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RE: Inductance measurement



Original poster: "John H. Couture by way of Terry Fritz <teslalist-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>


Matt -

This is a good question and not easily answered. There are many ways to do
the job and each meter manufacturer probably uses a different method to
measure inductance. I notice the meters are normally rated (+/- 5%) so the
accuracy is fairly good whatever method is used. Several years ago when I
was working on the homemade Induction meter I show in one of my books I came
to the conclusion the Maxwell bridge circuit would be the best to use. The
equation is
     L = Ra x Rc x Cb
This equation does not involve frequency so the measurement is good at any
frequency. The resistance and reactance can be made independent if Cb is
made variable. There are many other possibilities shown in EE books.

Two coils one with copper and one with nichrome wire would show the same
inductance with the Maxwell bridge provided the wire size, turns and
geometry of the coils were the same. This would be a good research project
for some coiler. I have never heard of any coiler who has ever made this
test. I don't believe that connecting different resistors in series with a
copper coil will be of any help. I have tried this and found the inductance
doesn't change with the Wavetek DM27XT unless the resistor is many ohms.

Inductance is defined as the flux linkages divided by the current causing
the flux. This definition is difficult to put into practice when designing
an inductance meter.

John Couture

----------------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, July 01, 2003 7:33 PM
To: tesla-at-pupman-dot-com
Subject: Inductance measurement


Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<Mddeming-at-aol-dot-com>

Hi All,
         Does anyone know how the resistance of a coil affects the measured
inductance using an LC meter (e.g. AADE LC IIB meter, Wavetek, etc.). I am
wondering if these meters really measure L, or do they measure delta Z and
infer L from it, with the built-in assumption that R<< XL ?
         If I have two identical solenoids, one wound with copper wire and
the other with, say, Nichrome wire, would they read the same inductance, or
would the larger phase angle fool the meter?

Matt D.