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Pulsed DC Electromagnet

  1. May 8, 2015 #1
    I need advice on assembling a pulsed dc electromagnet.

    I'd like to experiment with eddy current levitation. I've seen educational videos where a pulsed DC electromagnet will generate eddy currents in a plate of aluminum, which creates is own magnetic field, and causes the plate to levitate due to magnetic repulsion with the electromagnet.

    My idea is to use a pulse width modulator (PWM) as the pulsed power source at 12V and 30 amps, and feed it into a step-up transformer to increase the voltage which is necessary to create eddy currents, then have it go to the electromagnet, and back through a step-down transformer to return the voltage to 12V which then returns to the PWM. I don't know the exact voltage needed, but it appears to be higher than what comes out of the house mains.

    I thought of using microwave oven transformers (MOTs) as the step-up and step-down transformers, but after reading some of your treads that seems dangerous. I have a basic understanding of electricity, but don't have the hands-on experience. I'd like to put together something that is "plug and play", rather than solder breadboards with rectifiers, etc.

    This isn't an original idea, it's been done before, but it's been difficult for me to get advice on the nuts and bolts of how to put it together. Any help would be much appreciated.
     
  2. jcsd
  3. May 8, 2015 #2
    Is there any specific reason it has to be a pulsed DC electromagnet? Why not just drive it with a AC signal? That would simplify matters a lot.
     
  4. May 8, 2015 #3

    CWatters

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    I don't understand the need for the bit I have highlighted.
     
  5. May 8, 2015 #4
    This could be done with AC, but I want the magnetic field to always have the same polarity, say with North facing up, and South facing down. With AC the field would be constantly reversing every time the current changes direction.

    As for stepping the voltage back down to return to the PWM, that's a good question. I made an assumption here. I figured that if I increase the voltage to a level higher than what the PWM normally operates at that I would cause it to burn up. Since the PWM is basically a computer board with a couple of knobs and positive & negative terminals I thought a higher voltage might overload it.

    Good questions! Thanks for responding.
     
  6. May 8, 2015 #5
    Two things:
    - Aluminum is not magnetic. The levitation effect is caused through induction by the coil's field, which however means you need a constantly changing field (since induction operates on the *change* of the magnetic field)
    - There's no need for the magnetic field to be in one direction only. Lenz's Law ( http://en.wikipedia.org/wiki/Lenz's_law ) ensures that the induced field in the aluminum will always be opposite to the coil's field. In fact, if you somehow tried to yank the coil's current down in order to then go constantly back up (i.e. a sawtooth-shaped current), the coil itself will resist that through self-induction (Lenz's Law again) and cause a massive voltage spike that you don't want.

    So, you really do want to stick with AC.
     
  7. May 8, 2015 #6

    CWatters

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  8. May 8, 2015 #7
    I understand the basic principles of how a nonferrous metal will levitate. I wanted the polarity to be constant for an experiment. Since charged particles traveling through an homogenous magnetic field experience the Lorentz Force, I wanted to see if aluminum particles, charged by Eddy Currents, would be subject to the Lorentz Force if forced into motion. I didn't mention this at the outset, since it was way beyond the simple building of the electromagnet and I didn't want to clutter up the discussion. That's the reason I wanted the magnetic field only in one direction.

    My understanding of creating eddy currents by time-varying magnetic fields said that pulses of DC current will produce them. I didn't know anything about the collapse of the magnetic fields creating voltage spikes in the coil. It sounds like I would've fried my PWM power supply if I had tried it.

    Thanks for your input. I tend to agree with you that more hands-on experience is needed first. The really cool thing is that in two days I've learned more on this forum than I've learned talking with electricians in the last six to twelve months.

    You guys are great. It's horribly frustrating for someone like me trying to do something simple without someone to advise.

    Thanks again for your support,

    Skatterbrain.
     
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