Neodymium magnet Vs electro magnet strength.

Discussion in 'General Electronics Chat' started by tcmtech, Apr 4, 2014.

  1. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    This should be an easy one for someone here.

    I can't recall or find the formula for calculating the pulling/breakaway strength of an electromagnet based on its core size and number of turns Vs its input voltage and current.

    Say I have a soft iron core electromagnet that has a core cross section of 4 inches in diameter by one inch thick with a hollow 1 inch center hole and and electrical input of 36 watts.

    Any ideas what the theoretical gauss value would be for something like that and how would it compare to a neodymium of similar 4" x 1" x 1" size with a 13,500 gauss ~N42 - N48 value?

    Curious as to which would be stronger and by how much?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Watts don't mean much in terms of input, you need current to make a magnet calculation so you need resistance values too. The less resistance the better.

    Check the Wikipedia page on electromagnets. Ther you will find all of the equations and a photo of a 45T electromagnet - just a bit bigger than you asked for.
     
    Last edited: Apr 4, 2014
  3. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    Once I got my terms streight and found the correct online calculator I got my answer.

    My 12 volt 3 amp 300 turn electromagnet of similar dimensions to the neodymium magnet has about 3 - 4 times the Gauss value.
     
  4. Duane P Wetick

    Active Member

    Apr 23, 2009
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    If you can calculate the number of turns of wire times the amperes; AxT, this will give you a starting point for the the electromagnet's strength. For the neodynium-iron magnet, the lines/ square inch figure (gauss) is an area strength
    figure. Many factors determine the relative lifting strength of either magnet; geometry, surface contact, pole spacing etc.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Interesting. I haven't checked any of the numbers but that result seems reasonable. I would expect an electromagnet to overcome a permanent one. I suppose this helps explain why permanent magnet alternators are not commonplace.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Also for pure "lifting strength" an electro magnet benefits from using a pot type core. Like in a scrapyard crane or a surface grinder magnetic chuck. They allow both poles of the electro magnet to be used to do the work.
     
  7. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    Actually that is exactly what this relates to. Converting a normal wound rotor type heavy duty alternator over to a neodymium based PM one. I just was curious as to how much of a field strength loss I could expect using a large ring type neodymium that had the same basic dimensions as the original rotor coil.

    A buddy of mine and I did a conversion like this years ago using regular ceramic speaker magnets stacked together along with a nonmagnetic stainless steel shaft in order to get the most flux density as possible into the stator.

    At best the output was about 10% of what it had at the same RPM ranges as in its wound rotor based setup.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Not really. The main reason an electromagnet is used in an alternator is so that the output voltage can be controlled to the desired value by varying the current through the rotor and thus its magnetic field strength. That's not readily done with a permanent magnet rotor.
     
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  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As previously mentioned, With a varying rpm alternator as used in automotive use, you require a wound field to be able to control the output as required by load and rpm.
    In this case you are not concerned about the output frequency as it is converted to DC.
    This is the advantage to the alternator as opposed to the original DC generators used, is that high output can be maintained down to low RPM, (idle).
    Max.
     
  10. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    This is for AE wind turbine service where I need power output at all RPMs especially lower ones which a would rotor alternator can not do..

    Voltage regulation would be done by load dump or similar additional loading methods that I presently used with the units I have built that used large industrial PM type servo units as the generators.

    I just needed to find out how much of a possible field strength difference I could have between using a large neodymium Vs the normal stock would rotor at a specific input power level.

    At $200 a pop for the magnets I didn't want to burn up that kind of cash right now to find out I was way off on my expectations.;)

    BTW the alternator I was looking at converting is one of my Leece Neville 24 volt 120 amp units. In stock form it has a triple delta stator winding configuration to which when modified to a single series wye set up can put out up to 200 VDC at 10 or so amps continuous duty. :)

    I just wanted to know the theoretical loss I may see if I went to a PM rotor setup.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Usually for PM alternators they make a new one with the magnets inserted around the outer circumference of the rotor. Kind of like the way you did for the squirrel cage induction motor/gen mod you showed on ETO one time. They make the new rotor from aluminum and mount the magnets with the polarity alternating, + - + -....
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Some automotive pundent recently claimed every vehicle will have 1.5 kg of NdFeB magnets by 2020. I couldn't get GM to think about brushless DC motors 7 years ago, now they can't get enough of them.
     
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