Author Topic: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory  (Read 227 times)

Offline Benbmw

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I am planning to build a miniature DRSSTC and have been thinking of ways to overcome the high resonant frequencies inherent to small coils.

I am wondering how it would affect the performance to have a multilayer secondary or a “grouped” multilayer secondary similar to what is explored in this paper:
https://www.researchgate.net/publication/317145139_A_compact_multi-wire-layered_secondary_winding_for_Tesla_transformer

Other than that paper one of the only other discussions specific to this idea is on stack exchange where it is suggested that due to the non-transformer-like operation of a Tesla coil, any thing but a single layer winding would reduce performance:
https://electronics.stackexchange.com/questions/234726/can-a-tesla-coils-secondary-wiring-overlap

Do the explanations presented there seem to be accurate with y’all’s experience and the operating theory of Tesla coils? I’ll be honest that while I understand what he is saying, the explanation does not make intuitive sense to me.

Another idea I had for reducing secondary resonant frequency is to use some sort of ferrite core. The main objection to this is the extra loss added from the core. Most discussions also pointed out that the core is likely to saturate due to the high current in the primary; but what if the core only occupied the top half of the secondary to still increase its inductance while escaping most of the flux from the primary?

Feel free to weigh in on any aspect of this discussion and correct me where my assumptions are wrong!
« Last Edit: November 10, 2020, 10:23:05 PM by Benbmw »

Offline davekni

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #1 on: November 11, 2020, 03:46:31 AM »
Multi-layer winding in grooves as described in your first link is common for HV transformers (ie. TV flybacks) and can work for a Tesla coil.  However, it's lots of work.  Care must be taken at the transitions from the top of one groove to the bottom of the next.  Tesla coil secondaries often have hundreds of volts per turn.  If the wire at crossover touches the bottom turn of the old finished groove or the top turn of the new groove (after the new groove is wound), then there's multiple turns worth of voltage across thin enamel insulation, which will fail.

Multi-layer winding as in the second link does not work.  One is the issue mentioned about excess parasitic capacitance.  The other is similar to above (but much worse): many turns worth of voltage across thin enamel insulation.

I'd suggest thin wire and careful tightly-packed winding.
David Knierim

Offline Mads Barnkob

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #2 on: November 11, 2020, 09:50:12 AM »
The first link, with 3 wires in a grove, that would need extra insulation between those 3 turns in the grove as well.

As Dave suggests, we are usually just interested in lowering the resonant frequency and not so much constrained by physical dimensions.

So it is easier to just choose a thinner copper wire, wider secondary form, longer winding length or a bigger topload for more capacitance.

The high voltage per turn ratio can be mitigated with space winding the copper wire and add A LOT of varnish, thick epoxy layer or 10-20 layers of polyurethane, but space winding only brings the frequency up.
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Offline johnnyzoo

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #3 on: November 11, 2020, 06:13:37 PM »


The high voltage per turn ratio can be mitigated with space winding the copper wire and add A LOT of varnish, thick epoxy layer or 10-20 layers of polyurethane, but space winding only brings the frequency up.

A bit off topic but has anyone considered polyimide coating for tesla coil? It should have high dielectric strength as well as high temperature resistance. At least some polyimide coating compounds must be cured in oven which makes them less attractive to hobbyists but are there any that would cure in room temperatures?

Offline Benbmw

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #4 on: November 11, 2020, 07:12:52 PM »
Thanks Dave and Mads!

Mads when you say:

The first link, with 3 wires in a grove, that would need extra insulation between those 3 turns in the grove as well.

Why is it that those 3 “vertical” turns would need extra insulation? Wouldn’t it be equivalent to 3 turns side by side?

What I hear both of you saying is that it’s just not practical due to the high V/turn in Tesla coil secondary coils. I am still curious though, in theory is there anything that would prevent one of these schemes from working given enough insulation? I know the inter-winding capacitance increases; is it known how much of a practical effect this has on performance?

Like you all have said, space is typically not a constraint and therefore a single layer coil is the best solution, but I am still curious for the case that a compact coil is needed (say to integrate into another assembly) would this method work in theory.

The most promising scheme seems to be making maybe 3x3 bundles of turns spaced say 2 or 3 turns apart. At this point you have 6 turns of voltage between layers and some enameled wire can handle ~10kV, or some sort of insulator (thin Teflon sheet?) can be placed between layers.

Offline davekni

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #5 on: November 11, 2020, 07:31:49 PM »
The three vertical turns don't need extra insulation unless the cross-over from one set to the next touches.  Groups of 3 x 3 in larger grooves could also work, but would likely need to be filled with resin.  Even if wire enamel were rated for enough voltage, corona between wires will attack the enamel and cause eventual failure.  Such groups in grooves (segmented bobbins) are the common construction for HV transformers, potted or in oil to avoid corona.

Winding all the way to one end and then back to the start (as in the second link of your initial post), even with enough insulation, would work poorly.  The inter-winding capacitance would be much higher than top-load capacitance.  Frequency and impedance would be low, so require more secondary current to get enough voltage for break-out.

Concerning ferrite cores, that is possible in certain situations, such as Jan's impressive QCW coil:
https://highvoltageforum.net/index.php?topic=1073.msg7588#msg7588
That inspired me to build a ferrite-core QCW as well, with a replaceable core.  It's a work in progress:
https://highvoltageforum.net/index.php?topic=1268.msg9330#msg9330

The biggest issue with ferrite cores is their electrical conductivity.  I tried a ferrite core in my first SGTC attempt, wrapped in ~100 layers of 150um polyethylene.  Corona destroyed the polyethylene in short order.  Most power ferrites are MnZn, with conductivities on the order of 10 ohm-cm at room temperature.  NiZn ferrites have much lower conductivity.  However, being semiconducting ceramics, conductivity increases exponentially with temperature.  By 50-60C, even NiZn ferrites become conductive enough to short Tesla coil voltages.  Hysteresis losses will quickly heat the core to that temperature.

Ferrite conductivity is why they need to be potted in insulating material for HV use.  I didn't understand corona enough eight years ago to realize that high total insulation thickness is not sufficient.  Corona in the remaining air gaps destroys the insulation.  So the entire space needs to be filled with insulating material, oil or potting compound of some sort.  Vacuum is used to remove air bubbles.  (There's a thread or two here on vacuum-potting small HV coils - which I found quite informative.)
« Last Edit: November 11, 2020, 09:25:11 PM by davekni »
David Knierim

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Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
« Reply #5 on: November 11, 2020, 07:31:49 PM »

 


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