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Re: Continued Problems (fwd)





---------- Forwarded message ----------
Date: Tue, 05 May 1998 02:00:16 -0500
From: "Barton B. Anderson" <mopar-at-mn.uswest-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Continued Problems (fwd)

Bill and all,

Tesla List wrote:

> ---------- Forwarded message ----------
> Date: Sun, 3 May 1998 13:00:40 -0400 (EDT)
> From: fxphoto <fxphoto-at-centuryinter-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Continued Problems (fwd)
>
>  Let me preface this by stating that I am a relative "newbie" and I have not
> worked with a pole pig yet, so it could be my lack of understanding causing
> your cap to stand out like a sore thumb to me. You state that you are using
> a 0.025 uF 15kv (A.C. or D.C. rating?) cap in conjuction with a 5kva 14,400
> volt pole pig.  My understanding states that 14,400 (eff) x 1.414 (pk) x 2 (pk to
> pk) would equal 40,723 volts peak to peak and when tossed into a coil, the
> returned peaks would greatly exceed even this.

Ed, he's right. A 15kv cap might be kind of hard to protect.

> Mike Hammer has stated that above 12kv you should use three caps in series
> in order to limit the high voltage strain across each individual cap. I am
> just starting to understand why. I believe at 5kva, you are putting out around
> 0.347 A -at- 14400 volts which would need a total capacitance of around 0.064 uf at
> about a 60kvdc rating.

Bill, I think what your are doing here is calculating max current based on the rated
max V and KVA and then calculating C for max capacity based off of the transformers
impedance at max V. This doesn't mean one should necessarily stick with this
capacitance. For instance, I first tried matching my 10kva xfmr impedance to the
cap, which resulted in a .128uF cap at 13.27 joules which some list members
thankfully warned me of a melt-down (so to speak) as based on the overall size of
the coil. I half'd the capacitance to 0.064uF which pulled the joules to 6.64 which
I felt relatively comfortable to start with.

>  I am sorry that I do not understand the effect of the current control
> method that you are using has on Xc but it seems that Xc would change as you
> change the current output. If this is so, the capacitance would need to
> change according to the current output. Perhaps someone can jump in and
> explain this to me.

Xc = 1 / 2piFC: C is constant (whatever capacitance was chosen). 2pi is constant. F
is constant being the resonant frequency of the tank. Keep C and F the same, and Xc
doesn't change. Current isn't a part of this and doesn't effect Xc. Change C or F,
and you will change Xc, but then the tank will have a different resonant frequency
than the secondary. Overall resonance is lost until you retune to the new resonant
frequency. Increasing current to the cap allows "x" amount of current to be stored
and released at "x" voltage level. Resonance is that point where inductive and
capacitive "resistance to AC" (reactance) is equal. So, why doesn't Xc change with
current? Because Xl and Xc are opposite to one another. If current causes one to go
up, then that same current is causing the other to go down, thereby keeping Xc and
Xl in essence, unchanged. Well, somethings got to give right? It's ohms law now. Xc
and Xl is R. With "R" remaining constant and "I" increasing, then "V" has got to go
up as well as the power! Any comments? criticism?

>  Your 0.025 uf cap should work best at a current output of 0.140 amps , or a
> little less than half of your full 5kva output.

The 0.025uF will be at 2kva+ (based on the impedance dictated by current match of
the cap to the transformer), but these pole xfrmrs are capable of exceeding their
ratings and I think Ed is pulling it up to 8kva as measured from line input and
current draw without having the safety gap fire and without killing his caps.

>  Your safety gap is firing when the voltage reaches the breakdown voltage of
> the gap. I am not sure what 5/8" is voltage wise, but if someone said use
> 5/8" I would stick with it. By increasing the capacitance, you can increase
> the current throughput without changing the voltage (i.e. causing the safety
> gap to fire).

Makes sense to me. It would be interesting to know the breakover potential of 5/8"
(of course, environment plays a large role). I wonder if the tank voltage rise is
finding it's way across the cap during the break, thus firing the safety gap. If so,
good thing it's there. Maybe way off on that one. Just thinking.

Bart


>  Just my semi-informed 2 cents!!             Bill Turbett

> >>Date: Wed, 29 Apr 1998 12:52:12 EDT
> >>From: Esondrmn <Esondrmn-at-aol-dot-com>
> >>To: tesla-at-pupman-dot-com
> >>Subject: Continued Problems
> >>
> >>Sorry, this is going to be a long one.
> >>
> >>I fired up my 6" coil last night for the first time this year.  I am still
> >>having problems.
> >>
> >>Quick overview: Secondary 6" dia, 991 turns of #22 awg, 29.1 mh.  Primary 3/8"
> >>copper tubing wound in a 30 degree saucer shape, 3/8" spacing between turns,
> >>I.D. 8", O.D. 26",  5.25" tall, 14 turns total, 88 uh.  Secondary is mounted
> >>so the lowest turn is  3.5" up from the first turn of the primary - right at
> >>about turn 7 or 8.  Mark Rzeszotarski (sure hope I got that right Mark)
> >>calculated the coupling to be about .167, so I wouldn't think it is coupled
> >>too tightly.  Best tune is at turn 11.75 with a 5" x 40" toroid.
> >>
> >>Capacitor is a .025 uf, 15 kv Condenser Products unit with a spark gap mounted
> >>directly across it made of #10 solid copper wire with 1.0" dia loops on the
> >>ends.  Gap is set to 5/8".  Power is from a 5 kva 14.4 kv pole pig and is
> >>supplied across the spark gap which consists of a rotary with a static gap in
> >>series with it.  Current is limited to the pole pig via a Miller Thunderbolt
> >>welder with an adjustable resistor stack in series with the welder.
> >>
> >>I built this coil about two years ago and scaled things up gradually from neon
> >>sign transformer to the pole pig.  I was quite happy with the performance,
> >>getting nice white hot bolts up to 80" long at about 8 kva input.  Then the
> >>first C. P. cap blew up.  I replaced it and within not too long, lost the
> >>second one.  Several folks on this list suggested I install a spark gap across
> >>the cap unless I have lots of money - which I don't so I did.  Now, I can't
> >>input much power without firing the cap safety gap.  Last fall, last time I
> >>had this coil fired up, Malcolm, Dr. Cox and a few other helpful folks
> >>suggested I had a bad case of 60 hz resonance.  I have to ask a question here
> >>- are there systems out there that can be run at say 10 kva with 15 kv in and
> >>not fire a .25" safety gap across the cap?