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Re: gaps'n'caps

Tesla List wrote:
> 
> >From des-at-ellijay-dot-comFri Oct  4 22:07:02 1996
> Date: Fri, 04 Oct 1996 08:27:13 -0400
> From: Doug Swanson <des-at-ellijay-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: gaps'n'caps
> 
> Just a thought here...
> 
> In using a rotary gap, in moving the rotating electrodes past and away
> from the stationary electrodes, quench ought to be acheived when enough
> distance has separated the points.  I'm considering construction using
> the same RPM motor as has shown good results, using the same number of
> rotating electrodes, but by using simple geometry to come to this
> conclusion, increase the diameter of the wheel that these parts are
> integral to, in order to move each orbiting electrode a greater distance
> per number of degrees that the motor has turned.  This should give me
> less contact time, and a quicker quench as the electrodes move farther
> in their travels (circumference) as the radius increases.

Yes.

> 
> Also it seems that a larger disk should be steadier in its motion,
> considering gyroscopic effects. 
> Any thoughts on this?

Not necessarily so. A larger diameter rotor must be more closely
balanced
and "true" relative to the shaft in order to avoid excessive vibration.
Its particularly important to avoid rotational speeds which excite the
rotor at one of its natural mechanical resonant frequencies. This is
particularly true for "cold" rotors typically made from thermosetting
laminates, since these are rarely flat to begin with.

> 
> Also, I've considered using a porous (absorbent) material between cap
> foils, which when it soaks up the dial-x oil should provide adequate
> insulation (dielectric) between layers.  (thinking: if a pinhole in the
> polyethylene {original cap design} were filled with the oil, would it
> not maintain the integrity of the capacitor?)  Unless I'm overlooking
> something brutally obvious, it seems that something like cotton fabric,
> which by its nature is full of holes, could even be used as long as the
> oil completely soaked into it.
> 
Theoretically, this could work. The trick would be to remove all the
tiny air bubbles that would tend to get trapped in the fabric OR to only
run the cap at lower voltages. The problem with trapped air is that it
has less than half the dielectric constant (k=1) and only a fraction of
the dielectric strength of transformer oil. Any air bubbles trapped
between plates will see a much higher electrical stress. If the stress
is great enough, the air inside the bubble begins to ionize, creating
ozone, UV radiation, and localized heating. If the oil around the bubble
can't move because of the surrounding cloth matrix, this will eventually
lead to chemical degradation and dielectric breakdown of the oil. 

However, if you keep the capacitor voltage below the point where trapped
air ionizes (i.e., the "inception voltage"), the cap should last
indefinately. The inception voltage can be approximated by:

   Vinception = Vbr*(1+(Td/(k*d))

    where Vbr = 320 volts/mil (Miminum breakdown voltage of air at STP)
                  for small gap sizes
          Td  = Thickness of dielectric (plate seperation) in mils
           k  = dielectric constant (2.2 for Shell Diala-X)
           d  = air bubble diameter (mils)

For example, if you designed a cap with 32 mil plate seperation, and you
could somehow insure that the maximum air bubble diameter was 1 mil, you
could run the cap at 4975 Volts with NO internal ionization problems. If
you had one 5 mil trapped bubble, this drops down to 1251 volts! The
higher the applied voltage above this level, the more ionization occurs,
and the shorter the time to failure. Although the resulting dielectric
failure is not instantaneous, it would be inevitable. The above scenario
also helps explain why break-in at lower voltages is essential until the
larger air bubbles work their way out of homemade LDPE caps.

> This may be a "ridiculable" idea, (as was the hand-held calculator, 30
> years ago), and I am open to ridicule and suggestions as to whether or
> not this line of thought bears further investigation.  I can see a
> number of advantages to working with a fabric, over polyethylene...
> 
> doug

Hopefully, you'll not get ridiculed for asking questions on this
conference. Tesla himself made variable capacitors which used only oil
as a dielectic. Older style electrolytic capacitors used cloth to retain
the electrolyte and to seperate plates. Your idea's a good one - its the
implementation that would be tough!

Safe coilin' to ya!

-- Bert --