RE: Q Factor and Overall Efficiency

["John H. Couture" <couturejh-at-worldnet.att-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Monday, August 28, 2000 7:32 PM
To: tesla-at-pupman-dot-com
Subject: Re: Q Factor and Overall Efficiency


Original poster: "Malcolm Watts" <M.J.Watts-at-massey.ac.nz>

hi John,

On 28 Aug 00, at 0:50, Tesla list wrote:

> Original poster: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
>
>
> The Q factor tests have been discussed many times in past List posts.
> However, these tests give the Q factor in a low voltage condition. This is
> not the TC operating Q factor. The operating Q factor would give the
> secondary voltage with the equation
>     Vs = Vp * Q
> where Vp is the primary peak voltage.

I don't agree that this is valid for a disruptive TC and I
have stated why many many times so I won't repeat myself yet
again. Operating unloaded Q can easily be found by measuring
the decrementing waveform on an oscilloscope. I have found it
is pretty much the same as the results I've obtained in low
level tests.
----------------------------
It is a well known fact that increasing the primary voltage (Vp) will
increase the secondary voltage (Vs) of a disruptive TC with no other
changes. This means the Q factor would also increase. Does this mean there
is a flaw in what you stated in the past?

You indicated that you found the operating Q and the low level Q to be
pretty much the same. In your "Effective Resistance" post you said you did
not believe your coil with an unloaded Q of 300 (low level) would generate
2.7 MV (operating). This would mean you did not believe the low level Q was
not pretty much the same as the operating Q. Am I reading this conflict
correctly?

----------------------------- JHC

> To find the operating Q factor would require only a scope and the proper
> probe and the knowhow. To do the test find the half power points while the
> Tesla coil is operating. Then determine the Q factor with the equation
>     Q = Fr/(Fh - Fl)

I think there is some misunderstanding here. You can find Fh
and Fl in low power tests by sweeping a signal generator
feeding the coil above and below Fr but how do you get a
frequency sweep on a coil that is ringing all by itself? The
only resonable way to measure Q when there is a single
frequency present is to calculate Q off the decrementing
waveform (as you've already stated). Both methods are
equivalent and the second can also be done at low power if the
secondary is shock excited.
----------------------------
You are 100% correct in that the operating Q cannot be found by sweeping the
frequency of a coil that is ringing all by itself. The equation  Q =
Fr/(Fh-Fl)  cannot be used in this test. As you state the Q would have to be
found by the decrementing waveform (as I show in my TC Notebook). The
equation   Q = Pi/Log Dec     would have to be used. So now I have stuck my
foot in my mouth.

-----------------------------JHC

> where Fr is the resonant frequency, Fh is the higher half power frequency,
> and Fl is the lower half power frequency. The Q factor would be the
voltage
> gain from the TC input to the TC output. This gain could then be used to
> find the overall efficiency of the TC system. This would be done using the
> energy equation
>      Vs = sqrt(2J/Cs)
> Overall efficiency would be the Vs(half power) test volts divided by the
> Vs(energy) theory volts.

Am I correct in thinking you want to compare Vs (=sqrt(2J/Cs)
with Vs = Q.Vin?  You are comparing apples and oranges sorry.

Malcolm
 -------------------------
Overall efficiency is = energy out/energy in. However, because energy out
with a disruptive TC cannot be determined an overall TC efficiency can be
roughly estimated with the equation
  Vs = Vs test/Vs theory    where  Vs test = Vs = Q Vin  and
Vs = sqrt(2J/Cs).
Another method to estimate the overall efficiency of a TC is shown in my
Tesla Coil Construction Guide. Coilers often mention TC efficiency but do
not indicate how they determined this ellusive parameter.
----------------------------------  JHC

> This operating Q factor and TC overall efficiency may be a first in the
> design and engineering of Tesla coils. However, Terry Fritz may have
already
> done this? Have other coilers done this?
>
> John Couture
>
> ---------------------------