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Confused about voltage induced across inductors

  1. Dec 2, 2011 #1
    Confused about voltage induced across inductors :(

    Hi,

    As you may have gathered from the topic title, I'm rather confused about inductors :rolleyes: I have another topic going regarding boost converter design, which involves an inductor, but as this confusion/question/observation is really specifically related to an inductor and not the boost converter as a whole, I decided to start a new topic for it.

    My confusion is this: Why does the voltage induced across an inductor depend on the duty cycle of the applied DC signal and not the on/off frequency? It seems like the induced voltage is directly related to the rate of change of current, which is the frequency, because of this equation:

    [itex]V_L = L\frac{dI_L}{dt}[/itex]

    On the right hand side of the equation is the rate of change of the current, which is dependent on the frequency, right?

    I built a little circuit in Multisim and here are the results:

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_7149.png

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_7504.png

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_12719.png

    To my total confusion, changing the DC signal frequency does not change the voltage across the inductor. Doesn't this completely contradict the above equation??

    Changing the duty cycle, while keeping the frequency constant, resulted in:

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_13683.png

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_7149.png

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_6835.png

    http://www.roboticsguy.com/uploads/gallery/album_13/gallery_1_13_5867.png

    So changing the duty cycle changes the voltage induced across the inductor.

    What's going on here?
     
  2. jcsd
  3. Dec 2, 2011 #2

    berkeman

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    Staff: Mentor

    Re: Confused about voltage induced across inductors :(

    You are driving it with a voltage source and measuring the voltage. Drive it with a voltage source and measure the current, or visa-versa.
     
  4. Dec 2, 2011 #3
    Re: Confused about voltage induced across inductors :(

    Good point. The voltage is still changing, however, so why would that be?
     
  5. Dec 2, 2011 #4

    berkeman

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    Staff: Mentor

    Re: Confused about voltage induced across inductors :(

    I think it's an artifact of adjusting the "duty cycle" of the AC signal. If you plot the waveforms, it may give a clue.

    Also, you should probabaly be using sine wave excitation, if you want to clearly see the variation of current with frequency.
     
  6. Dec 2, 2011 #5
    Re: Confused about voltage induced across inductors :(

    Remove the inductor and re-run the simulations. That should give you a clue.
     
    Last edited: Dec 2, 2011
  7. Dec 2, 2011 #6

    berkeman

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    Staff: Mentor

    Re: Confused about voltage induced across inductors :(

    :biggrin:
     
  8. Dec 2, 2011 #7
    Re: Confused about voltage induced across inductors :(

    Lol, stop making me feel like an idiot :rofl:
     
  9. Dec 2, 2011 #8
    Re: Confused about voltage induced across inductors :(

    I'm still really confused on why the boost converter is dependent on duty cycle and not frequency, though. That's what I was trying to figure out through this "experiment." If anyone could shed some light on why that is so, that would be great :smile:


    EDIT: My boost converter question: https://www.physicsforums.com/showthread.php?p=3646504
     
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