Re: Coil Efficiency (and true wattmeter)
From: David E. Sharpe[SMTP:sccr4us-at-erols-dot-com]
Reply To: sccr4us-at-erols-dot-com
Sent: Tuesday, July 29, 1997 11:51 PM
To: Tesla List
Subject: Re: Coil Efficiency (and true wattmeter)
> Interesting equation, and certainly a different spin. I agree with your
> proposal that the amount of power transfered to the base of the
> resonator is a critical parameter. However, I'm not sure I understand or
> agree with some aspects...
> Why Vcap (RMS) and not Vgap (gap firing voltage)? Shouldn't the energy
> "per bang" only be a function of the cap voltage when the gap fires, and
> not a [complex!) function of the cap charging/discharging waveform?
> While the tranfer efficiency will certainly be less than 1, why derate
> by k? Although k governs how long the energy transfer takes from primary
> to secondary, other system losses during this interval will govern how
> much initial primary "bang" energy actually makes it to the secondary. A
> lossless system could transfer 100% of the initial primary energy to the
> secondary for any non-zero value of k. Indeed, for moderate values of k
> and good quenching gaps, there's evidence that the energy "transfer
> efficiency" from primary to secondary LC systems can exceed 50-60% in
> well designed coils.
> I'm also curious about the last paragraph discussing magnetic and
> capacitive energy transfer. Is there empirical evidence that
> primary-to-secondary capacitance is a significant contributor in the
> transfer of energy between the primary and secondary LC systems? While a
> physically "large" primary can act like a local ground plane, and can
> contribute a significant portion of the secondary:toroid's effective
> capacitance, this effect should not contribute to the primary:secondary
> energy transfer process.
> Safe coilin' to you!
> -- Bert H --
The Vcap (rms) is referring to the firing voltage during the
period of the AC wave that the gap can fire. In Tesla's CSN, he
derives a first order approximation of the power thruput of his
system, the end result that only 50% of the sine wave (half power
points and ^) on both alternations are really available for effective
power transfer. If numerical integration is performed between
45 and 135 degrees permitted firing envelope, the effective
rms charging (gap firing) voltage on the tank capacitors is
approximately 0.9 of peak value, NOT 0.707.
I may be wrong, but k is a measure of magnetic coupling efficiency
between primary and secondary, and thereby ultimate magnetic
power transfer capability. I also agree and stand corrected
that the previously posted equation would be a best case
approximation, parasitic losses in the tank circuit would certainly
reduce transfered power.
Early magnifier work with free coils by the TCBOR suggests that a
significant amount of energy is in fact coupled into the base of
the resonator by capacitive (ie 'E' field) coupling. With the
spark gap avalanching in tens of nanoseconds, a capacitance of
several hundred pf will appear as a low impedance path between the
primary and secondary with the step rise front
current (and voltage) wave at the instant of the gap turning on.
John Freau and I collaborated on several experiments where significant
power was being coupled into the resonator capacitively by a primary
coil which was open circuited and being driven by a separate tank
circuit. I can not give absolute values (how would you measure? :^C)
but a ballpark guess may be in the 20-30% based on these experiments.
DAVE SHARPE, TCBOR