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Increasing QCW streamer length

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Steve Conner

Thu Jul 17 2014, 09:51AM

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

Yes, but the length of streamer from a QCW depends on what happens throughout the whole burst, not just the end, so you can't understand it by considering just the zero phase shift case.

The only reason to shift "both" bridges against the current would be if you had a halfbridge.

BSVi

Thu Jul 17 2014, 12:40PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

In my opinion, secondary, loaded by streamer this big, is mostly active load. We use it's resonant nature only to ignite streamer, then we simply pump power into streamer.

At this stage we need high coupling to quickly transfer power so streamer stays hot. Hot streamers branches less.

IMHO, the biggest challenge in QCW design is to make so that streamers are not branching.

As for phase shift tuning, I tune it so at lowest frequency (at the end of the bang) it outputs 100% power. I tried different settings and different ramp profiles (exp and log of different powers). Linear ramp and this settings is the best combination as I found out.

teravolt

Thu Jul 17 2014, 02:00PM

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

BSVI are you doing frequency shifting or phase shift?

BSVi

Thu Jul 17 2014, 03:19PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

teravolt, i'm doing phase shifting

Ash Small

Thu Jul 17 2014, 03:48PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Goodchild wrote ...

From what I have found the limiting factor is usually how well you can keep the coil in tune. As sparks get bigger and the secondary get more out of tune with the primary and requires more and more power to increase spark length.

So if you start off with it 'out of tune' so that, as streamer length increases, in comes into tune, you'd be able to get bigger sparks?.....Or would you not be able to grow the sparks in the first place?....As long as you can get sparks they should grow as it comes more into tune?.......Depends how far out of tune it is to start with, I suppose?

Goodchild

Thu Jul 17 2014, 04:04PM

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

So to illustrate my point I have generated a simple LT SPICE simulation that models a basic ZCS dual resonant transformer with a resistive load on the output. Yes, yes the resistive load is not perfectly accurate I know, but it will suffice to illustrate this concept. The driving source is a behavioral voltage source running primary current feedback such that it will ZCS. The output is +-100VDC. Also all tests are performed with a 1mS burst.

I have set up LT SPICE to step through several coupling coefficients so that we can observe power at various points in the circuit. No other parameters are changed.

The lower window of traces are output power (as defined by V * I of R1 the load resistance) we will assume that higher power here represent more output power for driving the sparks and as a result longer sparks.

The middle window is input power (as defined as V * I of B2). This is the input power to the system. It should be noted that because my setup is simple and has no bulk capacitor it can be assumed that all reactive AND real power must flow through this behavior voltage source B2.

The final windows at the very top is the difference of input power to output power and is a measure of the reactive power element of the system (i.e. power that doesnâ€™t go towards making bigger sparks but instead just sloshes around in the primary LC and bridge).

The last bit of information to note is the trace colors. Green is the lowest coupling (0.1) blue is the next highest (0.2) red (0.3) and so on as the traces get smaller and smaller.
So for better illustration of this topic I will pick two couplings and compare them side by side: 0.1k and 0.4k

Letâ€™s start with input power:
0.1k = 12.448Kw
0.4k=3.282Kw

Next output power:
0.1k= 6.742Kw
0.4k=3.114Kw

I can see why it could be thought that lower coupling could produce bigger sparks, because input power and output power are both indeed larger with lower coupling! However this can be misleading, we must look at what part of this power is reactive and not used for spark production.

Reactive Power:
0.1k= 5.706Kw
0.4k=0.168Kw

Now it can be seen that the amount of reactive power in the system is much lower with a higher coupling. However this is still not a fair comparison, because a looser coupled system will natural try to draw more power. So we must run an additional simulation.

We will now tweak the drive voltage such that the 0.4K system also draws around 12.4Kw and then look at reactive power again. After increasing the bus voltage to +-195VDC the 0.4K coupled system now draws 12.479Kw making it a valid comparison for the early 0.1K test.

0.4K at +-195VDC:
Input Power = 12.479Kw
Output Power = 11.838Kw
Reactive=0.641Kw

Now 0.641Kw is still much lower than the 5.7Kw seen with the system coupled at 0.1k for the same input power.

Lastly we can calculate the ratio of transferred to total power for both systems:
0.1k = 54.2%
0.4k=94.88% (100V bus)
0.4K=94.86% (195V bus)

So as an extra validation it can be seen that the ratio holds true even at varying power and bus voltage for the same coupling coefficient.

So in conclusion by lowering coupling it can be misleading, because input and output power will grow without varying any other parameter , leading to bigger output, However it is much less efficient from a reactive power stand point.

I hope this helps to clear up any confusion, sorry for the technical rant.

BSVi

Thu Jul 17 2014, 04:21PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

Excelent investigation, Goodchild! This is closely maches what i'm observing in real system. Seems that increasing bus voltage (and coupling) still the best way to increase streamer.

Maybe, it's even feasible to build buck-boost and use 1200v IGBTs to get ultralong streamers :)

As for tune, my experementation shows that you don't need good tune to have good streamers. That profs that secondary is almost resisitve load with low Q.

Goodchild

Thu Jul 17 2014, 04:56PM

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

BSVi wrote ...

Excelent investigation, Goodchild! This is closely maches what i'm observing in real system. Seems that increasing bus voltage (and coupling) still the best way to increase streamer.

Maybe, it's even feasible to build buck-boost and use 1200v IGBTs to get ultralong streamers :)

As for tune, my experementation shows that you don't need good tune to have good streamers. That profs that secondary is almost resisitve load with low Q.

BSVi, I agree, this is what I observed with my QCW as well. As an alternative to making the bus voltage higher when raising coupling, the tank can be made lower impedance. This is useful if you are already running near the max bus voltage the system can handle. The trade off however is having to deal with larger currents in the primary LC.

The tuning part is curios; I also observed this with my system. I could change tuning by full turns and still produce much the same output for the same input power. I would assume part of this is due to the tank LC being high impedance, but I imagine coupling also plays a part. I need to think on this some more.

On another topic:

By chance have you measured the frequency/phase shift from X meter of spark from your system? Iâ€™m currently working on a spark mode for QCW sparks. And I need more data than what I can gather from my system.

Here is what Iâ€™m doing. I measure the L , C, and R values of the secondary using a high frequency LCR bridge at resonance. (Calculated values would also work) This gives me the base resonant frequency of the secondary with no spark loading. Then what I do is look at the frequency/phase shift of the secondary (via a base current CT) with various lengths of spark loading down the secondary.

Using this data we can then graph frequency shift per unit length of spark loading. This will give an approximate mathematical model of the sparkâ€™s parasitics. We can then use that model to find approximate L and C values per unit length of spark.

BSVi, would you be interested in doing a measurement of frequency/phase shit for various lengths of sparks on your system? In addition provide your secondary parameters? I would be interested to do a comparison.

EDIT:
Iâ€™m not sure if you have accesses to a deep memory O-scope, but this works very well for capturing a whole burst so that frequency/phase shift can be analyzed for the whole period of a single burst.

teravolt

Thu Jul 17 2014, 07:13PM

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

BSVI, since you phase 2 dual half bridges ( a form of PWM as Steve said) do you try to sync the phasing to the ring up in the primary because when I watch your video some times the spark starts to leave but does not seam to go the full distance? Question, should the energy lead the ring up in the ramp up via PWM or phasing? like in a car you can advance or retard your ignition to affect efficiency. In newer cars they have variable timing to increase efficiency. Can this type of logic be applied to a tesla coil. does that make sense?

Steve Ward

Fri Jul 18 2014, 07:26AM

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

First, excellent work BSVi

.

Second, this whole coupling vs power confusion i think can be answered by saying "the copper losses are significant". I think Usprings analysis agrees completely with my line of thought: we simply get better results from higher coupling because it lowers the primary resistive losses by requiring less stored energy. Storing more energy is required when you want a bigger spark; its just the price you pay for using resonance. If we could avoid the resonant transformer, the efficiency should be better still, but good luck making any other type of HV transformer operate at >300khz and produce the required voltage.

Eric, you missed in your simulation in 2 ways. The first one is that you modeled the secondary circuit incorrectly as a series LC, which confuses the analysis. Secondly, you didnt pay any attention to the resistances of the coils, which i think its equally important to the design as the coupling or impedance of the tesla coil as a whole. Simply ignoring it will be missing the point.

In summary, if you want more power you need to lower resistances by using bigger wire and less of it, which results in lower coil impedances. Want proof?

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