small DC motor lost some magnetism
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  1. #1
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    Default small DC motor lost some magnetism

    I have a pattern cutter (oxyacetylene). The torch moves as a speed-controlled DC motor turns. In the end of its rotor is snapped a knurled steel lug. The DC motor is permanent magnet type. Some of the magnetism goes into the lug, enough to make it "stick" to the side of a template.


    This thing was dropped on the ground in farm country in a shed that later was used to house chickens. Needless to say, it became befouled with chicken poop. The motor bearings when I got it were siezed. By working it with Kroil I was able to get it operative again. But the bearings were totally crunchy. So I took it apart and replaced the bearings. Now it's quiet but when I removed the rotor, it lost about half of its magnetism. So I want to remagnetize the motor.


    Problem is I don't know how.


    I'm considering winding a couple pounds of light gauge magnet wire onto a hollow spool big enough so I can put the motor (about the size of a soup can) fully inside the spool, then touch the ends of the wire to a car battery.

    Is there a better way to make a one-off remagnetizing jig?

    metalmagpie

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    See about a "magneto remagnetizer"...someone should have one semi locally, and/or that motor is small
    enough to flat rate it out/back.

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    The change in magnet flux has ruined the motor. Unless you can have engineered specs achieved in re magnification the torque/speed will surely be wrong.

    That's a lot of screwing around time which figures into money

    So buy a new replacement motor and be ahead

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    That will magnetize it in the wrong direction. You need a fixture with poles in the right places. Don't try to do it yourself because now it probably has remaining poles, just weak. If you try to magnetize it, all you will accomplish is to destroy the evidence. After you magnetize, you must have a keeper that completes the magnetic path while you transfer the rotor to the motor. You need to slide the rotor from the keeper to the housing while keeping a magnetic path. Breaking it, even for an instant will let it slip back to the weakened state.

    For decades I repaired GE tachometer generators, which are DC generators, the same as DC motors used in the other direction and zapped a few magneto magnets along the way. Your only realistic possibility is to find someone equipped to do it.

    Bill

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    I bought this Mendell Magnetizer to magnetize tools: Mendell Magnetizer. Anybody used one? I don't have any manuals or description of how it should be used for magneto magnets re-magnetizing (the task it was originally designed for, apparently).

    I'd be happy to do it for you if somebody explains to me how to do it properly.

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    you could pull the alnico magnets out and replace them with neodymium magnets of half the thickness and space them out to get the required field strength.

    magnets such as these
    N45 2 in x 1/4 in x 1/8 in .

    can be used for a motor as small as a 1" diameter rotor (12 magnets would fit around a 1" diameter, and this approximates an arc pretty well.)

    neos are so much stronger than ceramic or alnico, you will need to experiment with shims and changing the number of magnets to get the flux density right.


    or you need to turn a diameter the same as the rotor.. cut slots in it and wind a lot of wire on it.. magnetize the magnets.. and as you slide it out pull the new rotor in behind it.

    this could be problematic.. you might need some kind of an intermediary keeper because the shaft and commutator of the rotor may be as long as the magnet is, so you need something to fill up that space completely as the shaft and commutator are directly where the coil for the magnetizing solenoid is..

    i would just replace the magnets with neos.


    another option would be use the motor's rotor to remangetize the motor, but i'm not sure if that actually works. the rotor is mostly air and you may not be able to get the required field strength due to limitation of how much current you can push through the brushes. you could solder a wire on the end of the commutator (where the brush doesn't run) to get the required amps needed.

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    I sort of thought I could remagnetize it with it fully assembled. No plans to remove the rotor ever again.

    metalmagpie

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    Quote Originally Posted by metalmagpie View Post
    I sort of thought I could remagnetize it with it fully assembled. No plans to remove the rotor ever again.

    metalmagpie
    i edited my post after you replied.

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    Quote Originally Posted by metalmagpie View Post
    I have a pattern cutter (oxyacetylene). The torch moves as a speed-controlled DC motor turns. In the end of its rotor is snapped a knurled steel lug. The DC motor is permanent magnet type. Some of the magnetism goes into the lug, enough to make it "stick" to the side of a template.


    This thing was dropped on the ground in farm country in a shed that later was used to house chickens. Needless to say, it became befouled with chicken poop. The motor bearings when I got it were siezed. By working it with Kroil I was able to get it operative again. But the bearings were totally crunchy. So I took it apart and replaced the bearings. Now it's quiet but when I removed the rotor, it lost about half of its magnetism. So I want to remagnetize the motor.


    Problem is I don't know how.


    I'm considering winding a couple pounds of light gauge magnet wire onto a hollow spool big enough so I can put the motor (about the size of a soup can) fully inside the spool, then touch the ends of the wire to a car battery.

    Is there a better way to make a one-off remagnetizing jig?

    metalmagpie
    Are you sure the knurled head that rides on the pattern you are cutting is not magnetic, on a lot of swing arm pattern cutters the tracing wheel (knurled head part)is magnetic its self. Some times there are different diameters as well.

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    Quote Originally Posted by dana gear View Post
    Are you sure the knurled head that rides on the pattern you are cutting is not magnetic, on a lot of swing arm pattern cutters the tracing wheel (knurled head part)is magnetic its self. Some times there are different diameters as well.
    I am dead positive. I have several tracing lugs of different diameters. The ingenious part of the motor design is the way the magnetic field is carried out of the end of the rotor through the tracing wheel to let it move steadily along the edge of a template. You could work around one of these machines for years and never realize that the tracing lug wasn't magnetic.

    My swingarm tracer is called an Ultra-Graph. ESAB still sells these machines. I might buy a new motor if I could ever find a real ESAB parts guy.

    metalmagpie

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    I hate to post a link to a YT video but this one shows exactly the operation that I need done: YouTube

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    This is sort of OT, but still in the re-magnetization arena. I have a Brown & Sharpe 599-7742-7 Heavy Duty Dog-House Permanent Magnetic Base Holder that has maybe 3 lb attraction when turned on. I assume the rotor needs to be re-magnetized just like metalmagpie's motor. It would be really nice to get it working as new.

    Is anyone aware of a reference, either online or print, that discusses this technology in more detail?

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    Quote Originally Posted by metalmagpie View Post
    I hate to post a link to a YT video but this one shows exactly the operation that I need done: YouTube
    Unless the motor is a configuration I have not seen, the video is nonsense. That isn't the way to remagnetize it. Besides, it would magnetize the bearings, too.

    Wheels17, to remagnetize your base, put in the "on" position, check the polarity of the two poles with a magnetic compass, and place it on a magnetizer with the north pole on the base to the south pole of a magnetizer such as the Mendell mentioned earlier. Zap it and you are through.

    Johansen, You probably could remagnetize the motor by putting a pulse through the armature, but it would be risky. A large very short pulse might work. You couldn't use the brushes in their normal position because they are oriented to make the armature pull toward the field pole. You could rotate the brush holder to a position where the armature was aligned with the field or solder wires on two brushes and apply a little voltage which would pull the armature into alignment.

    Once again, I do this for a living. The way to do it is to make a core that fits the field with a coil around it and hit it with a big pulse. Then you use the armature to push the slug out. I will try to post a picture later.

    Bill

    P. S. For a while I was getting GE 5PY59 tach generators constantly. They invariably had been run to death, the bearings were dust. It turned out they came from a large pornography printer, used to read the speed of the press. Business must have been good.

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    Obviously I am no expert, but my reading has led me to believe that many small motors are first assembled and then magnetized. Also, the remagnetizing force can be too small but not too large.

    Perhaps the bearings in the motor on the video do get magnetized. It isn't clear to me that being magnetized would impair the performance of a ball bearing anyway.

    It looks to me like the company who made that video does that for a living too.

    metalmagpie

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    Is the original motor an actual alnico magnet? That would explain it losing strength. I have pulled ceramic magnet motors apart and had no significant change in strength. There is an air gap, and the rotors do not carry nearly as much field thrugh them as solid "iron" would, due to the space for windings. So the difference is not so much, and ceramic is less fussy than alnico.

    And, the betting is on the thing indeed being magnetized assembled. Much less hassle that way.

    I don;t know any factory manager who wants magnets stacked up all charged and in the open, even in motor housings already. Too much chance of contamination or problems. Better if done after assembly so nobody has to mess with them.

    We had a big TECO-Westinghouse generator that we had to put together because it was shipped unassembled (it was a "no-bearing" generator, the bearings were in the frame it went into). An 18" diameter rotor full of Neo magnets around the OD. We had to use a hoist to let it into the housing, because it had a couple of tons of pull (or so) when partly inserted. Even at a smaller level, too much chance of damage during assembly trying to get the keeper out and the rotor in. Damage to the device, or to the workers. That rotor was plain scary.

    Speakers, for instance are magnetized after assembly. They are set (in one factory I visited) on a little platform, and some huge thyristors fire and let a really big pulse of current flow through coils in the platform. The whole 40 lb speaker will jump, and after that it is magnetized.

    The steel structure is not a problem, the field is so large during the pulse that the steel is saturated, and drops out of the problem. The magnetizing coils have no core as far as I know.

    2 pole motors can be done that way. 4 pole are more complex, but just as do-able, with the right setup.

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    I remember these profile cutters when they were being used......the magnet hold wasnt that great,and only reliable in straight lines,other wise you had to hold the unit against the template,and especially on corners ..In any case you had to watch every second ,and help the magnet nearly all the time......Magic eye units killed em overnight.

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    Quote Originally Posted by john.k View Post
    I remember these profile cutters when they were being used......the magnet hold wasnt that great,and only reliable in straight lines,other wise you had to hold the unit against the template,and especially on corners ..In any case you had to watch every second ,and help the magnet nearly all the time......Magic eye units killed em overnight.
    Mine held onto templates just fine - until I took it apart. And the Ultra-Graph is still in the ESAB product line. This particular machine is about the best of its type. It can turn on your torch, light it with a spark, and start the oxygen and the motor at the same time.

    If I can't find a way to remagnetize my motor, I plan to try putting a hefty neodymium magnet on top of the knurled lug. The speed dials won't be right, but I never trust those anyway.

    metalmagpie

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    I found a cheap mag/demag unit. So when it shows up I'll get some more data points.

    metalmagpie

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    Quote Originally Posted by MichaelP View Post
    I bought this Mendell Magnetizer to magnetize tools: Mendell Magnetizer. Anybody used one? I don't have any manuals or description of how it should be used for magneto magnets re-magnetizing (the task it was originally designed for, apparently).

    I'd be happy to do it for you if somebody explains to me how to do it properly.
    I think the use model for a magnetizer is simple. Put item into area of max flux density. Energize. Done.

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    Quote Originally Posted by metalmagpie View Post
    I think the use model for a magnetizer is simple. Put item into area of max flux density. Energize. Done.


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