Primary loss measurements

["Lau, Gary" <Gary.Lau-at-xxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Hi All:

I'd like to report on a series of experiments I've made recently.  I've been
trying to quantify various losses related to the primary and nearby objects.
To do this I measured the ringdown time of a low voltage-excited primary
tank circuit, with no secondary.  In place of the spark gap I used a small
N-channel MOSFET with a very low Rds-on parameter of 0.01 Ohm, and activated
the FET with a square wave from a signal generator.  A 9V battery and 10K
resistor across the FET charged the cap up to 9V for each cycle.  The cap
was a pair of .015uF metalized polypropylene caps, totaling .03uF.  The
primary is a flat spiral of 1/4" copper tubing, 69.5uH.  Tank frequency was
110KHz.

If the circuit is a three element series R-C-L loop, then the envelope of
the ringdown follows V = Vo e^(-tR/2L), with R being all of the AC and DC
resistance and losses in the circuit.  If I measure the time t1 it takes for
the envelope to decay to 1/e or 0.368 times initial amplitude Vo, then R =
2L/t1.

With just the primary 17" above a concrete basement floor (and nothing else
near it), R = 0.229 Ohms.  Doubling the height off the floor made no
difference, so I conclude that 17" proximity to the floor had no losses
associated with the floor.  I consider this the base level measurement with
no proximity-based loss components.

If I add two 15/30 NST's on the floor below the primary, R rises to 0.236
Ohms.  Incremental loss R is 0.007 Ohms.  If I put my coil back together
with its usual 15/60 NST, static vortex gap, PFC caps, and Maxwell cap all
below the primary, R goes up to 0.244 Ohms.  Incremental loss R is 0.015
Ohms.

If I then add a 6"x23" toroid 24" above the primary (again with no
secondary), R rises still to 0.253 Ohms.  Incremental loss R is 0.024 Ohms.
Lowering the toroid to 14" above the primary increases R to 0.310 Ohms.
Incremental loss R is 0.081 Ohms.

Lowering the toroid further really kills the ringdown, as does a shorted
strike rail.  A small handful of metal screws directly within the primary
made no difference to losses.

It remains for me to convert these resistance figures to Watts of losses.
Should it turns out that these incremental losses consume only 1W of power
out of a kilowatt, then its all just academic, but interesting none the
less.  They are certainly small compared to gap losses.  I also plan to make
a similar primary out of heavy stranded wire to see if that is actually
significant towards losses.

Regards, Gary Lau
Waltham, MA USA