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| Ideas for measuring Inductance in dependence of DC Current |
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| 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. |
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