Re: [TCML] Optimizing coil performance

["DC Cox" <resonance@xxxxxxxxxxxx>]

To get high performance, always use the largest coilform dia possible.  For
pole xmfr units at 10 kVA I use min of 18 inch dia and perferably 24 inch
dia.  This increases the inductance which increases voltage output with the
same dI/dt rates change of current.

The equation is V = -L x dI/dt and L for an inductor increases as the square
of the radius, so jumping from 8 inches up to 24 inches will give you 900%
more potential difference --- this assumes all other factors remain the
same.

Most pole xmfr coils use an asynchro gap, usually ten 1/2 dia electrodes
running at 1725 rpm, for a break rate of 287 pps.  The rotary electrodes
should be min of 1/2 inch dia tunsten and I use 1 inch dia stationary
electrodes.  Spark gap quenching (turn-off) is critical to keeping the
energy ringing up in the secondary.  If quenching is poor it all dumps back
into the primary.  Many experimenters use too small rotor electrodes and
quenching is poor.  Also, don't use steel bolts as rotor electrodes.  Steel
work fine at 60 Hz but is very poor at RF frequencies.  Remember, the part
of the circuit that limits your max primary current is any series connection
or material that doesn't transmit RF currents efficiently.

With an 8 inch dia sec you need to experiment with sec elevation above the
horizontal pri plane.  Usually around 3.75 to 4 inches with 1.25 pri-sec
spacing works good.  Again, with an 18 inch dia inductor the vertical
spacing works best with 6.75 inches and 9 inches is required for any 24 inch
dia sec.  I discovered years ago this relationship is a near linear function
both upwards and downwards.  I did a lot of experiments with various size
coils using a large brass ead-screw setup to actually raise the sec up and
down while it was running.  I was quite surprised how high the sec needs to
be elevated to achieve maximum spark length.  You can experiment with your
setup using 1/2 - 3/4" chunks of plywood to raise the sec small increments
at a time.

At high power, with a larger sec dia, a 34 x 8.5 inch toroid (or similar)
will also increase your output spark brightness (more capacitance = more
current).  It also allows the potential to reach higher values prior to
breakout.

Hope this tips give you some starting points.

Dr. Resonance




On Wed, Aug 13, 2008 at 12:41 AM, David Kronstein <david_kronstein@xxxxxxxxx
> wrote:

> Hi all,
>
> I'm getting my Tesla coil up and running again, and I'm revisiting the
> design trying to improve performance. Currently the coil struggles to get
> 7-8' arcs with 10kVA input, with the spark gap firing at a capacitor voltage
> of around 20-24kV. The current specs of the coil are:
>
> Power:    10kVa 14.4kV pole pig, arc welder as variable inductor ballast
> Capacitor:   0.136uF 24kV MMC (originally 0.204uF)
> Spark gap:   240bps SRSG
> Prmary:    3/8" copper tube, 3/8" spacing, 13" ID flat pancake, currently
> tapped at 6.7 turns
> Secondary:    20AWG, 8.5" dia, 48" winding length
> Toroid:      8" x 24"
> Fres:     ~100kHz
>
> What would you guys recommend changing or upgrading to get better
> performance? I'm currently thinking of getting a variac to control input
> power, because the variable inductor affects optimum RSG phasing.
>
> Regards,
> David Kronstein
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
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