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Re: Desktop Bipolar Coil



Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 6/28/03 5:18:21 PM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

>Thanks for the comments. I hope I was not misunderstood regarding spark
>length; I am only seeking to attain 18 inch sparks from a 2 foot coil. The
>calculations were best case with the power level I plan to run to show 18
>inches should be possible. Should 7.5KV -at- 30 be too little I can always get
>a 9KV -at- 30 and throw another cap on the MMC.
>
>My reason for using the SRSG is that I found a really cool little sync motor
>in a surplus store and was looking for something to use it on. Probably
>overkill but isn't that why we build these things in the first place. :-)
>
>=========================================
>Dave Kyle


Dave,

Theoretically speaking, a bipolar coil should give sparks that are
about 1.41 times longer than a classic TC for the same input power.
This is because a bipolar coil can be thought of as a twin coil
of sorts, and a twin gives sparks that are 1.41 times longer than
a classic TC.  This is because a twin is really two coils in a way.
There are actually two coils each giving sparks according to
the formula results for 1/2 the input power.  The spark lengths
are then added together.  To calc the output of a twin, take 1/2
the input power, then apply the  formula:

   spark length inches = 1.7*sqrt input watts.

Then double the result.

For example for a 7.5/30 nst, first take 1/2 the power or 112.5 watts,
and apply the formula:

   length (inches) = 18", then double this for 36" sparks.

Or alternatively, use the direct twin spark length formula:

(twin) spark length inches = 2.4*sqrt input watts.

Again for the 7.5/30 nst, the wattage is 225 watts, so
   sqrt of 225 = 15,   and 15 x 2.4 = 36".

It must be realized however that for a bipolar, these theoretical
spark lengths may not be achieved for a variety of reasons as
Ralph and others have mentioned.  It may not be possible to
"guide" the sparks upwards and around and outwards using
electrode arms to direct the sparks towards each other, without
encountering large corona losses, which may compromise the
spark length.  Also, it may be necessary to use smaller than
normal toroids, or perhaps none at all.  In addition, bipolar coils
are often small, and small coils may have difficulty achieving
the formula's predicted results.  Nevertheless, your goal at 18"
sparks is only 1/2 the "theoretical", so it should be achievable,
especially from a two foot coil.

John