Ideas for measuring Inductance in dependence of DC Current

[Ysjoelfir]

Hey everyone, for my bachelor thesis I am working on some experiments with inductors.
Right now I am thinking about a way to measure the decrease in inductance while a certain DC current is flowing through the inductor. Basically I want to know which DC current is needed to decrease the inductance to 90%.
Since it is pretty hard to measure inductance while the inductor is being powered with DC only, I am thinking about sending a DC with superimposed (precise) AC through it, calculating the inductance from AC measurements while increasing the DC ratio and watching the drop in inductance.

We have a (actually several) Fluke 5220A at hand which I thought to utilize to get the DC part for the experiment. But how do I add the AC content? I thought about modulating the input voltage, feeding it a variable DC with superimposed AC from a frequency generator. Has anyone experience with that?

Also, I THINK that the 20A maximum current from the 5220A is potentially not enough since the inductors I am working with are specified to a minimum of 48A AC continuous. Any ideas for that case?

[TheUnnamedNewbie]

It is important to keep in mind that if you do this, you need some way to isolate the impedance seen to your DC source.

Two very important questions here: What kind of values are you planning on measuring, at what frequencies? That will heavily define what you can and can't do.

[Ysjoelfir]

I want to measure the Phase angle, AC voltage and AC current. Frequency would be just 50Hz. By sampling those values I am able to calculate the inductance without having to worry about influences from the DC part, since the power meter that I use to measure those values can be AC coupled.

The expected inductance would be in the ballpark of 300 to 500µH, if that helps somehow.

[langwadt]

basically something like this:  https://www.frostburg.edu/personal/latta/ee/qsk5/pindiodes/spstswitch.jpg

with the diode replaced with you inductance and the other components adjusted to match your frequency ranges

[capt bullshot]

That's the way TE manufacturers propose:
https://www.voltech.com/Articles/104-160/3_Applying_DC_Bias_Current_During_an_LCR_Test

Basically, either use a large enough inductor to decouple the DC from AC, or use their specially for this purpose made DC current source (with high AC impedance). One could imagine to homebrew such a source, especially as you want to measure the impedance at 50Hz, so the frequency response requirements aren't too difficult. Then just use another amplifier (e.g. an audio power amp) to couple the 50Hz into the inductor.

Another idea came to my mind: If you have two identical inductors available, connect them in series and use your bog standard low impedance power supply to push the DC current through both. Then do the impedance measurement AC coupled across one inductor (the LCR instrument will see both inductors in parallel) and multiply the result by two.
 

[duak]

A different and more direct way to do this is to apply a constant voltage to the inductor while measuring the current.  As the inductance falls due to increasing current, the dI/dt will also increase.  If you can digitize the measured current, you can calculate the inductance at every current level up to the limit of the power supply.  This method does not require other inductors or capacitors, but does require a controllable power supply and some sort of high current measuring instrument.

[Ysjoelfir]

Just to give you a quick update on this matter: I tried different methods, some I thought about myself and some from your suggestions. many didn't work as I intended, for example I couldn't find any way to get the LCR bridge to recognize the (value-wise) relatively small inductor between those caps - It decided that the arrangement was purely capacitive, which I somehow predicted but hoped that this wouldn't happen with appropriately sized caps.
However, the simple "put an bigger inductor in series" method described in the voltech document worked extremely well, I wound around 15 meters of 2.5mm² wire onto a random toroidal core that was flying around in the lab and got around 80mH, so roughly 200 times the inductance I try to measure.  Using that thing I came pretty close to what I wanted to archive, however, I noticed that I have to increase the inductance even more since I still get aroung 3% difference with the unloaded L-L combination idling with the DC supply connected in comparison to the inductor measured alone, without any connection to anything.

Anyway, more suggestions and discussions are always welcome I am eager to try out any way that I can reproduce here in the development department.

[capt bullshot]

Just to give you a quick update on this matter: I tried different methods, some I thought about myself and some from your suggestions. many didn't work as I intended, for example I couldn't find any way to get the LCR bridge to recognize the (value-wise) relatively small inductor between those caps - It decided that the arrangement was purely capacitive, which I somehow predicted but hoped that this wouldn't happen with appropriately sized caps.

Yes, might be an issue. My LCR wouldn't like that setup (from my post) at all, because it expects the DUT fully floating in respect to quite anything.
So a few more ideas:
- try splitting these capacitors for a Kelvin connection - use one capacitor for each of the four connections
- use one of these power analyzers that has the capability to calculate the DUT impedance together with an appropriate source (e.g. function generator and audio power amp) instead of an LCR bridge.

Hope you get the point from my sketchy drawing

The method pointed out by duak is promising also, some time ago I've used a capacitor and thyristor discharged into an inductor to determine the inductors saturation current. With some more math and some effort to create reproducible "shot" I'd think one could calculate inductance over current from that data.

[T3sl4co1l]

Tip: if you have two identical inductors, you can connect them in series, using a bypassed (low impedance, voltage source) supply to set the bias current.  Then the middle node is simply the impedance of two inductors in parallel, each going to ground (since the bias supply is at AC ground).  Any impedance measurement method can be used, with a coupling capacitor if necessary to remove the last bit of DC offset.  For example, resonating the inductor with a capacitor, and measuring the impedance peak (peak frequency and capacitance gives inductance; amplitude ratio or bandwidth gives resistance or Q).

The complement of this, is quite commonly used in RF circuitry: varactor diodes are commonly available in a series-opposed configuration, so the middle tap can be biased to set capacitance, while the end terminals are tied with some impedance (an oscillator or tuned amplifier).  Also works equally nicely to measure, for example, type 2 ceramic dielectric saturation.

Tim

[Marco]

The fluke trans impedance amplifier seems capable enough to superimpose a 50 Hz AC current signal on the DC signal. So all you really need is an extra high impedance current source to put in parallel with the fluke a shunt and an oscilloscope.

[Doctorandus_P]

Use a DC voltage and your inductor which get switched to GND with a beefy MOSFET.
Then turn the MOSFET on with short pulses and measure the current through your inductor (with a non-inductive power transistor).

This is a nice and easy way to measure the inductor saturation characteristics.

On a sidenote:
Some 100 years ago, (before most elecronics) there were special transformers / inductors especially designed to switch AC currents.
When the switch is "off" then it acts as a large inductor (Several Henry's, just like a regular transformer).
When the switch is "on" a DC current is pushed through a separate winding to saturate the core of the transformer, which allowed a large current to pass through the AC winding.

[Ysjoelfir]

Another update: I tried the ideas you proposed and especially the idea duak wrote seems to be a nice idea since It can involve one of the devices we produce - which is always nice if you present the project to your boss Also, all PSUs we have at hand max out at 20A, so that is a bit of a problem which could be solved with a capacitor bank discharging into the inductor. So I guess this method could be a good idea, I will definitely try that out. Should be pretty close to what Doctorandus_P suggested. (Nice sidenote btw! I love to learn about those ancient technologies!)

The quadruple capacitor + LCR meter version I didn't try so far, but I soon will. Have to do some other measurements right now, like measuring the self resonance frequency. I will try biasing the inductors while doing so to see how the resonance peak changes. Could also be a way to tackle the problem. Basically as Tim proposed.

[Conrad Hoffman]

This is a common measurement, or at least it was. Search out the General Radio Corp. Incremental Inductance Bridge, 1633-A. You can probably download the manual to see how they did it. Sometimes things were more straightforward with old floating bridges, rather than modern LCR meters, but the old ways might give you some ideas.

[voltsandjolts]

Cross-reference:
https://www.eevblog.com/forum/projects/inductor-saturation-tester-alternative-route-to-dump-the-excess-energy/