DIY vacuum-pump? (for vacuum impregnating)

wrote:


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.
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Steve W wrote:

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
Outcome: Two coils, more to come, new wire to buy, ...
But I don't think I'm stupid. I'm only nuts!
Nick
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If you want to recover your costs, I think that Hemingways no longer have a supply. You could be the supplier of coils to all of Europe
(And sell oil pumps to Myfords :-)
Mark Rand RTFM
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Mark Rand wrote:

I already see me accepting 99% returns. "The primary is burnt". I now have 0.08 Ohm on the primary winding, next one will be 0.06 ohms. Doesn't run on a simple breaker.

Nick
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Nick Mueller wrote:

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).
Peter Fairbrother
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wrote:

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.
Mark Rand RTFM
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Mark Rand wrote:

OK. That might be the "bumping" I found in an description. But they didn't give any numbers.
And I just wanted to build a new engine ...
Nick
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Or was "bumping" the description of shaking/knocking the part to loosen/remove any bubbles that were sticking to it?
Mark Rand RTFM
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Mark Rand wrote:

That was bumping the vacuum. But on my first coil (layers not well wound) that moved the wires around under the final insulating layer. Some lessons to learn, but getting better ...
Nick
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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?
Wes
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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.
Mark Rand RTFM
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Mark Rand wrote:

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 have). 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 .. 100°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 wrong.
Nick
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Peter Fairbrother wrote:

That was my model too.

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!
Nick
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Old fashioned cam operated fuel pumps were very good for providing pressure and vacuum - if you can still find any!
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Nick, the English term would be varnish rather than paint. Martin
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>> Two fridge pumps used in series as a vacuum pump might give somewhere
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Peter Fairbrother wrote:

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.
Strange!
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.
Nick
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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 :-)
Mark Rand RTFM
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Mark Rand wrote:

With this test, nothing except the manometer was connected. But an idea I had yesterday was, that some condensed water inside the pump evaporated? No, can't be. Boiling point / temperature (ca.) 200 mbar / 50°C 20 mbar / 20°C
But you are right, the vanish might boil at the (wanted) pressure.

You didn't see my welds. :-))
Nick
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On Tue, 18 Sep 2007 13:23:09 +0200, Nick Mueller

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 major problem.
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.
Jim
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snipped-for-privacy@yahoo.com wrote:

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..140°C. I'll see what it does at a very low pressure. It is called "Aquatherm".
Nick
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