Re: MOT charging reactor PS

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> Does anyone know a quick and dirty method for measuring inductance with
> a multimeter?

Not with a multimeter by itself, but with an AC source and a resistor....

Say you've got a 10 Henry inductor (approx)... At 60 Hz, that's an
impedance of around 3.8 K ohms (50 Hz=3.17K).  Hook it in series with a
known resistor of approximately the same magnitude (say, anywhere from 1 to
10 K) across a low voltage AC source (like a 12V filament transformer).

measure the voltage in 3 places: across the resistor (Vr), across the
inductor(Vl), and across the source(Vs).  Note that Vr+Vl will be more than
Vs....

Now, it gets a bit tricky

Calculate the current flowing through the series combination: I = Vr/R

Calculate the reactance of the inductor using the current you just
calculated: X = Vl/I

Turn the reactance into inductance: L = X/(2 * pi * f) where f is line
frequency (2*pi*f for 60 Hz = 377)


The foregoing assumes that the inductor is a pure inductor and has no loss.
 If you want to determine the loss, you'll need to a bit of math and use
the source voltage (Vs) as well...  Consider the inductor as the sum of
some resistance and inductance (Rx + Lx).  The math is left as an exercise
for the reader...