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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi To clear some misunderstanding:
-The Q and Q-factor in my last post were the same thing. I made this change to distinguish from the reactive power, which is also called Q and measured in VAR.
-Of course you can design the primary tank circuit for a certain optimal Q-factor, but you must then make sure that the real working Q of the tank circuit really matches the calculated one by tuning. Because as I said, the real working Q heavily depends on spark loading.
This suggest I should add some capacitance for the desired theorical Q of 10
But this is weird because I am getting better output when detuning the primary to a higher L, which rather suggest I should decrease C to keep the primary resonant frequency at the correct value.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Hi Tom, thanks ! I've been away for almost a year. I now have some free time and I'm back to HV fun :)
@Dr.DC
Hi Jan, After carefully re-reading the whole thread, I thought that I possibly missed something important
Let's start back here:
Dr. Dark Current wrote ...
1. the doubled peak voltage is 4960 V * sqrt(2) which is ~7 kV, but this is just theoretical, lets assume 6 kV 2. the 572B has a plate dissipation of 160 watts, it is safe to assume plate input of 4 times the plate dissipation which is 640 watts 3. the peak input power is 8/3 times * average input power which is 1700 watts, so with a Q of 10 you need 17 kVAR of peak reactive power 4. at this peak power the rms voltage on the primary circuit is 6 kV / sqrt(2) which is around 4200 volts, you use this to calculate the C and L (from their reactances)
Dr. Dark Current wrote ...
3. I = V/X (X.. Xl, Xc) = Q/V (Q as in reactive power)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi
PhilGood wrote ...
But this is weird because I am getting better output when detuning the primary to a higher L, which rather suggest I should decrease C to keep the primary resonant frequency at the correct value.
What is wrong in my calculations ?
As I said, you NEED TO detune the primary to get the correct impedance, there is no "correct" value other than the one which gives biggest sparks! This value of primary tank resonance is somewhere below the secondary resonant frequency, but you must determine the exact frequency experimentally!
For the calculations of C, Q, etc. you must guess the running frequency. The running frequency is NOT the secondary resonant frequency. For the calculations the usual guess of running frequency is around 0.7 * calculated secondary resonance.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The correct calculations:
Note: I don't know the resonant frequency of your secondary without any extra top load added, so lets assume 2.171 MHz (then you can correct it)
Note: These are simplified calculations, the real thing may behave slightly differently, this is because of the not very well-known behavior of plasma sparks and their influence on the RF power envelope and detuning.
Note: In my calculations, Q=reactive power [VAR], Q-factor="Q" of the tank circuit [-]
1. The plate dissipation of the 572B tube is 160 watts, so assume average plate input power of 160*4 = 640 watts. 2. The peak power for a half wave doubled waveform is 8/3 times the average power, this is 8/3*640 = 1707 watts. 3. The RMS tank voltage during the peak is ideally (transformer output voltage)*2. If we assume some voltage drops on the capacitor and transformer impedance, I will use a correcting factor of 0.9. So Vrms_tank = 2480*2*0.9 = 4464 volts. 4. For a Q-factor of 10, we need Q=17 kVAR of reactive power. 5. The running frequency is approximately 2.171 MHz * 0.7 = 1.52 MHz. 6. C = Q/(2*pi*f*Vrms^2) = 17000/(2*pi*1.52M*4464^2) = 89 pF. For such a small capacitance, you must take into account plate and wiring capacitance. 7. L = approx. 120 uH +-40% for tuning (from the Thompson equation). The coil will get good output with these values or values with a higher Q-factor, however the heating of the coils will be increased.
The design of coils for such a high frequency is not an easy task, as eddy current losses and skin effect losses become severe.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Hi Jan,
Thanks for the detailed answer. VTTC's are a fascinating subject for me, and I really appreciate that you spend time to educate me :)
Dr. Dark Current wrote ...
Note: I don't know the resonant frequency of your secondary without any extra top load added, so lets assume 2.171 MHz (then you can correct it)
I guess you are talking about that 15cm piece of copper wire I added to the topload to simulate sparks capacitance, right ? Without it, secondary resonant frequency is 2.368MHz
After carefuly reading your calculations:
I understand steps 1 to 4
Step 5: "5. The running frequency is approximately 2.171 MHz * 0.7 = 1.52 MHz."
I guess this is because of sparks and plasma detuning secondary. If yes I understand why I should not add any additional top load when measuring my secondary resonant frequency because this is already taken into account with this 0.7 correction factor.
Steps 6 and 7:
I think I did the right calculations. I got wrong values because I missed step 5 (and also beacause I used a peak RMS voltage of 4200V where it should be 4464V)
C = Q/(2*pi*f*Vrms^2) is equivalent to C = 1/(2*pi*f*Xc) with Xc=V/I and I=Q/V. But your single formula is much more friendly than my 3 steps calculation !
I redid the maths with 1.52MHz and 4464V and found the same value of C=89pF. Same for L with L = Vrms^2/(2*pi*f*Q) = 4464^2/(2*Pi*1520000*17000) = 1.22e-4 = 122µH
If I redo the maths with a 2.368MHz resonant frequency (measured without the additional copper wire on top load), I get C=82pF and L=112µH (160µH needed with the +40% for tuning). It seems these are good values to start with
I also understand that even with calculations, there will be a good part of guessing, tuning and experiments (and that's part of what makes Tesla Coils fun). But I would just like to get as close as possible with calculations (and as you can see I was really very far!)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, especially at those high frequencies, the approximate calculations can be more off. Another thing to note is that when you use these calculations, they tend to work good with a coupling coefficient "k" of around 0.3.
You say:
wrote ... I guess this is because of sparks and plasma detuning secondary. If yes I understand why I should not add any additional top load when measuring my secondary resonant frequency because this is already taken into account with this 0.7 correction factor.)
I already explained it but I will explain it once more. The spark capacitance detuning is just a small part, most part of this frequency drop comes from adjusting the required impedance of the tank circuit. Close to resonance, the impedance is very small, so you deliberately detune the tank circuit until it has the correct loaded impedance seen by the tube.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Hi Jan,
Thanks for these additional infos !
I will use JavaTC to design my new primary for a k of ~0.3 with the needed inductance
Thanks also for explaining again the reason for detuning the primary, indeed I didn't get it right earlier, I now understand.
It is obvious you have a great experience with VTTC's!
Just curious, did you find all these calculations and informations by experience ?
Because I really spent nights searching infos about all the calculations you explained me in this topic, but I could not find anything like this and there isn't much VTTC's documentation available.
It seems that most people who build VTTC's don't go through all these calculations, most of them use Steve's formulas from his VTTC FAQ, maybe they don't need more because they mostly replicate some setups that are known to work fine with "standard" values ?
I'm thinking to code a javascript web application to help in all the calculations you explained me, something like JavaTC but much more simple and dedicated to VTTC specific calculations.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, all these calculations are my own work. From all the VTTCs I built, there is a lot of useful data. Of course the formulas are theoretically based, but I use only those which fit all or most of my VTTC data. So practical experience is a must when verifying those equations. Nevertheless, the capacitive plasma discharge is such a complex object, that it is almost impossible to theoretically describe, so most formulas are just approximate with smaller or bigger error depending on the individual coil parameters. This means that tuning must always take place for the best results.
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My first coil : 572B VTTC
