Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Thu Apr 13 2006, 02:12PMJust a question regarding frequency splitting within a two coil coupled system.
With two coupled coils, there is a frequency split which occurs that is depending on the coupling factor. The higher the coupling, the greater the frequency split. That we know. Now, with DRSSTCs, it has been stated that tuning the primary circuit to the lower frequency pole of the frequency split is beneficial for DRSSTC systems.
The question is, and its not as easy or straightforward a question as it sounds, is what exactly is occuring when you "tune" the primary to the lower pole of the split frequency. Firstly, as you decrease the natural resonant frequency of the primary circuit, the lower pole frequency will move as well, so you will never match the natural primary frequency with the lower pole frequency.
Does tuning the primary circuit lower in frequency (closer to the lower frequency pole) force the feedback to operate the DRSSTC at the lower pole frequency? To basically kind of fall in place?
What are your thoughts?
Dan
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Steve Conner, Thu Apr 13 2006, 02:53PM
I'm not telling, but you can buy my book that explains it for only $39.99
Ha-ha only kidding.
It's true that you will never match any of the pole frequencies with the primary frequency.
Now, as you change the tuning of the primary relative to the secondary, both pole frequencies move, and (this is the important bit) their relative magnitudes also change, as follows:
If you tune the primary to a lower frequency than the secondary, both pole frequencies move downwards. The magnitude of the lower pole increases and that of the upper pole decreases. (assuming the magnitude you're measuring is primary current: for secondary voltage the opposite is true.)
If you tune the primary to a higher frequency than the secondary, exactly the opposite happens. Both pole frequencies move upwards, and the primary current is heavier at the upper pole than the lower one.
Now, a simple feedback DRSSTC will operate at whichever pole has the highest loop gain. So the pole you end up operating on depends on whether you tune the primary above or below the secondary, whether you use primary or secondary base feedback, and whether streamer loading ever pulls the secondary below the primary, causing it to swap poles mid-burst. (I like to call that mode-hopping.)
In contrast to that, my PLL driver operates at whichever pole I tell it to, which made the whole puzzle that bit easier to unravel. I figured it out while designing and testing the driver, and tried explaining it to a bunch of other people, but I don't think any of them ever understood. I found that with this driver, I got best results operating at the upper pole frequency with the primary tuned lower than the secondary. That isn't possible with an ordinary feedback driver, unless you use secondary base current feedback.
Under extremely heavy streamer loading, both poles flatten out and the response looks like that of a 4-pole bandpass filter. It doesn't really matter which "pole" you drive at under those conditions.
Steve Conner, Thu Apr 13 2006, 02:53PM
I'm not telling, but you can buy my book that explains it for only $39.99
Ha-ha only kidding.It's true that you will never match any of the pole frequencies with the primary frequency.
Now, as you change the tuning of the primary relative to the secondary, both pole frequencies move, and (this is the important bit) their relative magnitudes also change, as follows:
If you tune the primary to a lower frequency than the secondary, both pole frequencies move downwards. The magnitude of the lower pole increases and that of the upper pole decreases. (assuming the magnitude you're measuring is primary current: for secondary voltage the opposite is true.)
If you tune the primary to a higher frequency than the secondary, exactly the opposite happens. Both pole frequencies move upwards, and the primary current is heavier at the upper pole than the lower one.
Now, a simple feedback DRSSTC will operate at whichever pole has the highest loop gain. So the pole you end up operating on depends on whether you tune the primary above or below the secondary, whether you use primary or secondary base feedback, and whether streamer loading ever pulls the secondary below the primary, causing it to swap poles mid-burst. (I like to call that mode-hopping.)
In contrast to that, my PLL driver operates at whichever pole I tell it to, which made the whole puzzle that bit easier to unravel. I figured it out while designing and testing the driver, and tried explaining it to a bunch of other people, but I don't think any of them ever understood. I found that with this driver, I got best results operating at the upper pole frequency with the primary tuned lower than the secondary. That isn't possible with an ordinary feedback driver, unless you use secondary base current feedback.
Under extremely heavy streamer loading, both poles flatten out and the response looks like that of a 4-pole bandpass filter. It doesn't really matter which "pole" you drive at under those conditions.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Thu Apr 13 2006, 05:15PM
Thanks. Okay, that makes more sense now. I did simulate it (see attached), so the upper pole's magnitude does increase when primary frequency is tuned higher than the natural frequency.
Thanks again!
Dan
]1144948550_15_FT7499_freq_split01.pdf[/file]
HV Enthusiast, Thu Apr 13 2006, 05:15PM
Thanks. Okay, that makes more sense now. I did simulate it (see attached), so the upper pole's magnitude does increase when primary frequency is tuned higher than the natural frequency.
Thanks again!
Dan
]1144948550_15_FT7499_freq_split01.pdf[/file]
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
JimmyH, Fri Apr 14 2006, 12:38AM
I'd agree with all that, if we were running steady state, but the whole point of the DRSSTC is to not be anything like CW :P
If you use primary feedback (which, I would argue is the best and safest way, since it is the only way to ensure you switch after the 0 crossing), you switch when the primary current crosses 0.
If you look at the primary current, it is not a single sine wave, there is some beating going on. If you do a Fourier transform, you'll find that it is oscillating simultaneously at both frequencies, and therefore, you are driving it at both frequencies at once (although they aren't driven equally).
Right, and the since the lower pole is bigger, the primary drives the lower pole more. It 'beats' less, but in the transient response, it is still exciting the upper pole.
I would call that "driving the lower pole", and I think this is Dan was looking for.
Say you had it set up so that the poles were nearly equal in magnitude (ie forming full notches) , but streamer loading pushed the balance a bit, it would just drive the other pole slightly more. There is generally not enough time for a full "mode hop" in a DRSSTC.
JimmyH, Fri Apr 14 2006, 12:38AM
Now, a simple feedback DRSSTC will operate at whichever pole has the highest loop gain. So the pole you end up operating on depends on whether you tune the primary above or below the secondary, whether you use primary or secondary base feedback, and whether streamer loading ever pulls the secondary below the primary, causing it to swap poles mid-burst. (I like to call that mode-hopping.)
I'd agree with all that, if we were running steady state, but the whole point of the DRSSTC is to not be anything like CW :P
If you use primary feedback (which, I would argue is the best and safest way, since it is the only way to ensure you switch after the 0 crossing), you switch when the primary current crosses 0.
If you look at the primary current, it is not a single sine wave, there is some beating going on. If you do a Fourier transform, you'll find that it is oscillating simultaneously at both frequencies, and therefore, you are driving it at both frequencies at once (although they aren't driven equally).
If you tune the primary to a lower frequency than the secondary, both pole frequencies move downwards. The magnitude of the lower pole increases and that of the upper pole decreases. (assuming the magnitude you're measuring is primary current: for secondary voltage the opposite is true.)
Right, and the since the lower pole is bigger, the primary drives the lower pole more. It 'beats' less, but in the transient response, it is still exciting the upper pole.
I would call that "driving the lower pole", and I think this is Dan was looking for.
Say you had it set up so that the poles were nearly equal in magnitude (ie forming full notches) , but streamer loading pushed the balance a bit, it would just drive the other pole slightly more. There is generally not enough time for a full "mode hop" in a DRSSTC.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
teravolt, Fri Apr 14 2006, 02:19AM
In my SSTC I can controle what ever frequency I chose. I noticed that if the tesla is sligtly detuned, eather pole becomes dominate. If it is detuned will the will the stremers be afected in amplitude if the frequency is set to the pole with the greatest amplitude? If the frequency is set to the center frequncy and boath poles are equal isn't more eficiant at F resonant because coupling is gratest?
teravolt, Fri Apr 14 2006, 02:19AM
In my SSTC I can controle what ever frequency I chose. I noticed that if the tesla is sligtly detuned, eather pole becomes dominate. If it is detuned will the will the stremers be afected in amplitude if the frequency is set to the pole with the greatest amplitude? If the frequency is set to the center frequncy and boath poles are equal isn't more eficiant at F resonant because coupling is gratest?
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Fri Apr 14 2006, 02:49AM
Cool! Thanks for the comments again everyone. I did manage to take some measurements with a spectrum analyzer this evening on one of my DRSSTCs and was able to get a nice double-hump response and I did indeed verify that i could reverse the magnitudes off the upper and lower pole by adjusting primary tuning.
Thanks again
HV Enthusiast, Fri Apr 14 2006, 02:49AM
Cool! Thanks for the comments again everyone. I did manage to take some measurements with a spectrum analyzer this evening on one of my DRSSTCs and was able to get a nice double-hump response and I did indeed verify that i could reverse the magnitudes off the upper and lower pole by adjusting primary tuning.
Thanks again
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Steve Conner, Fri Apr 14 2006, 10:51AM
Woo! That's cool that you were able to get experimental results that agree with the theory. It's even cooler that you have a spectrum analyser
JimmyH: I like to think that burst lengths in a DRSSTC are long enough for it to be considered as "sort of steady-state-ish". To deal with the non-steady-stateness of things, I imagine that the drive from the inverter isn't a single frequency, but a band of frequencies, on account of the burst not being infinitely long. (The fewer cycles in the burst, the wider the band of frequencies.) Some of the energy in that band ends up exciting the other pole from the one you're trying to drive, and this is what causes any ripples in the envelope you might see, as the two modes beat against each other.
In the coil I have just now, with a high operating frequency, longish burst, and tight coupling, the unwanted mode hardly gets excited at all. There are no visible ripples or beats in the RF envelope. But I imagine as I build a bigger coil it'll need looser coupling to avoid flashovers, and a shorter burst length in terms of cycles, both of which will get the unwanted mode excited more. So I may need to learn how to drive both modes together, or it may turn out that my PLL driver can't do that and I need to use Steve Ward's design
It's interesting to note that Jimmy's original DRSSTC used a drive frequency midway between the two poles to excite both modes equally, and used the beating between them to his advantage, to get complete energy transfer like in a classical Tesla coil. But nowadays we seem to try and drive one mode much more than the other.
Steve Conner, Fri Apr 14 2006, 10:51AM
Woo! That's cool that you were able to get experimental results that agree with the theory. It's even cooler that you have a spectrum analyser
JimmyH: I like to think that burst lengths in a DRSSTC are long enough for it to be considered as "sort of steady-state-ish". To deal with the non-steady-stateness of things, I imagine that the drive from the inverter isn't a single frequency, but a band of frequencies, on account of the burst not being infinitely long. (The fewer cycles in the burst, the wider the band of frequencies.) Some of the energy in that band ends up exciting the other pole from the one you're trying to drive, and this is what causes any ripples in the envelope you might see, as the two modes beat against each other.
In the coil I have just now, with a high operating frequency, longish burst, and tight coupling, the unwanted mode hardly gets excited at all. There are no visible ripples or beats in the RF envelope. But I imagine as I build a bigger coil it'll need looser coupling to avoid flashovers, and a shorter burst length in terms of cycles, both of which will get the unwanted mode excited more. So I may need to learn how to drive both modes together, or it may turn out that my PLL driver can't do that and I need to use Steve Ward's design
It's interesting to note that Jimmy's original DRSSTC used a drive frequency midway between the two poles to excite both modes equally, and used the beating between them to his advantage, to get complete energy transfer like in a classical Tesla coil. But nowadays we seem to try and drive one mode much more than the other.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Steve Ward, Fri Apr 14 2006, 06:14PM
Actually, on my larger coils, im not detuning much at all (just enough to compensate for streamers)... in fact, now im using beats as the shut off point for my drive. The only reason i detuned was to eliminate notches so that i could constantly increase primary energy in order to get a bigger bang. But with the big coils, i found i can get plenty of bang energy in just a few cycles (maybe 7-9 cycles). So now i just tune it to notch and turn off the drive at that point. At high power, the notching isnt so apparent, but there definately is a drop off in primary current at the end. I gained much of this insight from my pspice simulations. As mentioned before, a notch indicates that secondary voltage (energy) is max, making it the most effective time to shut down the inverter. This tuning method has gotten me to 12' sparks now.
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.
Steve Ward, Fri Apr 14 2006, 06:14PM
It's interesting to note that Jimmy's original DRSSTC used a drive frequency midway between the two poles to excite both modes equally, and used the beating between them to his advantage, to get complete energy transfer like in a classical Tesla coil. But nowadays we seem to try and drive one mode much more than the other.
Actually, on my larger coils, im not detuning much at all (just enough to compensate for streamers)... in fact, now im using beats as the shut off point for my drive. The only reason i detuned was to eliminate notches so that i could constantly increase primary energy in order to get a bigger bang. But with the big coils, i found i can get plenty of bang energy in just a few cycles (maybe 7-9 cycles). So now i just tune it to notch and turn off the drive at that point. At high power, the notching isnt so apparent, but there definately is a drop off in primary current at the end. I gained much of this insight from my pspice simulations. As mentioned before, a notch indicates that secondary voltage (energy) is max, making it the most effective time to shut down the inverter. This tuning method has gotten me to 12' sparks now.
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.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
JimmyH, Fri Apr 14 2006, 06:36PM
Well, it certainly depends on the coil. In my two DRSSTCs, there is a very clear beating going on, although for some coils that drive for many cycles, it may not be as apparent. Even those coils I would have a hard time believing are going to chose one pole, and then 'hop' to the next. Either way, ignoring the transient response is ignoring a huge part of it.
It's not the fact that you have a short pulse that happens to have some energy in the other pole, it's due to the drive frequency being synched to the primary frequency(ies), so when the primary has energy in more than one frequency, it will drive both.
Your PLL also puts out a fixed frequency, so you're going to see less beating for that reason also.
What I think is interesting is that you get the same beating if you drive both poles equally as you do when you drive at the zero between them.
It's also worth noting that this approach is more difficult to transfer enough energy with. When my CT broke, I started tuning for max spark length (which happened to be the lower pole), rather than for the notch, and I got a big jump in max spark length. It went from ~50" or something to 74" just by changing the drive frequency, and eventually got up to 97" after playing with it more.
JimmyH, Fri Apr 14 2006, 06:36PM
[JimmyH: I like to think that burst lengths in a DRSSTC are long enough for it to be considered as "sort of steady-state-ish".
Well, it certainly depends on the coil. In my two DRSSTCs, there is a very clear beating going on, although for some coils that drive for many cycles, it may not be as apparent. Even those coils I would have a hard time believing are going to chose one pole, and then 'hop' to the next. Either way, ignoring the transient response is ignoring a huge part of it.
To deal with the non-steady-stateness of things, I imagine that the drive from the inverter isn't a single frequency, but a band of frequencies, on account of the burst not being infinitely long. (The fewer cycles in the burst, the wider the band of frequencies.) Some of the energy in that band ends up exciting the other pole from the one you're trying to drive, and this is what causes any ripples in the envelope you might see, as the two modes beat against each other.
It's not the fact that you have a short pulse that happens to have some energy in the other pole, it's due to the drive frequency being synched to the primary frequency(ies), so when the primary has energy in more than one frequency, it will drive both.
In the coil I have just now, with a high operating frequency, longish burst, and tight coupling, the unwanted mode hardly gets excited at all. There are no visible ripples or beats in the RF envelope. But I imagine as I build a bigger coil it'll need looser coupling to avoid flashovers, and a shorter burst length in terms of cycles, both of which will get the unwanted mode excited more. So I may need to learn how to drive both modes together, or it may turn out that my PLL driver can't do that and I need to use Steve Ward's design
Your PLL also puts out a fixed frequency, so you're going to see less beating for that reason also.
It's interesting to note that Jimmy's original DRSSTC used a drive frequency midway between the two poles to excite both modes equally, and used the beating between them to his advantage, to get complete energy transfer like in a classical Tesla coil. But nowadays we seem to try and drive one mode much more than the other.
What I think is interesting is that you get the same beating if you drive both poles equally as you do when you drive at the zero between them.
It's also worth noting that this approach is more difficult to transfer enough energy with. When my CT broke, I started tuning for max spark length (which happened to be the lower pole), rather than for the notch, and I got a big jump in max spark length. It went from ~50" or something to 74" just by changing the drive frequency, and eventually got up to 97" after playing with it more.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Steve Conner, Fri Apr 14 2006, 08:00PM
I think driving both poles equally is pretty much the same thing as driving the zero midway between them, for short transients at least? I guess it's not quite the same, since driving at the zero gives practically no output in the steady state.
I suppose it is possible for the feedback driver to respond to a pattern of zero crossings that actually contains two frequencies and reproduce the same two frequencies in its output, so both poles get driven. But I always saw that as a disadvantage, since the feedback might go funny at the notches.
Steve Conner, Fri Apr 14 2006, 08:00PM
I think driving both poles equally is pretty much the same thing as driving the zero midway between them, for short transients at least? I guess it's not quite the same, since driving at the zero gives practically no output in the steady state.
I suppose it is possible for the feedback driver to respond to a pattern of zero crossings that actually contains two frequencies and reproduce the same two frequencies in its output, so both poles get driven. But I always saw that as a disadvantage, since the feedback might go funny at the notches.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Part Scavenger, Sat Apr 15 2006, 12:07AM
/me bows to the brains in this thread.
This is great stuff!
Part Scavenger, Sat Apr 15 2006, 12:07AM
/me bows to the brains in this thread.
This is great stuff!
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Sat Apr 15 2006, 12:39AM
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.
HV Enthusiast, Sat Apr 15 2006, 12:39AM
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.
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.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Steve Ward, Sat Apr 15 2006, 10:50PM
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.
Steve Ward, Sat Apr 15 2006, 10:50PM
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.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Sat Apr 15 2006, 11:09PM
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!
HV Enthusiast, Sat Apr 15 2006, 11:09PM
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!
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
teravolt, Sun Apr 16 2006, 05:32AM
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.
teravolt, Sun Apr 16 2006, 05:32AM
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.
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
HV Enthusiast, Mon Apr 24 2006, 06:45PM
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)
HV Enthusiast, Mon Apr 24 2006, 06:45PM
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.
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)
Re: Tuning and Frequency Splitting within a DRSSTC (or two coil) system
Terry Fritz, Mon Apr 24 2006, 10:22PM
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.
Terry Fritz, Mon Apr 24 2006, 10:22PM
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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