DIY vacuum-pump? (for vacuum impregnating)

Not sure what "PU in water" means but if it needs 120..140 deg C cure it sounds like the right stuff.
Jim
Reply to
pentagrid
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PU is PolyUrethane and water is DHMO. :-) It is polyester (and not PU as I wrote) suspended/emulsified in water. Can be thinned with tap water. Like those Aqua-paints. Solvent free. That stuff doesn't get try (or verrrry slowly) without heat.
It seems to be "Dolphon BC365" sold under a different name. Dolph's has hits with "Aquaterm", but maybe that comes from their Italian sister.
Nick
Reply to
Nick Mueller
snip
The usual trouble is that the soldered joint between the winding wire and the thicker lead out wire forms a bump which disturbs the smooth surface. This scheme eliminates both the joint and the thicker wire
Pull out the first foot or so of the thin wire off the reel and fold it back and forth on itself several times so that the wire from the reel now has a multi stranded tail - typically 7 or more strands.
Twist most of this fairly tightly so that the single wire that continues back to the reel is mechanically supported. Do not twist the last bit just before it continues as a single wire.
Now when you start the first layer of the HV wind this last bit can be teased out flat and taped down so that it projects no higher than the remainder of the layer.
For uniform field, 30Kv/cm is the generally accepted gradient. 20mm balls are adequate for up to 30Kv, proportionately less for lower voltages.
For air close to normal temperature and pressure, breakdown voltage is proportional to pressure and inversely proportional to absolute temperature.
Needle gaps or sharp corners break down at much lower voltages.
Jim
Reply to
pentagrid
I've never used this stuff but anything containing with water is normally unsuitable as an impregnant.With oil filled transformers very great care is taken to see that the oil is really dry and free from even the tiny amount that it can naturally absorb! Water contamination both reduces the dielectric breakdown strength and can cause long term corrosion of fine wires.
Your varnish is clearly engineered for impregnation purposes but I'm not too sure about its long term compatibility with very fine wires. Everything depends on getting rid of the residual water. This has to exit as vapour from the centre of your windings so you need a very good long high temperature vacuum bake.
For low temperature experimental work wax is pretty convenient.
Jim



Reply to
pentagrid
Me too! :-)
It is sold as impregnating varnish. The dielectric strength dry/whet is 1900/1850 Volts/mil. Funny, quite the same strength with water in it. But I realized that this varnish simply might be the wrong type and I need something different. :-( They sell different stuff for VPI. The one I have is not listed for VPI. That wasn't clear when I ordered it. Anybody needs 0.75l of varnish. :-)
OK, I'll look for something different, but I'll give it two more tries with the better vacuum. Maybe the water is boiling out and I can re-apply pressure with the coil completely submerged to force it back into.
:-( I didn't intend to heat the vacuum chamber.
Nick
Reply to
Nick Mueller
Learned a new trick! Just tried that (modified) and it works great. Now I don't have to fear to break the tiny wire when putting the coil into the housing and connecting the thick and stiff HV-cable to it.
Thanks!
OK, will build that one too ...
Still takes me 1 1/2 hour for a coil. I even missed The Simpsons! Need a food-pedal for the winder motor.
Thanks again! Nick
Reply to
Nick Mueller

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