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Re: Barts Coil Desing (was flashing vs. whatever)
From: Barton B. Anderson[SMTP:mopar-at-mn.uswest-dot-net]
Sent: Thursday, November 27, 1997 7:15 PM
To: Tesla List
Subject: Re: Barts Coil Desing (was flashing vs. whatever)
To Bert, Malcolm, and All
> >>(SNIP from Bert)
> > > How big were you planning on making your secondary and toroid, and what
> > > voltage/current is your power source??
> >(SNIP from Bart)
> > Bert,Yes, it's a large value cap! Here's my project: (100KHz, 17KW)
> > Items completed:
> > Secondary: 12.5"OD, 33.2" H. 783 turns. #18 polythermalese magnet wire (1/4
> > Wave of 2460ft).
> <(SNIP from Bert)
> The physical wire length is about 2460 feet (at 100% winding density),
> but your resonator's natural frequency (without a top-load) will be
> about 150-155 kHz. This corresponds to a quarter-wavelength of about
> 1600 feet. A coil will always have a self-resonant frequency that's
> higher than the equivalent length of straight wire because of lower
> propagation velocity (much less than the free-space velocity of light).
Bert, that's true. However, as you mentioned further in the post, the loaded
frequency of the resonator is "near 100KHz". My goal is to match the x-former
impedance to the tank circuit then match the resonator loaded frequency to the tank
frequency, both of which came out close to 100KHz.
> >(SNIP from Bart)
> > Primary: Flat pancake 3/8" soft copper tubing. Currently 8 turns (may be
> > expanded), 3/8 spaced, 23" OD. Adjustable up/down first winding of sec. by 9
> > inches each direction.
> >(SNIP from Bert)
> You'll need to provide at least 1.5" - 2" of clearance between the
> innernmost turn and the secondary to avoid secondary:primary flashovers.
> With a ctr-ctr spacing of 0.75" (3/8" spacing between turns), this will
> bring your outside primary diameter to about 27". A pancake coil with a
> 15.5" inner diameter and 3/4" ctr-ctr spacing will give you about 49 uH
> at turn 8.
>
I have to make a correction on my diameter, a typo to be exact. The OD is 29 not 23.
Sorry about that. But it does come close. There's actually about 2.5" spacing
between secondary and 1st turn of primary.
> > Toroid: 8 x 30".
> >(SNIP from Bert)
> This is a good size for this coil - you might even want to increase the
> diameter a bit to 36" - 40". A 30" diameter toroid should add about 25
> pF of top capacitance (discounted for the effects of secondary
> shielding), which should drop your secondary's loaded operating
> frequency down to around 100 kHz.
>
Yes, 100KHz. This is the loaded resonator frequency I've been shooting for. The
toroid isn't the best built. I used flexible water evacuation piping and wound with
doublesided carpeting tape, then aluminum, then copper taping. These combinations of
layers aided to smooth it out quite nice. I may add a final layer of aluminum foil
(I still need to add a conductive center).
> <SNIP from Bert)
> I suspect that because of the problems you had in estimating too low an
> operating frequency, this also caused you to need an abnormally high
> tank cap value. To bring your system into tune, you'll only need a bit
> over 0.05 uF. I'd also recommend adding a few more turns to your primary
> to give you a bit more tuning range.
>
Bert, Malcolm, All,Here, I run into a conflict. When I calculate the cap off of
transformer impedance matching for 60Hz, I come up with 0.256uF.14.4KV x .707 =
10,181Vrms
10KVA / 10,181Vrms = 982mA
10,181Vrms / 982mA = 10,368ohms
C = 1/(2pi x Z x F)
C = 1/(6.28 x 10,368 x 60Hz)
C = 0.256uF
However, if I design off the resonator specs, I come up with 0.05uF as the cap of
choice (using Ed Sondermans Excel spreadsheet). Here's where I need come
clarification. I know I can swap C for L to keeping mutual reactance the same
lowering my cap value, which is what Malcolm is suggesting, and may very well be
what I end up doing. But what of the capacitor matching to the line frequency? Is
this all hogwash or did I just make some wrong assumptions?
> >(SNIP from Bart)
> > I have a contact at CSI. He offered to build the cap (all polypropylene) for
> $300 (might be the way to go). He said this particular process takes about 6 weeks
> to build one of these caps (unlike the paper and poly type used for wicking the
> oil).
>
> <(SNIP from Bert)
> Hmm.. sounds too cheap for this size cap. Are you sure he understood
> that it's for disruptive Tesla Coil operation off a 14.4 KV RMS source??
>
I explained it to him, and he has actually built caps for Tesla Coil use (he was
working elsewhere at the time). So, I assume he understood. I understand your
"Hmm..", it is a deal!!
<(SNIP from Malcolm)
>Whoa!! IMHO your secondary is *far too short* for the voltage that
>your primary energy will develop in it!!! Go for a *much* smaller cap
>and higher primary inductance. I wouldn't build a secondary less
>than ten feet high for that cap value. I might be shot down in flames
>by coilers who have built such high energy coils but I'm placing my
>bets.
>> What size toroid are you contemplating using?
Malcolm, as mentioned, the toroid is 8 x 30. Interesting point though on the size of
the secondary for the "POWER" of this coil as based on the cap size of .256uF. It's
interesting to me because I have ran across articles identifying to stay away from
long narrow forms. Also, to consider the ratio of primary to secondary depending on
the type of primary in use. And with flat pancake primary's, the ratio between the
two should be near 1:1. My ratio is 1.5:1 which I felt was a pretty good one. Maybe
not?
By the way, BIG THANKS to everyone for the postings on "flashing vs. aluminum foil".
It's clear that flashing is the best choice for the job for both the electrical and
construction considerations
Heat wave in MN (40+),
Bart