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Inductor testing frequency
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pete_dl:


Hi,
     I want to measure the precise (within 1%)  inductance of an inductor sold as 5H. I am looking at hand held LCR meters and the lowest frequency they commonly test at is 100hz. I will be using the inductor in a LC circuit with resonance at 20hz. I have read that inductance does change with frequency, but I would like some advice whether a 5H inductor measured at 100hz will indicate a much different figure at 20hz ?

Many thanks,
                          Peter.

T3sl4co1l:
Yes.  Typically, the equivalent circuit of an inductor is:

( DCR + Lnom ) || Lcore || Zwinding

( '||' means "in parallel with", and '+' means "in series with".)

Lnom is an ideal inductor, the nominal value.  Lcore is a lossy inductor corresponding to eddy current losses in the core.  Zwinding is an RC element (or higher order networks) representing the capacitance and loss of the winding.

DCR + Lnom gives the LF cutoff, where it goes from resistive to inductive.  Lcore and Rwinding give the peak impedance (parallel resonance at some midband frequency).  Cwinding dominates above the peak, then at high frequencies, Zwinding dominates (if it's just RC, then just Rwinding; more likely it's some complex peaks and dips, which we try to not care about).

The equivalent inductance of that network, will vary with frequency.  At ~DC, the inductance is large and complex (which is to say, DCR dominates).  In mid band, inductance is usually falling gently (due to core loss) -- how much depends on how good the inductor is.  Above the impedance peak, inductance is negative (i.e., it's capacitive instead).

Assuming you're talking about an iron cored inductor, I would be shocked if you can even find one that is that precise, and I strongly suspect L will depend on frequency enough that doing it at 20Hz will give a different result.

You may well get lucky in that the DCR and core loss effects cancel out, and you get the right inductance by coincidence.  But that means your measurement is part inductance, part DCR and part core loss, and all those sources of error are in your measurement.

What do you need an inductor of that value and tolerance for, anyway?

Tim
pete_dl:
Thanks for the reply tim.

I am trying to design a very energy efficient circuit to drive a piezoelectric actuator with a 20hz sine wave. I have managed to drive the actuator with a DAC and opamp, but i want to get more energy efficient.

I read papers describing energy recovery schemes that dumps the stored charge from the piezoelectric actuator into an inductor in order to create the other half of the sine wave. basically a LC resonator with the actuator being the capacitor.

What I was hoping was I could buy an  inductor of roughly 5H, measure it accurately using a meter, then tweak the capacitance of the circuit with extra capacitors in order to achieve the 20hz resonance. I calculated that I need to measure the capacitance and inductance to within 2.5% so as to get the resonance within 1hz of my target.

If I cant rely on an inductance meter, then should i tweak the capacitance to maximise how long 1 full charge will ring for in the LC circuit with a scope ?

Pete
T3sl4co1l:
Just use a class D audio amp.

Tim
pete_dl:
A class D amp would be better than my opamp. However, energy will be lost in the low pass filter and also the energy stored in the piezoelectric actuator will also be lost each cycle. The LC resonator seems to be ideal (with my very limited knowledge of electronics) but I have not calculated how much will be lost in the resistance of the components.

Pete

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