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Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
as you said, the vacuum is there to remove air entrapment. Once the blubbling stops, you can vent the box. Just make sure the windings are not drained by rotating the whole thing.
Usually, any roughing pump that reaches 10^0 mbar will do. Water jet pumps however (as they reach 15-20mbar depending on the medium temperature) will lead to doubtable results.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
hboy007 wrote ...
as you said, the vacuum is there to remove air entrapment. Once the blubbling stops, you can vent the box. Just make sure the windings are not drained by rotating the whole thing.
Usually, any roughing pump that reaches 10^0 mbar will do. Water jet pumps however (as they reach 15-20mbar depending on the medium temperature) will lead to doubtable results.
thank you, how about a hand held automotive pump?
should i drill a hole and insert a nipple or somthing to connect to the vacuum. and after just plug it ?
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
any leakage will make the hand held pump unusable.
Can you install a foil collar around the transformer? Depending on the size of your box, you could fill it up completely with oil, then pump out the dead volume and let the displacement work for you.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
hboy007 wrote ...
Can you install a foil collar around the transformer? Depending on the size of your box, you could fill it up completely with oil, then pump out the dead volume and let the displacement work for you.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The object is basically to pump out the air. Keep pumping until all bubbling has stopped, then, when you release the vacuum, the oil level drops as the oil fills all the voids where the bubbles were.
You will be surprised by how much the level drops, so ensure you have a good 'head'.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
You can do a better job by evacuating the box dry, holding it that way for a while, then letting it suck in previously dried and degassed oil. Like the pros do. Of course that needs more equiment, time, and trouble. Maybe not necessary if voltage stresses are modest and the project doesn't need to work for decades.
1) to better extract moisture from inter-layer insulation, magnet wire insulation area, etc. which starts in equilibrium with the room air humidity, and would otherwise contaminate the oil.
2) When pumping on an oil-filled pot, the pressure at the bottom can be no less than the hydrostatic pressure of the oil. Say, 9 mbar (7 torr) under 100 mm of oil, for a bubble radial expansion factor of 5. Why waste your pump capacity? A 2-stage mechanical pump should get you to 1 mbar (10x expansion) in seconds. If you're patient, maybe down below 50 microns (1/16 mbar; 25x expansion -- if there were bubbles).
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Vacuum pulling/sealing
