Re: Coil Efficiency (and true wattmeter)

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Tuesday, July 29, 1997 9:12 AM
To: 	Tesla List
Subject: 	Re: Coil Efficiency (and true wattmeter)

Tesla List wrote:
> From:   David E. Sharpe[SMTP:sccr4us-at-erols-dot-com]
> Reply To:       sccr4us-at-erols-dot-com
> Sent:   Monday, July 28, 1997 10:25 PM
> To:     Tesla List
> Subject:        Re: Coil Efficiency (and true wattmeter)
> I have previously posted preliminary information on measured
> streamer lengths (MAX) to a ground target versus input power.
> I agree with Malcolm that efficiencies from AC line to coupled
> power into resonator is very low (typically less than 20%).
> I furthermore agree with the original post by Greg Leyh (HI)
> that the power into the tank circuit really drives the system,
> but I suggest that the COUPLED POWER into the base of the
> secondary (regardless of magnifier or standard coil) be the
> metric that determines ULTIMATE SPARK LENGTH OF_ANY_SYSTEM!!!
> My spin is therefore:
> Power efficiency = .5 * Vcap^2 / C * BPS * k / PacIN
> Where:
>         Vcap = Adjusted RMS voltage based on permitted firing
>                window on AC wave (kV)
>         C    = Tank capacitance (uF)
>         k    = Coupling coefficient (Primary to secondary)
>         Pacin= TRUE AVERAGE INPUT AC POWER, measured via
>                true wattmeter (optoisolated or otherwise)
> The ONLY assumption in the above equation is that the
> predominant coupling mechanism by FARADAY's LAW is magnetic.
> I will, however, admit that a significant amount of power
> transfer occurs CAPACTIVELY between the primary and secondary.
> Measuring that contribution would be difficult.


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 --