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Voltage/Length (fwd)
From: Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: Monday, January 19, 1998 1:21 PM
To: Tesla List
Subject: Re: Voltage/Length (fwd)
Robert W. Stephens wrote:
> I am in the process of constructing a HV capacitive divider column
> probe to be able to measure both peak voltage and waveform shape on
> Tesla coil outputs up to either 500 kV or 1 MVolts (haven't decided
> how tall to make this sucker yet as one will be an inside capable
> tower, the other only outside capable given my present state of the
> lab).
[snip]
> Greg Leyh has, as I understand actually measured the output terminal
> voltage of his older 45 kW Tesla coil as no more than some 500kV
> while producing streamers between 20-30 feet in length (if memory and
> attention serve me correct). His finding follows my own measurements
> albeit at somewhat reduced streamer lengths and input power levels to
> his own very impressive achievements.
> Greg, would you like to share your own measurement experience again with
> this list?
The 550kV for my coil is an estimate based on actual measurements.
Here's how I came up with that:
I ran the coil at 1 pulse per second, by putting a big resistor
string in series with the DC power supply. A grounded sphere was
brought towards the toroid, until it was just close enough to draw
an arc, at 1pps. This arc distance was about 2 feet.
Then with the coil off (toroid grounded), HVDC from the test supply
was applied to the sphere, with the sphere in the same position.
It took about 275kVDC in order to bridge the same gap. At this voltage,
I believe that the meter is good to +/- 20%. At 1pps however, the
primary voltage is half of normal, due to the resonant charging circuit.
So, for normal operation, the output voltage is probably about
275kV x 2 = 550kV.
I feel that this number is roughly correct, as it correlates well
with the secondary current, which measures about 25A peak during
operation, 10 to 15A during 1pps tuning.
The secondary is 20000 Ohms at resonance (2pi x 62kHz x 52mH),
and 20000 x 25A = 500kV. PSPICE gives values anywhere from
300kV to 750kV, depending on what secondary model I use!
I feel that I still don't have a good model.
Multi-Megavolt estimates of output voltages are wildly optimistic
for most coils. A simple energy balance calculation can show that.
Consider a coil with 100pF of sec capacitance, and 3MV of claimed
output voltage. The claimed stored energy in the sec would then be
450 Joules, and with an coil efficiency of say 60%, gives a primary
cap energy of 750 Joules!! (most coils run at less than 80 Joules)
Now at 240 breaks/sec that's 180,000 watts of req'd wallplug power!!
And the stored energy and power requirements would increase from here
with the square of the claimed output voltage.
The Multi-Megavolt ratings do sound better in advertisements however.
-GL