Has anybody got plans for a vacuum pump? Or at least a clue what pressure I need for vacuum impregnation coils?
My injector pump is only making -0,85 bar and I think this is not enough.
*) I'm winding small ignition coils for a new 4 stroke. One part is impregnating the coil in vacuum (8 kV!) with some special paint. The other one is casting the coil into a container. I did that with 2K silicone. When I injected the silicone from the buttom (buttom up) the cap popped off and spread the silicone all around. :-)
Have you any articles on the coil construction? I'm building a four stroke but abandonded spark ignition because I couldn't find any home brew in this space.
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).
Edwards are the classic vacuum pump manufacturer and turn up fairly frequently on Ebay. However they're over kill for a one off impregnation cycle.
For occasional use, provided you've got a zero leak setup, an old sealed unit refrigerator compressor run as a vacuum pump is good enough. It can also be further improved by using your injector pump as a backing pump on the refrigerator outlet.
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.
I have some sort of vacuum pump driven by a single phase motor that is not used. I haven't studied the thread properly as its too early. Will photograph it today. Bob
No articles. It's a bit complicated. The coil I have built (version 2) is quite small. OD
23mm, length 40mm. Sizes for just the coil. Some pictures:
The test setup with my first coil (OD 30mm):
The V2 coil compared to the V1 together with the housing:
And the finished coil:
Neat, isn't it? :-))
Problem is, without electronics, it will not work. The coil is charged with
20 A for 0,5 ms. It can't be loaded for a longer time, because of its low resistance of the primary winding. This coil has a energy of 50mJ, in cars you find 120mJ. To make the coil even smaller, I'll have to find out what the minimum energy in the spark is. Maximum voltage is about 8kV.
Electronics is more ore less a IGBT and a suppressor diode to limit the primary induction voltage. I had 400V induction voltage but now will switch to 500V. A IGBT is fast enough, no problem. Initially I thought it won't work. Can't scope the high side of the coil. No 10kV HV probe here and to expensive. Just bought a 2.5kV probe for the primary side.
The processor is a Propeller from Parallax. Software now is ***very*** primitive. This is next to be done, beside making better coils. Anyhow, it's running on a single 87ccm 4 stroke.
Here is the data for the coil: Core is a Micrometals P2448-240. primary winding is 100 windings of 0.95mm, secondary is 1500 of 0.2mm. The later being double isolated (IIRC 3.5kV). and isolation layer (Polyester sticky tape, 0.06mm, 4.5kV) between each layer of windings. I tried TEX-E wire for the 0.2mm wire but it makes you poor, is thicker and doesn't make things better. The isolation now should be on the save side, so the coil can be used "open" (no spark plug connected). If someone is verifying my data and finds some errors. Yes Amp-windings is high! Design isn't finished, because electronics and coil have to match and its a bit of try and error. Tweak this parameter, tune there ...
I'm expecting to get about 50000 sparks per minute. But I'll see...
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.
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 ...
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