DIY high voltage transformer isolation with mylar/kapton foils.

[Fflint]

I'm in the planning phase of building a high voltage transformer for a vacuum tube amplifier.

I have various resources from "back in the day" when special paper was used to isolate between winding layers, but I'd like to use a modern material. Mylar or kapton foil, but I'm not sure how to decide on the thickness required. Even the thinnest available (7 micron) of these, in theory should be enough if we take into account just its electric properties, but I suspect it is "not enough". Even with paper, much thicker layers are used than needed if we just consider electric properties.

So I'm starting this thread to ask for any "rules of thumb" etc regarding high voltage transformer isolation using these modern materials.

The transformer will be on a laminated core similar in size to EI-230, 50Hz, 3000VA, primary of 240V, secondary 2880V, approximately 1.4V per turn, and one layer in the secondary will have 167 turns, so the highest voltage between adjacent layers ~480V. According to my literature one should use 1mm thick mineral oil soaked paper between these layers. But I'd prefer mylar, or kapton. Anyone has any idea on what thickness to use (please quote sources if possible)?

[Roehrenonkel]

Hi Fflint,
 
i guess the given paper-thicknes is more for mechanical than for dielectric strength.

Good luck

[jbb]

Mechanical strength is a big factor for taping. There will always be some lumps and bumps (eg first and last turn of a winding layer) and the tape needs to be strong enough to not rip.

Additionally, a tape layer could have a teeny pinprick flaw in it, so it’s common to use multiple turns of tape; the odds of such flaws occurring right on top of each other is very low.

[uer166]

Use something well defined and used in the wild such as 3M 56 polyester tape. Calculate number of required layers based on datasheet spec and type of insulation required (basic or reinforced).

~500V is not thAt large of a working voltage, you may actually want to size each insulation layer for the full ~3kV.

[T3sl4co1l]

Just get the regular 3M #56 or whatever tape, of adequate thickness, and lay down enough of it to get the required rating in the number of layers.

Mains-frequency transformers will be pushing hard to exceed the rating of a single layer of tape between individual layers, even winding very thin wire.  Between sections, where isolation is required, several layers of tape are most likely required anyway (reinforced type insulation), on top of enough to achieve the total isolation rating.

Check the datasheets for dielectric rating.

Thickness of PET or PI for a given rating is much less than the paper (plain or vulcanized) of ye olden days, saving space, and the operating temperature can also be higher (particularly for silicone adhesive polyimide, and suitable enameled wire of course).  One downside is the higher capacitance, which can be mitigated using more thickness than required for isolation purposes.  Capacitance is irrelevant for power transformers of course, but is a design parameter for output transformers.

Tim

[jonpaul]

hi: Just ONE or need many for production?

2400V power HV transformes are used in Microwave ovens, HV PS, Ham radio TX and Tesla coils, amont other.

Easy to find used or as scrap (MOT) or at Ham Radio fleas.

Try a local Amateur Radio org for the next meeting.

Foil = thin metal.
You need a tape or sheet insulator.

Paper used is
fish paper" or "kraft paper"
Avail in many thickness and de cut sizes or tapes.

USE PAPER ONLY FOR VARNISH IMPREGNATED TRANSFORMERS.

An HV transformer at mons ><2400V is NOT safe unless properly impregnated.

We have used 3M56 Mylar tape for decades, 1 mil mylar, plus adhesive = 2.2 mils overall, readily avail.

Kapton is VERY hard to cut and easy to tear. Also costly.

In  layer wound HV trsf, the highest insulating stress is at the ends between layers and at the entire wdg start><finish><core.

Use appropriate safety rated bobbin.

THIS IS A LETHAL TRANSFORMER use appropriate HV precaution.

Bon chance

Jon

[CosteC]

According to my literature one should use 1mm thick mineral oil soaked paper between these layers. But I'd prefer mylar, or kapton. Anyone has any idea on what thickness to use (please quote sources if possible)?

This sounds like extremely old rule. Today wire is far better than 100 years agou, so better is wounding technology. Rou can also get enamelled wire in at least 3 grades of insulation strength. 500 V layer to layer is not horribly a lot.
There are also various transformer insulation tapes, with varying mechanical, thermal and electrical strength. Any transformer wounding company has catalogue of of-the-shelf tapes you could easily select something from. Tape I normally use for switching mode transformers withstands 5 kV/layer.

[jonpaul]

1 mil will not meet safety isolation but OK for interlayer. However basre paper/plastic is useless in prod, we all use UL rated tapes like 3M56 = yellow mylar, 1mil + adhesve = 2.2 mil thickness

Jon