Author Topic: Magnets  (Read 10785 times)

Offline Silvio Klaic

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« on: February 18, 2013, 03:48:04 PM »
It's difficult to find good information about magnets and how they behave.
So here I will put some info about them which I found over the years.

The best start is to look K&J Magnetics FAQ. On their site you have also other useful stuff like magnetic calculators etc.
Here are my expanded data from theirs FAQ which I have done thru measurements and experiments:

  • Both poles of magnet are equally strong, but only when measuring in magnetic neutral environment. If you don't measure in magnetic neutral environment results will differ from pole to pole.
    For example if you measure poles of magnets in nature on earth northern hemisphere where is south magnetic pole, you will get data with stronger south pole than north.

  • Stacking magnets will get you slightly higher magnetic field, but there is limit how much magnets you can stack.
    Magnetic conductivity of used material is not that good; therefore there will be resistance to magnetic field.
    You can look values of permeability for different materials:
    MaterialRelative permeability
    Neodymium magnet1.05
    So as you can see neodymium magnet has slightly higher permeability than air, but only small fraction than regular steel/iron.

    Here are some measurements which I was performed on cylindrical neodymium magnets stacked together on top of each other (in series).
    Diameter of magnet 5 mm, length 5 mm, grade N35, maximum strength 3900 gauss.
    Measured only on magnet south pole with gauss meter HGM0200 (resolution 10 G, accuracy +/-2%):
    Number of magnetsField strength (gauss)Strength increaseLength (mm)Max strengthMax strength increase
    6401010,47%304400   11,36%
    Stacking magnets together does not give you too much more power and stacking more than 9 magnets is a total waste.
    So no, you can not get more than 25% of initial (single magnet) field strength in any combination. Therefore combining magnets to get stronger field have no sense.

  • The bigger the better? Not really, but it depends for what you need them.
    If you are interested in building magnetic motor, holding weight or something where magnetic force is important, then this doesn't apply.
    Bigger magnets have much less force per volume/surface than bunch of small ones.
    Here is calculation for square magnets grade N35. Each is 10mm thick, with magnetization thru thickness.
    Price and all data are from company that sells them in my country, so price can be variable to other places, but point here is to show difference in strength.
    Pull force is calculated using The Original K&J Magnet Calculator.
    height (mm)width (mm)max strength (gauss)pricepull force (lb)surface (mm2)pull force/surfacepieces for 3600 mm2 total force (lb)diff total forcetotal pricediff total price
    10543000,56 €4,9500,09872352,8314,04%40,55 €8,30%
    401040004,34 €27,674000,0691759249,03192,25%39,03 €4,25%
    501043005,33 €31,95000,06387,2229,68169,55%38,40 €2,55%
    601042006,36 €35,796000,059656214,74152,01%38,13 €1,85%
    402038008,43 €42,18000,0526254,5189,45122,33%37,92 €1,29%
    5025400012,64 €52,6112500,0420882,88151,516877,82%36,41 €-2,76%
    4040400015,34 €59,5416000,03721252,25133,96557,22%34,50 €-7,84%
    6030390018,51 €63,2718000,035152126,5448,50%37,03 €-1,11%
    10020390022,80 €67,7720000,0338851,8121,98643,16%41,03 €9,59%
    5050360024,39 €73,9625000,0295841,44106,502424,99%35,12 €-6,21%
    6060360037,44 €85,2136000,0236694444185,210,00%37,44 €0,00%
    This data can be easily tested and some of my tests confirms that this is close to real thing.
    So what it shows is that single big magnet is several times weaker than bunch small ones for same volume and surface.
    Also you can see that price of single big magnet vs. smaller ones for same amount volume/surface is very close (10% difference).
    However if you compare forces, then you need 3 times less smaller magnets to replace big one, thus total price, volume and surface is 3 times smaller for same amount of force.
    Conclusion: smaller is better!

Further tests which are currently in progress, result will be posted upon completion:
  • Magnetic force between two magnets at different angels passing by each other on rails.
  • Suppression of magnetic field on one side by redirecting it using different materials aka "magnetic shield".
  • Forces between magnets in linear motor with "shielded" magnets.