Author Topic: SiC PSFB QCW coil  (Read 1955 times)

Offline dr. kilovolt

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Re: SiC PSFB QCW coil
« Reply #20 on: May 15, 2020, 08:56:56 PM »
Hi Uspring

Thanks for the simulation. Actually the first 1-2ms could be missing on my graphs, maybe related to your requirement of 50V initial voltage? I'm really surprised Your model works so well (except the arc length of course) for QCW sparks. I might give it a play :-)
I can produce quite short ramps (~5ms) with my coil. With these short ramps, the OCD always activates at 100A near the end of the ramp. I believe without it the primary current would get very close to Your calculated 115A. With increasing ramp length and decreased branching, the peak current tends to drop somewhat.


By the way, I was trying to fine-tune the ramp shape, it's unbelievable how subtle differences in power ramp speed have large impact on spark growth, especially branching. You have two power envelopes which look very similar macroscopically, only one has almost unnoticably faster rise near the end. With the first one the arcs do not branch near the end, with the second one they tend to. It is really a kind of magic with such arcs.


-Jan

Offline Steve Ward

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Re: SiC PSFB QCW coil
« Reply #21 on: June 14, 2020, 08:30:55 PM »
What a fantastic build, and performance!

The ferrite slug is interesting, I'd considered it but never had the right parts around to do a decent job.  It should about double the primary inductance versus without the slug, have you measured with/without to check the difference?

Do you have any dimensions or sketches/pics of the coil assembly?  Was potting necessary or just more "overbuilding" to ensure you wouldnt have flashover issues?  Im not sure how long the ferrite rods are relative to the secondary coil, i guess if they are a decent portion of that length, some epoxy to handle the high-stress areas would be a good call.

Offline dr. kilovolt

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Re: SiC PSFB QCW coil
« Reply #22 on: June 14, 2020, 10:10:08 PM »
Hi Steve! Thanks for the compliments.

The dimensions of the coils and ferrite are approximately as in the attachment. As the ferrite fills the space inside the primary quite nicely, the increase in inductance is IIRC around 4-5 times.

I don't know if the potting is necessary, but as time advances I'm becoming too lazy to do a "testing" build, especially as I'm a perfectionist and I woud make the build "almost final" anyways...  ;D  ??? I know there's a lot of sacrifice if something doesn't go right, but I can't help myself.
And that actually happened with this coil. Originally I had a classic external primary and the secondary was stuffed full of ferrite toroids and potted. Because of the large permittivity of the potting compound and relatively large conductivity of ferrite, a quite large current was coupled to the ferrites, which resulted in internal arcing from the bottom of the ferrite tower to secondary cold end, making a carbonized track. For low duty cycles during testing that didn't produce much flames, but at full power it would.

Anyways, the coil is not designed optimally, the primary is tuned higher than the secondary for optimal power transfer. A better, more efficient design would use a lower impedance tank circuit and lower primary tuning. But modifying the potted primary is impossible. Better have a too high impedance tank, which can be tuned to the desired power, than a too low impedance one which would not allow to bring the current down...  ;D

Offline davekni

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Re: SiC PSFB QCW coil
« Reply #23 on: June 15, 2020, 12:03:28 AM »
This potted-primary coil is very intriguing, and performance is amazing.  It's enticing me to try a QCW coil some day.  So, here's my thought for getting the advantage of potting without losing as much flexibility:

The basic idea is to make a potted secondary winding form with a large cavity for the primary and ferrites.  To avoid corona, the cavity would be lined with grounded vertical wires.  Picture a circular bird cage without the circumferential wires.  I'd start with some sort of thin container such as the bottom 80% of a pop bottle up-side-down.  Drape vertical wires over the bottle, then pot that assembly inside the secondary pipe section.  The wires are inside the potting to avoid corona to the wires.  They provide a grounded environment for the primary, but don't form any loops to block magnetic fields.

Does that sound workable?  Any alternatives or variations that would be better?

Did you use vacuum during potting to remove bubbles?  I'm struggling with how to apply vacuum to larger structures like this while providing room for expansion before bubbles eventually pop.
David Knierim

Offline Steve Ward

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Re: SiC PSFB QCW coil
« Reply #24 on: June 16, 2020, 07:35:08 PM »
Maybe something like paraffin wax is a good-enough solution?  The phase change absorbs a lot of heat, but im not sure if the localized heating would cause cracking of solid wax elsewhere, compromising the insulation properties.  On the other hand, am i naive in thinking it might offer some self-healing anti-corona properties as the wax exposed to RF corona discharge would rapidly heat, melt and expand to fill any air void?  Might also turn it to carbon and make a mess...

I experimented with putting an entire coil assembly under mineral oil.  Mineral oil appears to withstand about 6X the electrical stress as air.  I even managed to make tiny ~.75" long branching "free-oil" streamers inside of the oil bath, they appeared mostly white in color and were highly fractal, similar to an acrylic Lichtenberg figure.  When the breakout point was extended outside the oil, the discharge would surface track some 8" or so back to ground potential along the surface of my acrylic oil enclosure.

The required voltages aren't too big (50-60kV...70kV), so its interesting to ponder the design of an epoxy/wax/oil-submerged "Tesla coil", and how small it might possibly become with the use of ferrite to boost the inductance and maintain reasonable operating frequency and keeping wire size manageable. 

High Voltage Forum

Re: SiC PSFB QCW coil
« Reply #24 on: June 16, 2020, 07:35:08 PM »

 


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