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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm going to begin vacuum filling epoxy my transformers with epoxy. It appears I can reach 5 pascals with my planned setup.
But before I buy it, is 5 Pa low enough ? can degassing epoxy be done in 45 minutes at this pressure. degassing would be nice but non essential in my current designs due to FEA showing a good field design.
The essential requirement is to draw water-thin epoxy into a coil already wound on a bobbin. The coil is ultra fine wire (0.005"), but there are gaps to all locations in the coil.
Registered Member #2989
Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94
Some ramblings for you You can get two part epoxy in the same bag so there is no air when mixing it. If the stuff is runny enough you could have it going up the way so less likely to trap air. ie put in a syringe and force it up into a mold. As for a vacuum I don't think it will make a huge difference, 5Pa 25Pa etc it's going to be almost the same, only thing if your epoxy has a lot of air in careful it doesn't foam up fast and coat every thing. You can vacuum a bit then let it sink back etc so it doesn't over flow while de-gassing. As some epoxies cure slower at different temperatures maybe you could get more time to de-gas by changing the temperature. As a side note I have seen coils wound that run the wire through a bath of varnish before going on to the core, mind you I can't imagine working with very fine wire.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Blackcurrant wrote ...
As a side note I have seen coils wound that run the wire through a bath of varnish before going on to the core, mind you I can't imagine working with very fine wire.
Ive done it. Here:
But this "wet-winding" decreases packing efficiency and causes bunching at certain regions.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
You don't need much vacuum at all, I took a job in a coil winding factory once, about 15 years ago, one winter, for a couple of months. Ended up redesigning their vacuum impreg system.
It takes a matter of minutes. You just wait until the bubbles stop coming out.
You can get good results using a glass bell jar, as you can see when it stops bubbling, but what I did was put a thick piece of plexiglass (can't remember what type)over a round cutout in a steel chamber.
You can just use rubber gaskets to seal it.
The maximum pressure you can reach with vacuum equipment is less than 15 PSI.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
5 Pa should be fine, anything < 1 torr should be plenty for getting air out. It depends on the epoxy how long it will take, but dont expect it to stop bubbling at your minimum pressure. Generally Ive gone down to ~0.5-0.2 torr, let it sit for 5 minutes, back off to 1 torr, and if the epoxy stops bubbling after backing off on the pressure, its degassed enough.
Repeat for vacuum potting the transformers.
Edit:
Another thing to remember is the weight of the fluid. Assuming your epoxy is about as dense as water, 100mm of epoxy will have a pressure of 7 torr at the bottom of the cup, so no point in multiple orders of magnitudes below that.
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Vacuum drawn epoxy
