K M Light Bulbs & Khz

From: 	Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: 	Friday, December 12, 1997 10:56 AM
To: 	Tesla List
Subject: 	K M Light Bulbs & Khz

> From: 	John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
> Sent: 	Monday, December 01, 1997 11:27 PM
> To: 	Tesla List
> Subject: 	Re: Recycled: Measuring Coupling Coefficients
> At 10:16 PM 11/30/97 +0000, you wrote:
> >
> >From: 	Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
> >Sent: 	Sunday, November 30, 1997 8:59 AM
> >To: 	Tesla List
> >Subject: 	Re: Recycled: Measuring Coupling Coefficients
> >
> >John,
> >Terry originally thought the same as you about the operating
> >frequency, but found that in fact the method is not dependent
> >on frequency. In fact, as the title of one of his posts pointed
> >out, this method is perhaps the best for most coilers as it does
> >not require very much in the way of test equipment, and yet yields
> >answers at least 1% accurate if you use a heating element or 
> >high power resistor as the current limiting element (as opposed
> >to a light bulb, which is non-linear and introduces a bit of
> >error at 60 Hz).
> >
> >Greg Leyh uses a variation on this method, where he applies 120
> >VAC directly to the SECONDARY and then measures the voltage at the
> >primary and the current at the powered secondary. It is always
> >useful to have information on several different ways to do things.
> >Then use the method that works best based on the equipment you
> >have at hand.
> >
> >Always exercise caution when dealing with the 120 VAC power
> >lines!
> >
> >Hope this helps.
> >Fr. Tom McGahee
> >
> --------------------------------------------------------
>    Fr. Tom, All -
>   I agree that the K factor (coupling coefficient) appears to be independent
> of the frequency for Tesla coils. However, further research indicates the K
> factor is a number that depends on the type of operation. Refer to Terman's
> Radio Engineers' Handbook. For example, in radio circuits the K factor
> varies with the frequency because of the variable  tuning capacitor.
> Changing this capacitor changes the operating frequency and changes the
> coupling. This means that a Tesla coil with a fixed capacitor would have a
> fixed coupling  and be independent of frequency . However, if the TC
> capacitor is changed and the operating frequency changed the coupling would
> also be changed without changing the physical characteristics or geometry of
> the coil. Has anyone ever checked this?
>   The mutual inductance test for K factor introduces two additional
> variables, Lp and Ls, to the voltage and current that is necessary to
> determine the Lm. This test would be less accurate compared to the other
> tests using only two variables. If all variables are  measured to the same
> accuracy the other tests would have twice the accuracy of the mutual
> inductance test.
>   As for the light bulb shown in the Tesla Coil Design Manual for the mutual
> inductance test, the non linearity of the bulb is not involved. This test
> uses a fixed current that is independent of the non linearity of the bulb.
> The fixed current is obtained by varying the primary voltage with a variac.
> This has the advantage that the mv in the secondary is the uh of the mutual
> iinductance so calculations are not necessary. This test is also safer
> because it operates at low voltage.
> Comments welcomed.
>   John Couture

Sorry it took me so long to get around to answering this post. I have been
up to my armpits in writing Christmas letters, and am just beginning
to come up for air.

As regards the light bulb, I am afraid that it WILL cause an error in the 
measurements. An applied 60 Hz sine wave AC voltage will NOT result in a 
sine wave shaped CURRENT through the bulb. This is due to the fact that
the resistance of the bulb is a function of applied voltage. Unlike a 
simple resistor, the light bulb's resistance is nonlinear. It CHANGES
its resistance constantly with an applied AC. At low frequencies like
60 HZ the effect is quite noticeable. Put 120 VAC across a resistor
and you will get a sinusoidal current. Put that same 120 VAC across a
lightbulb and you will get a distorted waveform that is somewhat sinusoidal
but not 100%. Note that it is the CURRENT that gets distorted. If you want
to see this on a scope, then put a 100 ohm fixed resistor in series with a 100
watt lightbulb. Observe the voltage waveform across the resistor and you will
see that it is slightly distorted. You can see this best on a dual trace
scope by superimposing the original 120 VAC waveform and the waveform across
the resistor. Scale and superimpose. In this case the resistor is being used 
as a current sensor for the non-linear lightbulb.

Now, since all these neat measurements assume that we are feeding in
an undistorted SINEWAVE, and we are actually feeding in a slightly
DISTORTED sinewave, we will, in fact, have a small error. Not a huge
error, but enough to create a percent or two of error. So, a resistor
is better than a lightbulb. But I still use lightbulbs because they are
so darned convenient. But I am aware that there is a slight error.
And so I live with it and go on about life with great joy anyhow :)

Next point. If the formula does NOT contain anything in it that is
directly related to frequency, then it does NOT matter at what
frequency you make the measurement. Period. I will, however grant that
our human brains have a propensity to think that we will get better,
more accurate results if we test at a frequency near that at which
the circuit will eventually be operated. Any perceived greater 
accuracy is NOT however due to the mutual inductance or K issue at
hand. That having been said, let me ALSO say that it is usually easier
make AC measurements at 1 KHZ than 1 HZ for rather obvious reasons. BUT,
if I had a really good volt and current meter that gave me accurate
measurements at 1 HZ, then I absolutely COULD do my measurements at
such an absurd frequency. And they would be as accurate as my measuring
equipment allows. Period.

60HZ is nice because it is FREE. I use the electric company's super 60HZ
low impedance signal generator that is available at my friendly 
neighborhood wall outlet. How nice and convenient. Not to mention cheap.
And accurate enough for my needs. Most meters are optimzed for 60 HZ
anyway :)

I only mention the above things because I don't want anyone out there on
the list scratching their heads trying to figure out whether they should
use 60 HZ or 213.765 KHZ as their test frequency when determining 
mutual inductance and K. Use what you got. They both work fine! But if
you want maximum accuracy, use a fixed resistor rather than a lightbulb.

Hope this helps.
Fr. Tom McGahee