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Re: Gap Losses
Hi all,
Last one from me before going on vacation. Just like expand
on this excellent post from Richard...
> I still can't see how presicely measuring the the input energy (even to
> the nearest microwatt) Would give us even the remotest idea of gap
> losses when there are a lot of circuit losses starting at the power cord
> in the wall through the ballasts, transformers, variacs, capacitor and
> primary circuit. The gap is of course the biggest loser of energy, but
> the ballast system would be right on its heels.
>
> In my water arc explosion work, (uses a simple capacitve discharge
> circuit with a switch or a gap), I have found that the gap or switch
> losses are often between 30 and 40% of the input energy. and the
> inductive tank circuit (wiring and capacitor) is good for another 10%. I
> have changed the wiring by 1 foot plus or minus and get another gain or
> loss of 5%. The stuff is just too touchey to pin something down relating
> to the gap alone with generalizations made from one measurement way back
> at the head end. System efficiency would demand a precise measure of the
> output energy (spark) which can't be done by the use of common
> instruments.
>
> Using the ammeter in the power transformer secondary circuit won't do
> much for us either, as the gap is a dead short across the transformer
> secondary when firing. It might tell us more about the percent impedance
> of our transformer than gap losses.
>
> True gap losses can only be obtained with a wideband CT in series with
> the tank circuit components and a simultaneous voltage measurement across
> the gap. Integrating these with time will show the actual energy expended
> in the gap which can then be subtracted from the input energy to yeild
> still another inaccurate idea about how much energy reaches the Tesla
> primary. (Circuit losses and magnetic losses in the Tesla primary circuit
> are still ignored.)
>
> The gap losses are ever changing throughout its conduction period. The
> drain on the power line or mains is increased during this time by
> shorting of the power transformer. Resonant energy from the discharging
> capacitor in the tank circuit is just one of the currents which flow
> though the gap when it is firing.
>
> Richard Hull, TCBOR
It has become abundantly clear to me during my experimentation this
last week that while absolute losses in various parts of the circuit
are difficult to quantify, there are several rules that come into
play:
Power losses in all normal resistive parts of the tank scale
with Ip^2.
Power loss in the gap scales with Ip.
Outside of the tank circuit (transformers, power cord etc.) all
losses are related to line current squared.
For the tank alone: using the best possible components you can get
causes the gap to be the dominant loss. Using poor capacitors will
cause them to be a major loser (this has been seen when people switch
from homemade caps to commercial pulse caps). Insufficient copper area
in the primary coil and poor interconnects will cause them to be bad
losers. In all cases, reducing peak tank currents as far as possible
will reduce all losses and stress on components most likely to fail
(caps) and reduce gap electrode wear. The best way to go about this :
design a primary with the highest possible surge impedance. The best
way to go about this is to run at a looow frequency with moderate
tank capacitance and high tank inductance with a high cap voltage to
maintain bang size. Running caps in parallel is a great idea to
reduce current per capacitor (I^2*R loss). This might sound counter-
intuitive when you consider that Xp = 2PIfLp but the big bonus in low
frequency running is that cap reversal frequency is greatly reduced
which reduces dielectric losses. You have to up power, you have to up
input voltage.
Also, I think it may be time to re-examine some gap
arrangements. I know cylindrical series gaps are widely used and
the use of blowers might well be improving their conductivity. BUT:
for every series gap in the circuit you are suffering a voltage drop
which equates to running a lower voltage with just a single gap. I
think the quench characteristics of a jet of air through a single
long gap vs the series arrangement should be compared with the
series gaps. With a single wide electrode spacing it may be possible
to effect an even better quench. And the use of a highly pressurized
enclosed gap might be far better again, esp if a gas flow can be
maintained through the space between the electrodes. Perhaps someone
who has the gear can look into this. I will in time.
If you are able to monitor the tank circuit _on its own_
with an oscilloscope, look for the shape of the ring down. If it is
triangular (linear), your gap is the major loser. If it is logarithmic
(curved), you can do much better in the cap/wiring dept. I found out
the hard way.
Finally, raise the primary coil well above the ground to
minimize shunting losses in the ground/building floor.
Many people's experience have contributed to the above
conclusions (to name some would be unfair to others) and I want to
acknowledge them all and thank them for sharing their ideas and
experience.
Bye for a week or so,
Malcolm
PS - input on the above welcomed of course.
PPS - many thanks to Bert Hickman for input on the modified Q equation
for a primary where the gap is the major loss. Xp/Rp no longer applies
in this situation as Rgap varies sinusoidally with tank current.
MJW