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Re: Spice simulation pictures
Tesla List wrote:
>
> >From rwstephens-at-ptbo.igs-dot-netSat Oct 19 22:22:57 1996
> Date: Sat, 19 Oct 1996 19:45:18 -0500
> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Spice simulation pictures
>
> All,
>
> Yesterday I (rwstephens) wrote:
>
> >I was able to swing the 100 pF load to about 85 kilovolts RMS (120 kV
> >peak). The secondary winding got so hot in just 120 seconds of operation that
> >I shut the system down for fear of melting the PVC tube on which it was
> >wound! My circulating current was 8 amps RMS according to the
> >measured output voltage across a capacitive reactance of some 10.6 K
> >ohms, but the heat produced in the wire felt more the equivalent of
> >maybe 30 amps RMS. Can anyone explain where my figuring leads to
> >such an error? The interconnect between the top of the secondary and
> >the drum capacitor was #14 PVC covered solid wire. It got bloody hot
> >too. If I had had one then, that's where I would have liked to place an RF
> >thermocouple ammeter.
>
> The answer to my own question occured to me last nite. And of course
> I feel foolish for not thinking of it. My math is fine. I had
> forgotten about skin effect! I just calculated how much of the
> actual cross section of that #14 AWG wire is carrying my 8 amps of RMS
> current. Assuming a skin depth of 0.1 MM at 150 kHz then only 24.3% of the
> copper is carrying the RF current. If we scale up the 8 amps proportionately
> times 4 , (4 times 24.3 % is 100%) that is the equivalent current of 32 amperes
> RMS in the whole of the #14 guage wire and I had guessed the equivalent of 30
> amps at 60 Hz from experience based on the heat felt in the wire by
> my hand. I'd say my guess was pretty good after all.
>
> Ahhhh, a mystery solved. : )
>
> rwstephens
> half asleep
Robert,
Well, I figured you knew the answer all along, and were just wanting to
give the rest of us a test! :^)
Unfortunately, when I did my original post in reply, I tried
"faking-out" my spreadsheet model using some of your coil parameters,
and managed to mess up on the calculations big time, since other
parameters were still from my coil! Right concept - _wrong_ numbers -
never blindly trust what a spreadsheet says!
Straight #14 AWG wire at DC has about 2.53 Ohms/1000 feet. At 150 kHz,
simple skin effect increases this to about 6.5 Ohms/1000 feet. Proximity
effect from coiling the wire further increases this to about 11
Ohms/1000 feet. Thus Rac/Rdc = 11/2.5 or about 4.4X the DC resistance
(current flowing through only about 23% of the copper). The 8 Amps of RF
current is like passing about 35 Amps DC. The #14 AWG at 150kHZ is
conducting about as well as 20-21 AWG does at DC.
Now that I'm sitting on the coil - turn the power on, please!
-- Bert --