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Re: Help in calculations
On Sun, 28 Apr 1996 22:06:49 -0600, Tesla List
<tesla-at-poodle.pupman-dot-com>, you wrote:
Hi Malcolm,
I said,
>> Now for the real question. Does the voltage vs time on the cap look
>> like a straight line? My gut says that this is false, so I'll look at
>> it with my 16nf cap and 60 mA neon tonight.
and later posted. upon scoping the primary with my 15kV 60mA neon:
looks like a sine wave until the gap fires, then almost a sawtooth.
Three firings per half cycle with the primary, 3 or 4 with the
secondary installed using my 90CFM cylindrical gap, 7.5KV (half
secondary of my neon), 16nF of primary cap and 13 turns on my primary.
>> I'll stick by this number until someone shows me where the extra
>> energy is coming from. If you run the numbers using Tesla's values, I
>> think you will find that his actual power input was much lower than
>> everyone says. If you thing about the neon acting as a perfect
>> transformer until it reaches it's current limit. Until the voltage
>> reaches the firing point of your spark gap, you are "wasting" (not
>> using) power. At this point the gap fires, discharging the cap. The
>> neon now wants to be at (say 11.4kV) into 0 volts (the cap), it goes
>> into current limiting and acts as a current source for the portion of
>> the AC cycle that puts the cap voltage above you gap firming voltage.
>> The remainder of the half cycle is again unused power.
>>
>Since the transformer may be modelled (at 50/60Hz) as a perfect
>voltage transformer with a humungous inductor in series,
Or a smaller "leakage" inductance in series with the primary.
Visualize the physical construction of a shunt limited transformer.
Primary in the center with shunts to the lower half of the core,
secondaries on either side of the shunts with a magnetic path to the
lower half of the core.
Now (in your mind's eye) short out the secondaries, the magnet field
from the current flowing in the secondaries tries to cancel out the
flux in their section of the core. This action in a normal transformer
would lead to a lower flux in the primary, decrease the back emf, and
increase the current in the "normal" primary. But, there are the
shunts between the primary and the secondaries, these limit the amount
of flux that can be canceled (that portion that bypasses the secondary
can't be canceled). This action effectively puts the limiting/leakage
inductance in series with the primary!
> and taking
>into account the capacitive load, you can see that the charging
>waveform is going to be a sinusoid.
Go look at the waveform of your primary circuit, after the first spark
gap firing, the charge rate is not perfectly straight, but it's not a
1/4 sine either.
>Also, during the time that the
>inductor is passing charge current to the capacitor, it is storing
>energy. This happens whenever the (ideal) transformer voltage exceeds
>the terminal voltage of the capacitor. For corona production, a
>static gap conducts for about 200uS or so before extinguishing with
>typical systems and not much quenching. For arc production, the gap
>extinguishes much faster (energy drain). Point is, even 200-300uS is
>only a fraction of the actual charge time, so the inductor energy is
>mostly used to charge the cap. So I think you can safely double the
>throughput that you think is happening if the spark gap is set wide
>enough
If you are thinking about a SMPS we don't have a free-wheeling diode
to ground, the inductor would be dumping half of it's power back
through the secondary-into the primary.
>(bearing in mind that the transformer is overvolting in a big
>way).
I have to start agreeing with you here. I saw 7.5kV RMS/10kV peak with
just the neon connected to the cap. With my spark gap to my primary, I
saw 15kV to 20kV peak! I'd wired it up as cap in parallel with the
neon, shunt gap to hi-side of the primary, low side of primary
grounded.
> You are in fact using nearly a half cycle of the mains to
>charge the cap.
Nope, I my case about 1/6. The first third-of-a-half-cycle is just
charging, the second third gap firing, the final third of a half cycle
the cap is following the neon-secondary voltage back down to ground.
> This mechanism is known as resonant charging and
>happens because of the large phase shifts between the circuit
>inductances and capacitances.
It would be if I had a large series inductor between my neon and my
cap. I don't, yet my voltage goes from 10kV peak to 15kV - 20kV, which
indicates that something other that a simple leakage inductor in
series with the primary is occurring. Don't you love it when your
lumped-circuit model beaks down;) 0h-bad pun:(
*************************************************************************
> As usual, things like this point the way to better circuits, in
>this case, a perfect transformer (pole pig) and an external inductor
>on the secondary side can be used in a voltage multiplying scheme
>assuming the primary cap can take the voltage. This scheme has the
>advantage of removing the overvolt burden from the transformer
>windings and placing it across the choke. I suspect an undamped
>inductor on the transformer primary does much the same thing as an
>ideal transformer is a lossless component, only in this case, the
>secondary windings do produce the extra voltage.
**************************************************************************
>
Malcolm,
go reread the above paragraph ( between * s). Did you just
describe a Magnifying Coil?:) Only in this case, the primary and
secondary coils are working at the mains frequency.
>Open to other ideas on this,
ditto
jim
p.s. I just build a rotary gap using a 1HP 1750 rpm (tesla
invented) induction motor. Bad choice, I'm getting a beat period of
about 20 seconds! I'm going to have to get an Asynchronous motor.
Thanks skip for your fine building description and to rws for
construction hints. Robert, you WERE right about not using an
induction motor!
cheers,
jim