Author Topic: Regarding the Ward QCWDRSSTC  (Read 876 times)

Offline TDAF

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Regarding the Ward QCWDRSSTC
« on: February 10, 2018, 07:36:05 AM »
In his write-up of the Fat Coil
Ward said that he used Ferrite transformers for coupling the Full-Bridges
My question is where can i find such ferrite cores which can withstand that kind of power??

Offline Mads Barnkob

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Re: Regarding the Ward QCWDRSSTC
« Reply #1 on: February 10, 2018, 08:16:58 AM »
I got some different types for sale, they are fairly priced, so do some calculations and see if you can use them.

If you need a lot, we can work out a combined price :)

https://highvoltageforum.net/index.php?topic=51.0
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #2 on: February 10, 2018, 12:16:38 PM »
Sure Thing :)
Which of these cores would you recommend for the use I have in mind?
any other feedback would be appreciated
Also, can i get some more data on these cores like the inductance per turn and  the effective cross-sectional area
« Last Edit: February 10, 2018, 01:16:26 PM by TDAF »

Offline Steve Ward

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Re: Regarding the Ward QCWDRSSTC
« Reply #3 on: February 18, 2018, 07:13:46 PM »
Just for reference, each transformer (there are 8 total) has a core cross section of 20x20mm.  The outer dimensions of the core are 80x80mm if i remember correctly.  So they really are quite modestly sized ferrites, and easily available even from places like Mouser/digikey if you know how to find them :P.  I would favor cores with the lowest losses, since this application is rather high frequency.

The primary wind of each transformer was 10 turns of litz wire (looks like ~14awg equivalent) which was salvaged (along with the cores) from inverter-based welding machines.  Your secondary turns will depend on the impedance of the primary you are driving.  With my 8 transformers, i had essentially 4 secondary turns, which are all in series so the total transformer ratio is 10:(4 * 8 ) or 10:32 (so 450V in gave 1440V driving my primary LC).  The resonance capacitor is on the output side of the transformer bank.  The machine works at ~350 to 300khz, which i think put the flux density in the ferrites at about half of saturation level.  When designing transformers there is a trade between copper loss and core loss.  More turns means less magnetizing force on the core for a given volt-second applied which reduces core loss, but increases copper loss.  My design is not optimized but was based on materials available... i could have fit a LOT more copper in there if i tried. 

Pic of transformer bank: https://www.flickr.com/photos/kickermagnet/15051691754/in/dateposted-public/
« Last Edit: February 18, 2018, 09:47:32 PM by Steve Ward »

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #4 on: February 19, 2018, 08:08:25 AM »
Now, I by all means am an amateur
Pardon the dumb question but
how did you match the impedance of the primary LC with the transformers??
also, are there any other better methods for current sharing??

Offline Hydron

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Re: Regarding the Ward QCWDRSSTC
« Reply #5 on: February 19, 2018, 09:39:14 AM »
I don't think there is explicit impedance matching going on with the transformer - they are stepping up the bridge output (which looks like a voltage source) to drive the LC tank with a higher voltage than is otherwise possible with 600V IGBTs, while allowing easy forced current sharing between a number of small full bridges. Better let Steve comment though!

As for other methods for current sharing, you can split the MMC capacitor into parallel sections either side of the primary coil to allow the use of a few small bridges with forced current sharing and no extra transformer/chokes. This is what I'm doing for my next coil, and judging by his Flickr photos, Steve did it for one of the Tesla guns.

Offline profdc9

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Re: Regarding the Ward QCWDRSSTC
« Reply #6 on: February 19, 2018, 08:29:43 PM »
I have some big ferrites from Surplus Sales of Nebraska I have used for making power ferrite transformers:

https://www.surplussales.com/Inductors/FerPotC/FerPotC-5.html

The U/I core has a pretty large cross-section, about 27 by 25 mm.

Dan

Offline Steve Ward

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Re: Regarding the Ward QCWDRSSTC
« Reply #7 on: March 26, 2018, 04:32:37 AM »
Like Hydron said, the transformers basically just increase the bridge output voltage by the turns ratio.  This transformation of voltage IS impedance matching in essence: i chose a what would be considered a "small" primary capacitance of 13.5nF (about 34 ohms) so that i could use relatively thin primary coil conductors for convenience, but since the effective driving voltage is 1440V rather than typical 350V, the machine processes the "right" amount of power.  Now, the "matching" part is, how did i choose the turns ratio?  Basically, the "impedance" of your H-bridge is Vbus/Imax, you are trying to find the best driving voltage that maximizes the bridge current without demanding "too much" current, so Imax is the maximum current your design is targeting.  If i used a greater turns ratio for higher driving voltage, i would risk blowing out the IGBTs from current overload, or the annoyance that the current limiter has to act, which lowers the power.  If the turns ratio was too low, the bridge current would not reach Imax at the highest supply voltage and that would result in less power.  The thing about tesla coils is that their effective impedance is strongly dictated by the streamer resistance, so you can only experimentally determine the correct impedance matching with actual sparks being produced (or more advanced simulation/theory to account for spark mechanics).

As to the impedance of the transformers themselves, they do contribute some stray inductance and a series resistance to the tank circuit of the DRSSTC.  The stray inductance effectively lowers the coupling coefficient of the "tesla coil", and resistance just makes heat.  Ideally, the transformers would not contribute any inductance, but in practice it is relatively easy to construct transformers that offer relatively small stray inductance relative to the primary coil, so their contribution is simply tuned out by adjusting primary L or C for optimal spark performance anyway.

Sectioning of the primary capacitor is a pretty solid way to force current sharing and i've used it on many occasions.  It does limit your MMC options, but usually it seems to work out fine.  This method should be more efficient.  I liked the flexibility that the transformers offered for the Fat coil, so I explored that idea there instead.

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #8 on: March 27, 2018, 12:56:24 PM »
Hmm....
interesting...
Now, sorry for the digression
but, in your Flickr I see you use an iron powder toroid and the buck inductor
I haven't been able to find a toroid which won't saturate at the currents you run
what is this toroid??
where can I procure such a toroid?

Offline Steve Ward

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Re: Regarding the Ward QCWDRSSTC
« Reply #9 on: March 27, 2018, 06:41:40 PM »
The buck power inductor was scored for free at my first job at an aerospace company designing motor controllers.  They had these huge inductors being thrown away because they were not properly inventoried (along with loads of IGBTs, capacitors and other cool parts i was lucky enough to sort through).  The inductor is overkill, but since it was the right inductance for the job, i used it as is.  The core is 2 of this type stacked, and i think about 24 turns of 00 AWG teflon wire resulting in about 220uH of inductance with very high saturation current >400A:

https://micrometalsarnoldpowdercores.com/pdf/T650-34-DataSheet.pdf

I inquired about the cost for these cores maybe 8 years ago, and the price was reasonable at something like $40 each.


Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #10 on: April 16, 2018, 02:04:07 PM »
Sadly, the prices since then have doubled to $80  :( :(

Offline profdc9

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Re: Regarding the Ward QCWDRSSTC
« Reply #11 on: April 16, 2018, 05:30:31 PM »
I wonder if it would be any cheaper to stack several smaller toroids.  Look at surplussales or Amidon:

http://www.surplussales.com/INDUCTORS/FERTORO/FerToro-1.html
http://www.amidoncorp.com/product_images/specifications/1-27.pdf

You could stack several smaller toroids to get the same inductance while still staying under the saturation field.  The cross section of the combination of toroids increases linearly when you stack.  This might allow you to use many cheaper toroids.  You just have to cram all of the wires into the toroid core window.

Dan

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #12 on: April 16, 2018, 06:43:38 PM »
Sadly, I live in India not USA
The shipping is way too exorbitant and my parents refuse to pay for it :(

Offline profdc9

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Re: Regarding the Ward QCWDRSSTC
« Reply #13 on: April 17, 2018, 04:44:19 PM »
Perhaps its possible to make the transformer air cored.  Basically make a large diameter flat coil so that loop encloses a lot of area and achieves a relatively high inductance.  For example, a simple approach would be to take a multiconductor cable with large wires (for example a cable used for running large 3-phase equipment) and coil this multiconductor cable into a flat multiturn coil, using each of the conductors as a winding.  The insulation on the wires in the cable would need to be rated for high voltage.

Because of the skin effect, it might be desirable to carry the current for a particular winding over several smaller wires.  This would increase the surface area of the collection of wires to reduce the skin effect, like Litz wire.  The smaller wires for the primary winding could be interwoven with the wires of the secondary winding, reducing the leakage inductance as well.   The main problem with this idea I think is that the leakage inductance with an air core is going to be higher, and the amount of wire needed is going to be greater so the ohmic losses are greater.  Carrying the current of each winding over several interwoven wires might help offset both of these problems, the only disadvantage being is that strong insulation will be required between the turns if there is a high voltage.

Dan

Offline Hydron

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Re: Regarding the Ward QCWDRSSTC
« Reply #14 on: April 17, 2018, 04:55:00 PM »
Air core isn't an option sorry - you're after high coupling in this instance which isn't achievable without a core. Maybe you can find something suitable in a scrapyard or something?

Offline profdc9

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Re: Regarding the Ward QCWDRSSTC
« Reply #15 on: April 17, 2018, 05:25:00 PM »
Well, it's been tried, look at page 43 of this DTIC

http://www.dtic.mil/dtic/tr/fulltext/u2/a338885.pdf

They were able to get a k=0.88.  Perhaps not high enough for this use, but should improve at higher frequencies at 300-350 kHz (this transformer was made for 1 kHz operation) because the eddy currents tend to concentrate the field on the surface of the conductors, reducing the magnetizing current inside the wire.   They used long flat sheet conductors separated by kapton layers, however, and interleaved them. The gaps between the wires have to be kept as small as possible to limit the volume the magnetic field between the turns.  Nevertheless, if you can get ferrites, it will be much easier to do this as it will concentrate the field lines between the two windings, and will work much better than air core.

Dan

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #16 on: April 17, 2018, 05:29:53 PM »
Air core isn't an option sorry - you're after high coupling in this instance which isn't achievable without a core. Maybe you can find something suitable in a scrapyard or something?
I live in a small town
No scrap yards

Offline TDAF

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Re: Regarding the Ward QCWDRSSTC
« Reply #17 on: April 17, 2018, 05:30:28 PM »
I am able to procure samples of t520-2
YAY

Offline profdc9

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Re: Regarding the Ward QCWDRSSTC
« Reply #18 on: April 17, 2018, 05:36:13 PM »
Keep in mind 2 mix iron powder has a low permeability, it is made for RF use, mu=10 vs mu=33 for the type 34 suggested by Steve Ward, and wind accordingly.

Dan

Offline Teravolt

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Re: Regarding the Ward QCWDRSSTC
« Reply #19 on: April 17, 2018, 06:22:04 PM »
Hi there all and TDAF, since you are short of materials try stripping a ordinary transformer and using the core. if you trying to filter the output of the buck that may work. another place is that newer microwave ovens use a flyback transformers that have a core. do you have any pictures? and what kind of resources do you have?  a qcw is kind of a task with limited supplies.

High Voltage Forum

Re: Regarding the Ward QCWDRSSTC
« Reply #19 on: April 17, 2018, 06:22:04 PM »

 


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