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Hi Alex,
Sounds like an interesting coil!
Glass plate or glass bottle capacitors have a long history in HV
applications. Tesla himself used heavy glass water bottle immersed in
oil for his large Colorado Springs system. Soda-Lime window glass has
excellent dielectric strength (~13 kV/mm) and a relatively high
dielectric constant (between 7 and 8). Unfortunately, soda lime glass is
also fairly lossy when used as a dielectric at Tesla coil frequencies.
Its loss tangent ranges from about 0.02 - 0.04, making it about 300-600
times as lossy as modern HV capacitors made using Low Density
Polyethylene (LDPE) or Polypropylene film. Its dielectric loss is
actually about 50% higher than Mylar (PET), which is not recommended for
Tesla Coil use because of its excessive losses. Also, glass becomes more
electrically conductive when heated via ionic conduction. This can
actually lead to overheating, thermal runaway, and dielectric failure,
particularly for glass capacitors used in higher power systems.
Single-strength glass plates are about 3/32" thick, or about 2.4mm. The
transformer will deliver a nominal peak voltage of 7kV, and during
operation, your capacitors will see a equivalent stress of ~14 kV (RF
peak-to-peak) during operation. Using 13 kV/mm, the DC strength of each
plate would be about 31 kV, so you should be OK (at least for short-term
operation).
However, if your plate capacitor is configured as a tightly-packed
sandwich, the interior plates will become hotter than the outer ones due
to the longer thermal path to ambient air. Assuming your HV transformer
delivers about 500 watts (depending on your ballast inductor), your
capacitor may need to dissipate an average of 10-20 watts of heat in
operation - and possibly twice this if you're really pushing your
transformer. If the plates were all in free air, convection cooling
would likely be sufficient to prevent overheating. However, extended
operation with the plates sandwiched together may cause the interior
plates to overheat and fail.
Some other changes you may want to consider:
1. Make sure you have a safety gap across your rotary gap to prevent any
gap misfires from allowing resonant rise (at mains frequency) from
overvolting your capacitor or HV transformer.
2. Consider increasing the spacing between primary and secondary
windings or their relative position (i.e., move the primary so that it
is lower relative to the secondary). Either will reduce the coupling
coefficient(k), but will likely require retuning the system. A lower k
is actually desirable with spark gap coils, which typically operate best
with k in the range of 0.1 - 0.2. This will likely fix your
primary-secondary breakdown problems as well.
3. Consider adding either a small corona ring (1/2" copper or aluminum
tubing) above the top of your secondary winding to prevent the winding
and coil form from being damaged by corona and spark tracking. This will
still allow you to keep the "look" of your coil.
4. Add a larger toroidal topload to further improve performance.
However, this will significantly change your coil's appearance.
BTW, the first Tesla Coil that I built (when I was a pup way back in the
late 1950's) used double-strength glass plate capacitors driven from a
15 kV 30 mA NST. The caps generated horrendous amounts of ozone, and
lots of noise from the aluminum foil rattling against the glass as they
charged and discharged. They also generated the most beautiful blue glow
around the edges of the foil plates, with lots of little longer
bluish-white spark tendrils. Unfortunately, most plates eventually
failed, perhaps at defects or bubbles in the glass. The breakdown and
arc-through usually fractured the plate.
If you get a chance, please post some links to images of your system.
Also, do you remember the title of the book where the coil was described?
Play safely and best wishes,
Bert
--
Bert Hickman
Stoneridge Engineering, LLC
http://www.capturedlightning.com
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b alex pettit jr via Tesla wrote:
Hello All, I have been in the process of getting an old build
operational again.I constructed this RSGTC back in the mid '80s based
on info from a book published in 1920. It is of classic design in
thatthe Secondary is wide and short by current day designs ( 8" dia
x 15" hi ) and has no top load - just a small 1" brass ball on a
short post. It creates a very large brush discharge from the top turn
and equal length sparks from the ball.It looks like a true antique as
the components have nicely finished wood bases and boxes. It was
built using a surplus military grade 5KV 90 ma transformer and is
throttled by a tapped inductor of my design on the AC input.
Thecapacitor is a 2" stack of 'single strength' window glass and
aluminum foil whichwas immersed in hot beeswax such that the edges of
the glasspanes are sealed .. The foil comes to within 1/2" of the
edges of the glass. The cap measures 0.025 uF. Input 5KV max. ARSG
runs at 360 BpS. The coil resonance measures 290 KHz and this agrees
with the JavaTC analysis. Coupling 0.3 k I have Never attempted full
power operation as there were always pri to sec arc-overs.I finally
fixed this ( after 30 years ! ) by sleeving the upper turn of the
primary with plastic tubing. Question : Whats the probability that
the Capacitor withstand full 5 KV transformer output ?
Regards,AlexOrlando FLKK4VB
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