Author Topic: QCW with replaceable ferrite-core primary  (Read 321 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.

BTW, completely unrelated except for modeling, but I'm hoping to do some simple one-dimensional gas pressure/flow modeling for a  supersonic ping pong ball launcher.  Had the Guinness speed record for a while, and plan eventually to reclaim that record.
/>
/>
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?)
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
https://www.youtube.com/KaizerPowerElectronicsDk60/join - Please consider supporting the forum, websites and youtube channel!

Online johnf

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Re: QCW with replaceable ferrite-core primary
« Reply #4 on: October 18, 2020, 08:25:41 PM »
Dave
I saw a vacuum cannon with ping pong balls at Richard Hulls place a few years ago from memory it went at around 2000 ft per sec and had no trouble going through water filled soda cans barrel was about 16 feet long. I think it used sacrificial thin mylar windows. Always wondered about doing it with golf balls??

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #5 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.

John,

Golf balls could be launched with gun powder and a bit of packing to insulate the ball.  Ping pong balls take more finesse, so a more fun challenge.  Also more impressive to see what a 2.7g ball can do, going through 3/4" plywood (nominal 3/4, actually 17mm).  Our (a father-son project) official record is 806m/s.  We since hit 873m/s (2764f/s), shown in the later videos.  (The "smarter every day" youtube channel has a video of supersonic baseball launching.)
David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #6 on: October 20, 2020, 05:08:29 PM »
You should check, whether it is inside the rules of table tennis to punch a hole into the opponents racket.

Wrt simulation: A one dimensional equation is:

d^2y/dt^2 = c^2 * (1+dy/dx)^(-gamma-1) * d^2y/dx^2

The derivatives are partial derivatives. y is the displacement of air molecules relative to their initial position x. So y(x,t) is 0 for t=0. Gamma is the adiabatic exponent and c sound velocity. This equation differs from the Navier Stokes version in the way, that it describes displacements instead of the state at a fixed point of space. The pressure, e.g.

p = p0 * (1+dy/dx)^-gamma

which is a function of x, is at the place y(x,t) + x. p0 is the atmospheric pressure.

For small dy/dx the first equation becomes the usual linear wave equation. That won't work to describe your experiment.

I'd consider using helium to break records. Air mass in the barrel, which also has to be accelerated is probably larger than the ball mass.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #7 on: October 21, 2020, 05:00:34 AM »
"d^2y/dt^2 = c^2 * (1+dy/dx)^(-gamma-1) * d^2y/dx^2"

For discrete simulation, would the array of elements be specific quantities (fixed number of moles) of gas along the length, where the discrete gas quantities change position and length as the simulation progresses?  Could that handle a moving boundary condition at one end (position of the ball)?  I'll need to think about the physics in more detail, but that's a ways out.  Want to finish Tesla coil projects first.

Yes, helium is one of the changes we made to improve speed beyond the record "Myth Busters" TV show had achieved a couple years earlier.  The down-side of helium is its high gamma, but low density more than compensates.  Our unofficial max speed of 873m/s is faster than the existing official Guinness record.  The official record is about to increase, however, as someone recently hit 924m/s in preparation for an official record attempt next month.



David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #8 on: October 21, 2020, 07:17:07 PM »
Exactly, the idea is to divide the gas column into initially equally long sections. Every section contains the same amount of gas throughout the calculation. Pressure gradients provide forces on the sections and pressure itself is calculated by the length of the section and the adiabatic compression law. The boundary condition to the ball is given by its mass adjacent to the gas volume.
The boundary to the gas supply is not clear to me though, as there is additional gas mass entering the system. I need to think about this. Perhaps you can't avoid using Navier Stokes. It's more complicated, since you have to solve a system of coupled partial diff eqs.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #9 on: October 21, 2020, 08:21:55 PM »
My intention is to simulate the entire system, so no gas entering.  Time=0 will be when the membrane breaks.  Part of the length will have vacuum, and the rest will have the same initial pressure.  The ball is in the vacuum portion.  However, there will be one or two diameter changes along the length.  I also hope to include flow resistance along the pipe (turbulence), at least some crude approximation, as a refinement after initial runs.
David Knierim

Online johnf

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Re: QCW with replaceable ferrite-core primary
« Reply #10 on: Today at 09:14:24 AM »
Vacuum ie no air
What turbulence???
we are at the crossover between laminar flow and molecular flow--two very different sets of equations / mismatch of the two

good luck Dave

Online klugesmith

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Re: QCW with replaceable ferrite-core primary
« Reply #11 on: Today at 05:33:39 PM »
Carrying on off-topic,  professional hypervelocity guns generally use hydrogen instead of helium.
https://en.wikipedia.org/wiki/Light-gas_gun
Works better, and the fire hazard can be managed.  Just like hydrogen that cools electric generators in most large power plants.

Glad to hear about simulation and modeling, and that a contest has been going on.
Am reminded of the Punkin Chunkin' contests, which came to my attention before anyone had brought pneumatic cannons. In those days the categories were things like elastic slingshots, trebuchets, and centrifugal hurlers.
/>At the time I had been launching small pumpkins (150 mm; 6 inch) from a steel mortar, propelled by 2 liter soda bottles popped with carbon dioxide.  A representative air time is 9 seconds.  Barely pre-Internet, it took a lot of work to find thermodynamic charts for CO2, to get numbers for simulation. 
Could tell about testing gases with different gamma values, but let's have a discussion thread with appropriate title.
« Last Edit: Today at 05:53:22 PM by klugesmith »

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Re: QCW with replaceable ferrite-core primary
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