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I'm not sure how relevant the upper vs. lower pole distinction is to Odin. The bursts are short enough that the system doesn't really settle to a steady state.
You're probably right.
If secondary res frequency shifts due to arc load matters, as a consequence the driving frequency also matters. I guess the PLL starts out with a calculated f value and then adjusts f to have ZCS. Initially there won't be ZCS if running at the upper pole, since with strict adherence to ZCS you'll always end up at the lower pole. For short bursts probably performance depends on your PLL start f.
Do you see any PLL f shifts during the burst and if yes, do you know, whether they are due to the effort of the PLL trying to get to ZCS from its initial value or to an actual change of the ZCS f? In the latter case the ZCS f change should have roughly the same magnitude as the shift of secondary fres.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Hello again everyone,
I will be the first to admit when I’m wrong on something; I absolutely love when it happens because that’s when I learn stuff. I have been talking with another 4hv member over email and during our dialog and SPICE trades a fundamental shift in understanding happened for me.
For starters Steve Ward was correct, my original SPICE simulation was indeed flawed in one way and that was the placement of the lumped capacitance (secondary + topload) capacitance. This should indeed be in parallel with the secondary and this makes a major impact in the operation of the system.
So this time I ran a stepped AC analysis on the revised DR circuit and stepped though couplings 0.1, 0.2, 0.4, and 0.5. Now 0.1k indeed has the largest gain for the system at around 53.3dB while 0.5K is 40.2dB. This is a difference of 13.1dB and I would consider that significant.
Now the next thing we have all been talking about is why this is the case? My explanation is the result of damped oscillators. Both the secondary and primary are damped oscillators. The secondary is damped mainly by the spark loading. The primary is also damped partly by spark loading via the mutual coupling between the two resonators (lower coupling means less influence of Rspark on the primary Q). In addition both circuits have a DC and AC resistance that also damps and limits the max power that can be ringed up in either circuit.
If we take a look at this first simulation that has no limiting resistances (primary or secondary) it can be seen that as coupling is lowered voltage gain of the system increases. As Udo pointed out copper losses will ultimately limit the lower limit you can set the coupling, before gain starts to drop off.
In this second simulation, the primary has a modest 20mOhms of resistance added for AC/DC copper losses and the secondary has 80Ohms added. Now at around 0.1k a drop in gain can be seen and going lower that this would degrade system performance.
The caveat is tuning sensitivity as many have pointed out. I personally think that this is why BSVi experienced poor performance when lowering coupling. To use my simulation as an example; in order for 0.1K to have better or comparable performance to 0.5K the system must stay in tune to within ~350Hz this is incredibly tight! This can be a problem if you have a QCW like BSVi’s that shifted 38KHz during the burst.
Lastly I will present a simulation based on the parameters from BSVi’s setup. I first ran BSVi’s coil through java TC to gets some rough approximations of the electrical parameters of the coil. I used the data provided in the first post on this thread as inputs.
Next I ran the stepped AC analysis with the coil tuned at resonance and found that the optimal coupling for max gain while having a frequency shift of 38KHz is about 0.45k. BSVi’s coupling (calculated) is around 0.534K and this still provides good gain, only differing from the optimal by about 0.7dB. Max optimal gain is around 85.6dB. Anything below this optimal coupling would have degraded the Q of the primary and lowered the overall system gain. This analysis is on par with what BSVi observed when lowering coupling.
So my conclusion: the optimal coupling for the system is all relative and there seems to be a “sweet spot†where going higher or lower would result in poorer performance.
EDIT: As curiosity got the best of me I went on to collect more data on BSVi’s coil from SPICE and evaluated it over the entire coupling range. I collected and graphed both peak gain along with the +-3dB frequency window in which this gain resided. The results are very interesting.
For starters Steve Ward was correct, my original SPICE simulation was indeed flawed in one way and that was the placement of the lumped capacitance (secondary + topload) capacitance. This should indeed be in parallel with the secondary and this makes a major impact in the operation of the system.
I also agree with Wards point wrt to how where arc load happens. But from the perspective of a simulation the important parameter is the secondary Q. It doesn't make much of a difference whether it is introduced by a series or a parallel resistance. The values of the resistances are quite different for a given Q, though, since Q=R/sqrt(L/C) for a parallel resistor and Q=sqrt(L/C)/R for a series one.
If you want ZCS, there is not much of a choice, which frequency you run at. Even if you have a large frequency window for a big coupling, it does not necessarily imply, that the coil will run within it. If, e.g. the primary and secondary are tuned to the same frequency fres, the coupling will cause 2 frequencies (actually 3, but I will disregard that here) at which there is ZCS. One is below fres and one above. The larger the coupling is, the further they will move away from fres.
A large coupling will increase performance by making the secondary see more of the primary field, but it will also decrease performance by moving the ZCS frequency away from the optimal point, i.e. the secondary fres. These effects almost cancel each other.
So what is the advantage of a large coupling? Since the operating frequency and the secondary fres are already quite far away from each other, the change of secondary fres due to arc load does not make much difference anymore (relatively). The coil is less sensitive to arc detuning.
Registered Member #1637
Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
SimpleDriver is now up and running. I'v also added QCW mode to my bluetooth interrupter to control it with smartphone. Coil itself is not tuned well yet, but it works. I'm working on documentation now. Here is a short overview video (yes, worst-english-you'v-ever-heared warning):
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Does the touchscreen on your phone work near a Tesla coil? :O
Uspring: When damping due to streamer loading is taken into account, you do not have 3 discrete frequencies any more. It starts to look more like a filter with a passband.
Uspring: When damping due to streamer loading is taken into account, you do not have 3 discrete frequencies any more. It starts to look more like a filter with a passband.
Yes, there's only one ZCS left, if Qsec drops below 1/k, depends somewhat on tuning. Another advantage of high coupling is, that you can reduce the number of primary turns which lowers primary losses and leads to faster primary current rampup.
Registered Member #1637
Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
Does the touchscreen on your phone work near a Tesla coil? :O
It depends on touchscreen technology. Some screens are suspeciblie to noise from tels coils. For those screens I have special mode that disables touchscreen while coil is running. When this mode is activated, hardware buttons should be used to stop coil.
Older single-touch and virtual multitouch (as in my LG P500) capacitive screens and, especially, resistive touscreens works just fine with coil nearby.
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Increasing QCW streamer length
