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Re: Interesting but not so impressive results...


From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Sunday, September 07, 1997 7:32 PM
To: 	Tesla List
Subject: 	Re: Interesting but not so impressive results...

Tesla List wrote:
> 
> From:   Rodney Davies[SMTP:rgd872-at-anu.edu.au]
> Sent:   Sunday, September 07, 1997 8:24 PM
> To:     tesla-at-pupman-dot-com
> Subject:        Interesting but not so impressive results...
> 
> Hi All,
> 
> Well, tonight I finally got around to arcing up my old 12" coil...
> I'm using a 500W bar-heater as a current limiter, and a pole-pig 10KVA
> 11KV SWER transformer.
> The capacitor is approx 0.02uF. (home made rolled job).
> 
> Here's some brief specs -
> Secondary -
> 900+ turns, self-resonates (unloaded) at 77Khz.
> 1mm enamel coated wire, space-wound at 21 turns/inch.
> PVC former diameter - 12.5"
> Aspect ratio - 1:3.75
> 
> Primary -
> 5 turns of 3/8" copper pipe spaces at 3/4" between turns (archimedes).
> 
> spark gap at this stage is a single static.
> 
> >From tonights effort, I figured I was getting around 5-7KV out of the
> transformer, probably about 200-300Watts...
> 
> The coil is currently unloaded, so the wire was merely sticking up in the
> air from the top of the secondary... (yup, I know...should be loaded).
> 
> Well, I did actually, with various round metal objects, but that decreased
> the output significantly, so I left it unloaded since that was where the
> longest arc was occuring...
> 
> Now here's the dissappointment, I was only getting about 4-5" of arc...
> 
> Yup, you're right, she's way out of whack! out of tune, out of
> everything...even the capacitor is too low, should be around 0.1uF...
> Anyway, plenty of time for the fix-ups...
> 
> The major point of tonights exercise was to observe the wave of the
> secondary coil... Here's what I did, and I know what you'll say - don't
> arc from the windings!!!
> 
> Ok, I held an earthed wire in my hand, holding it approx 6" away from the
> side of the secondary and observed the corona.
> There was a definate faint corona occuring, but the interesting part is,
> what sort of corona I obtained whilst moving the earth wire up and down
> the length of the secondary.
> 
> Ok, about 5" from the bottom, the corona was bright and dense and the arcs
> that did occur were fairly violent... I then moved the wire further up the
> secondary and observed that approx 1/3 the way up, I had the strongest
> corona and most violent arcs.
> Then, I moved the wire further up and noticed that the corona almost
> dissappeared at about 2/3rds the way up, but then became brighter within
> 4" from the top.
> 
> Perhaps a diagram may help explain...
> 
>         Secondary.
> 
>         _________
>         |       |   <---- started to become brighter.
>         |       |
>         |       |
>         |       |
>         |       |   <----- virtually no corona
>         |       |
>         |       |
>         |       |   <----- brightest.
>         |       |
>         |       |   <----- earth wire ---+
>         |       |   medium brightness    |
>         ---------                       ---
>         Base.                            \\\
> 
> I guess this is an indication of what the wave is doing...
> 
> Also, there could be alot of contributing factors -
> 1/ zero quenching on the single-static gap
> 2/ completely out of tune
> 3/ No toploading
> 4/ primary circuit no where near in resonance
> 5/ secondary re-coupling energy back to primary
> 
> etc..etc...
> 
> But, I'm sure I'll fix it up soon, just need more money...
> Some, if not most of you will agree, big coils are an expensive hobby! :-)
> 
> Anyway, I was just wondering if anyone would like to elaborate on the
> corona effects I observed with the secondary?
> 
> Yes, I know, never arc to the windings...but it's a really bodgy coil, so
> I guess it really doesn't matter...
> 
> Thanks Guys!
> 
> Have fun!
> 
> Catchya!
> Rod


Rod,

You're clearing driving the resonator at a significantly higher rate
than it's 1/4 wave frequency. From your description of it's behavior, it
sounds more like you are approaching 3/4 wave mode - i.e., your tank
circuit is driving the resonator at around 3X the secondary's
self-resonant frequency. Under these conditions, the first voltage
maximum will occur about 1/3 the way up the resonator, going to 0 at
about 2/3 the way up, and then going to another voltage peak near the
top. Your primary looks like its running at around 245-250 kHz or so
depending upon the diameter of the innermost turn - I assumed about 16".
Comparing the primary versus secondary self-resonant frequency, you
appear to be in the ballpark at around the 3:1 ratio needed for 3/4 wave
operation. Don't increase the power level under these conditions -
interturn arcing may damage an otherwise nicely constructed resonator. 

Now here's the kicker - when I estimate your secondary's self-resonant
frequency, I come up with something closer to 130 kHz. You didn't say
how long the wire was coming off the top of your coil, but is apparently
is long enough to add a fair amount of top-load capacitance. Also, while
you were making your corona measurements, your body capacitance was also
increasing the resonator's effective self-C. The combination apparently
lowered the secondary Fo to a point closer to the 1/3 that of the
primary. BTW, did you actually measure your coil's self-resonant
frequency to be 77 kHz? If so, under what conditions?

Going forward, try lopping about 11" off your resonator length, add
about 8 - 10 more primary turns, a 6" x 24" flex-duct toroid, and add a
series vacuum gap and you'll be good to go with a hot-running coil
running at about 110 kHz! BTW, is blodgy a technical term in Australia?
;^)

Safe coilin' to you, Mate!

-- Bert H --