# Re: Mutual Inductance and Coupling Measurement

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From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: 	Saturday, August 09, 1997 12:51 PM
To: 	Tesla List
Subject: 	Re: Mutual Inductance and Coupling Measurement

Edward V. Phillips wrote:

>         Here is a pretty fundamental way to measure the mutual
> inductance, provided you have a millivoltmeter.  Feed a known
> 60 cycle current (say power line in series with 100 watt light
> bulb) through one winding and measure the induced voltage
> across the other.  Suppose, for instance, that the mutual
> inductance is 100 microhenries.  This represents a reactance
> of 0.0377 ohms at power line frequency, so the voltage will
> be omega x M x I = 377 x 100 E-6 x 1 =37.7 millivolts.
>         Having measured M the coupling factor k is the
> mutual inductance M divided by the square root of L1 x L2,
> where L1 and L2 are the winding inductances.
> Ed

Your method is also my favorite way for measuring M and k,
since all you need is a Fluke 77 and a 120V 'suicide cord'!
I connect the 120VAC directly across the secondary, however,
in order to get maximum primary signal level.  The induced
primary voltage usually falls in the 0.20V to 0.40V range. A
calibration chart placed near the coupling adjustment cranks
makes it easier to dial in the coupling.

This is also a great way to measure the secondary self inductance,
as the drive freq is very well defined. And since the current in the
secondary is only 6.15A at 120VAC, ohmic heating is not an issue.

I wonder though if the coupling between the pri and sec at 60Hz is
the same as at 60kHz?  Could the reactive currents in the secondary
at resonance shape the magnetic field in a different way?

-GL

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