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Re: Museum Coil Revisited

Tesla List wrote:
> 
> > > Subject: Museum Coil Revisited
> > Subject: Re: Museum Coil Revisited
> > > Subject: Museum Coil Revisited
> 
> Subscriber: ccurran-at-execpc-dot-com Tue Dec 31 09:33:03 1996
> Date: Mon, 30 Dec 1996 18:33:28 -0600
> From: Chuck Curran <ccurran-at-execpc-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Museum Coil Revisited
> 
<BIGggg Snip>

> Bert:
>         You've made some interesting comments, things I was wondering about but
> not to sure of as I was crawling around the coil at the Museum.  Another
> point that I expect to hear some discussion take place on is the L/C
> ratio in his primary.  Most of this group have been working with primary
> coils with 10-12 turns and a resulting smaller capacitor for a given
> resonant frequency.  

Yeah... after looking at the performance of Robert Stephens' MTC at 7
KVA and the performance shown by Cox'es coil, there's no doubt that you
can get high performace out of either approach. The biggest advantages
of larger multi-turn primaries seem to be in lowering the value of tank
capacitance and reducing maximum primary/gap current. The first is
clearly a cost savings in the cap itself, and potentially in the size of
the HV transformer. Reduced gap currents permit smaller electrode size,
and reduce quenching and cooling headaches... however, its not at all
clear that there's any performance advantage one way or another for a
properly-sized and quenched rotary.  

> The Museum coil had a five turn primary coil and it
> was tapped at something less than this--I didn't note exactly where it
> was.  Their cap was pretty big for the estimated 166 Khz resonant freq.
> My present understanding was that the larger primaries with 10 -12 turns
> coupled better with the secondary and the resulting smaller cap made
> life easier on the spark gap system.  Gotta ponder this one abit now!
> 

Yeah, nothing's ever easy, and there's always tradeoffs!! :^) You're
still going to have to beef up your rotary no matter which option you
choose.


> >
> > Chuck, could you answer a few more questions about this system:
> >
> > - Approximately what diameter was the rotor, and of what material?
> 
>         The rotor was "about" 10 inches in diameter and it was a phenolic, or
> plastic of some sort.
> 
> >
> > - Was it insulated or hot, and how many electrodes were on the rotor?
> 
>         Six electrodes and insulated from the motor shaft.
> >
> > - Was it a single or a multiple/series gap system?
> 
>         Double gap system, like many of us have already made.  The two
> stationary electrodes were positioned perpendicular to the disk surface
> and located near the O.D., one on either side of the rotating disk.  The
> two stationary electrodes were co-axial with each other, while each
> rotating electrode was mounted through the disk.  Hope that makes some
> sense.

Perfectly! This confirms that Cox was doing nothing "fancy" with series
breaks, just doing high current gappin'. Makes me wonder what these
coils could do with a better quenching rotary!

> >
> > Also, there's a bunch of us in Illinois who'd like to make a field trip
> > to see your new coil in operation when you're up an running. We could
> > also make a stop in Milwaukee to see the (hopefully repaired) pair of
> > coils in operation. Any idea when the Museum people thought they'd have
> > the other coil operational?
> 
>         I asked the same question and the stage manager said in just a couple
> of weeks.  I'd be happy to contact the Director that set up my visit and
> see if he would let several more people measure and poke their coils.  I
> would expect they would be more than agreeable.  He asked me to visit
> before 10:00 AM or after 4:15 PM since their shows run all day--the
> first time the stage hands let me go and look immediately after the show
> which is what I think could be set up.
>         I'd be happy to show you my progress on my new coil, but it won't be
> 100% done for some time yet, probably around 3/1/97.  Cripes, I need
> about 2 more people to help me get that secondary out of the
> basement--how's your back?  I did get the primary frame all assembled
> this last week and Tom Brown, V.P. at Plastics Caps called me this
> morning and went over my request for a quote.  Real nice of them, but
> the quote came in for a .05mfd-at-15,000 VAC when I asked for 30,000 VAC.
> Price was $554 which is about what I expected, but Condenser Products
> had their .05mfd.-at-20,000VAC priced at $450.00 last summer.  Gotta do
> some more diggin on this one.  Let me know of your interests and I'll
> help if I can
> 
> Chuck
> >

Thanks Chuck! 
Sounds like we should plan to set something up after March, and tie it
in with a visit to see & examine the museum coils. Again, thanks for all
of the information you've gathered on the museum coils!! Its a shame Cox
isn't on Chip's list...

FWIW, David Flinn (one of our Chicgo area coilers) says that the
president of Plastic Caps is interested in potentially establishing a
limited inventory of "off the shelf" standard value TC caps, and is
willing to stock limited quantities. This would save the 6-week
build-time from CP. They'll also derate and warranty their caps like
Condensor Products. Plastic Caps is the outfit Cox apparently uses for
his reliable museum systems (and I think these are also the ones he's
advertising in the TCBA). It'd be nice having a "local" source for these
puppies if they're priced competitively and warranted the same as CP!

Safe coilin' to you, Chuck, and Happy New Year!

-- Bert --