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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Goodchild wrote ...
Capacitive spark loading really only becomes a problem when you are trying to push a massive amount of spark out of a very tiny secondary like a QCW as the spark capacitance overwhelms the secondary self capacitance.
Lowering the secondary capacitance is usefully for like what plazmatron did when winding a small secondary and not wanting to use a bunch of tinny wire.
Other than these two reasons I don't see any reason to add capacitance to the secondary circuit. A mid sized or large sized DR doesn't have ether of these problems and on top of this I agree with Steve in that it would add a massive amount of energy that could be very bad when it has a heavy ground arc.
This is very true. The only reason I used a secondary MMC in mine was, I didn't want to wind a secondary with very fine wire. I have done this before, and even with a coil winder, it is an unpleasant experience when you get 3/4 the way along the former, for the wire to break.
So basically I used a secondary MMC for lack of any other reasonable choice. In a mid to large sized DRSSTC, just using more inductance is the reasonable choice, and designing massive secondary MMC's is not.
Likewise, Steve Ward used an MMC on his coil, as it was a more reasonable choice, than having an enormous toroid on there, since his reason for adding secondary capacitance was not to lower the frequency, but to prevent the huge sparks from detuning the small coil.
Lets remember that Steve Wards 3 main reasons for adding the secondary MMC had nothing to do with lowering the resonant frequency, but mostly preventing streamers from loading the secondary out of tune.
The problem of detuning was the one I was trying to address. A measure of streamer detuning is the term Q * Cstreamer/Csec, i.e. detuning gets worse with larger Q and larger proportion of streamer capacitance to toroid (and secondary MMC) capacitance.
So if you e.g. double your secondary capacitance, frequency and Z will decrease by a factor of sqrt(2), Q will increase by sqrt(2). The detuning term Q * Cstreamer/Csec will decrease by sqrt(2), i.e. less detuning. That is probably what Steve Ward observed. If you double secondary inductance, Q will decrease by a factor of sqrt(2). Q * Cstreamer/Csec will also decrease by sqrt(2). That is the same amount of decrease as if we had changed the capacitance, so it doesn't really matter whether we increase secondary inductance or capacitance. Both decrease the effect of detuning due to streamers.
It is a nice feature, that as coils get bigger, also their sensitivity to streamer capacitance decreases.
Edit: Which of course must be since their streamers are larger.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
That is the same amount of decrease as if we had changed the capacitance, so it doesn't really matter whether we increase secondary inductance or capacitance. Both decrease the effect of detuning due to streamers.
Well, of course lowering the frequency of the coil will make a constant added streamer capacitance less significant. But what if you want to keep the frequency if your coil constant? Then, there is only one solution, and that is to add capacitance.
Furthermore, raising the secondary inductance and lowering its Q does not sound like a good path towards better performance. We already do this with most DRSSTCs, and i suspect this puts them at a disadvantage because eventually the secondary impedance limits how much power you can put into the sparks. This ultimately happens no matter what (provided your driver is good enough), and this is what makes coils get bigger and bigger in order to make bigger sparks. Adding bigger toroids, without compensating the coil's inductance to keep frequency constant, often gives the result of no improvement. I think this is because lower frequency powered sparks probably eat up more charge per cycle, so in the end you've gained nothing. I think only by lowering the inductance, and raising the capacitance (keeping Fres fixed) can you see an increase in capacity for a given tesla coil, in terms of spark production.
So with the original question in mind, i think internal secondary capacitance could be applied to coils of all sizes, particularly if the designer is trying to extract the maximum possible spark from a given coil size. I would not claim that this would produce the most efficient tesla coil, however, because there would likely be some voltage limitations imposed on the system. The lower top voltage would have to be compensated for with more cycles for spark growth. I think this would lower efficiency, because spark propagation is more efficient with higher voltages.
Another issue to watch out for, the type of capacitor used is probably pretty important. Very lossy ceramics will be... very lossy. Id probably stick with PP film since they are so easy to come by, self-healing, and very low loss, and provide a wide range of capacitance to choose from.
Furthermore, raising the secondary inductance and lowering its Q does not sound like a good path towards better performance. We already do this with most DRSSTCs, and i suspect this puts them at a disadvantage because eventually the secondary impedance limits how much power you can put into the sparks. This ultimately happens no matter what (provided your driver is good enough), and this is what makes coils get bigger and bigger in order to make bigger sparks.
I would like to know why you believe, that secondary impedance limits the power you can put into sparks. AFAIK low Q will change the requirements on the driver, i.e. the ratio of bus voltage and primary current. This can be compensated by choosing a higher primary inductance and consequently a lower primary MMC if you want to keep fres. Your tesla gun is a good example for getting huge sparks out of tiny coils I'm amazed about how you kept the sparks at the place you wanted them to be.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I would like to know why you believe, that secondary impedance limits the power you can put into sparks. AFAIK low Q will change the requirements on the driver, i.e. the ratio of bus voltage and primary current.
Good catch, this is a confusing topic even for me. Say you want to get more power through your DRSSTC, but you already maxed out the supply voltage and drive cycles. One option would be to lower the tank impedance, remove inductance and add capacitance on the primary side. This raises the primary Q, causes it to ring up to higher currents (more power out of the bridge), and will hopefully make the sparks bigger. In practice this works, but becomes less efficient as you store more and more energy in the primary. If the majority of your energy is stored in the primary, then its likely that a good chunk of it is simply going to be returned to the DC bus at shut down, rather than going to the arcs.
I think there is an ideal range for the Q of the primary and secondary for maximum system efficiency. The Q of the whole system is important as it tells us the *steady state* power output, and i think this is the point Upspring was getting at. I think the transient mode of operation is more complex, and the total system Q is insufficient information, and the Q of the primary and secondary should be considered individually.
In practice this works, but becomes less efficient as you store more and more energy in the primary. If the majority of your energy is stored in the primary, then its likely that a good chunk of it is simply going to be returned to the DC bus at shut down, rather than going to the arcs.
My coil, which is basically a DRSSTC-3 clone, runs in the transient mode, i.e. it ramps up to high primary currents until breakout occurs. This then pulls the secondary into resonance with the primary frequency, which causes the primary to dump almost all of its energy fast into the secondary. Primary current then drops to a low value and stays there, so that longer burst don't cause a significant increase in spark length. Shutting the coil off at that time doesn't waste a lot of primary energy. This seems to suggest, that spark length mostly depends on the the max primary energy but looking over the available data of other coils, this is not really true. Some coils seem to be able to grow sparks after this initial dump. They are more QCWish.
I think there is an ideal range for the Q of the primary and secondary for maximum system efficiency. The Q of the whole system is important as it tells us the *steady state* power output, and i think this is the point Upspring was getting at. I think the transient mode of operation is more complex, and the total system Q is insufficient information, and the Q of the primary and secondary should be considered individually.
Yes, my comment was motivated by by this steady state equation, which describes the power transferred to the secondary:
P = Ipri^2 * k^2 * Lpri/Lsec * Rarc
Rarc is the arc loading resistance (parallel to the secondary). It holds only for the case, that the coil is in tune. For detuned coils it is lower. You can push any amount of power into the secondary provided your bus voltage is high enough to get enough current. But you can reduce the bus voltage requirement by decreasing Lpri.
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Secondary MMC - thinkable for normal mid-sized DRSSTCs?
I'm amazed about how you kept the sparks at the place you wanted them to be.