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: 	Friday, August 01, 1997 2:06 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:   Tuesday, July 29, 1997 11:51 PM
> To:     Tesla List
> Subject:        Re: Coil Efficiency (and true wattmeter)
> > >
> > <SNIP>
> > Dave,
> >
> > 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 --
> Bert
> 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.
> Regards


Thanks for the clarification on Vcap - now I understand what you're

There's a common misunderstanding of the effect of k - for coupled
resonant circuits, the energy transfer "efficiency" is actually a
function of the Q's of the primary and secondaries. "Lossy" systems will
lose energy all during the time that the primary energy is being
transferred to the secondary. Loose coupling means that the amount of
time it takes to transfer all the primary energy to the secondary is
longer. With lower Q, this means that more is dissipated while the
transfer is occuring. leaving less that eventually gets to the
secondary. Thus, most coilers assume that the reason for poorer
efficiency is low k, while the REAL reason is dissipative losses (mostly
in the primary circuit, and mostly in the gap). 

In the electrostatic coupling experiments, was the base of the test
resonator grounded, and what kind of resonator output did you see? Did
it seem to be like an impulse excitation to the resonator the instant
the gap fired, or did it more follow the primary voltage swings? Sounds
like an interesting experiment!

Safe coilin' to you, Dave!

-- Bert H --