Increasing QCW streamer length / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Increasing QCW streamer length

first previous 7 8 9 10 next

Move Thread

LAN_403

Ash Small

Mon Jul 28 2014, 11:57AM

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

Steve Ward wrote ...

I think the secondary resistance and impedance plays a role here too. The EMF driving the secondary sees more leakage inductance in the secondary, which has nearly as much impedance as the arc has, so wouldnt this cause a reduction in power as well and help explain the sensitivity to K?

Also, there has been some question about energy storage and i pose this hypothetical to maybe help think about the issue. Imagine you have 2 secondary coils, one with 20pF of effective capacitance and one with 40pF. Assume the Fres is identical. We know from measurements that it takes at least 40kV to begin spark production, so as i see it, the 40pF resonator requires twice the drive current (all else being equal) to achieve just 40kV (which is about half the peak voltage most QCWs will develop). If the coils had the same resistance, then significantly more power would be wasted in the 40pF resonator. However, the winding should only need half the inductance, which means less length and thicker wire. I suspect the resistance could go down sufficiently so that both secondaries have the same losses while producing 40kV. But still, there will be twice the primary amps required to drive the 40pF load. Whats the benefit? The same spark looks like a smaller load to the 40p coil and consequently the inverter can drive more power into the system and make bigger plasma because the source impedance is effectively smaller.

Bigger coils have higher energizing costs, and i think its really quite significant in terms of primary current during operation. Figuring out the right amount of energy storage for your tesla coil design i think is key to optimizing its efficiency. Seems obvious, and overlooked.

Your example of two different secondaries with the same Fres reminds me of another thread here some time ago, which I'm not sure you were involved with, Steve.

The subject was the effects of voltage and current on streamer growth. While it seems obvious that higher voltage leads to bigger streamers, the effects of higher current, the thread concluded, can also have a significant effect on streamer growth, due to the heating effect and greater ionization. Put simply, the greater heating and ionization of the air due to higher current makes it easier for the next cycle to ionize more air, leading to increased streamer growth.

This, I assume, further contributes to the load looking even smaller, as far as the higher capacitance, lower inductance secondary is concerned. Also, the capacitance of the streamer has less effect on the Fres of the secondary than the same size streamer would on a lower capacitance, higher inductance secondary, I think.

I hope I'm not going off topic here.

Steve Conner

Mon Jul 28 2014, 02:20PM

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

Uspring wrote ...

In a QCW setting, any obstruction of the power flow to the secondary, may it be from lower coupling or higher leakage inductance, will cause the primary to ramp up to higher currents, since it is less loaded. Leakage inductance is not consumptive, so the additional input power will go the secondary.

Something is wrong with this argument, as it implies that the harder you try to prevent power flowing into the secondary, the more power will flow to the secondary. The logical conclusion is that putting the secondary in the next room would give 20ft arcs.

I assume this is not the conclusion you intended, so can you explain?

Goodchild

Mon Jul 28 2014, 03:16PM

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

Steve Conner wrote ...

I assume this is not the conclusion you intended, so can you explain?

You have hit the nail on the head. This is my main reservation on the whole lower coupling = bigger sparks argument.

EDIT:
BSVi, what are you using for an RF ground? I didnâ€™t see a mesh or anything of the sorts in your photo.

Uspring

Mon Jul 28 2014, 06:50PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Something is wrong with this argument, as it implies that the harder you try to prevent power flowing into the secondary, the more power will flow to the secondary. The logical conclusion is that putting the secondary in the next room would give 20ft arcs. I assume this is not the conclusion you intended, so can you explain?

Steve, Eric, you're being too realistic

The hitch is, that I've neglected the effect of primary copper losses. To quote myself:

In a QCW setting, any obstruction of the power flow to the secondary, may it be from lower coupling or higher leakage inductance, will cause the primary to ramp up to higher currents, since it is less loaded.

In a lossy primary, current won't ramp up forever, even if there is no secondary at all. To quote myself again:

Now consider a lossy primary. Current will also increase here with lower coupling, increasing power input, but also energy is burned in the tank. If tank loss resistance is large enough, power output decreases, even though power input increases.

That is going to happen at some current level. Just for entertainment consider a superconducting primary and caps and fat IGBTs. Set that up in a lab, that is electrically inert and start the burst. After some time you'll have megamps circulating in the primary. Finally the secondary next door will notice and only stop growing arcs until the gigawatts in the other room have found a place to go.

The upshot is, that a low k will increase power throughput only as long that primary losses allow for this, i.e. up to some current level.

Steve Conner

Mon Jul 28 2014, 07:21PM

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

Well, the other problem is that the lower the coupling, the narrower the system bandwidth and so the more vulnerable it is to detuning by streamer load.

I take this to mean that the lower the coupling, the bigger the topload required for a given size of spark.

I have no idea what happens when you apply infinite power to a superconducting coil that's badly out of tune.

Probably a semi-infinite explosion.

Ash Small

Mon Jul 28 2014, 09:03PM

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

Steve Conner wrote ...

I take this to mean that the lower the coupling, the bigger the topload required for a given size of spark.

I'm beginning to form the opinion that a larger topload will always result in a larger spark, assuming copper losses in the secondary don't dominate. This was the case with Odin, and we've seen recently in other threads that adding capacitance, in the form of a string of capacitors inside the secondary, also improves streamer length.

This also helps with detuning due to streamer load, and assuming secondary copper losses don't dominate, the voltage will still ring up, regardless of winding ratio.

I'd be interested to hear about any instance when this might not hold true.

Steve Conner

Mon Jul 28 2014, 09:44PM

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

The way I see it, the topload size is not unlike the "loading" control on the old ham radio antenna tuner. There is an optimal size that extracts maximum power from the driver.

If the topload is too small, the streamers will detune the coil too easily and the primary current will ring up uselessly and trip the current limiter.

If the topload is too big, the primary current will get excessive and trip the current limiter, before the output voltage gets big enough to grow the desired size of streamer.

The optimal size of topload probably depends on several variables.

Ash Small

Mon Jul 28 2014, 10:19PM

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

Steve Conner wrote ...

The optimal size of topload probably depends on several variables.

Copper losses?

(I don't expect an answer, Steve. I think I'm starting to get the idea. It's probably time I built a TC.)

Uspring

Tue Jul 29 2014, 08:12AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

A large topload or equivalently, a secondary MMC, as well as a large coupling are beneficial wrt arc detuning problems.

Wrt excessive primary current, top load might have some effect. But primary current also can be controlled by primary inductance or coupling. For Odin, the extra turn probably was partially responsible for the success. I assume, that you added the extra top load in order not to have to fiddle with the primary MMC (?). Another rationale behind the extra top load is not clear to me.
The upper pole operation of Odin should make it less susceptible to arc detuning.

Maybe a big top load serves as a backing energy store to feed the arc. Mostly, though, the energy stored in the secondary tank is much less than that in the primary.

Edit: In the steady state the energy ratio between secondary and primary tank is Qsec/Qpri, which might be around 1. Initially, during primary rampup, it is likely less than that.

Steve Conner

Tue Jul 29 2014, 09:19AM

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

In the case of Odin, I added the extra topload because it seemed like the output was being limited by detuning, and the topload was smaller than what I saw used on other coils.

I added the extra primary turn because I felt that the primary impedance was already too low, so I didn't want to lower it still further by adding extra capacitance.

I'm not sure how relevant the upper vs. lower pole distinction is to Odin. The bursts are short enough that the system doesn't really settle to a steady state. My previous coil Mjollnir had at least 3x more cycles per burst, so there was a clear transition into steady-state behaviour and the PLL driver performed better. Odin was supposed to be a straight scaling up of Mjollnir, I'm still trying to figure out why the behaviour is different.

When designing a DRSSTC I would try to get Qsec and Qpri roughly equal. I'm not sure how close it is possible to get, since streamer load varies quite widely over the operating range of the coil.

first previous 7 8 9 10 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob