Re: Rotary Gap Construction?

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Matt,

At 04:22 PM 3/20/2002 -0600, you wrote:
>Hello hello,
>	My last coil used a static spark gap with marginal results at best, so I'm
>planning out a means of building a rotary gap.  Since many of you seem to be
>experts in construction, I was wondering if anybody would mind throwing in a
>few engineering suggestions.  I'm open to anything, and have a substantial
>budget to work with.  I'm looking around for 2400-4800rpm variable motors
>(most of them I can just control the angular velocity with a variac, right),
>I'm assuming that this should be enough if I have about 6-8 presentations
>every rotation.

There are synchronous and non-synchronous motors.  Sync motors run in
perfect timing with the AC line voltage and are usually considered to be
better than async motors.  It all depends on a lot of things but if you are
powering your coils from NSTs then sync motors would be best IMHO.  If you
have a big pole pig system then you can crank up more power so it is not
such a big issue.

So you should decide definitely if you want a sync or non-sync (async) system.

>
>What I'm having difficulty with, as of now is:
>1) My idea calls for using a conductive circle with bolts attached as the
>electrodes.  I would then have two stationary electrodes (one at the 9:00
>and one at the 3:00 position).  The problem I see in this is that the
>charge, as it passes through the metal disc, will also partially hit the
>motor.  This is bad.  Is there a way to, say, using electrical tape, wrap
>the circle around the center with tape, or would this not work?

Usually the disk is mounted on non-conductive material that insulates the
high voltage from the motor.  Someone did get it to work with a solid
conductive disk but that scares me =:O  I would not let the high voltage
get to the motor shaft or frame.  It is just not right.


>
>2) This would be slightly more complicated, but I suppose I could use a
>belt, run off the motor and connected to the electrode plate, thus the motor
>would be insulated from the electrode plate.

That should work too.  Be careful as some belts have conductive carbon in
them.  Probably no shock hazard since the resistance is high but the
current can damage the belt.

>
>3) I'm also worried about the heat from the sparks melting the bolts... I
>don't think those viewing the demonstration would appreciate a "hot dross
>bath"... is there something (like tungsten?) that I should coat the
>electrodes in?

You can easily get "pure" tungsten rods 1/8 inch in diameter from any
welding supply store.  Avoid the thorated and other types since they
contain low-level radioactive materials that you just don't need to deal
with.  

>
>As of now, I'm looking at the following for my setup:
>a combination of the following transformers:
>One 15kV 60mA neon
>Two or three 12kV 30mA neons
>Combination of the above... suggestions welcome here too.

That strongly suggests you should be using a synchronous gap.  It may have
up to twice the power.

>Everything through a 20A 0-120V variac.

perfect!

>
>2 .018uf capacitors (in parallel)

That cap will run very well with a 15/60 NST and a sync gap.

>12 turn flat primary spaced 1/2"
>8" x 32" secondary, 24AWG topped with a 14x4 aluminum toroid (may build a
>larger toroid).

I assume you have seen the many on-line programs to check the tuning and all.

>
>The circuit is in the simplest design... Vac through primary transformer in
>parallel with main spark gap. (two chokes to protect transformer).  Tank
>circuit standard.

I don't think chokes help much:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/rcfilter/rcfilter.html

This is a popular filter to protect the NST:

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

>
>Thanks much, I'm still fairly new to coil construction, so I apologize in
>advance for my ignorance :).  Strangely enough, none of the professors here
>have had much HV experience, so I'm more or less on my own with this
>project.

Neat!  You probably have lots of other questions so ask away...

Cheers,

	Terry


>
>><>Matt Meyer
>meyerml-at-stolaf.edu
>
>><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>
>Matt "Woody" Meyer         Ellingson 120       507-646-2883 (CUTE)
>
>St. Olaf College Physics Major
>St. Olaf Admissions Tour Co-Coordinator
>2001-2002 Ellingson Hall Residence Life Junior Counselor
>