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Secondary Frequency(fwd)
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From: Mark S. Rzeszotarski, Ph.D. [SMTP:msr7-at-po.cwru.edu]
Sent: Friday, August 21, 1998 8:00 AM
To: Tesla List
Subject: Re: Secondary Frequency(fwd)
Hello All:
Antonio wrote:
>I found 24.4 mH. How did you compute the Q?
>(I am finding much more (624), considering only
>the skin effect, but your value looks more
>realistic.)
To estimate Q accurately, one must consider D.C. resistance, skin
effect and proximity effects. The D.C. resistance is easily measured with
an ohmmeter or estimated from wire table info. At radio frequencies, skin
effect becomes important. This is seen even at frequencies as low as 50
kHz, so tesla coilers should use appropriate wire sizes to account for this
effect. Skin effect is caused by the electromagnetic fields established
within the wire causing the current to bunch up along the outer surface of
the conductor. That is one of the reasons why hollow copper pipe works well
for primaries. If a solenoidal coil geometry is used, these electromagnetic
fields from each wire also affect nearby turns, causing a further
constriction of the current to a region along the inner surface of the
solenoid. As a result, the total A.C. resistance due to all of these
effects is typically 2-4 times the DC resistance. Proximity effects are
lessoned by space winding the wire, but then inductance falls, resonant
frequency rises (more skin effect) and Q is proportional to frequency and
inductance, so there are some tradeoffs here. (Can you say computer model?)
I used experimental data published by Medhurst to estimate proximity
effects, along with known skin effect formulas. I tested the algorithm
using some excellent experimental data from Malcolm Watts from a few years
ago, along with some of my own experimental data.
Regards,
Mark S. Rzeszotarski, Ph.D.