RE: Re: tank circuit of VTTC

[davidt-at-pond-dot-net (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hello,

--- Original Message ---
"Tesla list" <tesla-at-pupman-dot-com> Wrote on 
Wed, 23 Aug 2000 08:01:29 -0600
 ------------------ 
Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-com> 

>>>>>>>>>Snip<<<<<<<<<<<<<<<<<<<<<<

Herwig,

No problem! At 10 kV plate voltage, your peak circulating tank
current
will be over 41A, and almost 25A at 6 kV. Since you will be running
off
DC the RMS value of the circulating primary tank current will
be about
0.707 of this, or about 29A and 17.5A RMS respectively. The real
problem
is that, because of skin effect, the skin depth of your copper
conductors is only be about 0.15 mils (0.004 mm), and only 0.19
mils
(0.0051mm) for aluminum At the operating frequency of your coil
(about
300 kHz). 

Because of skin effect, a circular #10 AWG conductor that has
almost
8200 square mils of conductor area at DC is reduced to about
48 square
mils of effective area at 300 kHz, making it behave as though
it had
170X the resistance of the same wire at DC! To the RF current,
it's
dissipating the same amount of power as the equivalent DC current
flowing through a 32 gauge wire. It's going to get very hot if
you run
it at 29S RMS. I'd suggest beefing up the tank circuit by going
to
larger diameter copper tubing or flat copper strap, and possibly
silver
plating it as well. You are likely going to melt the PVC insulation
on
your primary under extended running conditions. 

..............................................

Well, I would have to take issue with this. I have used #12 PVC
insulated wire on my primaries without any problem. Again, with
up to 3 833C's filted DC 2+ KW in. "I" believe #10 would be fine,
if you go and use a larger diameter pri conductor, then you may
run into problems with size.

...............................................

Flimsy capacitor leads won't cut it either. At these currents,
simple
joule heating of the leads and capacitor plates can conduct and
generate
substantial heating. For a single string, it'd be similar to
heating
both leads of your MMC capacitors with a soldering iron while
the unit
is running.  

However, I have no doubt that a properly constructed MMC could
safely
handle this task. However, your MMC would need to be constructed
of a

.........................................

Sure can. Been, there, done that. My current MMC has been in
use for over a year and a half, with many hours of run time (I
have used my Audio Mod Vac coil for mood music in my shop while
working on other projects ;-)). MMC is a single strand of Panasonic
0.015 uF caps, it does get warm, and I sugest using two strings
for longer life. 

.........................................

significant number of parallel chains to divide the current per
capacitor lead to a more manageable value. A single string won't
work...
at least not for for very long. :^). 

.........................................

Long is relative :-)

.........................................

Let's look at the MMC dissipation tests that Terry performed
awhile back
using a 3 Amps RMS 370kHz CW RF source. The experimental data
is
reproduced below, with the caps sorted in best-to-worst order:

* GE 42L32222 3.23 Deg. C (big cap but must be polypropylene)

* Phillips 6.17 Deg. C (56nF version)
* WIMA FKP1 6.8 Deg. C (medium size polypropylene) 
  Arcotronics 14.9 Deg. C (Smaller tubular poly caps)
  Panasonic 19.04 Deg. C (medium size polypropylene) (old test)
  Panasonic 19.74 Deg. C (new test)
  PHILLIPS 367 KP/MMKP 39.76 Deg. C (4.7nF version small poly
cap)* 
  FCI 0.015uF 69.52 Deg. C (big cap so probably polyester) 
  Sprague 430P 116.6 Deg. C (big cap so probably polyester)

Now, assuming you wanted to keep the capacitor temperature rise
to less
that 10 degrees C., you will need to design your MMC so that
each cap
sees no more that 3-4A RMS, using capacitors from any of the
top three
vendors above. This implies that you'd need to use at least 4-6
parallel
capacitor chains to handle 17 Amp RMS and 7-10 parallel chains
for the
29 Amp case. This also assumes the caps are separated enough
to permit
individual free-air cooling. You may be able to squeeze more
out of the
MMC by blowing air across them, beefing up the leads, or immersing
the
unit in oil. You MAY also be able to get by with a bit lower
design
voltage on the MMC - perhaps going to 25-30 kV - to save on cost.

.....................................

I generally run mine at 1.5 X Plate voltage.

.....................................

However, there's not very much empirical experience with the
long-term
performance of MMC's in a CW RF environment...

.....................................

Like I said: I'v been running them for over a year and a half.
It is very true that while running with just filtered DC everything
gets hot after several minuts, I would not recommend running
a setup like mine (#12 pri, single strand MMC, 2-3 388C's -at-5KV)
for more than 2-3 min runs, on filted DC alone. I must point
out that the meathod of Audio modulation I use (grid), I run
the tubes at almost cutoff, with 40-50K Resitance in the grid
leak, and the effective "pulsing" of the tube must cut the duty
cycle down. That must be how I can get 10 to 20 minute runs (
not full power with longer runs, of course). I did "burn" up
a secondary once, it seems to get the hotest af anything (besides
tubes ;-)). My Schematic can be seen at:
http://216.36.6.9/webdoc2.htg/Tube_coil/Audio_mod_sch_sm.jpg

Regards,

David Trimmell
www.ChaoticUniverse-dot-com


Hope this helped, and good luck in your design!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com