Re: The VTCC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "K. C. Herrick" <kchdlh-at-juno-dot-com> 

On Thu, 25 Dec 2003 11:07:03 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > Original poster: Ed Phillips <evp-at-pacbell-dot-net>
 >
 > Tesla list wrote:
 >  >
 >  > Original poster: "K. C. Herrick" <kchdlh-at-juno-dot-com>
 >  >
 >  > Hi Steve-
 >  >
 >  > I enjoyed seeing the photos of your VTCC.

[snipped]

...a Marx-like primary--where a set of resonating
 > capacitors is
 >  > repetitively charged in parallel from the mains & then discharged
 > in
 >  > series thru the primary coil.  I've always been reluctant to work
 > with
 >  > 5-12 KV of mains-ac lurking around!
 >  >
 >  > Regards,
 >  >
 >  > Ken
 >
 >         That idea will take lots and lots of capacitors.  In place
 > of the spark
 > gaps, how about a motor-driven multi-contact mechanical switch?
 > Such
 > things have been used for a similar purpose. If designed right you
 > could
 > eliminate the resistors.
 >
 > Ed
 >
 >
 >

Ed, Steve Ward (& all)-

Funny you should suggest that, Ed...  I've been toying with that very
idea.  I'll ask Terry to put up http://hot-streamer-dot-com/temp/tspk16s2.pdf
which is a schematic of what I've had in mind.

I show 12 capacitors (each one actually 4 in series-parallel for reduced
electrical stress) mounted circumferentially on a rotating disk.  Each
connects to both a pair of brushes & a pair of gap-contacts (each such
pair shown schematically only as arrows).  The disk assembly is to be
mounted directly under the secondary and, so mounted, incorporates not
only the capacitors of the primary LC resonator but also one of the coil
turns (of which the capacitors are a part).

The mains connects at the left, charging a pair of storage capacitors
thru current-limiter L1.  L2 and the associated rectifier are for
resonantly-charging the capacitors.

As the disk rotates, the capacitors are alternately charged in parallel
from the mains, thru the brushes, and discharged in series into the
primary coil thru the gaps.  Notice that every other charging event puts
opposite-polarity charge on the capacitors.  That is to minimize
material-transfer across the gaps due to net dc current flow.

If the assembly were ~14" in diameter, to accomodate my existing 12"
secondary, then 3 1/2 turns of 3/4"-diameter primary "wire" spaced at
1.5" would yield about 3.3 uH inductance.  To resonate at ~140 KHz (my
secondary's Fr), I would need about 0.36 uF.  For twelve
capacitors-groups in series, each one would then be about 0.36x12=4.4 uF.
  All of those in parallel, for charging, would constitute 52 uF.

I've simulated the charge/discharge cycle with 160 V-pk mains.  At 100
"sparks"/sec, I get the C's charged to ~450 V each.  But with only 12,
4.4 uF capacitor-groups, the rms mains current is a low 8.5 A.

If, instead, I use 24 groups, then 0.36x24=~8.6uF and 8.6x24=~207 uF to
charge up.  Using 207 uF in the simulation, I draw 14 A rms from the 117
V mains, for a more respectable net charge to dump into the primary.

I show a "main" gap at the right, if such is to be used while setting the
rotating gaps to essentially zero width.  A rotationally-synchronized
main gap could allow for more convenient adjustment of spark duration,
but it would add complexity.

Tuning is to be accomplished using a flexible turn made from half-hard
copper sheet-stock.  It is to be bowed out, or not, using a threaded
nylon rod passing along its axis.  A traveling nut-arrangement would
connect to the end of the flexible turn, pulling it along & flexing the
turn.

With 24 capacitances, the disk would turn at 300 rpm for a 100/s spark
rate.

Will anyone take on this kind of design as a project, or at least
embellish this design with more or better ideas??

Ken Herrick