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Inductance of VR crank sensor

  1. Dec 14, 2012 #1
    I helped my colleagues simulate inductance of vr crank sensor in Maxwell but my simulation result is quite different compared to the test one.

    Our sensor is quite simple, just a piece of permenant magnet, a piece of flux concentrator made of normal steel, and one copper coil hooked outside of the flux concentrator. It is that simple. My colleague used LCR meter to test the inductance and it was around 300mH. However, my result was around 400mH.

    It is LCR HiTESTER 3522-50: http://www.hioki.com/product/352250/index.html

    One thing I cannot understand is I do not know how LCR meter measures the inductance. What I learned at school is that inductance always varies with the current. With coil terminals connected to LCR meter, I do not know what current is there.

    The internal resistance of coil is 1000ohms. I do not have any test experience. Can you give me a hint?


    Also, my colleague told me if the sensor is covered with an AL case, the inductance goes lower, around 200mH. Do you know the reason?

    Many thanks.
     
  2. jcsd
  3. Dec 14, 2012 #2

    jim hardy

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    Go back to your very basic physics. What's inductance?

    Inductance is "Flux linkages per ampere", or L = n x Phi / i

    Your inductance meter applies some sort of current
    which will induce eddy current in the iron, and in the aluminum,
    which by Lenz's law cancels out some of the flux.

    In other words, your inductance meter mistakenly thinks it's applying the only current. It applies an exciting current allright, but the resulting eddy currents also cause flux. Their flux is in opposition. Lenz: Induced current opposes the change in flux.

    I once observed voltage across an inductance with an o'scope while measuring it with an inductance meter. The meter must have had a computer inside for it applied a variety of waveforms to the device under test, none of which made any sense to me.

    If you figure out how your inductance meter works please enlighten me.

    My best insight to inductance was gained by applying a triangle wave current and observing the resulting voltage wave.
    A perfect inductor would deliver square wave voltage (di/dt constant on each half of triangle wave).
    Deviation from that square wave is the effect of your eddy currents and will be dramatic with applied frequency. The shoulders round off as eddy currents settle out with each slope reversal.
    My automobile tire iron made a decent inductor core only up to around eight hertz. Faster than that it became resistive. Above ~600 hz it was magnetically not there any more.

    Have some fun with your setup while the curiosity is fresh! Post some 'scope shots.

    old jim
     
    Last edited: Dec 14, 2012
  4. Dec 17, 2012 #3
    The reason why I ased here is because I have no clue how LCR meter excites the inductor.

    Thank you.

     
  5. Dec 18, 2012 #4

    jim hardy

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    well from looking at the manual which is available from that link you posted,

    it's not quite clear .

    They say it can apply voltage or current excitation over a wide frequency range that is set by front panel control
    but they didn't say anything about waveform


    so were it mine i'd take an o'scope and see .
    If it applies a nice sinewave, which sounds logical, they probably assumed that we'd assume that.

    One experiment is worth a thousand expert opinions.

    As i mentioned - my tester was quite cryptic.

    old jim
     
  6. Dec 20, 2012 #5
    Jim you are absolutely right. I went to the lab with my colleague and found out that it was tested at 1000Hz and I used magnetostatic solver!

    Sadly the result doesnt change that much from 1000Hz to 10Hz or 5Hz. And we cannot find the cable to connect the LCR meter to scope. I think the current magnitude also affects the final result.

    Thank you.

     
  7. Dec 20, 2012 #6

    jim hardy

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    I'd say that's a good thing .

    Perhaps they have a core thats immune to eddy currents. I dont know much about ceramic magnet materials, are they similar to ferrite cores in that regard?

    Inductance and magnetism is interesting and it's fun to experiment. I hope you get a 'scope working and explore properties of some devices. And some core materials.

    Likely that LCR meter of yours is intelligent.
    I am often outsmarted by test equipment. That's why i like to know how it's testing a device.

    old jim
     
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