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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, while experimenting with my non-interrupted DRSSTC running on the lower pole, I noticed that with increasing bridge voltage from zero, the tank current quickly rises but then it kind of "flattens out" and there is not much more increase of output current with higher bridge voltage - the tank impedance increases. I guess this might have something to do with the spark loading decreasing the secondary resonant frequency and pulling the coil more in tune, decreasing primary Q.
Now my question is - what is the expected behavior on the upper pole? Applying my spark loading theory, the tank current should be low until a certain voltage, at which the sparks become big enough to detune the secondary coil further away from the primary tank frequency, increasing its Q and current. Does this sound about right or am I completely off?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
As far as I know, the two poles behave the same with respect to resistive damping.
But detuning by streamer capacitance affects the two poles oppositely. It increases the tank impedance if you're running on the lower pole, and decreases it if you're running on the upper one.
Or maybe I've got that backwards The graphs recently posted by Uspring show the effect. Basically the effective transformation ratio changes according to the difference between the primary and secondary resonant frequencies, which is why you can "tune" a DRSSTC by adjusting the primary tap.
I'm not sure which one is better. You can argue that a system running on the lower pole is adjusting itself to better match the spark as it gets bigger, hotter and lower impedance.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve, do you think the dynamic tank circuit impedance has an effect on the spark appearance? Assuming the same driving frequency, which pole should be better to get sparks as straight as possible (either by using the ramp up from mains waveform, or QCW modulated supply)?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't know! And I wouldn't be surprised if my QCW approach, with the current control loop, preferred the opposite pole to Steve Ward's approach, controlling DC bus voltage.
However, the upper pole is still preferred because it lets you achieve a higher operating frequency, and hence straighter sparks, with whatever resonator you had handy. This is why Steve and Eric use it.
The upper pole also allows tighter coupling without flashovers, because it has a concave voltage profile: the voltage gradient is lowest at the bottom of the resonator and increases towards the topload. The lower pole has a convex one, making flashovers worse. I worked this out from theory, and Richie B. confirmed it by experiments with a voltage probe.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Theoretically the lower pole also has the most gain. I have also used the natural resonant freq on my QCW with similar performance to the upper pole.
I plan to wind a higher freq secondary though and try tuning to the lower pole and see how this effects performance. I have a feeling that the capacitive loading of QCW sparks could be a lot higher that a regular DR making lower pole tuning a lot more practical.
But detuning by streamer capacitance affects the two poles oppositely. It increases the tank impedance if you're running on the lower pole, and decreases it if you're running on the upper one.
Or maybe I've got that backwards. The graphs recently posted by Uspring show the effect. Basically the effective transformation ratio changes according to the difference between the primary and secondary resonant frequencies, which is why you can "tune" a DRSSTC by adjusting the primary tap.
You got that right. I expect the primary current to rise as a consequence of detuning if running at the upper pole.
Steve also wrote:
The upper pole also allows tighter coupling without flashovers, because it has a concave voltage profile: the voltage gradient is lowest at the bottom of the resonator and increases towards the topload. The lower pole has a convex one, making flashovers worse. I worked this out from theory, and Richie B. confirmed it by experiments with a voltage probe.
You got me thinking on that one. This makes perfect sense to me. At the upper pole the magnetic fields of primary and secondary are in opposite phase so they cancel each other to a certain extent in the region where they overlap i.e. at the bottom of the secondary. Effectively that shortens the secondary coil, so that it has less inductance which makes its (coupled) frequency go up. That is the reason, why the upper pole is above the secondaries uncoupled frequency.
The phase difference between primary and secondary depends on the secondary loading, though. Under heavy loads it will go to 90 degrees, so that the cancellation is reduced.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, I agree. As you showed in that earlier post, I believe that under heavy loading, the coil ends up in the same operating point, no matter which pole it started from.
The question is, do our coils ever get to this "heavy loading" condition? Could we find out by measuring the primary/secondary phase shift on the fly with a DSO?
An observation that might be relevant: Years ago, when a DRSSTC was still called an "ISSTC", everyone used secondary base current feedback. I showed that it couldn't give reliable zero current switching, because the primary-secondary phase shift varied a lot as a function of spark loading, and I tried to persuade everyone to shift to primary current feedback.
So maybe this proves that we do have "heavy loading".
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve McConner wrote ...
Yes, I agree. As you showed in that earlier post, I believe that under heavy loading, the coil ends up in the same operating point, no matter which pole it started from.
But this is true only when the resonant frequencies of the two individual circuits are the same, right? When we tune for the upper or lower pole, we usually do this by actually detuning the primary tank to one or the other side, so the conclusions might not be true.
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DR tank impedance, lower vs. upper pole
The graphs recently posted by Uspring show the effect. Basically the effective transformation ratio changes according to the difference between the primary and secondary resonant frequencies, which is why you can "tune" a DRSSTC by adjusting the primary tap.
