HV transformer isolation

[T3sl4co1l]

PTFE tape has nil thickness to begin with, and is porous and cold-flows under pressure.  In short, it's useless here!

Actually using masking tape isn't a showstopper here -- as long as you only need class A insulation, and take adequate precautions to deal with the possibility of it catching fire (it's not exactly 94V-0 grade!).

"Electrical tape" is definitely not the thing here, as it is also very soft, especially when hot.  Or duct tape, or anything else rather on the gooey side.

It's well worth getting some legit yellow transformer tape!  It's only $20, and a small roll will supply many transformers.  I forget which Scotch part number is usual, but any moderate temperature (105C+), polyester tape will do.  The adhesive is usually acrylic based.

It goes on kind of loose, not the stickiest thing, but the adhesive softens on heating -- usually when the assembly is impregnated with varnish, then baked to cure it.  But even if you've just got solid layers of polyester film separating the layers, without resin, you'll stand a much better chance of success!

Tim

[fcb]

We use AT4004.

[james_s]

A trick that is used in some HV transformers is to use a bobbin that splits the winding into multiple side by side windings that are in series. When dealing with high frequency it becomes even harder to prevent corona and arcing, you may want to pot the entire winding using a casing similar to those used for CRT flyback transformers. For prototyping and experimentation immersing in mineral oil is a practical approach. It can be messy but it is a very effective insulator. I've wound HV transformers that I placed in a glass jar of mineral oil with the leads feeding through insulators in the lid. Just don't use stranded wire leads, the oil will wick up the wire.

[T3sl4co1l]

Bank wound construction (or "pi wound" like the old chokes) is fantastic for decreasing capacitance.  Downside: more leakage inductance.  Well, in short, it's a method to raise the characteristic impedance of the winding.  At high turns ratios, it's impossible to get a Zo anywhere near what you're using it for, so every little bit helps.  (Such a construction might achieve Zo ~ low kohms, whereas you might be using it for ~mA at ~kV = Mohms!)

CCFL transformers are cheaply available, constructed in this way.  I don't know how much OCV they generate, but it could be a few kV.  I've seen small Tesla coils made with them.

Tim

[Neukyhm]

I have finished winding the first layer of wire, I have left 3 layers of epoxy dry (the first attempt had one). Now I'm thinking what to do before starting the second layer of wire, more epoxy layers? What about applying some layers of PVC black tape? Is it a good insulator?

Notice that the first layer of wire covers the 41.2mm of the coil former, but the second layer just takes less than the half of it. I have estimated the voltage difference between the first and second layer of wire at it highest value to be around 3kV.

[Gyro]

You seem to be specializing in tape that plastic deforms (after your plumbers tape). PVC is also soft and stretchy, although thicker than the ptfe, so will also tend to thin out under compression, temperature performance is poor too. What you really need is Polyester/PET/Mylar. Try ebay - even if you don't have it immediately to hand, it's better to wait and do the job properly.

[james_s]

I wound some transformers a few years ago and used polyester tape, I think I got it from tapecase.com.

[Neukyhm]

  I forget which Scotch part number is usual

I think you are talking about Scotch 23, is it good?

I would wind a few layers of it above the 3 layers of epoxy. The epoxy and the Scotch tape should be strong enough to withstand around 3kV@110kHz.

[Neukyhm]

Ok I have now some Scotch 23, before using it I would like to hear your opinion, is it good? What's the best way to use it? If I wrap some layers of it around the epoxy, it should be able to withstand those 3kV.

[T3sl4co1l]

I suppose if you're going to use epoxy at all, it might be nice to apply a layer of goo, then wrap it with tape so the tape smooshes it in place.  But that's messy and hard to do.

I'd just recommend winding dry, cover each layer with two overlapping layers of tape, and make sure the tape goes past the edge of the winding so you don't get arcing around the edges (creepage).

Tim

[Neukyhm]

I suppose if you're going to use epoxy at all, it might be nice to apply a layer of goo, then wrap it with tape so the tape smooshes it in place.  But that's messy and hard to do.

I'd just recommend winding dry, cover each layer with two overlapping layers of tape, and make sure the tape goes past the edge of the winding so you don't get arcing around the edges (creepage).

Tim

Will do. Also I need some assistance understanding the Scoth 23 datasheet, page 3, graph "Dielectric Strength Versus Thickness",  I don't understand why dielectric strength (intensive magnitude) depends on thickness (extensive magnitude)  . Also I don't know how this tape's dielectric strength is better when thinner.

[T3sl4co1l]

Dielectric strength is better when thinner, something about the molecules being lined up, tighter, stronger -- but mind that the thickness drops too (that's where it's being elongated from), so the total voltage per layer of tape may be less!

Oh wait, splicing tape?  This isn't the yellow mylar stuff.

Try... uhh... Scotch 56?

Tim

[Neukyhm]

Dielectric strength is better when thinner, something about the molecules being lined up, tighter, stronger -- but mind that the thickness drops too (that's where it's being elongated from), so the total voltage per layer of tape may be less!

Oh wait, splicing tape?  This isn't the yellow mylar stuff.

Try... uhh... Scotch 56?

Tim

Scotch 23 seems better than 56 (also 23 is thicker, but it's not a problem to me)

Damn, 3M has a lot of tapes.

[james_s]

You might check out the forum at 4hv.org, lots of guys there making HV transformers. I wouldn't bother using epoxy at all unless you vacuum pot the whole winding. HV transformers require careful technique to prevent arcing and corona, and the higher the voltage and/or frequency the more difficult it gets.

[T3sl4co1l]

23 is squishy, a property several posters have enumerated is undesirable!

Tim

[Neukyhm]

You might check out the forum at 4hv.org

People over there are suggesting me to use Kapton tape, uh... now I really don't know what to use XD

It seems that, If you search on the internet, you find people using everything. Some say winding a lot of layers of PTFE tape works fine, others use Kapton, others prefer Scotch. 

[T3sl4co1l]

Well, if it helps, I'm an engineer, and most of them are not...

Kapton(R) (polyimide) is even better, having higher operating temperature (if the kind with silicone adhesive) and better corona resistance.  Major downside is ease of tearing and cracking, but that shouldn't be a problem in a transformer, where the wires don't have to move.

Tim

[ahbushnell]

This Engineer agrees with you.  Kapton is good stuff for high voltages.  There are other materials also.  Note if you use oil insulation it's best to pull a vacuum on it to get the bubbles out. 

[David Hess]

I like to refer to the silicon adhesive Kapton tape as "aerospace tape" to go along with my "aerospace epoxy".  It is marvelous stuff but I have seen it replaced with Mylar tape (but not the stuff for transformers) to save in costs and the yellow Mylar tape they use for transformers seems pretty good also.

[MadTux]

Polyester (Polyethylene terephthalate) sheet is usually used for isolation in between transformer winding layers.
https://www.google.com/search?q=polyester+transformer+tape
Most parcel packaging tapes are made from the same material and can be used for that application (at least I used it)
Most important aspect is not to wind completely to the end of the transformer core so that no discharge can happen at the air gap (as dielectric breakdown voltage of air is much lower than the one of PET polymer):

good:
-----------------------------------------------------
              °°°°°°°°°°°°°°°°°°°°°°°°
-----------------------------------------------------

bad:
-----------------------------------------------------
 °°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°° ZZZ
-----------------------------------------------------     Z
 °°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°ZZZZ
-----------------------------------------------------
If the tape isolation wrap gets warped, wrinkled and ugly, you can use a heat gun to shrink it back into niceness, btw.

For isolation of complete transformer, I wouldn't use epoxy, much too expensive, difficult to apply, hard remove for repairs, modifications or to reclaim parts. Go with plain candle/paraffin wax and simply cast it into the transformer housing. Prior to casting, heat it well to boil off any moisture and then cast it while it's nice and hot and has low viscosity, so that it can seep into all the small spaces. If mechanical strenght/melting point is too low for your application, you can actually dissolve polyethylene plastic (plastic bottles etc) into your molten paraffin wax (takes some time). The long polyethylene molecules (PE is like parafin, but with much longer hydrocarbon chains) increase strenght/melting point.

If something goes bad, simply use a heat gun or dump the whole assembly into a pot of already molten wax.

Used this to for isolation on capacitor/diode voltage multipliers, up to maybe 100kV (judging from spark lenght) and it worked great.

[David Hess]

Polyester (Polyethylene terephthalate) sheet is usually used for isolation in between transformer winding layers.

I assume the common yellow polyester (same thing as Mylar in case anybody was unaware) transformer tape uses acrylic adhesive.  I have noticed before when unwinding it after it is old that the adhesive is dry and flakes off.

For isolation of complete transformer, I wouldn't use epoxy, much too expensive, difficult to apply, hard remove for repairs, modifications or to reclaim parts. Go with plain candle/paraffin wax and simply cast it into the transformer housing.

Epoxy has another problem which may have to do with water absorption.  As Tektronix discovered when they changed from wax to epoxy during the era of tube oscilloscopes, some epoxies become lossy as they age which can completely disrupt operation of self oscillating inverters.  My first choice is wax as well for all of the reasons you identify.

Prior to casting, heat it well to boil off any moisture and then cast it while it's nice and hot and has low viscosity, so that it can seep into all the small spaces.

I have thought about doing this with wax under a vacuum.

If mechanical strenght/melting point is too low for your application, you can actually dissolve polyethylene plastic (plastic bottles etc) into your molten paraffin wax (takes some time). The long polyethylene molecules (PE is like parafin, but with much longer hydrocarbon chains) increase strenght/melting point.

There are a lot of different grades of wax also and combining two in the right proportions can tailor its characteristics.  The problem I have usually seen is the wax separating or cracking if it is part of a large volume but I do not have much experience with using it yet.  There is a fair amount of information online about it.

[Neukyhm]

Some of you are suggesting to use Kapton. I have found some cheap polyamide tape, but it's not genuine DuPont Kapton, will it do the job?

Genuine DuPont Kapton is really, I mean, REALLY expensive. I will take a look to polyester tapes as well.

[T3sl4co1l]

Polyamide != polyimide (and yes it matters, and yes chemists are that pedantic..).

Though I don't actually know what polyamides would be made in tape form.  Probably the most common is the aramid fiber Kevlar (and relatives).  Hmm, there's probably fiber-reinforced tapes though.

Anyway, I don't have a problem with cheap tape, as long as it'll do what's needed.  Test it if you like.  Polyimide doesn't melt, it just kind of chars -- at nearly red heat!

Mind you aren't getting the full temp rating if it's only acrylic adhesive.

Tim

[Neukyhm]

Polyamide != polyimide (and yes it matters, and yes chemists are that pedantic..).

Thanks for that, I didn't realize, even big suppliers like RS use polyamide when referring to Kapton 

I have bought that fake Kapton and I will test a few layers of it with 20kV before using it in my transformer.

I'm also investigating these polyester tapes, they are very affordable compared to genuine DuPont Kapton, and they seem good according to the datasheet

[T3sl4co1l]

2.5 and 3.5 mil?  You may find that tape hard to work with, actually, compared to thinner stuff like #56.  You can always put more on, but you can't shave it down.

Tim