Re: Dwell time, etc

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 3/28/06 10:42:50 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:


> Hi,
> ----- Original Message -----
> > Original poster: FutureT@xxxxxxx
> snip
>
> >Typically the gap does not quench because
> > the electrodes pull apart and break the spark.  Rather the
> > electrodes may still be aligned when the gap quenches.
> > This is because the cap runs out of energy and the voltage
> > drops until quench occurs.  A rotary gap determines when
> > the gap will fire, but not when it will quench.  Quenching is
> > more determined by other aspects of the Tesla coil although
> > teh gap does have some effect.
> snip
>
> I agree with the above.
>
> What I then wonder is what cause the ring of fire affect.
> Is it too small a cap for the ballast? and hence recharge is so rapid that
> the spark is reignited and quench is then by separation?


Robert,

Yes, I believe that is the basic cause.  A slow rotary speed can
cause the same effect esp is the cap is somewhat small.  I've
also seen power arcing (ring of fire) when no ballast was used at all.

> An other question. Some contributors suggest that dwell time can be adjusted
> to produce longer sparks. If quench time is not influenced by dwell time how
> can adjusting dwell effect performance.


I don't think the dwell time can truly be adjusted to produce longer
sparks except maybe in some special situations.  I've done a lot
of tests in the past with various mechanical dwell times, and it didn't make
any difference in spark length.  If the dwell is extremely long,
the gap can re-fire while still aligned causing very inefficient
operation.  It's usually impossible to shorten the mechanical
dwell time enough to truly hasten the quenching.  In some
very large coils such as Greg Leyh's coil, the frequency may
be low enough that the rotary mechanical dwell may begin
to approach the desired optimal electrical quench time, so
the rotary may assist in quenching.  Greg suggested that this
is the case with his coil.

> A noticed in a crude simulator (dead short for the SG) that under certain
> circumstances the charge voltage was dependent on the dwell time (period of
> the short). in say going from 1/100 of a cycle dwell time to say 1/256 the
> charge voltage changed by say 20%. I did not investigate this effect only to
> make the dwell time as small as possible so I do not know it if was an
> artifact of the simulation or a particular set of conditions that are
> sensitive to the dwell time.


If the simulator is simulating electrical effects, then I can understand
the advantage of a short electrical dwell which would give the
capacitor a longer time to charge.  But if only the mechanical dwell
is being affected, and the electrical dwell is remaining the same,
then I don't see why it should matter what the dwell time is (with
the caveats I mentioned above).


> I did consider that the longer the dwell the
> greater the ballast inductor's charge voltage(hence its stored energy) under
> the appropriate phase conditions.  So that when quench does occur the cap
> voltage is charged to a greater voltage then if the dwell had been of
> smaller duration.


Yes that's what I mentioned above.  But I don't think mechanical
dwell usually affects electrical dwell,  (again unless it's extreme
so re-firing can occur, etc).

> Is this the reason some experimenters suggest dwell time effects
> performance. I should add that its difficult to see how if the spark does
> reignite why it would not be drawn out in to a ring of fire regardless of
> the presentation time of the electrodes).


I think that provided the gap does not re-ignite, the mechanical
dwell shouldn't matter.  I agree that a re-igniting gap should
produce the ring of fire.

John


> Robert (R. A.) Jones
> A1 Accounting, Inc., Fl
> 407 649 6400