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Increasing QCW streamer length

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BSVi

Wed Jul 16 2014, 12:35PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

I have measured bus voltage before and after discharge. Before it is 379V and after - 300v. So I dont run out of voltage.

Considering capacitance of 13600uf, bang+losses energy is 364J.

Dr. Dark Current

Wed Jul 16 2014, 01:38PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve Conner wrote ...

Looking at it in terms of power and energy, when the coupling is loosened, more energy is stored in the primary as reactive power and returned to the DC bus after the burst ends. That is where the "missing" energy goes.

Yes, but we're talking about QCW coils, where the bursts are so long that the system can be analysed in a steady state.

teravolt

Wed Jul 16 2014, 04:50PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

you guys know more than I on this but I would think that more coupling is better because I would think that you want to convert your energy storage into spark. Instead of storing it in the primary. Aren't drsstc's secondaries allowed to self resonate a little therefore less coupling.

After the cap sags a little the mains will start taking over with ripple so you probably will get more than the calculated joules. the question is how long dose it take for the caps to sag to that point

Goodchild

Wed Jul 16 2014, 09:31PM

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

Steve Conner wrote ...

I agree with Eric, except in a DRSSTC, the impedance of the primary also limits the rate of energy transfer. I think it's better to limit it by using a higher impedance primary and maintaining tight coupling, than by loosening the coupling.

I'm interested by this spark length limit where the tank circuit refuses to ring up any further. I would like to find a way of predicting it in advance, rather than finding out after the coil is built. Do you have any measurements you could share that might help?

Steve I agree, I usually build higher impedance tanks to help control current in most of my DRs as well.
In a QCW though having the impedance too high will result in not being able to ring up additional energy without added bus voltage. I think this is one of the issues BSVi was having.

Dr. Dark Current wrote ...

Yes, but we're talking about QCW coils, where the bursts are so long that the system can be analysed in a steady state.

QCWs and DRs are the same mode of operation (both transient, not steady state by any means); the only difference is how the energy is added to the tank circuit.
In a regular DR itâ€™s not typically controlled, except by coupling and tank impedance (these are not variable parameters). In a QCW we control power flow via some modulation scheme. This still only controls the rate at which energy is allowed to flow into the LC, it doesn't change the fundamental operation of the Tesla coil resonator.

Think of it this way, if you kept lower coupling you would slowly build up more and more current in the primary LC. Once you had the secondary completely out the current would be massive in the primary and almost all of it would recycle back to the bus caps (some lost to resistive loss). With no secondary in place your coil will still have a massive amount of reactive power flowing in it.

One the other hand the tighter you couple the coil the more of this energy is transferred into the secondary LC. This is what we want! A larger current in the primary doesnâ€™t constitute a larger output in a loosely coupled transformer.

Dr. Dark Current

Wed Jul 16 2014, 11:08PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Of course the system must be analyzed with the secondary coil in place. The ring up is much shorter than the burst length, in other words, the energy stored is much smaller than the energy transferred. So my argument for larger sparks with looser coupling still holds true... (the ring up will be longer, but still it is comparing numbers like 0.1ms and 20ms).

Goodchild

Thu Jul 17 2014, 06:44AM

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

Dr. Dark Current wrote ...

The ring up is much shorter than the burst length, in other words, the energy stored is much smaller than the energy transferred. So my argument for larger sparks with looser coupling still holds true... (the ring up will be longer, but still it is comparing numbers like 0.1ms and 20ms).

I still don't quite believe that, do you have any experimental data that shows this? Because classical transformer theory tells a different story.

As you lower coupling, energy that is normally transferred to the secondary ends up as reactive power in the primary. This is transformer operation regardless of whether or not it's a Tesla coil. Lowering coupling only slows down this process

The only reason we don't normally run higher couplings in Tesla coils is to control the output dv/dt so that the coil won't flash over. However in QCW we have control over the dv/dt and as such we slow it down significantly, thus allowing higher coupling without exceeding the dv/dt that the coil can stand off without flashing over.

Steve Conner

Thu Jul 17 2014, 07:28AM

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

Jan's point (which is valid) is this:

During the burst, only real power can be transferred into the tank circuit, because we use feedback to get the drive voltage in phase with the current. Therefore the only way for energy to be recycled is by returning to the DC bus after the burst is finished. This can be significant in a DRSSTC, but there is no way a QCW tank circuit can store the amounts of energy we are talking about.

Phase shift modulation might well alter this argument, as the voltage and current are no longer in phase and reactive power flow is possible. Loosening the coupling will change the bandwidth, so you may need a different amount of modulation to get the same current envelope as you had before, and a different amount again to get the increased current required for looser coupling to show increased spark length.

I think loosening the coupling also makes detuning by streamer load more significant.

Uspring

Thu Jul 17 2014, 07:58AM

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

Eric wrote:

I still don't quite believe that, do you have any experimental data that shows this?

Primary capacitance is 12.5nF, frequency about 400kHz. With 200A current, that would amount to a cap peak voltage of 6.4kV. The energy content of the MMC is then about 0.25J. Compare this to the total bang energy of 364J. Input power mainly goes right through the primary into the secondary.

Steve wrote:

I think loosening the coupling also makes detuning by streamer load more significant.

Yes, and that is the reason why it is a better idea to reduce primary turn count, then to loosen coupling in order to increase primary current.

Still this leaves the question open, why the energy argument does not seem to work to explain the reduction in arc length when coupling is reduced.

Steve Conner

Thu Jul 17 2014, 08:49AM

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

Maybe he forgot to re-adjust the phase shift modulation as I suggested above. I think on changing the coupling of a phase shift modulated QCW, the optimum modulation envelope would change, possibly by a lot.

Well, that assumes that phase shift modulation is achieved by detuning the frequency. Phase shifting one side of a full bridge against the other can be seen as a form of PWM, I'm not sure if it implies a change in frequency at all.

Uspring

Thu Jul 17 2014, 09:25AM

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

I was thinking more about the end of the burst, where there is probably no shift at all.

I have a problem of understanding how the phase shift is implemented. Are the two half bridges shifted against each other or a both bridges shifted against the current. I wouldn't know, why one would do the latter. Just to make sure.

Well, that assumes that phase shift modulation is achieved by detuning the frequency. Phase shifting one side of a full bridge against the other can be seen as a form of PWM, I'm not sure if it implies a change in frequency at all.

I tend to believe, that shifting voltages against currents in the tank implies a frequency change. For phase lead to higher frequency and more change for a low primary Q.

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