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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
so when pulling a vacuum for oil filling purposes, do I pull the vacuum while the transformer is under oil? then let the oil follow the leaving air? then hold it for a few minutes for good measure?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
do I pull the vacuum while the transformer is under oil?
Yes, wait until the bubbles stop, then let it up to ambient pressure, and the oil will fill the voids. You'll be surprised just how much oil will be sucked in, you need quite a bit.
EDIT: Some systems pull the vacuum first, and then cover the xformer in oil, and then let it up to ambient. These systems are supposed to be better.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Ash Small wrote ... EDIT: Some systems pull the vacuum first, and then cover the xformer in oil, and then let it up to ambient. These systems are supposed to be better.
Yup, for example high voltage utility transformers and x-ray transformers. The rectangular steel tanks of pad-mounted utility transformers are designed to withstand full vacuum inside.
Suppose we want to hold the vacuum for a while at a modest 0.2 torr, to extract not only air but absorbed moisture in insulation materials. Any bubbles would expand by a factor of 15 in diameter and 3800 in buoyancy. Well, with an absolute pressure of zero (perfect vacuum) over the oil, we would hit 0.2 torr at a depth of 3 mm below the oil surface. Under 6 inches of oil the pressure can never be less than 10 torr (1/76 atmosphere), at least in Earth gravity.
Carl Pugh told me that where he worked, they would admit dielectric oil to an evacuated tank through a fine spray nozzle at the top. A handy opportunity for final drying and degassing of the oil. I figure that since it's already in a vacuum, the oil mist would promptly settle rather than be sucked into vacuum pump.
Ash: Thank you for the report of a successful commercial process where the oil goes in first. I think there are many applications where that simpler approach is good enough.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
klugesmith wrote ...
Ash: Thank you for the report of a successful commercial process where the oil goes in first. I think there are many applications where that simpler approach is good enough.
Actually the process I described used a 'varnish' (possibly polyurethane?) which was heated to ~100 C?, then the Xformers were vacuum impregnated, then the Xformers were baked @ 150-200 C? for eight hours, possibly to soften the varnish so it became less viscous, I don't know?
It was that varnish that never seems to fully cure.
I'd certainly boil the oil and put it under vacuum first.
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Vacuum Pumps - I need an education
