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Re: Safety Gaps


Hi Vivian, and all,

I don't know what size your main capacitor is,  but with such a powerful
HV supply,  and a low break rate,  you risk overcharging it in the time
between presentations.

If the capacitor is small compared to resonant size (ie. C<38nF)  it will
charge to a very high voltage between presentations.  Since the electrodes
are far apart at this time,  the voltage has to jump the safety gaps,
(with quite a roar I imagine ?)

If your capacitor is very small (ie. C<<38nF)  the natural resonant
frequency will be such that there are many oscillations in the time
between firings of the rotary.  I have some voltage plots which show this
occuring with a 100BPS synch gap.

There are three possible solutions:-

1. Add more capacitance.  A larger capacitor will charge to a more
realistic voltage,  without overshooting between presentations.  You will
get a similar overall power throughput due to the additional capacitance,
even though the voltage is lower.

2. Change to a higher break rate such as 200 BPS (still synchronous.)
Since another presentation has now been added between every 100BPS
presentation,  these will catch the voltage rise before the safety gap
fires,  and make use of the stored energy by sending 200 bangs into the
primary as opposed to 100.

3. Lower the supply voltage,  or add more ballasting to limit the current.
   This will make sure that the capacitor does not charge too rapidly,
but I guess you really don't want to do this one :-)

I opted for a 200 BPS rotary design,  which I have tested at the lashup
stage,  and it seems to work quite well.  My power supply is 10kv/224mA
and I got serious overcharging problems with a 100BPS rotary.  The firing
angle of the rotary was really critical,  if the safety gap was not to
fire.

Are you going to be at the Corby Teslathon on the 29th ?,

If so I would be glad to discuss this one furhter.

						- Richie,

						- In sunny Newcastle.


On Mon, 17 May 1999, Tesla List wrote:

> Original Poster: "Vivian Watts" <V.C.Watts-at-btinternet-dot-com> 
> 
> Hi All,
>     I know this has been asked many times as I've read previous posts but
> I'm having difficulty setting my safety gap with my rotary.  You may recall
> I blew my Neon a while ago while running my new Sync rotary so I don't want
> to do the same again.  I tried setting the S-gaps so they just don't fire
> with no primary circuit connected.  However connecting the primary circuit
> causes the S-Gaps to fire at about 50% input voltage and no main gap
> running.  So I assume I'm getting resonant rise and the S-gaps then fire.
> However when running the rotary the S gaps still fire at 50% input.  I tuned
> the Rotary phase with about 5% input voltage to the primary and rotate the
> rotary phase to the centre of max arcing range.  This seems to correspond
> with the electrodes lining up with the white part of a strobe disc I had
> attached.
> 
> My safety gap is standard, with a separate gap from each HV terminal to
> ground.  Also a separate gap across the rotary.  All are 1/4" brass round
> nose shape.
> 
> I originally set the S-gaps to 1/4" with the rotary S-gap at 1/2".  With no
> main gap the safeties did not fire so being worried about over-volting I've
> closed the gaps down  as described above.
> 
> I'm now running two 5KV 120ma Neon's in anti-phase with centre ground.
> 
> regards
> 
> Viv Watts UK.
> 
> 
>