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What about polystyrene sheet for transformer layer insulation?

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jpsmith123

Thu Nov 12 2009, 09:40PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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It seems that "high impact" polystyrene sheet material (in 0.020" thickness) is available fairly inexpensively; e.g., this place:

Although its dielectric strength is lower than PE or PP, its dissipation factor is very low, and it's not "slippery".

I'm wondering, has anyone used it in a transformer?

LutzH

Fri Nov 13 2009, 02:56AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Hello:

I am not sure if this is a consideration in your design, but I gave up on this material for bobbin application due to low melting point, and chemical solvent compatability figuring that it may have a problem in oil due to any aromatic hydrocarbon traces in the oil. It is the main ingredient in the old Q-dope anti-corona varnish, so it will handle HV well though.

One plastic which is being used more and more for very HV application is Polycarbonate, or Lexan. Lawrence Livermore, and Los Alamos labs are using it in HV pulse equipment. Its insulating value is phenominal better than Teflon in many applications, plus it is strong as hell, and not slippery. I am curious if folks in the forum have played with this material? Thin enough sheets may work, also for bobbins also maybe?

rp181

Fri Nov 13 2009, 03:20AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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where did you get that? lexan is very common is structural systems, sure it wasn't for that?
Bobbins from lexan would be strong, and cheaper than teflon bobbins.

jpsmith123

Fri Nov 13 2009, 03:51AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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I don't think the melting point of polystyrene is a problem (IIRC 240 degrees C or so)...maybe you're thinking of the "glass transition temperature" of about 100 degrees C?

Hopefully it will not get to 100 degrees C.

In any case, I'm working on making some CPVC bobbins (the single layer PE coil former effort is on hold right now), and the CPVC is only good for about 100 degrees C as well. (Moreover, the thermal expansion coefficients of CPVC and PS differ by less than 10%).

I'm actually making two sets of bobbins - one set will be wound with 30 gauge magnet wire and epoxy encapsulated, and the other set will be wound with 26 gauge teflon wire and not encapsulated; both will be conventional multiple-layer windings.

Unfortunately it's taking me a long time; in part because I'm new to using a lathe, and in part because I have to stop and order parts to make a vacuum chamber, lathe accessories, etc., as I go, and also because the design continues to "evolve".

In retrospect I probably should've just put the whole thing in oil, but I'm too far along now to go back.

Edit:

As far as chemical compatibility is concerned, as I understand it, PS is ok with epoxy, which is what I was intending to use.

LutzH

Fri Nov 13 2009, 07:30PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Hello:

By golly you are right, I made an assumption error due to the recomended operating temperature range of a PS product, thanks for clearing this up, I will maybe re-visit the material.

Reference the polycarbonate, yes it is the same material as Lexan, the only thing here is that I would not feel comfortable with using Lexan until I could determine the effect of any added UV stabilizing agents which may be added to the name brand product.

When reading the HV pulsed power articles I remember they refered to it as "Polycarbonate", not Lexan, so maybe there could be an issue with UV stabilizers etc. What I do remember very clearly was that the Polycarbonate had a higher hold off voltage than just about any other polymer out there, in the government work. This excited me because the physical properties of polycarbonate are close to ideal for many HV applications, its hard, it does not creep, its machinable, it can be glued, and perhaps most import it is very tough, and resistant to cracking. Pending further research Polycarbonate is close to the top of the bobbin materials that I am considering. If I can find more literature to confirm its HV properties, then it will be my #1 choice. If the UV inhibitors do not mess up Polycarbonate's HV properties then Lexan would be perfect since the UV inhibitors may be of benifit with corona situations.

I will try to find the LL article to post it, but it has been a while so I will have to wait until I get my home PC going again in the next few days. The study in question related to insulator spacing, and cylinders, for a pulse HV particle accelerator, it had a table of polymers, and some oil/gas specs also, for now I have only my hand written note, with a smiley face after Polycarbonate.

I found a bunch of generic Polycarbonate products at the same companies web site from the previously posted PS link, this material sounds real good, maybe just the ticket for winding a secondary on for the Tesla folks :)

Proud Mary

Sun Nov 15 2009, 07:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A bit old fashioned, perhaps, for 4HV ,but Kraft paper (the kind of coarse brown paper still used for parcels in the Old World) remains a standard for transformer insulation.

At low frequencies, it is hard to beat Kraft paper/vegetable oil (sunflower) dielectrics.

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