[Prev][Next][Index][Thread]

Gap Dwell Times (formerly: Beating Solved)

Tesla List wrote:
> 
> >From MALCOLM-at-directorate.wnp.ac.nzTue Sep 24 22:28:55 1996
> Date: Wed, 25 Sep 1996 10:04:47 +1200
> From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> To: tesla-at-pupman-dot-com
> Subject: Re: Beating Solved
> 
> This caught my eye....
> 

> 
> The optimum dwell time for a coil is approx:  Tdwell = 1/2*k*Fr
> I would say that if your dwell is currently 40uS and the optimum is
> 20uS, it is currently exactly wrong.
>     However, it doesn't actually matter a great deal if the coil is
> throwing attached sparks since the loss of sec. energy prevents the
> energy returning to the primary and assists quenching (by proxy).
> 
> Malcolm

Malcolm -

  I did some research this weekend, pouring through a Corum brothers
monograph and some old posts to this newsgroup and found out that the 
optimal dwell is 1/(k*Fr), *not* 1/(2*k*Fr). In other words, the spark
gap should conduct for only 1/2 of the superimposed beat-frequency 
envelope. Primary current peaks at that point, and maximum energy is
transferred into the secondary. (I hope this is correct...perhaps
Richard H. can jump in on this one....????)

  While I don't have the exact specs (yet...) as far as k and the beat
freq for my coil, 40uS is a bit long for a 230KHz (approx.) coil, unless
my coupling was in the dumper, which it isn't. 40uS at 230KHz yields
a k of only 0.108, which definately would be in the 'dumper', and the
coil would probably NOT toss out it's healthy 6-footers.

  Yeah, *any* timing on a rotary gap *will* work, as the idea is to
create the RF oscillations. Worst thing to happen is that the coil
isn't running at peak efficiency, and more line inductance (current
limiting) is needed.

  Problem is that most coilers go with what they can get their hands
on, and make the coil run as best as possible with the stuff on hand.
For me at least, that meant using the 1800-RPM motor which was already
laying around, and 3/8" tungsten-carbide tips which I was able to
purchase dirt cheap. It works. Not optimally, but it works. 66+ inches
from a 22" tall winding ain't to shabby, even if the dwell is "exactly
wrong".

  I could have re-designed the gap, and used 5/16" tungsten rod instead
of the 3/8" tips. At 1800 RPM, that would have given me the 20uS dwell.
By staying with the 3/8", and moving to the 3600 RPM motor, I not only
get the dwell, but also increase the BPS from 240 to 480. Thus, *twice*
as much energy is commutated into the system, which hopefully will
result in hotter (not necessarily longer) sparks.

- Brent