50 'CPractical thermal resistance= 80'CGel Time= 40 minCuring time= 50 minminimum curing temperature : 10'Cmaximum curing temperature : 40'Caccording 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?

author: jonpaul
author_rank: Super Contributor
timestamp: August 13, 2021, 10:43:53 am
content: 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 ChanceJon