DC Motor Magnet Lifespan

Discussion in 'General Electronics Chat' started by KCHARROIS, Apr 7, 2015.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Hello,

    I will be machining something similar to a DC motor but will only turn 180 degrees. I will have a coil in the centre and 2 magnets on the rotating piece. So my question is without considering wear and tear on everything but the magnets, how long until the magnets lose their magnetic strength or should it even be considered? And what about 2 magnets facing each other north - north or south - south?

    Thanks,
    KCHARROIS
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
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    Pics help out a lot ..
     
  3. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Shouldn't need a picture, just imagine a normal dc motor... What is the lifespan of a magnet in that motor, meaning until it has no magnetic strength?
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,061
    3,836
    I was just using some motors with perm magnets over 30 years old from an old HP pen plotter. No problem. Some degrade over time but that is because of oxidation I would guess (or demagnetized by something else). Most permanent magnets you buy today are made with pretty good technology to prevent oxidation or other issues - they will last 10+ years.
     
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
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    Most of the rare earth magnetic materials retain there magnetic field for many decades (or more), some of the earlier motors recommended that a iron keeper be inserted when removing the armature, but I have not had that problem.
    Max.
     
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  6. MrChips

    Moderator

    Oct 2, 2009
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    Permanent magnets are, well, permanent.

    If you subject a magnet to high temperatures or physical shock the magnetic domains can randomly realign themselves and thus reduce the overall strength of the magnet.
     
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  7. Glenn Holland

    Member

    Dec 26, 2014
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    The life span of magnets depends on the material and also the conditions in which they operate such as a motor, lifting magnet, or a magnetic holding table such as those used on a surface grinder.

    Alnico can experience accelerated decay (demagnetization) if the reluctance is cycled (by repeatedly increasing and decreasing the air gap) or subjected to an opposing field. Ceramics are better, but they can be demagnetized if misused by cycling the air gap. This commonly occurs when people play with magnets by sticking and removing ferrous objects. "Rare Earth" magnets are extremely rugged and they are used in applications with very high opposing fields such as motors.

    In all cases, if a magnet is to be used for a critical application that requires constant flux, keep it away from ferrous objects until it is ready for installation in the final product.
     
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  8. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Great! Thanks for the replies. I guess its safe to say if i remain at normal room temp and have no corrosion they should last over ten years. So the second part now is what if I have a 2 magnets positioned facing each with the same poll like north and north or south and south. Will this degrade there magnetic strength quickly? Or is it likely that one magnet's poll's will change?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
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    This is the condition in most DC motors that poles alternate around the stator. So you end up with poles opposing on either side.
    The only motors I have heard of that seem to suffer when dismantled for some reason is Stepper motors.
    Max.
     
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