I have two primary circuits that I can use with my secondary and driver.
One has high impedance, 5 nF and 58 uH, and another has lower impedance,
22 nF and 14 uH. With the first system I don't have breakout at the
secondary. Tuning the system between the resonances, without feedback,
the measured output current shows a series of complete beats, with
slowly decreasing amplitude. With the nominal design for 10 cycles for a
full current beat, I see two beats with 10 cycles with the driver set to
produce between 10 and 20 cycles (energy going out and back in).
Yes, but note that the energy mainly goes back into the driver, i.e. the
power supply caps, on the second beat. This is due to a phase change of
180 degrees between driving voltage and primary current between first
and second beat.
This is undesirable and can be avoided with primary feedback. Then energy can
only go one way, i.e. from the driver to the primary.
With 30
to 40 cycles I see four beats. With 50 to 60 cycles I get six beats, and
so on. The amplitudes of the beats decay slowly due to losses. This
agrees with a linear model. If I tune the driver at the resonances, I
get ramps of increasing current until breakout occurs. A breakout point
results in essentially the same behavior, with small streamers with the
tuning between the resonances and longer streamers with tuning at both
resonances. Too long bursts don't increase the streamers beyond a limit,
due to detuning.
I don't think it is detuning. If you are running at the lower pole, i.e. at
300kHz - 5% (due to coupling), the streamer capacitance will pull the secondary
into tune with this frequency. That will lower your primary current. This implies,
that you will stay way below the max current rating of 90A and get weak streamers.
Lowering the driving frequency to keep the current up doesn't really help either,
since your primary tank will get severely out of tune. You can put lots of current
into a primary (series) tank only close to its resonance. You'd need to reduce
primary fres to get the current up again.
If you run at the upper pole (300kHz + 5%), the streamer capacitance will
move the secondary fres even further from the driving frequency. That will
allow the tank current to rise, but since you're away from your secondary resonance,
little of the primary energy will go to the secondary.
With the low-impedance primary there is an important change. Tuning
between the resonances I get breakout easily. If I just extend the burst
length I see that the primary current increases as a kind of ramp after
a first beat, and streamers grow with burst length up to a limit caused
by detuning. If I tune at the resonances, I get less impressive results
and much larger primary current.
Yes, the lower primary impedance will cause a much higher primary current.
But you will probably have to limit burst length in order to stay within your
bridges current limit. The primary current will rise later in the burst due to
detuning.
I've done some simulations and they indicate, that you need to limit burst
length to 50us in order not to exceed the 90A limit.
So far I am confirming my idea that tuning between the resonances leads
to a more efficient drsstc, even with breakout and longer bursts than
the minimum. But the voltage peak at the end of the first current beat
must be enough to start significant streamers.
I've also tried to do a simulation with streamer loads. The best I could
achieve was to use the 58uH primary with an increased MMC of 7nF
and driving it at about 250kHz. The simulation shows an increase of
arc length of more than 50% against the simulation with your settings.
Primary current would be limited to less than 90A. Burst length should
be 100us or more.
Maximum output voltage is not necessarily decreased. More energy in the
primary tank leads to a higher magnetic field if the geometry isn't
changed.
That would imply more secondary voltage.
Surely. But the energy stored in the primary circuit without current
beats is mostly wasted. I am still insisting on at least a first beat,
extracting the energy in the primary circuit to start streamer
formation, as happens in a conventional Tesla coil.
The beat will transfer energy from the primary to the secondary
mostly at the time when the primary current drops. One way to get a beat is to tune
the coil as you do. The other way is to detune it, i.e. run at a lower
frequency as secondary fres and wait until primary current ramps up
far enough to cause arc breakout, which will then pull the secondary
into tune. This then also causes a (single) beat.
The advantage of this is, that the duration of rampup can be arbitrarily
chosen by detuning and is not directly related to the coupling. Coupling would need to
be very low for slow beats. And longer burst times put more energy
into the arc.
Udo
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