Re: Dwell time, etc

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi Gerry, all,

That's also my understanding. The mechanical dwell time on most 
rotary gaps is several times longer than the energy transfer time of 
the Tesla coil system they're driving. But if the coil is properly 
tuned and heavily loaded by a nice display of streamers, the  energy 
in the primary will be used up quickly and the spark in the gap will 
quench by itself before the mechanical dwell time is up. My gut 
feeling is that the electrical circuit constants have more of an 
effect on the quenching than anything you can do mechanically.

My old DC spark gap system was quite good in this respect as it used 
a small capacitor with a high breakrate and high ~20kV charging 
voltage. The rotary gap would arc while the electrodes were still 
more than 1/2" apart and then quench before they had even presented. 
If the rotary gap speed was turned down far enough, the capacitor 
would recharge before the electrodes had time to move apart again, 
and i t would fire twice per presentation.

When I increased the tank capacitor size from 12nF to 37nF it no 
longer did that. The gap ran a lot hotter and brighter and I started 
to get trouble with trailing arcs. The rotary gap would suddenly take 
on the appearance and sound of a catherine wheel firework and the 
fuses in the power supply would pop. Adjustments to the tuning never 
made much difference. I put it down to a mismatch caused by trying to 
ram too much energy into an undersized secondary.

That rotary gap was pretty poor though, I now have a nice one with 
1/4" tungsten electrodes and a proper balanced rotor made by Finn Hammer :-)

Steve Conner
http://www.scopeboy.com/


Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "Gerry Reynolds"

Hi John,
< br>I calculated the dwell time on my 8 inch 1800rpm SRSG with four 1/8"
flying electrodes and thought the dwell time was like 3 or 4 times
the energy transfer time. This seemed long enough for energy to flow
back into the primary and reignite the gap. However, it also seemed
like if a breakout occurs, the energy is quickly removed from the
secondary so dwell time would not be a problem. Is this correct
thinking or did I make an error somewhere??

Gerry R.


>Original poster: FutureT@xxxxxxx
>After the gap fires, the cap is mostly empty of voltage. It takes
>time for the current to charge the cap to reach enough voltage
>to fire across the gap again. This is similar to how a static
>gap quenches, then fires again when the voltage reaches
>a high enough level to jump the gap. In the rotary, by the time
>the cap has enough voltage to jump the gap, the electrodes should
>have passed out of align ment, so no spark will occur until the
>next time the electrodes align. The uncharged cap is holding
>down the voltage during the initial cap charge time because the
>cap is large enough in value so it takes enough time to fill. Imagine a
>bucket of water with a small hole drilled two inches away from the
>bottom (on the side of the bucket). This hole represents
>the still-aligned electrodes of the
>rotary. If you start filling the bucket using a water hose, the
>water will not pour out of the hole right away. First the bucket
>will need to fill to the level of the hole. But during this time
>the electrodes move out of alignment thus "closing" the hole.
>Then the bucket can fill completely.
>
>John