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Re: Measuring Coupling Coefficients




From: 	Mark S. Rzeszotarski, Ph.D.[SMTP:msr7-at-po.cwru.edu]
Sent: 	Wednesday, November 26, 1997 1:18 AM
To: 	Tesla List
Subject: 	Re: Measuring Coupling Coefficients

Hello All,
        Terry said in part:
>        I have been playing with measuring coupling coefficients lately.  I
>was just wondering if anyone has a relatively easy way to accurately measure
>this elusive factor?
>        I have tried measuring the coils in series so that the inductances
>add and then subtract and then applying the formula M=(La-Lb)/4.  However,
>due to the large difference in the primary and secondary inductances (two
>orders of magnitude) this formula can introduce substantial error.

        There are at least three ways to measure coupling, one of which you
identify above.  Another method uses the open circuit, short circuit
inductance measurement of two coils.  I find the same problems with that
method as well.  The best method is based on the way Tesla measured it at
Colorado Springs (of course), here reproduced from a snippet by Greg Leyh in
the archives:

Here's an unsolicited suggestion on how to make a solid measurement of the
mutual inductance, using only a variac and a voltmeter:

Apply as much 60Hz AC current to the secondary as it will comfortably take.
(consult wire tables)
Measure the 'open circuit' voltage of the primary.

The mutual inductance = Vp / (Is * 377),  derived from Vp/Is = 2*pi*F*Lmutual.

I usually omit the variac, and just throw 120 volts across the secondary,
which yields about 0.25 volts RMS on the primary for k=0.15 .
It's a quick measurement method, with about a 1% accuracy in my case.


Regards,
Mark S. Rzeszotarski, Ph.D.