Thanks Sandy - The neat part of what Nick's doing is that he is home-brewing
the components! Also if you price the Hemingway parts, I think it works
out at around £60/$120US so that's quite a budget to build your own and
learn something on the way.
So I didn't save a penny. :-))
wire, varnish, cores, isolation material: 100.- EUR
geared motor (for the coil winder): 25.- EUR
casting compound (PU; 1kg, will last) 27.- EUR
rod of POM (2m long, will last some time): 18.- EUR
HV-probe for the scope: 40.- EUR
Two coils, more to come, new wire to buy, ...
But I don't think I'm stupid. I'm only nuts!
One rule-of-thumb way to think about it is to assume that the air
remaining will compress to atmospheric pressure and leave holes (voids)
in the filling material. Thus -0.85 bar will leave 15% voids.
You need about 5 mbar (0.5% voids) or so to do vacuum impregnation
professionally, though reasonable results can be had at pressures up to
about 50 mbar if a layered approach is used (impregnate only one or two
layers at a time).
The "paint" normally bubbles in the vacuum, as dissolved gas is given
off. This also allows any remaining air in voids to dissolve in the
"paint" once the external pressure is raised, so leaving no voids at all
(in theory ...).
Two fridge pumps used in series as a vacuum pump might give somewhere
around 10 mbar, or even less, depending.
Probably available for free, but use an oil recuperator (the pump outlet
goes straight up into the bottom of a container with holes in the top
and perhaps filled with some - ???what do you call those things people
sometimes use to clean dishes, they are made of plastic or metal ribbons
about 1/10th inch wide and look like doughnuts without holes??? - when
the motor is switched off the oil which has collected in the container
drains back down into the pump).
When we did VPI on coils at GEC Machines, the P bit was significant as well...
Vacuum Pressure Impregnation.
Vacuum to get rid of most of the air and 4-5bars to force the resin into the
coils. Just atmospheric pressure can leave significant voids that will lead to
partial discharge erosion of the insulation on HT windings. This applies to
Nick's first impregnation phase.
So basically you are using the elevated presure to crush the bubbles? Why
4-5 bar, that is only 72 psi, most home air compressors can do 110psi / 7.5
bar. Do those little crushed bubbles end up trying to expand again at
normal presures and elevated temperature?
Those are just the pressures that were used! a lot of the reason for the
elevated pressure isn't to crush the bubbles, it's to force the resin into the
voids that exist in the winding. If you have a multi-layer coil, possibly with
an outer insulation and some inter-winding insulation, you have to overcome
the viscosity of the resin.
What I have read (not (yet) my experience; need a modified container) is:
The vacuum container has an in feed for the varnish (I learned that word)
from the bottom through a valve. Start to vacuum the container and open the
valve, so the varnish gets sucked into the container. Now the coil is
standing in varnish, but just for a few mm. As vacuum increases, the
varnish gets sucked upwards in between the wires. Keep the level of varnish
low in the beginning. If the vacuum is complete, the level has to be
*below* the top of the coil. As soon as the level gets above the coil,
bubbles are trapped. "Bumping the vacuum" worsens things in this case. I
*think* it helps to have an air tight outer insulation layer (as I do
But this requires a better coil body as mine is for now. More holes (or the
usual two slits) on the end disks to ease the varnish coming in.
Also heating up the coil to 50 .. 100Â°C (but below curing temperature) helps
by decreasing the viscosity of the varnish.
But this is just my understanding and model how things work. I might be
Good numbers thanks. Can't impregnate layer by layer or I'll get mad.
I found an article describing vacuum impregnation. So I'll have to change my
setup and feed the "paint" (what's the right word? Mine is PU) from the
bottom through a valve.
I'll try a similar setup. Have a fridge pump (airbrush compressor) and will
connect the injector to its back-end and see what I get.
Also thanks to the other repliers!
Tried something similar:
airbrush-compressor (AKA fridge pump) and my injector on the outlet-side:
Only a minor decrease in pressure (-0.9 bar; without injector: -0.88 bar or
so). Even if I do have a leak on the fridge-pump's outlet-side, I would
expect more, because the injector is very good at making volume at
about -0.5 bar.
Did I make a stupid error, or does that only work with two fridge-pumps in
series? My fridge (one I lent to someone) just let escape the magic smoke,
so maybe I have a second one.
I wonder if you are suffering from the fact that the resin is producing
condensable vapours. If it is, you may need to introduce a gas ballast
(controlled leak) part way into the cycle. This may be hard to arrange without
building a pump from scratch. Another project :-)
With this test, nothing except the manometer was connected.
But an idea I had yesterday was, that some condensed water inside the pump
No, can't be. Boiling point / temperature (ca.)
200 mbar / 50Â°C
20 mbar / 20Â°C
But you are right, the vanish might boil at the (wanted) pressure.
Difficult to be sure but it sounds to me like leaks.
'Fridge pumps can pull a pretty good vacuum but their
displacement is only a tiny fraction of the scraping vane type
normally used as backing pumps. Even the smallest leak can be a
Physically locating the leak is often near impossible
and it's simpler to coat the whole of the suspect areas with a
low vapour pressure grease or wax. Heat the whole of the suspect
area before coating it. If the pressure goes up you've got
trapped high vapour pressure crud which needs fixing first.
If the "varnish" you is a true solventless varnish
engineered for impregnation it will have a low vapour pressure
and is an alternative leak sealant.
Procedures in Experimental Physics. John Strong.
ISBN 0-917914-56-2 is a mine of information on vacuum techniques
and has lots of other interesting stuff in it.
I'm getting a decent vacuum pump in the next days, so I'll see.
Thanks a lot to the one! (Don't know whether he wants to be named here).
Will make a new vacuum chamber now. With an inlet valve for the varnish at
the buttom, so I can suck the varnish in with already having vacuum in the
chamber. Also, I will be able to de-gas the varnish in advance.
The one I have is a PU in water that is cured at 120..140Â°C. I'll see what
it does at a very low pressure. It is called "Aquatherm".
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