Determining Current Through A Coil After Passage Of Magnet

Discussion in 'Analog & Mixed-Signal Design' started by positive8, Sep 27, 2016.

  1. positive8

    Thread Starter Member

    Sep 27, 2016
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    Here's a very simple one that baffles me (I'm a digital guy).
    Situation:
    For example, a neodymium cylindrical magnet of , say, 5000 Gauss field, passes through a 500-turn coil of AWG 32 magnet wire at a velocity of 10 m/s. The resistance is approx 10 ohms, additional impedance should be nominal. The oscope trace nicely matches the results expected by the standard equation, EMF = N(delta T)/(delta time). However, current (measured by galvanometer, multimeter, whatever, AC/DC) seems to be in the microamp range, rather than substantially higher as I would expect from i =( N/R)(delta T)/(delta time), which equation is available in standard references and also immediately derivable from the EMF equation.

    Is the instrumentation I'm using unable to read the transient pulse of the current or am I missing something basic here? TIA
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    What are you using for a LOAD on the coil?
     
  3. positive8

    Thread Starter Member

    Sep 27, 2016
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    Nothing, really. The coil is simply connected to a ammeter - digital or analog. Very low impedance, so wouldn't I be expecting a large transient current pulse?
     
  4. KL7AJ

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    How fast can your ammeter respond?
     
  5. positive8

    Thread Starter Member

    Sep 27, 2016
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    I have 3 off-the-shelf models, not top-notch, but not minimal-function types either. I wondered if that was the problem, so I used analog ammeters (both DC and AC) and a supposedly very sensitive analog galvanometer - all of them just twitch, never reaching a few milliamps.
    Is it possible that a) ammeter response isn't fast enough or b) the amount of current, being highly transient, doesn't provide enough energy (RMS) to do the work required to move the galvanometer coil, even though the peak would be consistent with the equations?

    If this is the problem, would I be able to determine total current output by charging a capacitor with this pulse, knowing the voltage as I do, and then measuring capacitance?
     
  6. Kermit2

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    Feb 5, 2010
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    What are the size/length relationship between the coil and the magnet?
    Is the coil much shorter than the magnet or the other way around?
     
  7. positive8

    Thread Starter Member

    Sep 27, 2016
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    In this case, they are the same length , slightly more than 1 cm. I get a very clean, single sinusoidal waveform on my oscope that matches the standard equation. I've varied the velocity of the cylindrical magnet from 1 to several m/sec. The air gap between the outer edge of the magnet and the winding is at most 2 mm.
     
  8. BR-549

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    Sep 22, 2013
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    What is causing the 10 meter/sec velocity?
     
  9. positive8

    Thread Starter Member

    Sep 27, 2016
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    Gravity (which makes it easy to calculate velocity), or I simply blow into the acrylic tube, propelling the cylindrical magnet through it. (Let me stress this has NOTHING to do w/coil or rail guns, just an attempt to make an energy harvester).
     
  10. BR-549

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    Sep 22, 2013
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    So, you are letting the magnet free-fall for 1 sec. (30 feet) before it enters coil?
     
  11. positive8

    Thread Starter Member

    Sep 27, 2016
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    No, for free-fall I've used at most 3 6-foot sections of acrylic tubing in series. Obviously, some friction from occasional contact with inside of tubing and air resistance , but not much. Blowing hard into a 3-foot section of the pipe can propel the magnet at significant speeds.
    I can calculate the velocity of the magnet from the stored traced of its passage on the oscope, knowing the length of the coil, and guess-timating the point at which the magnet starts to generate EMF when in proximity to the coil but not actually entering it,and also residual EMF when it's exited it.. I'm very sure I'm getting reasonably accurate velocities.
    Regardless, I still can't get the current reading predicted by theory at any speed.
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Guessing
    Severe impedance mismatch
    Try loading the coil with a resistance equal to that of the coil and put your low resistance current meter in series with it.
     
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  13. positive8

    Thread Starter Member

    Sep 27, 2016
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    Holy smokes, I'll bet that could be it. Will try it, for sure. Very appreciative. Will post results.
     
  14. Kermit2

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    Betting your voltage falls to a much lower value too.
     
  15. Kermit2

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    With the big air gap and counter EMF produced in the coil, most of your magnetic flux is probably being repelled when using the ammeter alone.
     
  16. positive8

    Thread Starter Member

    Sep 27, 2016
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    I would think so, given the added resistance..............
     
  17. BR-549

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    Sep 22, 2013
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    OK. How are you expecting to see the current? From the reading on an analog meter, or a digital readout?

    You are measuring the voltage from the coil on the scope right? I am assuming that you have a multimeter on current mode or an ammeter across coil. And the scope probes across coil. What peak voltages are you getting on scope?
     
  18. positive8

    Thread Starter Member

    Sep 27, 2016
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    Ah. Do you think that's a big air gap? Looking at magnetic field charts supplied by the manufacturer of the neodymium magnets, the field force is highly concentrated in the leading edge where the cylinder wall adjoins the plane surface of the magnet top. Using the magnetic field strength calculator on the K and J Magnetics Website, it calculates 0.5+ Tesla field at 2mm from that leading edge. This correlates reasonably with the EMF measurements I'm reading.
     
  19. positive8

    Thread Starter Member

    Sep 27, 2016
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    Right, on an oscope. And as you say re the ammeter (usually with oscope probes detached), using several different digital ammeters, off-the-shelf crude analog ammeters and a good galvanometer. All under-report current, well below what would be expected from induction equations.

    For a 500-turn, 0.75 in diameter coil (AWG 32 mag wire, 14 ohms) 1 cm in length, I'm getting a sinusoidal waveform with VPP of about 14v at around 3+ m/s. This matches theory pretty well
     
  20. BR-549

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    Sep 22, 2013
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    Can we see the coil?
    And magnet.
     
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