Re: TC efficiency, was Math help...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

John C. and all,

I suspect that the nonlinear power vs spark length characteristic is more
due to mechanisms underlying streamer and leader propagation/maintenance
than being an indicator of Tesla Coil inefficiency at higher power levels.
It simply takes increasingly larger amounts of power to develop and
maintain longer leaders. The current, channel temperature, and electrical
conductivity at the root going outwards must be greater for a long leader
than a shorter one. This helps compensate for the the increased voltage
drop that comes along with longer leaders. This voltage drop must be
counteracted by higher terminal voltage so that we can obtain sufficient
far end E-fields (>30 kv/cm) to support leader propagation and maintenance
(until we reach "steady state" balance between leader length and input
power). 

While topload energy scales as the square of output voltage, our ability to
overcome an increasing voltage drop in the leader is more a linear function
of output voltage for a given leader conductivity. A greater portion of the
available energy must go into heating and widening the leader root and
nearby channel versus shorter leaders. A leader having an overall voltage
drop that's twice that of a shorter leader will require twice the topload
voltage, or 4X the topload energy, to achieve the same E-field at the
leader tips. This is likely why John Freau's spark length formulas show
length to be related to the square root of power - quadrupling input power
doubles the length:

  L = 1.7*Sqrt(Power in) for a 120 BPS system

A coil generating longer leaders has much "fatter" and "whiter" arc
channels in the portion nearest the toroid, a clear indication that
significantly more energy is being dissipated in this portion of the
channel than for shorter leaders. The relationship has much more to do with
the "efficiency" of incinerating air than it does with the efficiency of
power transfer between TC primary and secondary of larger versus small
systems. In general, larger coils should be more efficient in transferring
power from the mains to the toroid. It's just not obvious if we only look
at the trend of spark length versus input power... 

Best regards,

-- Bert -- 
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "John H. Couture by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <couturejh-at-worldnet.att-dot-net>
> 
> John F -
> 
> A well known characteristic of Tesla coils is that the watts per foot of
> spark increases as the TC gets larger. This means losses increase and
> efficiency decreases as the TC is made larger. The range is about 200 watts
> per foot of spark for small coils to 2000 watts for large coils. This is
> based on measurements that  coilers make every day. Have you made
> measurements that would indicate the contrary?
> 
> John Couture
> 
> ---------------------------
> 
> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Wednesday, July 11, 2001 6:35 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: TC efficiency, was Math help...
> 
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <FutureT-at-aol-dot-com>
> 
> In a message dated 7/10/01 9:08:09 PM Eastern Daylight Time,
> tesla-at-pupman-dot-com
> writes:
> 
> > >
> >  >When we say the TC efficiency can be about 90/95% what are the input and
> >  >output conditions? Coilers talk about TC efficiencies but I have never
> seen
> >  >any published input/output calcs. In my TC Construction Guide page 14-4
> I
> >  >show a simple efficiency test I made with a small TC and the
> input/output
> >  >calcs were shown. Have any other coilers made these tests? The test
> showed
> >  >an overall efficiency of 56%. Larger coils have efficiencies much lower.
> >  >What are the input and output calcs for the 90 to 95% efficiencies? How
> do
> >  >they relate to the overall efficiency? How do these 90/95% efficiencies
> > vary
> >  >with TC size?
> >  >
> >  >John Couture
> 
> John, all,
> 
> The 90 -95% figure is only for energy transfer from primary
> to secondary for first notch quenching, not for overall TC efficiency.
> Many coils quench on the 3rd notch, so the transfer efficiency may
> be around 65% in that case.
> 
> Charging efficiency is often around 85% or so, and there's some
> loss in the secondary.  This brings the overall efficiency down
> to perhaps 50% for the overall TC for input vs output.  This is
> ballpark.
> 
> This issue of efficiency for small vs. large coils has been discussed
> previously on this list.  I would say that large coils are more
> efficient than small ones, mainly because various transformer
> % losses are lower in larger transformers.
> 
> John Freau