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Large DRSSTC: choosing secondary impedance

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Kizmo

Mon Apr 21 2014, 12:06PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I managed to have quite epic secondary failure last night and now im looking forward of winding new one. With topload the failed secondary was around 70kOhm impedance at resonance. From earlier conversations I recall several coilers stating that somewhat optimal secondary impedance for big coil is less than 50kOhm. Is this still true? Old secondary was 315x1270mm and I think i will make it a little larger for next coiling season.

Dr. Slack

Mon Apr 21 2014, 08:04PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

To an engineering approximation, 70k *is* 50k, just like secondaries have 1000 turns, +/- 3dB

HV Enthusiast

Mon Apr 21 2014, 10:20PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Can you provide more information regarding your failure?

Looks like you have a single point failure on your secondary which could have been caused by:

1. Faulted wire - kinked wire, overlapped wire, etc....
2. Too much coupling which caused an arc over at a weak point in your epoxy etc...
3. Dust / debris on your secondary causing an arcing problem (I once had a moth land on my secondary coil causing it to arc at that point killing the secondary)
4. Bad grounding (this can cause your secondary peak voltage to occur somewhere other than the top)
5. Improper primary to secondary spacing - voltage breakdown

Kizmo

Tue Apr 22 2014, 04:14AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

This coil is has relatively loose coupling, around .165 and there has *never* been single flashover over the period of 2 years i have run this thing. Failure started as turn to turn arcing ~100mm from the bottom end of the coil.

Im not too concerned about why it failed. When the failure happened we were ripping really loud and bright 3...4x secondary winding length ground strikes at nearly 15kW (yes, thats DC power, 28kVA..)

I suspect the failure has something to do with this

As interesting experiment im planning to build another secondary of same dimensions that has lower impedance, less turns and space winding at least the bottom end of the in hopes of adding more insulation between turns.

Steve Conner

Tue Apr 22 2014, 10:25AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree there is the possibility of some sort of "whiplash effect" where a really hard ground strike launches a travelling wave down the secondary. It may well be reflected from the bottom causing a buildup of voltage there. I think the best solution is to include some sort of impedance in the ground strike circuit.

Uspring

Tue Apr 22 2014, 11:45AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Since the primaries magnetic field is strongest at the bottom of the secondary, the voltage induced in the secondary might cause a particularly strong electric field near the bottom. Spacing the windings at the bottom of the secondary probably helps since it reduces the electrical field there and also improves inter winding insulation.

A side effect of this is, that it will reduce coupling.

Steve Conner

Tue Apr 22 2014, 12:16PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The voltage profiles in a DRSSTC are different to the ones calculated by TSSP/GeoTC, which assumes spark gap operation, exciting both resonant modes equally.

The voltage profile in a DRSSTC depends on which mode is excited. The lower resonant frequency gives a "convex" profile with a high E-field in the vicinity of the primary, as described by Uspring above. The upper one does the opposite, it gives a "concave" profile with a weaker E-field near the primary and a stronger one further up the secondary.

I proposed this years ago and Richie Burnett verified it by experiment. It's still a mystery to me how the primary manages to transfer energy into the secondary when the mutually induced voltage is opposing the self-induced one, but it's a fact that it happens.

Kizmo

Tue Apr 22 2014, 01:59PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

This was filmed after I realized the coil was not worth of repairing. It may look like it but there was no primary-secondary strikes, just more or less shorted turns arc welding each other.

Uspring

Tue Apr 22 2014, 02:25PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Steve wrote:

It's still a mystery to me how the primary manages to transfer energy into the secondary when the mutually induced voltage is opposing the self-induced one, but it's a fact that it happens.

Average power transfer is proportional to the sine of the angle between primary and secondary current. At the lower pole it's closer to zero and for the upper pole closer to 180 degrees. It's best to push a swing hardest, when it's at the bottom

Looking at the video, it seems though, that ground strikes are the culprit.

Goodchild

Wed Apr 23 2014, 03:34PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

This subject is also of interest to me. Because I too run coils with high impedance. My large coil for example has an impedance of near 80K at its 41KHz resonant frequency (this is without spark loading).

302.7mH secondary impedance (calculated)

Without spark loading at 41KHz itâ€™s 78K

With spark loading at 31KHz itâ€™s 58.8K

So after the secondary is loaded it drops down a lot, however when we get ground strikes I imagine this loading drops to nothing (because capacitance drops and frequency goes back up) and the impedance rises back up to near 77k. Am I correct in thinking this?

To my point, ever sense we have started using a counterpoise ground I have noticed that the coil is more prone to flashing over and burring up secondareis. I thought this was just a coincidence but your guyâ€™s comments have made me think otherwise.

The counterpoise ground helps to reduce radiated interference (lower impedance path to ground) however could it be a problem when dealing with ground strikes?

Iâ€™m also curious to where the magic 50K keeps coming from?

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