E-field Hotspots In Transformer Windings / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

E-field Hotspots In Transformer Windings

previous 1 2

Move Thread

LAN_403

jpsmith123

Fri Nov 07 2008, 07:04PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Hello PanosB,

I think the field enhancement in this case is more due to a "void" (where the air has a lower permitivity than the adjacent dielectric) than a "triple point" per se.

Regards,
JP

Finn Hammer

Sat Nov 08 2008, 08:18AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

The standard way to avoid high voltages between layers, is sectioning the winding into a stack of pie shaped windings. Each layer thus has only a few windings.
This is standard practice in high voltage distribution transformers, flybacks. Cut a flyback in 2 and see for yourself.
Also see this one, which still needs to pass the test.

Details from here and downwards.

Cheers, Finn Hammer

jpsmith123

Sat Nov 08 2008, 03:13PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Actually I don't consider 1 to 2 kv between layers to be all that "high"; the problem is that I was trying to avoid putting it in oil or impregnating it with silicone, epoxy, etc., yet still avoid corona. That idea now seems less feasible, and I suppose my next step will be to try to find a coil winding shop that can make a custom coil at a reasonable price.

Backyard Skunkworks

Sun Nov 09 2008, 03:55AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

What about bunching the turns up several layers thick without ever reversing the winding direction? Sort of how youd wind a TC secondary, except with the turns bunched up a few layers at a time. You would sacrifice a lot of space with this winding arrangment though.

Just a thought...

Finn Hammer

Sun Nov 09 2008, 09:08AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

jpsmith123 wrote ...

Actually I don't consider 1 to 2 kv between layers to be all that "high";

But you should!
The problems that you are facing are due to just that.
And you may very well find that the transformer shop, if you find one, will routinely wind it that way.

Cheers, Finn Hammer

Dr. Dark Current

Sun Nov 09 2008, 09:17AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Finn Hammer wrote ...

The standard way to avoid high voltages between layers, is sectioning the winding into a stack of pie shaped windings. Each layer thus has only a few windings.
This is standard practice in high voltage distribution transformers, flybacks. Cut a flyback in 2 and see for yourself.

This can be done only in diode-split transformers or lower voltage ones. The diodes greatly reduce the AC component on the highest potential sections, so the last-section-to-core insulation can be quite thin as it has to insulate mostly DC.

However if you were to use the same winding tecnique in a non diode-split transformer of significant voltage output, the insulation between last section and core would see full AC voltage which means big corona and dielectric losses.

I believe most of you have seen the failure of rectifiers in a diode-split flyback, where it literally lights up with corona and dies/bursts in flames a second later.

Finn Hammer

Sun Nov 09 2008, 01:21PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Dr. Kilovolt wrote ...

This can be done only in diode-split transformers or lower voltage ones. The diodes greatly reduce the AC component on the highest potential sections, so the last-section-to-core insulation can be quite thin as it has to insulate mostly DC.

Yes, but it also introduces DC to the windings, which means that you have to have an airgap in the core to handle the static flux. This works in a flyback with a few mA, but I doubt it will be any good in a power transformer.

Dr. Kilovolt wrote ...

However if you were to use the same winding tecnique in a non diode-split transformer of significant voltage output, the insulation between last section and core would see full AC voltage which means big corona and dielectric losses.

Big corona only if the distance to the core is too small. Again, in tv flybacks where every mm counts, due to price, this may matter, but it does not in a custom hobby transformer.

Dr. Kilovolt wrote ...

I believe most of you have seen the failure of rectifiers in a diode-split flyback, where it literally lights up with corona and dies/bursts in flames a second later.

Failure of the rectifier due to pulling more current than the device is rated for?

Cheers, Finn Hammer

jpsmith123

Sun Nov 09 2008, 05:05PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

"But you should! The problems that you are facing are due to just that."

Well in a purely tautological sense, yes. Practically speaking, it's only "high" because I don't have the means readily available to encapsulate it, and I want to eliminate the possibility of corona.

If I could take the bobbin I have and somehow add one dividing flange in the middle, or machine something similar from scratch, my problem would be solved, but I can't, as I don't have access to a machine shop.

"And you may very well find that the transformer shop, if you find one, will routinely wind it that way."

Probably not since if they are going to encapsulate, a straight layer winding would suffice and would probably be easier to make. But even if they do, it wouldn't matter to me since I'd be buying at a fixed price to my specifications, and I don't care how they choose to do it.

Dr. Dark Current

Sun Nov 09 2008, 05:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Finn Hammer wrote ...

Yes, but it also introduces DC to the windings, which means that you have to have an airgap in the core to handle the static flux. This works in a flyback with a few mA, but I doubt it will be any good in a power transformer.

Well, I have driven rectified flybacks to a few hundred watts, they heat up a lot but this is more likely because of big resistance of the secondary windings. I'm not sure but it's possible that halfwave rectification on the output halves the core's power handling capability though.

Finn Hammer wrote ...

Failure of the rectifier due to pulling more current than the device is rated for?

Probably, I don't remember the exact conditions, maybe it was combined with overvoltage (one diode dies by overcurrent > others see more voltage > they die by overvoltage)

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob