Using a vacuum to improve varnish impregnation of a coil 4

[DAlbertson]

I need to improve the varnish impregnation of a coil wound onto a bobbin (Ticona Vectra C130) with 30 layers of AWG 40 single-build insulation, around a solid core 1.5 inches long and 0.4 inches in diameter.

I'd like to have the varnishing process be scalable to a desktop-sized tank, processing between 100 and 200 coils at a time.

Ideally, the process would run at room temperature to keep the partial pressure of the varnish's solvent under control, but if it's absolutely necessary we can heat the tank and extract fumes.

A wire salesman mentioned an intriguing possibility: he said he had heard of vacuums being applied to aid in the impregnation. I have done this with composite parts in the past, drawing the epoxy resin into the carbon fiber fabric by surrounding the mold with a polymer bag and drawing the inside of the bag down to a near vacuum. A tube connects the supply of epoxy resin to the inside of the bag, and so the resin is transferred into the bag and fills the interstitial spaces in the carbon fabric very efficiently and completely. I believe woodworkers use a similar technique to laminate veneers onto base structures: the vacuum bag acts as a large-surface uniform-pressure vise.

Unfortunately, I don't see how to make this idea work without a tiny vacuum bag for each coil...impractical?

I'm probably not looking at the problem correctly. Can someone help me out by describing how this works? Also, if there are other technologies I should be looking at in addition to the vacuum I would be very grateful if you would point them out.

Thank you very much!
David

 

[electricpete]

For large rotating medium voltage machines, vacuum / pressure impregnation is a common approach.

To my understanding the process is something like the following:
1 - Heat the stator
2 - Draw a vacuum on the stator (draw air out)
3 - Allow the vaccum to draw resin into the compartment housing the stator, flooded to a level above the stator.
4 - Pressurize the space above the resin. Hold for up to 8 hours.
5 - When process is done (stator is cool, perhaps monitor insulation parameters to see when stabilized), drain and remove and put into preheated oven for about 10 hours to cure.
6 - If desired, repeat the cycle (2nd cycle doesn't do as much but provides some additional assurance).

Obviously you are not planning on repeating this complicated of a cycle. Just wanted to mention it... you can decide which parts might apply for you.

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[electricpete]

Steps 2,3,4 are done in a "VPI tank". (VPI = Vacuum/Pressure Impregnation)

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[DAlbertson]

ElectricPete,

Thanks very much for the process description!

I'm interested in a much smaller coil than you describe, about the size of the end joint of your little finger, but I don't see that the smaller size would greatly affect the overall idea.

I've got a couple more questions, if that's OK?

1. Given that we're discussing varnishing, and not potting, the varnishes are typically heat cured by evaporation of a solvent, correct?

2. When you select a varnish, do you need to be careful that the varnish's solvent won't disolve the existing insulation on the magnet wire, or is this pretty much a given?

3. The mass of these coils is so small that I was thinking of attaching their jigs to vibration motors, the idea being to aid in removing small air bubbles by vibrating the coil at 60 Hz. Sounds reasonable, or a waste of time?

4. Once the varnish impregnation process is complete, but before the coils are moved to the oven, the varnish is still quite liquid, right? We don't need to make provision to contain the varnish onto the coil, right? Just let the excess drip off for a few minutes, and then pop them in the oven?

I'm sorry for so many idiot questions! I'm trying to get a grip on how this might work, and foolish questions seem to follow me around!

Thanks very much for your help and patience!

Dave

 

[TStaples]

DAlbertson,

Electricpete is right on the money. VPI is the way to go. Adding vibration should not be necessary, as most of the potting resins (all that I have ever used anyway) are very low viscosity to facilitate penetration (wicking) into coil or stator windings. I've had good success with Dolph's resins.

I have used the vibration addition with heated encapsulation of polybutidiene resins, with no vacuum, and it was absolutely necessary to eliminate voids. Of course, these resins are typically much higher in viscosity than typical varnishes, and the path of resin entry/trapped air exit was limited and somewhat contorted.


-Tony Staples

http://www.tscombustion.com

 

[EdStainless]

Usually an electrical current is used to warm the windings and 'jell' the varnish. This will control how much coating you get. Yes, drain back time is needed, but it can be done in a second tank.
Vibration probably won't be needed. Drawing a vacuum first and then filling with resin should give you nice fill.
I have done this with both conventional solvent varnish and 100% solids systems.
You need to make sure that your varnish stays consistent by adding fresh at regular intervals and testing.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[sreid]

1)Modern varnishes cure by cross linking at elevated temperatures.

2)Usually not. The magnet wire varnish is not disolved by the encapsulation varnish.

3)Vibration can help with Thixotropix varnishes. Typically done only for MIL Spec work.

4)Correct.

A link to Dolph"s

http://www.dolphs.com/

Something else you might consider. You can buy magnet wire that has an outer bondable coating. This coating can be used two ways.

1)The wire is run through acetone while winding. This softens the bondable coating which then sticks together when the acetone evaporates.

2) The coils can be heated after winding which melts the coating. This can be done in an oven or by passing a high current through the coil (the current and time are found experimentally).

 

[DAlbertson]

Terrific; thank you all very much!

1. I've looked at the Dolph's link, and will call them tomorrow for more information.

2. I'm checking ThomasNet for VPI equipment manufacturers, and should have those narrowed down pretty quickly because I believe our application is small relative to some of the large machines I've seen so far.

3. I will try again on the bondable coating on the magnet wire, but I have already tried this and had no luck because of the small gauge we're winding: AWG 40. It seems that most manufacturers stop making the coating available at AWG 36 or 37.

These coils are so small; has anyone ever tried to keep a tiny flow of varnish running over the wire as it's wrapped? You'd need some sort of case to surround the bobbin as it's being wound so the varnish didn't get slung all over everything. A closed system, with a tiny spray jet inlet and a drain tube at the bottom of the case. It's just a thought; has anyone ever done this?

Thank you again for all the great information!

Dave

 

[Clyde38]

MWS has bondable magnet wire to 50 AWG....

http://www.mwswire.com/pdf_files/mws_tech_book/bondable_magnet_wire.pdf

 

[sreid]

You could check with MWS wire to see if they can make you bondable 40 ga.

http://www.mwswire.com/?gclid=CNaCi92pzY4CFQcdYwod8HnMBA