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Flashover over secondary PVC

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LAN_403

Lethal Shot

Sat Nov 07 2009, 03:15PM

Registered Member #1623 Joined: Tue Aug 05 2008, 03:31PM
Location: The Netherlands
Posts: 39

Hallo,

I'm trying to built a SSTC, but I'm stuck at a problem where I can't find any information about.
For some reason my SSTC arcs across the PVC of my secondary as soon as the voltage (of my variac) exceed's 80V.
The streamers from the copper tip are still there but streamers also form around the PVC of my secondary coil.
On this picture you can see what is does(the end of the secondary is just below the edge of this image).
StreamersrondPVC

Its quite amazing to see, but I rather not see it again.
It happend twice now and all the tape over the wire is to try to prevent it from happening (first there was ducttape in place but it arced nicely around the edges of it).
Does anyone know or has a suggestion what I could do to remove the problem?
Shorten the wire maybe, or add a topload?

Kind regards,

Remco

Steve Conner

Sat Nov 07 2009, 03:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's because you have that big expanse of bare PVC there. The windings should go right up to the top of the tube, they stabilize the electric field and prevent sparks. I suggest you chop the top of the tube off.

doctor electrons

Sat Nov 07 2009, 04:31PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Steve is right! Also, some of the grey pvc pipe does conduct electricity. Only at high voltages!
That may have something to do with the flashing. High voltages will ride on the "skin" (skin effect)
If the material has a high resistance. Chopping down the tube will surely help. Maby using the plain white
pvc pipe next time could benefit you as well.
Side note : I did a 4mhz coil that got really hot with that grey pipe. So hot it melted!! No fire or odd flashover though.
When i rewound the secondary on 2 white repair couplings (for correct diameter) my temp readings never exceeded 108 F.

Myke

Sat Nov 07 2009, 06:17PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

People usually recommend white PVC because black and gray PVC are colored with something that has a high carbon content.
The skin effect is dependent on the frequency of the signal. The higher the freq, the less current flows on the inner part of the wire.

Putting tape over the place it burned won't help because when the PVC burned, it left carbon all over the PVC. This makes it a lot more likely to jump back to the place it was previously arcing. Arcs can form though the smallest holes in insulators. Iirc, it's called the pinhole effect. There is air between the tape and the wire where it used to arc so there will be corona when you turn it on. The corona will eventually eat though the tape and arc to the PVC again.

Herr Zapp

Sat Nov 07 2009, 06:31PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

doctor electrons -

You wrote: "Also, some of the grey pvc pipe does conduct electricity."

How did you make this determination?
Can you provide some specific data to back up this assertion (bulk resistivity measurements, etc)?

Also, the term "skin effect" (as applied to radio frequency currents) relates to current distribution through the cross-section of a conductor, not to "surface tracking" on an insulator, which is a completely different phenomonon.

If the PVC coilform on your 4MHz coil got hot, it is likely that the cause was the relatively high dielectric loss factor of PVC, and not because the PVC was "conductive". Heating of the secondary windings may also have added some heat. A non-contact infrared optical thermometer can help determine if secondary heating is originating in the coilform itself, or in the windings.

Myke -

You wrote: " .... black and gray PVC are colored with something that has a high carbon content".

This is an often-repeated but untrue statement. No commercial PVC or ABS water or drain pipe material intentionally contains any conductive filler of any kind. It has been found that SOME (but not all) external ink markings on PVC pipe can be slightly conductive, depending on the pigments used in the ink.

Regards,
Herr Zapp

Lethal Shot

Sat Nov 07 2009, 06:36PM

Registered Member #1623 Joined: Tue Aug 05 2008, 03:31PM
Location: The Netherlands
Posts: 39

I've chopped my pipe so now it only sit just above the last winding.
I'm going to test it tomorrow and let you know the results.

doctor electrons

Sun Nov 08 2009, 04:07AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Herr zapp!
I will find some info on the grey pvc that i can post a link to. Good advice as to using the ir thermometer! I did actually. The heat was being
generated in the pvc itself. Measuring the surface temp of the secondary itself (copper) the pvc was about 90 f hotter. Myke is correct in his entire reply!
Good info Myke, thank you for that! To clarify, when i referred to the skin effect, in a tesla secondary or air core transformer. If the form can conduct, it will. The skin effect will happen because the material used in the form is not intentionally conductive. Electricity will take the easiest path which happens to be on the outer skin of the form. Obviously the forms outer surface offers less resistance than a path through the form. If you look carefully at Lethal Shots photo you can see that the flashover was traveling upward when its most logical path to ground would have been down into the windings. I bet someone here knows why!!
To Lethal!
That problem may persist because the varnish on the wire is burned off where the flash occurred. If you can pull a bit off and retune you should. The exposed area will form corona because it is also an easier path than the rest of the wire above it. And before anyone murders me on that, yes i do know that the resistance of 2" of thin copper wire is minute. But still restive, hv does not care about the math.
Found this! Didnt want to double post!

Lethal Shot

Sun Nov 08 2009, 10:19AM

Registered Member #1623 Joined: Tue Aug 05 2008, 03:31PM
Location: The Netherlands
Posts: 39

I've tested it with the copped pipe and it worked.
I could turn it all up to 180V this time only to have it end in a violent bang.
But at least the PVC problem is gone.
So I'm back to debugging everything again and thanks for all the tips and help.

Killa-X

Sun Nov 08 2009, 04:08PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Yeah...With my SSTC, i have 3mm of bare PVC between my top winding and the top of my secondary form.

You must admit though, That's a cool effect :D Lightning bolt on PVC :D

Herr Zapp

Sun Nov 08 2009, 07:41PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Lethal -

Just out of curiosity, was there a splice in the wire, or any other irregularity in the wire hidden under the tape, where the surface tracking originated? It's a little odd that the surface arcing was directed only to the left, as if there was a sharp point on the wire "spraying" corona in that direction.

Before and after winding secondary, did you carefully clean the surface of the PVC pipe? Any surface contamination (salt from sweaty hands, flux residue from soldering, etc.) can drastically affect the tendency of an insulator experience surface tracking.

Regards,
Herr Zapp

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