Re: Cap safety gap (renamed) (fwd)

---------- Forwarded message ----------
Date: Tue, 16 Dec 1997 16:57:05 -0700
From: "D.C. Cox" <DR.RESONANCE-at-next-wave-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Cap safety gap (renamed) (fwd)

to: Ed & List

It doesn't require a very high value resistance to spoil the Q enough to
prevent HF ringing.  We prefer a value of 250 Ohms.  A good plasma
technologist whom I work with on a regular basis at UW-Madison recommends
any value between 20 and 200 Ohms.  When using NST's be sure to have this
value in series with each bushing.  Also good idea to run it through about
60 turns air core wound on a 1 1/2" dia PVC tube -- this helps clip and
block the VHF signals that can really damage a xmfr.  We don't use a cap
safety gap at all but our caps are very conservatively rated at 50 kV pulse
duty and don't get near breakdown until around 110 kV.  We have never, ever
lost a cap in our commercial units that have been in service (in some
cases) for over 20 years.  It is very important to control the total
discharge gap opening -- usually less than 0.200 with NST's and less than
0.500 with pole xmfr operated devices in the 12.4-14.4 kV range.    

Hope this information is of assistance.


> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Cap safety gap (renamed) (fwd)
> Date: Tuesday, December 16, 1997 12:23 PM
> ---------- Forwarded message ----------
> Date: Tue, 16 Dec 1997 14:04:02 EST
> From: Esondrmn <Esondrmn-at-aol-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Cap safety gap (renamed)
> In a message dated 97-12-16 03:09:12 EST, you write:
> <<  I would like to hear other folks opinion on this subject.  I believe
> Mr.
>  > Cox and Malcolm suggested the addition of the safety gap across the
>  > Malcolm, you have the same C.P. caps that I have.  What kind of safety
> do
>  > you use, what transformer voltage, and how far do you open the gap?
>  > 
>  > Ed Sonderman
>  The point Mark made about the high frequency ringing with the gap 
>  across the cap is well made IMO. I never use a gap across the cap.
>  I *do* think it is a good idea as a *last* resort to stop the cap 
>  dying from overvoltage until BPS etc. is optimized.
>       I've never run a real high power supply like a pig. The 
>  only way a gap across the cap can fire is if the charging inductances 
>  allow the cap to ring up to that voltage in between breaks. It can't 
>  be energy coming back from the sec because you can't get more energy 
>  into the cap by that route than you started with. A gap across the 
>  transformer is a different matter entirely.
>      I've been mostly confined to neons and other transformers in the 
>  2kW range at most. I think the Q of the charging circuit should be 
>  lowered if the cap safety gap fires. It would also help if you can 
>  continuously vary the break rate. You should be able to strike a 
>  point for a given set of charging conditions where the safety gap 
>  stops firing entirely. At the same time, coil output should also peak.
>  Malcolm >>
> Malcolm,
> I would appreciate your thoughts on this.  The last time I had this coil
> up (outside), I tried many combinations of primary set up.  This includes
> welder settings, resistance in series with welder, rotary speed and
> settings.  I added 600 ohm resistors in series with each H.V. feed line. 
> bypass caps and no chokes in the H.V. feeds.  With resistive ballast
only, I
> get a popping sound coming from the rotary gap that doesn't sound good
and the
> gap firing is erratic.  When I add the welder in series with the ballast
> resistance, the popping in the rotary goes away and the system runs much
> smoother.  The discharge sparks are much hotter and louder than with
> ballast only - at the same primary voltage and current.  Is this an
> of a resonant charging condition that is allowing the voltage across the
> to ring up too high??
> Should I increase the H.V. series resistors from 600 ohms in each leg to
> 3,000 or 4,000 ohms?   At 500 ma (5 kva at 14.4 kv), I will run into a
> dissipation problem in a hurry along with concern about voltage breakdown
> across the resistor.  At 3,000 ohms, the resistor would be dissipating
> watts with 1,500 volts across it.
> The best performance in the above testing produced 57" sparks with the
> set at maximum, series resistance at 4.8 ohms, variac wide open and
> speed at approx. 400 bps.  Lowering the series resistance or increasing
> speed would cause the cap safety gap to fire.  Primary input energy at
> point was only about 3.5 kva.  I want to get the system dialed in to
where I
> can input all the power I have available - probably about 8kva.  I have
> produced 80" sparks in the past with this system.
> Your idea of lowering the primary Q goes back to the original design
where I
> had all the primary components interconnected with two parallel sections
> RG213.  Most of these recent problems have occured after I replaced all
> wire with 3/8" copper tubing - raising the primry Q.
> Comments?  Thanks,  Ed Sonderman