Author Topic: QCW with replaceable ferrite-core primary  (Read 950 times)

Offline davekni

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QCW with replaceable ferrite-core primary
« on: October 13, 2020, 01:50:43 AM »
Inspired by Jan's impressive ferrite-core QCW coil here:
https://highvoltageforum.net/index.php?topic=1073.0

and his wishing that the primary could be adjusted, I'm starting a project to make a QCW with replaceable ferrite-core primary.  The idea is to pot the inside of the secondary with a cavity for the primary.  However, a plain cavity would form corona discharge between the inside surface of the cavity and the primary.  So, here's the secondary form and cavity I plan to pot inside in a couple days:





The cavity is a Faraday cage with no closed loops.  All the wires connect at only the top.  Wires will all be inside the potting material.  At the bottom, I'll connect ONE of the wires to ground.  That way the internal electric field of the secondary will return to the Faraday cage within potting material.  Internal electric field around the primary will be due to only the primary voltage, low enough to avoid corona.  Magnetic field will be free to pass between Faraday cage wires.

Anyone see any issues before I mix and pour epoxy?  I will be using vacuum to remove as much air as possible, per what I've learned on this forum.

This is the first step of a long project.  Haven't built any of the electronics yet.  My aim is to explore lower frequency and longer ramp times than typical for QCW, to see if slow ramps can compensate for lower frequency.  Somewhere around 170kHz and 100ms ramp.
David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #1 on: October 17, 2020, 03:06:49 PM »
Lovely idea to improve on dr. kilovolts design by making the primary removeable with a Faraday cage. I'm wondering, whether an iron core actually increases coupling. Have you any estimates on this?

I'm certainly fond of any research on arc behaviour.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #2 on: October 17, 2020, 07:45:56 PM »
I made a mock-up by winding old power cords into roughly the intended geometry.  Coupling for that was 0.875.  Didn't measure coupling without the ferrite core, but would guess around 0.7 or just under.

Potting had some problems.  Developed air leaks around the top seal (bottom during potting) during vacuum degassing.  Managed to mostly patch those before epoxy cured, but it's not degassed very well.  May still be OK.  I read that QCW voltage is around 50kV.  Even if low-frequency raises that to 100kV, this potting may be sufficient.  Radial distance is 21mm epoxy plus 5.8mm PVC pipe wall.  Axial distance at the top is 30mm epoxy plus 6.1mm polycarbonate.

Uspring, Thank you for your modeling too.  I've done a bit of magnetics modeling, but don't know enough about arcs to simulate those.
« Last Edit: October 24, 2020, 07:48:06 PM by davekni »
David Knierim

Offline Mads Barnkob

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Re: QCW with replaceable ferrite-core primary
« Reply #3 on: October 18, 2020, 07:18:08 AM »
Interesting project, something that has been talked about, especially the internal primary coils and potting, but the shielding and other measures of actually making it feasible are new to me, looking forward to follow this.

In regard to low frequency QCW, there might be a little knowledge to get out of my failed attempt of making it simple: http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iv/

Regarding shielding around magnetic components, perhaps you can find inspiration in the Pearson Current Monitor 1330 I took apart, I was surprised that it was fully copper shielded in 2 layers with only a small slot cut through it:  https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=175696#post-175696 (I can make a repost here on hvf with full size pictures if you want?)
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Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #4 on: October 19, 2020, 05:32:05 AM »
Mads,

Thank you for reminding me about your simple line-power-cycle-ramped QCW.  I'd looked through that project when I first found this forum a year ago, but had forgotten that it was also low-frequency for a QCW.  For initial experiments, before I build a proper ramp generator, I'm thinking of trying a resonant ramp generator.  Charged bulk cap array feeds a large series inductor into another bulk cap array, with thyristor for trigger.  I have some MOT primary halves left from my eddy-current levitation demo.  Should make something similar to your line quarter-cycle without risk to other line-powered devices.  (I've seen a vacuum-tube coil powered by isolated line half-cycles as in your demo, behaving much like QCW coils.)

I use a somewhat similar slightly-over-one-wrap of insulated copper foil to shield the first stage of my DRSSTC current transformer.  Likely not needed.  For this QCW primary, copper foil could work around the circumference, but not at the top end.  Most of the primary magnetic field is passing out its top end.  There's no way to make a non-shorted turn in the plane where magnetic field passes through.  That's why the array of wires.
« Last Edit: October 24, 2020, 07:48:29 PM by davekni »
David Knierim

Offline Hydron

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Re: QCW with replaceable ferrite-core primary
« Reply #5 on: October 23, 2020, 05:36:02 PM »
Hey I've moved the ping-pong/pumpkin shooting stuff to https://highvoltageforum.net/index.php?topic=1274.0 - hopefully I did it right (I think I might have screwed up renaming the topic!).

Dave - some of your posts are relevant to both threads - maybe worth copying/pasting the bits into a new post there, sorry I couldn't figure out how to make two copies of said posts when splitting the thread.

Gotta say the pumpkin discussion led to watching some fun youtube videos - I liked the visual spectacle of rubber band/trebuchet ones more than gas launched though to be honest.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #6 on: November 22, 2020, 03:34:36 AM »
I've now wound a secondary around the potted core and a primary to go inside.  Here's some measurements:
     Secondary winding:
299mm high
160.8mm diameter
27AWG wire
~700 turns
33.19mH w/o ferrites
156.7mH with ferrites
60 ohms DC
     Primary winding (at least this initial version):
270mm high
90mm diameter
28 x 27AWG home-made litz wire
Four interleaved 7-turn windings.  Can be connected in series or parallel or 2S2P.
307uH with ferrites and series (4S) connection.  (19.2uH for 4P connection.)
     Combination:
0.909 coupling with ferrites (0.51 w/o ferrites)
124kHz upper pole frequency w/o top-load, with 7x lower primary tuning (50x capacitance - 2.4uH primary using 4S connection)
106kHz upper pole frequency with 620mm OD top-load

The big surprise (my dumb mistake to miss initially) was all the extra capacitance from secondary winding to internal Faraday-cage shield.  I was aiming for 170kHz with top-load.  Capacitance through the potting epoxy has fairly-high loss too, acting as around 900k-ohms across the coil.  Still shouldn't be a killer issue compared to ~50k arc impedance.

Some images of construction, starting with secondary around potted Faraday cage, up-side-down view:


Primary coil with four interleaved 7-turn windings:


Secondary inside primary Faraday cage:


Ferrite stack, made of a total of 44 E-55 core halves, 3C92 material, plus four flat E-64-50-10 cores for spacing:


Primary around ferrite stack, with leads extending through bottom plate:


Secondary on primary and ferrites:


More ferrites on top, 3F4 material 64 x 51 x 5mm plates:


With 620mm OD top-load, which I won't be using since frequency is already too low:


I had one other problem with the potting besides vacuum issues.  Three connections of the Faraday cage broke, presumably due to stress of epoxy shrinking slightly during cure.  I connected them at the bottom, but with parallel R/C impedance.  The impedance is to avoid a shorted turn in case one of the bad connections reforms with thermal cycling or age or whatever.

Ran a quick test driven more like an SSTC, using a quick half-bridge I constructed for example layout.  Ran it up to ~100kV peak.  At least there's no immediate insulation breakdown issues.

« Last Edit: November 22, 2020, 03:45:22 AM by davekni »
David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #7 on: November 23, 2020, 01:08:46 PM »
That's one huge coupling value. If you tune the primary tank to your secondary resonance frequency, the lower pole will be at 77 kHz and the upper one at 350 kHz. Choosing other primary fres's can extend this range. You could name the coil an AFTC (Any frequency Tesla coil).

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #8 on: November 23, 2020, 08:03:46 PM »
Yes, coupling is large.  That was one of my design goals - as large coupling as possible.

I must have a mistake in my posted numbers for you to get a 350kHz upper pole option.  124kHz is upper pole with huge (call it infinite) primary capacitance and no top-load.  If I were to move to exactly in-tune, upper pole increases by only sqrt(2) to 175kHz.  However, I want to keep the primary capacitance somewhat large.  That minimizes primary voltage and minimizes flux in the ferrite core.  The 2.4uF in my first test is too large.  As the arc load grew, my simple ZCS driver switched to lower-pole.  (Spice simulation shows this too.)  About 32nF would be in tune with the 4S (28 turn) primary connection (with no top load and no arc).  I plan to drop from 2.4uF to about 0.33uF for the next test.
David Knierim

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Re: QCW with replaceable ferrite-core primary
« Reply #9 on: November 24, 2020, 05:32:56 PM »
I took a k of 0.9, which raises the frequency by a factor of 1/sqrt(1-k) ~ 3. You probably used the k=0.5 as for the coil without core. It should be possible to run your coil at the known frequencies for straight arcs and also explore lower frequencies. Nice  :)
Exotic experiments would be running the coil with lower and upper pole at the ratio of 1:3 simultaneously. That are the fundamental and first harmonic of the driving square wave. You'd have a square wave TC then. (SQWTC).

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #10 on: November 24, 2020, 07:52:01 PM »
Uspring,
The 124kHz measured frequency (w/o top load) is upper pole, so includes the 1/sqrt(1-k) factor already.  This is with k=0.909.  The only tests I ran w/o the ferrite core were to measure secondary inductance and coupling - just for curiosity.  Nothing else is air-core.
David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #11 on: November 26, 2020, 12:07:11 PM »
David, I'm sorry to have bothered you with this. I was under the mistaken impression, that the location of the poles don't depend much on coupling, if resonance frequencies of the (uncoupled) tanks are sufficiently distant. That is only true for the lower pole. I had assumed the upper pole in your measurement to be near the secondary fres. Actually the upper pole seems to vary from 1/sqrt(1-k^2) of the secondary fres at very low primary fres to 1/sqrt(1-k), when primary and secondary fres are equal.


Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #12 on: November 26, 2020, 06:48:34 PM »
No problem!  I appreciate people questioning my results, as I make careless mistakes more often than I'd like.
David Knierim

Offline Mads Barnkob

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Re: QCW with replaceable ferrite-core primary
« Reply #13 on: November 29, 2020, 08:04:01 PM »
That amount of ferrite blocks o_O

The bottom cross reminds me of how higher coupling is achieved in induction stoves


But what are the purpose of the plane of ferrite blocks living on top of the secondary coil and between that and topload? Surely the ferrite was all there to get the highest possible coupling between primary and secondary coils?

With a mind blowing coupling factor of k=0.909, that is extremely close to k=1 where the coils would take up the same physical space. Really looking forward to see where this project ends :)
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Re: QCW with replaceable ferrite-core primary
« Reply #14 on: November 30, 2020, 06:37:23 AM »
The top ferrites make a small contribution to coupling factor.  They also allow the top-load to be placed close to the top of secondary winding.  However, since I already have too much capacitance, it may make sense to drop the top ferrites along with the top-load.  The ferrites are conductive enough that I need some form of corona ring to avoid arcing from the corners.  Without ferrites I might still need some small corona ring at the top, but it could likely be a non-shorted turn and still sufficiently effective.

Yes, lots of ferrites.  Cost was $158 for all the E-cores that make up the column and cross at the bottom.  The flat ferrites were in my old stash.
David Knierim

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Re: QCW with replaceable ferrite-core primary
« Reply #14 on: November 30, 2020, 06:37:23 AM »

 


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