Re: Remote Tuned Primary (RTP) ideas...
Tesla List wrote:
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> Hi All,
> One of my new projects (one of a zillion I have planned for the cold
> months ;-)) is to make a remote primary tuning device. It has two parts:
> The first is a way to adjust gap timing remotely. This one is pretty
> simple. Just a DC gear motor attached to a lead screw to rotate the motor
> on my rotary sync gap. There is a nut on the handle so when the screw
> turns, it turns the handle allowing about 40 degrees of movement. I will
> run the motor from a 12 volt gel cell battery and control it through two
> fiber optic cables for total safety (I hate having a wire hooked from
> primary stuff to a box I am holding :o) and to eliminate RFI. The fiber
> optic circuit is cheap and easy (low tech plastic fiber) and actually
> simpler than trying to safely bring out a regular control cable to a hand
> held box.
> The second part is more interesting. Having thought of many "great ideas"
> and having them all disintegrate for various reasons, I have settled on the
> following: I will have an "extra" MMC with four stings. They will have a
> value of 0.5, 1.0, 2.0, and 4.0nF (could be other values too). This device
> will be parallel with the regular primary cap. By selecting the strings in
> a binary fashion, I can select from 0nF to 7.5nF of additional capacitance
> in 0.5nF steps. Four fiber optic cables and simple circuits, as above,
> will select the capacitance from a simple binary thumb wheel switch. Piece
> of cake! Except for the four actual switching devices.
> My "pals" at Jennings could sell me vacuum contactors at $2000 a piece but
> that is kind of rough even for me. I have taken regular relays, pulled the
> covers off, and submerged them in oil. It works "sorta good" but the ~1mm
> distance between the contacts of most relays is a bit too small. I would
> probably have to use two in series which is not big deal (or series a DPDT
> relay's contacts - I just thought of that ;-)). The contacts still make
> good continuity and the oil and surfaces are not contaminated to any degree
> by the switching. A fairly typical relay (of high quality) will handle the
> voltage and current of just this small additional capacitance so that is
> not a problem either. I would have to vacuum impregnate the relay with oil
> to get it thoroughly saturated but that is easy. Soooo... I have three
> great questions for the collective minds of the great Tesla list to help me
> over some hard spots:
> 1. Is mineral oil the best thing to use for insulation? Or is there
> other substance (like silicone oil) that is really better?
> 2. There is one serious problem I have found. Most small relays have
> little isolation distance between the switch elements and the coil. An arc
> to the coil will probably blow up the control electronics (fiber optics
> prevents such a thing from blowing the operator ;-)). Some small "safety
> relays" have a large distance between the coil and the switches that should
> work good. They are usually pricey but they have cheap cousins. However,
> perhaps someone knows of a perfect relay for such a use?? Big power
> contactors would be great but they are really too big for such a use.
> 3. I would think there would be a cheap high voltage oil (or whatever)
> relay out there somewhere??? Kilovac makes HV reed relays (vacuum) but I
> think they would be over voltage and under current for this. Is there a
> better commercial relay I could get for not too much money that would solve
> the problem straight away??? I would think a small high voltage oil relay
> would be cheap and easy to make commercially??? I have thought of putting
> the relays in a super high vacuum but that seems far too hard for me. The
> specs are: 30KV isolation and switching, 100kHz, 2 amps RMS, and 70 amps
> peak. The fiber optic cable isolation allows these specs to be abused
> wildly ;-)
> Any help, ideas, etc. are welcome. It would really be cool to adjust the
> gap and primary tuning remotely from simple switches. One could really
> dial in a coil while at full power very easily with such a simple
> arrangement. It would be the envy of all your friends :-) It would really
> be useful for people that mount coils 40 feet in the air in large arenas
> and find the tuning is not the same as it was on the ground ;-)) The same
> controls would probably work with variacs and such too. Just needs a
> little research and development work...
> Terry - on the bleeding edge of Tesla coil technology...
BE CAREFUL, I tried to use a 120VAC solenoid to activate a
crowbar across the HV bus of my 5kVA coil about 4 years ago...
(discharging capacitors in a Tesla Equidrive configuration
with powering off system).
The EMI (and control coil flashing over) in the intense H field
of the tank circuit caused the crowbar to drop into the
energized tank circuit...KAAZZZAAM (and few other choice square
expletives :^C ). Took out my 12VDC control circuit
and fortunately nothing else (not even a 12V 10A Lambda
switcher powering the control circuit).
I do believe what you are proposing will work. A LEDEX
rotary solenoid (90 deg rotation) could operate a metallic rotary
cam on an insulated shaft, to make contact to two
isolated spring loaded fixed contacts under oil as an example.
I'm with you, even a 600VAC industrial contactor does not
have sufficient coil / armature to contact clearance to play
this game. A "special duty" contactor, like for plugging/jogging
service (elevators, dumbwaiters) are usually rated at 1.1 to 1.5kV
isolation, but would be risky. And control relays might be
humorous to try in a "dry run" low power application, but could be
a fiery climax....
Some vacuum bottles out of medium voltage switchgear (15kV class)
would be perfect for this application, but are likewise pricey.
But at 200-600A continuous rating (and 40-50kV BIL) would be
dam... near indestructable in this application.
Another idea might be to use solenoids to operate an enlarged version
of a Radio Shack knife blade switch under oil. Solenoids would
require enough pull in torque to overcome spring force (to hold
open or closed). You would want gravity / spring assist to open
(and open with no solenoid power [fail safe]), solenoid to elevate
knife blade into closed position. In this case the switch would
be mounted upside down to normal convention, with the solenoid and
insulated mechanical throw level mounted above the switch...
Just some thoughts...
DAVE SHARPE, TCBOR
Chesterfield, VA. USA