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Flux lines in diametrically magnetized cylinder

  1. Feb 15, 2012 #1
    Hey everyone,

    First I'd like to say my thanks to the community for all the information I managed to glean while lurking on the forum and doing my schoolwork over the last year or so. It's been an invaluable resource to me!

    Now, on to my question. (I see that it says not to post homework questions, but I don't think this really counts as it's just a question about the properties of magnets). I'm a Mechanical Engineering student working in a team on a wind generator for class. We were given the option of building our own alternator, and I decided to give it a shot a bit ahead of time. I've got a general design in mind, a vertical axis generator with static coils and rotating magnets. When considering how easy and time-effective it would be to machine my rotor and mount the magnets, I decided to go with cylindrical ones. I've also decided on air-core coils of oval/elliptical shape.

    Now, from what I understand the alternator will work best if I have the longer portion of the coil intersecting the flux lines of whichever magnet happens to be passing it at a 90° angle. My question is, if I use magnets with diametric poles

    (eg this: http://www.magnet4less.com/product_info.php?cPath=1_133&products_id=653),

    would I be able to get away with mounting my coils on the bottom of the housing with the circular faces of the magnet passing over them? My thinking is that since flux lines go from pole to pole this should work, but I fear that the effect will be much weaker than if the coils were mounted next to the rounded sides of the magnets.

    Please excuse any misguided assumptions or misinformation on my part, all that I know regarding the construction of alternators comes from a few days of haphazard internet research. I hope someone can shed some light on this for me!

    Thanks in advance!
  2. jcsd
  3. Feb 16, 2012 #2
  4. Feb 16, 2012 #3


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    I'm really not sure what you are saying. If you want an efficient generator then you don't just wave a magnet near a coil. A good generator design tries to ensure that as much flux as possible cuts the coil on each half of the cycle. This is achieved by having a core of iron through the coil and a rotating magnet with as little clearance as possible between the poles the faces of the core as they rotate past it.. Any deviation from this will result in flux leakage and a poor performance. The reason for using a 'normally' magnetised cylinder for a magnet is so that the above criterion can be achieved. You need a 'low reluctance path'.
  5. Feb 16, 2012 #4
    Thanks for your reply Sophie. I understand that there's a lot more to alternator construction if one is seeking efficiency. That said, I don't need mine to be optimal, I just need it to supply enough power at a relatively low rpm to charge some capacitors which will in turn light an LED. I think I was operating under the false assumption that the flux lines that needed to cut across the coils had to be between the two poles, not at the poles. It seems that I need axial magnets likes you said, which will actually simplify things a bit for me.

    Thank you very much for your help!
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