Measuring Inductor

[musicisair]

When measuring an inductor using an LCR meter what frequency should be used?

Also, what is the frequency manufacturers measure at? These things can be expensive so I'd also like to verify I'm getting what I've paid for.

Explain: I have a 12 gauge "3.0mH" air core inductor that measures 3.161mH at 100 Hz and 2.83mH at 10k Hz.

I'd like to know which frequency values are most relevant for passive crossover use, and at what frequency does software, like VituixCAD, expect in its simulations?

 

[rationaltime]

My thought is this.

When you design your passive crossover maybe you would care
most what are the component values at the crossover frequency.
Wouldn't you want to use that value in your simulations?

The variation you report would not make me anxious about inductor
value.

 

[NTK]

From the perspective of the circuit, usually what is important is the impedance across the inductor at the range of frequencies in the signal. Since the impedance across an inductor is: Z = j ω L = j (2 π f) L, it is directly proportional to frequency.

If the inductance change with frequency is relatively minor (as reported in post #1 where it changed from 3.161 mH at 100 Hz to 2.83 mH at 10 kHz — a ~10% reduction in inductance for a 100X change in frequency), its effect should be negligible to the operation of the circuit.

 

[solderdude]

When measuring an inductor using an LCR meter what frequency should be used?

Also, what is the frequency manufacturers measure at? These things can be expensive so I'd also like to verify I'm getting what I've paid for.

Explain: I have a 12 gauge "3.0mH" air core inductor that measures 3.161mH at 100 Hz and 2.83mH at 10k Hz.

I'd like to know which frequency values are most relevant for passive crossover use, and at what frequency does software, like VituixCAD, expect in its simulations?

3.0mH is usually for woofer/mid crossover so would pick the frequency closest to it in this case 100Hz.

 

[musicisair]

My thought is this.

When you design your passive crossover maybe you would care
most what are the component values at the crossover frequency.
Wouldn't you want to use that value in your simulations?

The variation you report would not make me anxious about inductor
value.

Not if the simulations use calculations that expect the entered inductance at a specific frequency. I couldn't find anything about that online though.

I guess I'm stumped on how to go about buying inductors if I can't know their inductance at the frequency I'm looking for ahead of time?

From the perspective of the circuit, usually what is important is the impedance across the inductor at the range of frequencies in the signal. Since the impedance across an inductor is: Z = j ω L = j (2 π f) L, it is directly proportional to frequency.

If the inductance change with frequency is relatively minor (as reported in post #1 where it changed from 3.161 mH at 100 Hz to 2.83 mH at 10 kHz — a ~10% reduction in inductance for a 100X change in frequency), its effect should be negligible to the operation of the circuit.

It's a bit of a case of getting what I'm paying for (and knowing what value to to buy for the desired frequency). You could easily spend $150 on an 12AWG inductor with a 2% tolerance... but at what frequency do you verify the accuracy?

Crossover simulators might be using this formula to compute the FR... in which case the Inductor value entered into the software would need to be a standardized value. Right?

 

[musicisair]

Update: I asked the manufacturer (Solen) and they they said they measure their air-core inductors at 1KHz.

@kimmosto does VCAD expect inductor nominal values to be measured at a specific frequency?

 

[RickS]

Update: I asked the manufacturer (Solen) and they they said they measure their air-core inductors at 1KHz.

@kimmosto does VCAD expect inductor nominal values to be measured at a specific frequency?

Sims are generally going to use the applied frequencies to the inductor so not really an issue.

More critical in woofer filter circuits is watching the DCR of the inductor. This sometimes dictates whether you apply an open core vs solid core inductor. You will have limited choices when you buy too. Am sure most meters likely measure at 1 kHz. A cheaper on may use 60 Hz. Rather than a meter would buy a DATS from Parts Express. It will do LCR measurements and more comprehensively than a meter (for only $130).

 

[musicisair]

Sims are generally going to use the applied frequencies to the inductor so not really an issue.

Does "applied frequencies" here mean that at low frequencies, the simulation will account for the inductor's slightly higher inductance, and at high frequencies, it will account for the slightly lower inductance?

More critical in woofer filter circuits is watching the DCR of the inductor. This sometimes dictates whether you apply an open core vs solid core inductor. You will have limited choices when you buy too. Am sure most meters likely measure at 1 kHz. A cheaper on may use 60 Hz. Rather than a meter would buy a DATS from Parts Express. It will do LCR measurements and more comprehensively than a meter (for only $130).

I'm using the DER DE-5000 (and performed open/short calibration before taking any measurements), as it was recommended by many here on ASR. It can measure at 100Hz, 120Hz, 1kHz, 10kHz, 100kHz.

Since these specific inductors I tested are made from 12AWG OFC, the DCR is quite low - specced at 0.30Ω. Actual, measured via the DE-5000, is 0.31Ω (DCR measurement accuracy is `+-1%+0.03Ω`).

These inductors looks to be out of compliance, as the inductance tolerance is +-1%. I have 3 of these 12AWG 3.0mH air core inductors and they measure 3.154mH, 3.155mH, and 3.151mH at 1kHz (measurement accuracy is `+-1%+0.005mH`), which is consistent, but all nearly 5% too high.
I guess I could unwind these a bit to get them closer to spec; in VCAD (using manufacture driver FR) the higher inductance causes a peak difference of only about -0.1db and a ~5Hz lower crossover point, though the DCR will also go down which may make up for the attenuation, which I didn't simulate.

Looking at the graphs in the review of the 3TD-X (which is what I'm planning on building), and thinking like a purist with a magnifying glass, I can't tell if attenuating the bass and reducing the XO-point would generally be undesirable or not (again, I know it's a miniscule change). That said, the woofer circuit's inductor in the 3TD-X Amir reviewed is actually a 14AWG 3.0mH air-core with a DCR of 0.42Ω (probably the EAV76-14-3000 from the Erse [RIP]); the build plans only list part values for their "Standard" crossover, which uses a 3.0mH laminate core inductor with a DCR of 3.649Ω. Once I build and measure I may look into adding an additional resistor in series to make up for the lower DCR. (link to the related build thread, just for posterity).

 

[musicisair]

I plotted the inductance measurements to attempt to extrapolate the point at which the inductance is 3.0mH: ~3167Hz

Measurements: {120Hz, 3.167mH}, {1000Hz, 3.159mH}, {10000Hz, 2.838mH}, {100000Hz, 2.832mH}

The logarithmic halfway point between 120Hz and 100000Hz is 3162Hz, and the halfway point between 3.167mH and 2.832mH is ~3.0mH.

My takeaway is that they do not list their inductors measured values at 1kHz, but instead report its "central" value (not sure what the right terminology for this is, is it "nominal"?).

I've asked Solen for clarification.

 

[Lambda]

uses this to measure inductance vs. frequency:

Thiele Small Parameters

ideally it shuld be flat and only induktive

 

[RickS]

Does "applied frequencies" here mean that at low frequencies, the simulation will account for the inductor's slightly higher inductance, and at high frequencies, it will account for the slightly lower inductance?

I doubt it but you really need to consider this holistically with real drivers. By applied frequencies, I meant its operating range. As for VituixCAD's sim, Kimmo is the man.

Try putting a 3.0 vs 3.3 mH coil in VCAD and see how little it affects the outcome with an ideal driver. then try with a real one. Then also remember this is a dynamic system with many variables and you are only looking at a snapshot. The speaker changes in different temps and humidity and as its drivers heat up and cool down. There is only so much a designer can reasonably control. Everyone has to determine their own point of diminishing returns.

I'm using the DER DE-5000 (and performed open/short calibration before taking any measurements), as it was recommended by many here on ASR. It can measure at 100Hz, 120Hz, 1kHz, 10kHz, 100kHz.

Since these specific inductors I tested are made from 12AWG OFC, the DCR is quite low - specced at 0.30Ω. Actual, measured via the DE-5000, is 0.31Ω (DCR measurement accuracy is `+-1%+0.03Ω`).

These inductors looks to be out of compliance, as the inductance tolerance is +-1%. I have 3 of these 12AWG 3.0mH air core inductors and they measure 3.154mH, 3.155mH, and 3.151mH at 1kHz (measurement accuracy is `+-1%+0.005mH`), which is consistent, but all nearly 5% too high.
I guess I could unwind these a bit to get them closer to spec; in VCAD (using manufacture driver FR) the higher inductance causes a peak difference of only about -0.1db and a ~5Hz lower crossover point, though the DCR will also go down which may make up for the attenuation, which I didn't simulate.

Most inductors are much worse tolerance. Much of my speaker work has used active crossovers. If you want precision, you may want to consider using. In any case, you can tweak either, passives are just much more work due to looser tolerances.

Looking at the graphs in the review of the 3TD-X (which is what I'm planning on building), and thinking like a purist with a magnifying glass, I can't tell if attenuating the bass and reducing the XO-point would generally be undesirable or not (again, I know it's a miniscule change). That said, the woofer circuit's inductor in the 3TD-X Amir reviewed is actually a 14AWG 3.0mH air-core with a DCR of 0.42Ω (probably the EAV76-14-3000 from the Erse [RIP]); the build plans only list part values for their "Standard" crossover, which uses a 3.0mH laminate core inductor with a DCR of 3.649Ω. Once I build and measure I may look into adding an additional resistor in series to make up for the lower DCR. (link to the related build thread, just for posterity).

As they obviously have spent more time with the design, the CSS guys can offer more insight into some of the crossover variants. Would be interested to hear. The rest of us can only extrapolate based on experience.

 

[musicisair]

Solen replied that they do indeed use measurements taken at 1kHz for their listed values, and that my measurement device must be inaccurate compared to the Stanford Research SR720 LCR meter they use in a metal-free room.

They actually went and measured an inductor from the same manufacturing date as the ones I have, and it measured much closer to spec: 3.0402mh@1KHz (which is above tolerance by only 0.34%). They also measured with a DATS V3 and said the results were even higher than my own measurements.

I've attached a plot of their measurements, which does not demonstrate the halfway phenomenon I thought I had discovered.

 

[musicisair]

remember this is a dynamic system with many variables and you are only looking at a snapshot. [...] Everyone has to determine their own point of diminishing returns.

Most inductors are much worse tolerance. [...] passives are just much more work due to looser tolerances.

Part of my thinking here is that measuring/documenting actual part values, and using parts with tight tolerances, means there will be fewer variables I will have to struggle with while I'm learning.

I also just enjoy diving headfirst into a narrow topic, getting lost in it, then trying to understand things a bit more on my way out.

Anyway, I do appreciate the reality check!

 

[SMPSGuy]

As Rick suggests. You have many variables. The windings will have both resistance and interwinding capacitance. Then the core, should you have one, will have all sorts of horrible things going on. The equivalent of resistance. but perhaps not quite, will be hysteresis losses that vary with frequency and magnetization level according to squares and cubes.

They really are quite horror show.

It is probably not applicable to your use case, there may be something in there, but...

https://bitsavers.org/components/siemens/1983_Siemens_Ferrite_Cores_and_Hardware.pdf

That's from back in the days when they used to publish meaningful data and you would still toast your head trying to work out how to apply it to your use case. I'm thick but I reached the conclusion that they don't really understand it either so you have to come up with your own perversion of something that makes sense to you.

"I also just enjoy diving headfirst into a narrow topic, getting lost in it, then trying to understand things a bit more on my way out."

That's the way.