From: Paul
Date: Tue, 12 Sep 2000 11:24:25 +0100
Subject: Re: [TSSP] Mystery of the missing loss
Terrell W. Fritz wrote:
> I examined the trusty o'l secondary and despite a few minor
> cosmetics it seem in excellent shape.
> Certainly no spots that would worry me in the least.
>
> The secondary is in excellent condition. I probed around for a shorted
> turn, but a simulation test with an extra shorted wire showed the dramatic
> effects such a thing would make. Since the inductance and other basic
> parameters are the same as years ago and the secondary is not normally a
> surface arcer or anything like that,
> I will rule out damage and shorts.
Phew! Thats good news Terry.
> Here is a little chart I made of the effective secondary
> resistance vs. frequency. I made it with the vacuum cap by varying
> the capacitance and finding the effective Rac.
>
> http://216.160.168.190/TeslaCoils/Misc/PaulNich/BRacvF.gif
Our model has had a few code fixes lately and is now predicting
Measured Model
f1 58 390 No toroid 150.5 kHz
f1 81 265 With toroid 96.8 kHz
Some rough calculations (Coil L=0.0741, Rdc=69.87)
No toroid: Q at 150.5 kHz, Rac = 172 ohms
Estimate Q = 6.283 * 0.0741 * 150500/ 172 = 407.
With toroid: Q at 96.8 kHz, Rac = 167 ohms
Estimate Q = 6.283 * 0.0741 * 96800 / 167 = 270.
To give Terry's observed Q factors we need
No toroid: Real Rac is 6.283 * 0.0741 * 150500/58 = 1208 ohms.
With toroid: Real Rac is 6.283 * 0.0741 * 96800/81 = 556 ohms.
At 150.5 kHz we have excess Rac of 1208 - 172 = 1036 ohms.
At 96.8 kHz excess Rac is 556 - 167 = 389 ohms.
Freq ratio 150.5/96.8 = 1.55,
Excess Rac ratio = 1036/389 = 2.66, pretty close to
the square of the freq ratio. Agrees qualitatively with
BRacvF.gif, although for some reason the Rac on the
chart are a lot higher, the trend is definately in agreement.
This is a great deal of extra loss to account for. It's going
to be really interesting to find out what's causing it!
This extra high loss is not without precedent:
Marco Denicolai's Thor system - Marco measures Q=222, calculations
range from Q=700 to more than 800. I've asked Marco to do a little
test: re-measure Q, then add a little more top cap to drop f1, then
measure again to see if the Q goes up.
> This secondary is a 1/8 inch thick cardboard impregnated with
> paraffin wax "sonotube" used for circular concrete forms.
Paraffin wax: Mainly C22 and C29 aliphatic saturated
hydrocarbons, loss factor: less than 0.0002 right up to 1Ghz,
by far the best possible wax. Melts 73 deg C. Breakdown 12MV/m.
Cellulose based paper, dry: loss factor 0.02 at 100kHz,
0.038 at 1Mhz. Poor, but in view of the small amount of material,
I can't see this being enough.
Polyurethane elastomer: loss factor 0.06 at 1kHz, that's the only
figure I have. Looks pretty poor, but again the volume is so small.
Terry, know anyone with an infra-red camera? Maybe you can get
some of those stick-on thermometer strips, the ones that change
color with temperature. Put a few hundred watts into the coil and
see which bits warm up!
Environment?
> I have played with some modelling and I think the "room" is far
> enough away that it should not be a factor.
I agree, but a check ought to be worthwhile.
An experiment: Set the coil up outside, elevated, no ground plane,
axis horizontal, no end-loading. Excite coil in middle through a
loosely coupled primary turn or two, resonate at half wave.
See if you can get a good Q. That should eliminate everything but
the winding and tube.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.