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EMF calculation

  1. Aug 30, 2011 #1

    I hope someone can help me. Looking at a magnetic generator, we will have a ring of magnets which will rotate, and then we have the copper wire static.

    I understand that if I have a ring of alternating magnets, all placed right next to each other, thereby creating a solid magnet surface for the copper wire to rotate against, the formulation for calculating the induced EMF = 2 x length into paper x radius to wire x Radial Flux Density B from magnets x rotational speed.

    Please see attached picture. The outer diameter is 300 mm and the height 50 mm. This design is different, as I have only 24 magnets, with a radial flux density of 1.4 Tesla. I think their size is approx length (29 mm) x width (14 mm) x height (50 mm). I then also have 18 copper wire coils.

    Is there a formulation I can use to calculate the possible EMF I can expect from this design?

    I need some help here, but I would like to be able to calculate how the EMF will change when I:

    1) Increase or decrease the size of the magnets (I expect here that the surface area of the magnet that get in contact with copper will have an effect)
    2) Increase or decrease the number of windings in one coil and thickness of the copper wire
    3) Increase or decrease the amount of magnets
    4) Increase or decrease the amount of coils
    5) Increase or decrease the air space

    Most of what I know by now I have learned on the internet or with other help, so please treat me kindly! I do not understand most of the symbols in formulations, so if you can help with a formulation with explanations of symbols, like with EMF above, that will be awesome!
  2. jcsd
  3. Aug 30, 2011 #2


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    Staff: Mentor

    I'm not seeing the attachment yet...?
  4. Aug 30, 2011 #3
  5. Aug 30, 2011 #4
    See page 7 in

    http://www.windenergy.nl/website/files/artikelen/AXIAL_FLUX_HowItWorks.pdf [Broken]

    for a 24-magnet, 3-phase alternator design.

    Bob S
    Last edited by a moderator: May 5, 2017
  6. Aug 31, 2011 #5
    Hi Bob S,

    Thanks! I have been suffering on the internet to find the right documents, but this one is awesome! In one of my other posts I was asking about the purpose of the steel in the design, and this document answered my question! It also one of my questions above, thicker copper wire will need less windings and thinner copper wire will need more.

    I do have 1 more question if you can help. I have spent most of the morning looking for sites, and found this one: http://www.6pie.com/faradayslaw.php.

    There is a formulation which states that V = -N * change in (( tesla * area meters squared)/ seconds). From this I can see that I can calculate the potential EMF if I have a magnet size and coil windings. Do you possibly know what the smallest magnet is that you can use in a magnetic generator design? Your document has a few magnet sizes in Appendix B, just want to know the starting point when looking at my design when it comes to magnets.
  7. Sep 5, 2011 #6
    Ideally, the path length in the axial air gaps (for the B field lines) should be less than 1/100 of the path length in steel. This means very small tolerances in the alignment of the rotor and stators. If you use smaller wire and more turns, you will generate more voltage, but the internal resistance and voltage drop will be higher. If the air gaps between rotor and stators are each 10 mils, then the sum is 40 mils (1 mm). If the total flux path length in iron is say 100 mm, that's only 100:1. It will work, but is less than ideal. The problem is that when current is flowing in the coils, the magnetic field produced by that current forces the primary field lines to find alternate paths through the stators. You need a very low reluctance path through the coils.

    Bob S
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