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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'd say that the spark length should increase if you decrease the coupling IF the over current protection doesn't kick in. Simply the input current and power are larger. The efficiency does decrease, but I'd think not to the point so that the power loss would dominate over the transfered power. That would be one very inefficient coil
I'd say that the spark length should increase if you decrease the coupling IF the over current protection doesn't kick in. Simply the input current and power are larger. The efficiency does decrease, but I'd think not to the point so that the power loss would dominate over the transfered power. That would be one very inefficient coil.
I completely agree with that. He observed decreasing arc length with decreasing coupling, though, which makes me believe in a lot of losses. On the other hand the coil performs nicely. I'm puzzled.
@BSVi: Can you tell us, what primary wire diameter you use?
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Dr. Dark Current wrote ...
I'd say that the spark length should increase if you decrease the coupling IF the over current protection doesn't kick in. Simply the input current and power are larger. The efficiency does decrease, but I'd think not to the point so that the power loss would dominate over the transfered power. That would be one very inefficient coil
I don’t agree with this statement matter in fact it should be the opposite. Spark length should get larger with an increased coupling. Lowering the coupling only makes the current large because more of the energy is staying in the primary circuit.
The desired outcome is to transfer the energy into the secondary circuit as fast as possible, such that it can be used to make sparks, rather than sloshing around in the primary LC loosing energy to resistive loss.
In a DR coupling is the only thing limiting the flow of energy into the secondary circuit and because we allow all of this energy to be available at once, the coupling needs to be kept low such that we don’t have an excessive dv/dt on the output, causing a flash over.
In a QCW however we are able to control power out of the bridge, only giving the resonator power as we feel fit for optimal spark growth. As such the coupling can be made much larger, because the coupling is no longer the only thing limiting the flow of power. This allows control of the coils output dv/dt to a rather fine degree.
From what I can tell this is why QCWs perform so well. By keeping the output dv/dt low we are able to grow the plasma channel over more time without flashing over at higher couplings. This is all due to controlling the rate at which power is fed to the resonator.
From what I have found the limiting factor is usually how well you can keep the coil in tune. As sparks get bigger and the secondary get more out of tune with the primary and requires more and more power to increase spark length.
The other limiting factor (and it looks like BSVi has hit this one) is the primary tank impedance. With a particular tank impedance configuration you will only be able to grow sparks to some length before you can no longer ring the LC up any further.
There are two solutions to this problem, either you must increase the bus voltage (if your setup can handle that) or you must lower the Z of the LC.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I still can't see how a higher input power (resulting from decreased coupling) doesn't result in longer sparks, because I think it does. The efficiency is not really that bad I'd say.
I think that it's best to look at any system involving power transfer from the point of powers and energies.
Its 2.5mm^2 solid copper wire (1.8mm^2). Primary gets warm, but not hot.
I did a quick check on the AC resistance and came up with at most 0.2 Ohm for the primary. That isn't really enough to explain the coils performance dependence on coupling by resistive losses.
Steve wrote:
Energy is equal to V*I*t. If you have 300V*200A*20ms that is 1200J of energy.
300V on a 12000uF cap will store 540J of energy. Can it be, that you run out of cap voltage?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
DR. DC, your argument would be true if the primary current were allowed to increase. However I think loosening the coupling gives less spark length increase per primary current increase, than maintaining the coupling and lowering the primary impedance.
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.
DR. DC, your argument would be true if the primary current were allowed to increase. However I think loosening the coupling gives less spark length increase per primary current increase, than maintaining the coupling and lowering the primary impedance.
Power transfer is proportional to k * Ipri and other things. For a given bus voltage Ipri is proportional to Qpri omitting losses in the primary. The equation for Qpri predicts a proportionality between Qpri and 1/k^2, if the operating frequency would not be affected by coupling. Since that is the case, also a weak dependence on k is possible. So we have to extreme cases:
a) Ipri ~ 1/k^2. Then power transfer is proportional to 1/k, i.e. it will increase with less coupling, although at the price of higher primary currents.
b) Ipri is constant. Then power transfer is proportional to k and will increase with more coupling.
Mostly the situation is somewhere between.
EDIT: The "other things" mentioned above makes this a lot more complicated as I've written. So please forget about the above. Sorry.
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 looking at 20ms bursts. Energy in the primary is a few J compared to hundreds in the burst.
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Increasing QCW streamer length
