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Re: NSTs and Cap charging tests (fwd)
---------- Forwarded message ----------
Date: Wed, 14 Apr 1999 22:44:06 +0100 (GMT)
From: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk>
To: tesla-at-pupman-dot-com
Subject: Re: NSTs and Cap charging tests
John, Reinhard, Tony, Terry, Malcom, and All.
John, your practical results seem very close to my simulations,
I really wish I had a chance to do more practical tests, as I think
there is much to be learned in optimising the charging system for AC.
John, Are you using a 120 BPS rotary for your tests ?
If so, I have not done many simulations with 100BPS (or 120 BPS for
60Hz.) I concentrated mainly on 200BPS for 50Hz, because I found
that the voltages are about 30% less, and I can get similar power
throughput with smaller capacitors due to the higher break rate.
(Will this give similar performance in practice ?)
Can you get a measure of the firing angle of the rotary by any means ?
I usually record the time from the zero crossing of the supply voltage
to the actual gap firing. Maybe the orientation of the motor or
electrodes indicates the firing angle.
The posting from Tony Greer is very interesting because this displays
something which I have seen in my simulations but have not heard of in
practice:-
Tony Greer wrote:
> ..... But when 96 volts was reached, input current
> SUDDENLY droped to about 16 Amps, and the output current droped from 200
> mA to 100mA, and became very steady....
> ..... but most noticeable was the different sound of the gap and
> streamers. At between 96 and 98 Volts, the noise became a steady buzz,
> like a single note. Probably firing in sync with the line, because
> raising the Voltage past 98 Volts would again cause the current to
> double and the noise would regain it's raspy sound.....
In one of my previous posts I commented on a few "special" capacitor
values
which would cause the static gap to fire smoothely as Tony described,
at all other values the firing was fairly chaotic. I never tried
simulations to find the effect of different supply voltages though.
I also found that it is easier to get a high power factor by using
a correctly set synchronous gap, than by using a static gap. The
power factor for the static gap only seemed to approach that of the
rotary gap, when the gap is set to fire at a VERY high voltage.
Like John, I found that it is possible to find certain firing angles
which draw alot of current but produce little spark. I found these at
timings near to the zero crossings of the HV voltage, where the
rotary begins to miss fire, and the safety gap kicks in ! In these
cases the simulation shows that the voltage builds due to resonant rise
and the voltage is near to zero when electrode presentations occur,
therefore gap firings take little energy out of the system. Current
is mearly sloshing in and out of the HV capacitor. Under these conditions
I noticed the voltage get very high between gap firings, (Another
reason for changing to a 200BPS rotary.)
For my new TC I have chosen a 200 BPS (50Hz) synchronous rotary, and
have already operated it with a 6kV 125mA neon transformer.
The firing angle was adjusted to obtain two equal energy bangs per
half-cycle. The capacitor voltage was checked with a scope also:-
Tank capacitor voltage waveform over one complete supply cycle:-
__
/ \ Both bangs are equal amplitude
/! / ! but there is always a voltage
/ ! / ! overshoot before the second firing
/ ! / ! of the gap. This is where the cap
/ ! / ! __ is charging on the downside of the
/ !/ !/ \ AC supply voltage.
\ !\ !
\ ! \ ! Since the rotation of the electodes
\ ! \ ! is synchronised to the supply freq.
\! \ ! the charging waveform is always the
\__/ same for every mains cycle.
Several parameters such as current draw, peak cap voltage, and firing
angle were all found to be within 10% of the simulated values. It's a
pity that PSpice cannot estimated spark lengths ! This system charged
a 44nF cap to 11400 Volts, 200 times per second, and produced 2 ft
streamers into the air. However I did not have time to try adjusting
the primary tuning. :-(
Does this sound reasonable in terms of performance ? Supply VA was
measured at 668VA.
I have got bigger sparks with the same transformer using static gaps,
but the firing was erratic and the supply VA was more than double !
With the rotary, the TC is running so much smoother, and the sound is
totally different. It produces a buzzy wine rather than white noise.
I plan to run the system from a 10kv / 224mA Radar power transformer,
and have detailed information if anyone would be kind enough (and bold
enough) to predict performance. I would also appreciate having someone
more experienced check it over, before I toast something expensive
due to a silly error. Any offers ? Please contact off list.
Richie,
- Finding it all very interesting
in sunny Newcastle.