ferrite core inductor distortion measurements?

[PeteMcK]

Has anybody come across any actual measurements of distortion in ferrite core inductors? I've come across plenty of assertion, but only one guy who actually measured an iron core inductor:
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It would be useful to have an idea of the actual magnitude of any hysteresis effects.

[mondogenerator]

interesting thread starter. Ive often wondered what ACTUAL measurement would reveal. Ive previously used ferrite chokes, with the provision that theyre high current types, and high enough that saturation is far less of a concern. Ive guess-timated around 5amp as a minimum.
I too would find measurements to be of interest, and to check my 'saturation safety margin' is adequate.

[PeteMcK]

just found this old (short) thread:
Articles on Inductor Distortion Tests?

[speaker dave]

Pardon me for interjecting without having direct experience, but here is a way for measuring inductor distortion. I think this will give a more direct visual indication than the Klipsch IM plots. I haven't done this but I'm pretty sure it would work.

See attached diagram. You need to drive the inductor with a power amplifier and an LF oscillator. You can test it from 100 Hz down. The lower the frequency the earlier you will see the saturation.

Basically you need to put a resistor in series and then drive the combination hard. You will watch the voltage across the resistor to see inductor current. The voltage across the inductor is essentially the total input if the sense R is small.

You must have an oscilloscpe that floats. i.e. it doesn't ground the amplifier. Connect carefully or you will short the amp.

You must be in XY mode rather than a time/sweep mode.

I'm not sure of what the distorted waveform will look like, but it will develop bumps or flats on opposite sides and this will indicate the distortion. Different cores will develop visible distortion at different levels. Ampere turns determine flux level so number of turns on a given coil will be a factor.

Can someone try this?

David S.

[simon7000]

Well I once used "200 Watt" inductors in a batch of loudspeakers. Once installed and at full power they caused the amplifiers to clip early and sounded absolutely awful. I replaced them with the copper foil coil types. They worked just fine.

[jcx]

you can make sensitive measurement with the parts the other way around - gnd the inductor and measure the V across it - its already gnd referenced

Lissagous plot is often used for visualizing reactive part nonlinearity

with protection you can use a soundcard to see distortion at much lower levels than a 'scope

[mondogenerator]

Good points there Dave!

I wouldve thought with the right equipment the test would be relatively simple, and your test method seems simple enough.

(I should have thought about the ampere turns, since im testing motors/generators all night.....

[speaker dave]

Originally Posted by jcx

Lissagous plot is often used for visualizing reactive part nonlinearity

I would have said lazyjouse but I didn't know how to spell it.

You can interchange the two parts but you still need to ground the point between them. If the scope is floating then it doesn't matter.

David

[gedlee]

I did see a comprehensive study once, but I don't remember where. My take away was that yes they are nonlinear, but only lower orders at very high power levels. By that time the SPL level is high enough that the ears masking would make these nonlinearities benign.

I no longer ever consider nonlinear artifacts that occur only at high power levels. Its the ones that occur at higher orders and low power levels that are the most audible. Basically, if the nonlinearity is audible at high power levels then you simply are not using high enough power capable devices. Fix that problem and the issue goes away.

Why can't you just look at the voltage or current waveform (depends on the setup) with a spectrum analyzer and note the harmonics? Thats certaily hos I would do it.

[simon7000]

Originally Posted by gedlee

I did see a comprehensive study once, but I don't remember where. My take away was that yes they are nonlinear, but only lower orders at very high power levels. By that time the SPL level is high enough that the ears masking would make these nonlinearities benign.

I no longer ever consider nonlinear artifacts that occur only at high power levels. Its the ones that occur at higher orders and low power levels that are the most audible. Basically, if the nonlinearity is audible at high power levels then you simply are not using high enough power capable devices. Fix that problem and the issue goes away.

Why can't you just look at the voltage or current waveform (depends on the setup) with a spectrum analyzer and note the harmonics? Thats certaily hos I would do it.

Actually the best results are when you drive with a current source and look at the spectra. Power handling is quite frequency dependent. So you usually need a power amplifier and high value resistor or feedback to make a current source.