Re: Non-Linear Coil Winding Experiment.
From: Skip Greiner[SMTP:sgreiner-at-wwnet-dot-com]
Reply To: sgreiner-at-wwnet-dot-com
Sent: Sunday, October 19, 1997 1:30 PM
To: Tesla List
Subject: Re: Non-Linear Coil Winding Experiment.
Tesla List wrote:
> ---------- Forwarded message ----------
> Date: Wed, 15 Oct 1997 07:30:22 -0400 (EDT)
> From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Non-Linear Coil Winding Experiment.
> Hello Terry and All,
> Terry Fritz has been experimenting with coils wound with non-linear
> winding pitch and observed that the distributed capacitance appears to vary
> with pitch. Being skeptical of his experimental setup, etc., I did a series
> of experiments last night which essentially verifies his findings.
> Distributed capacitance does indeed vary if the winding pitch of the coil
> I have a series of coils all closewound with enamel wire to a height
> of 10.5 inches using 3.5 inch diameter acrylic forms. I stacked 2-4 of
> these coils one on top of the other to simulate changing winding pitch along
> the height of the coil. I will present the findings here for a stack of
> three coils. The individual coil data is shown below:
> Coil AWG L Fres Cdis T B
> A 16 0.970 mH 2120 5.81 pF 0.75" 1.50"
> B 21 3.046 mH 1210 5.68 pF 0.75" 1.25"
> C 24 6.172 mH 855 5.61 pF 1.00" 1.00"
> AWG is the wire gauge for the closewound 3.5" x 10.5" coil
> L is the inductance in millihenries (BK Precision 878 meter, 1%
> Fres is the self-resonant frequency in kHz, measured using an HP
> 4193A vector impedance meter, verified using an oscilloscope.
> Cdis is the calculated distributed capacitance based on measured L
> and Fres
> T is the distance in inches from the top of the winding to
> the top of the coil form.
> B is the distance in inches from the bottom of the winding
> to the bottom of the coil form.
> Experiment #1 - stack coil A on top of B on top of C. Connect base of C to
> signal generator through a transformer, with the other transformer lead
> going to my good RF ground. The transformer is simply an iron powder toroid
> with 150 turns on the primary, connected to the signal generator, and one
> turn on the secondary, connected to RF ground and the base of the coil
> assembly. It's purpose is to reduce the 50 ohm output impedance of my
> signal generator considerably. It's frequency response is fairly flat up to
> 2.5 MHz or so. I measured resonance from 6 feet distance with a 'scope
> probe with 12" wire antenna attached. A digital counter was used for
> measuring frequency, and an HP vacuum tube oscillator was used to drive the
> system. I also measured the coil assembly base impedance and phase response
> using an HP 4193A vector impedance meter with the base of the coil connected
> to the meter. The top of the coil was always left floating in space, and no
> top toroid was employed. There is a gap between coils, and the connecting
> lead between coils was spacewound in this region to even out the taper
> between coils, since the base lead on each coil is approximately 3 feet
> long. Coils were placed 24" above the concrete floor in my basement, at
> least 6' from the test equipment.
> Findings: L=10.46 mH Fres=408 kHz, Cdis=14.5 pF, Zbase=26.6 ohms at resonance.
> Comments: L is greater than the sum of the individual inductances of the
> coils since mutual inductance between the coils is present. All coils are
> wound in the same direction. Cdis is the calculated distributed
> capacitance, based on the measured L and Fres. Zbase was measured using the
> HP 4193A. Note that the measured Cdis is somewhat less than the sum of the
> Cdis values for the individual coils.
> Experiment #2 - stack coil C on top of B on top of A. Connect the base of A
> to the signal generator. Repeat the above measurements. Now the 16 AWG
> coil is at the base, the 21 AWG is in the middle, and the 24 AWG is at the top.
> Findings: L=10.55 mH Fres=578 kHz Cdis=7.19 pF Zbase=26.6 ohms at
> Comments: L varies from the previous value above because the coils are not
> exactly centered on their coil formers (See measurements T and B above.)
> Note that Zbase has not changed! In addition, the resonant frequency has
> gone up considerably while at the same time Cdis has dropped to a value of
> about 1.5 times the single coil value.
> It appears that one can reduce Cdis substantially using a tapered
> winding approach. By using large wire size or turns spacing near the
> bottom, one can reduce the Cdis of the coil system. This is one of the
> things that Tesla was trying to do at Colorado Springs. Since the output
> voltage of a tesla coil is proportional to the square root of
> Cprimary/Csecondary, any reduction in Cdis is perhaps useful. One can, for
> example, use a larger Ctoroid to store more energy for those nice hot
> sparks, etc. Of course, the electrostatic properties of the toroid itself
> may also be altering the current distribution along the coil. The primary
> coil coupling would also have to be adjusted, but that is generally no problem.
> I expected Zbase to vary somewhat and was surprised to find it did
> not. However, this is in the non-spark breakout mode, and base currents are
> minimal. My guess is that Zbase will change a bit when the sparks start flying.
> More to ponder...
> Mark S. Rzeszotarski, Ph.D.
Mark and All
This is extremely interesting. I find a major similarity between Mark's
finding and what may be happening in a magnifier. After all, isn't the
secondary driver of the extra coil very similar to these stacked coils?
Isn't it possible that Tesla saw the possibility of reducing the Cdis in
this manner and the extra coil was an obvious (to him) way to get there.
I think that Malcolm has pointed out the possibility of using a tapped
secondary instead of a primary. This begins to make a lot of sense if
one were to change the wire pitch/diameter at the tap. Much more
investigation needed. Obviously the reduction in Cdis will contribute to
raising the output voltage and this is a simple way to achieve it.