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Tuning and Frequency Splitting within a DRSSTC (or two coil) system

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LAN_403

Part Scavenger

Sat Apr 15 2006, 12:07AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

/me bows to the brains in this thread.

This is great stuff!

HV Enthusiast

Sat Apr 15 2006, 12:39AM

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

wrote ...

I think the optimal tuning depends a lot on the system itself. As ive said many times before, i think only small coils should need to use the "detuning trick" to get a bigger bang before notching.

If i'm not mistaken, if we detune a coil (either toward the upper or lower pole frequency), the distance between upper and low poles should increase. Since this difference is equal to the beat envelope, wouldn't we be decreasing the time before a notch occurs? Of course, unless our intention is the make the beat frequency so high that it basically disapears.

Steve Ward

Sat Apr 15 2006, 10:50PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

If i'm not mistaken, if we detune a coil (either toward the upper or lower pole frequency), the distance between upper and low poles should increase. Since this difference is equal to the beat envelope, wouldn't we be decreasing the time before a notch occurs? Of course, unless our intention is the make the beat frequency so high that it basically disapears.

I cant say that i completely understand what is going on here, but this is how i see it. As Jimmy mentioned before, in the transient state, we are exciting both frequencies, and depending on the tuning we will excite one more than the other. With sufficient detuning, (and i only have experience with tuning the primary lower) we can basically completely eliminate the higher pole response, giving no apparent notching. This indicates that there is very little power transfer taking place (otherwise a notch would occur). But, eventually the secondary energy builds enough to achieve a small breakout, at this point i believe that the Qsec drops sufficiently to allow good energy transfer between primary and secondary circuits. Once this happens, the streamer grows quickly, draining almost all of the energy out of the system, producing what would normally look like a "notch" on the primary current waveform. This is how i tune my small coils. You have to be careful not to detune the primary too far, or else tremendous currents will build before the secondary breaks out. Otherwise, this tuning trick seems to produce the longest possible streamers for my smaller coils (i believe due to the large amount of energy stored in the primary by the time of breakout, serving to feed the streamer very quicky). Note, this is using primary feedback, i dont even think this detuning trick is possible with secondary feedback due to it naturally driving exactly at secondary Fres.

Hopefully that clears up what i said before.

HV Enthusiast

Sat Apr 15 2006, 11:09PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Okay, so that makes sense. In fact, i have already seen the results of this using a spectrum analyzer to look into the primary current waveforms (harmonics)

One thing i noticed, is that even when i detune quite a bit, there is still a noticeable upper hump present, so it doesn't disapear completely. However, as you said, its magnitude of the harmonics of the primary current at the upper pole is minimized by this detuning.

Next week i'm going to take some more measurements and vary the tuning across the entire gamut (on the primary coil) and then make plots of the harmonics in the current waveforms. This should show a nice trend as described. I'll post my data and waveforms when its complete. PSPICE can easily do this, but it will be nice to backup the simulations with real experimental data.

Should be an interesting test!

teravolt

Sun Apr 16 2006, 05:32AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

To tune my tesla I use a sweep generator and the notches or side bands are quite visible wich makes it easy to see it on a o-scope. If the side bands are not equal and the driver can be set to a spesific frequency and that frequency is set to the dominate notch the sreemers will be gratest if I understand correctly. The sparks may be longer but I would think that there is less current behind them when thay connect with a ground. In a DRSSTC doesn't this couses Steve connors mode hopping besause of the secondary feedback. Does it make it harder to breakout? In other words isn't the cupling afected when boath primary and secondary are not in tune and or the set frequency is not centered. If every thing is centered won't the sparks be hotter and and eficentcy improved. In a DRSSTC is the center frequency ever used because of mode hoping and if this is the case why not use a bandpass filter on the seconary cupling transformer before it gets to the feedback circuitry. Does anybody undrstand what I am saying or am I rambling? N.B.

HV Enthusiast

Mon Apr 24 2006, 06:45PM

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

wrote ...

Note, this is using primary feedback, i dont even think this detuning trick is possible with secondary feedback due to it naturally driving exactly at secondary Fres.

Yes, that is correct. It is only an option for primary feedback. With secondary feedback, no matter how you tune the primary (either above or below the natural secondary resonant frequency), the system will operate almost exclusively at the lower pole frequency of the coupled system. There is harmonic content at the upper pole frequency, but it is very low. I have verified this with both simulation and using a spectrum analyzer (coupled to a current transformer monitoring primary current)

Terry Fritz

Mon Apr 24 2006, 10:22PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

In my case, I use very small IGBTs at about 1500W:

I pretty much have to use primary feedback since I can't afford any extra heating do to missing switching at the zero crossing. If it accidently switches at full current, it will blow up. My little IGBTs fall out of saturation at about 750 amps. Goodness only knows how many thousands of amps those big bricks can take!!

But if you use really large IGBTs that can withstand a full current switching event (or many in a row), then you can go with the primary and antenna feedback. So you basically have to use large IGBTs that can't be easily broken at all.

Primary switching can follow the primary waveform very well but there always seem to be some time lag. Better circuits might fix that.

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