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Thread: took apart 3 phase motor. Rotor and Stator are worn. Wondering if its OK?

  1. #1
    Join Date
    Feb 2017
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    Rochester, NY
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    50

    took apart 3 phase motor. Rotor and Stator are worn. Wondering if its OK?

    I just bought this 7.5hp motor to use as an idler for the rpc I am building. It spun smoothly and each pair of windings showed open when checked against each other pair. There was a very light tick I could hear when spinning that I thought was a bearing. Took it apart and was kinda shocked at how caked the compacted sawdust was on the windings around the outside by the venting. The pic is after a considerable amount of blowing with compressed air.

    The surfaces of the rotor and stator didn't have sawdust but there are wear spots. I though it might be from compacted sawdust clumps getting in between and causing wear. The stator pics are the same wear section a different distance. One rotor pic shows the wear and the other close up pic shows the depth of the wear from the side.

    I am still waiting for the control panel I ordered to arrive, so I can't hook it up yet, and did not see it run. Paid $80 for it. If the wear will cause any problems then I am included to bring it back to the guy. I would just hook it up and see what happens but I have no idea if this much wear is really bad or a normal thing.

    DW
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    Last edited by dustin wassner; 07-17-2017 at 11:59 AM.

  2. #2
    I clean with mineral spirits or kerosene. You can buy rattle can winding varnish. I just take them to my motor shop and let them do it, only about 70 bucks for them to clean, varnish, bake and test stator.

  3. #3
    It's probably okay. Usually when you see that kind of marking it's because of a bearing problem. The rotor is usually a pretty tight fit to the stator so if the bearings are worn the rotor can contact the stator.

    Bad wear between the rotor and stator can be a problem, however. It'd be nice to see the motor run and run for a while under load. But $80 for a 7.4HP motor is a pretty cheap. I'd risk the $80 and use it after cleaning and installing new bearings.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  4. #4
    Agree with Mike - it's probably fine. I wouldn't bother putting varnish on it, either - it really needs to be between the windings, which you won't get. Clean it up with some mineral spirits, put new bearings on it, and give it a try.

  5. #5
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    Feb 2017
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    OK. thanks for the input. I just ordered bearings. I also noticed when it was disassembled that the rear bearing was replaced. Perhaps the one that got replaced was what caused the wear. I ordered cheap bearings off of ebay since I still don't even know if the motor will work. When bearings in these go bad, do they usually give a signal (screeching/ excess noise) or is it sudden? I ask because I would like to know what to keep an eye out for when using these cheaper bearings.

    A local motor shop told me the end bell journals are probably worn. I can't get a set of calipers in very well to measure, but the shaft has zero play. One journal has slight rub marks in it but it doesn't look like material is worn away...

    DW
    Last edited by dustin wassner; 07-19-2017 at 9:09 AM.

  6. #6
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    Bloomington, IL
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    What tools types will you potentially want to start? Might be a sign to just move to a 10hp if you plan to start any hard loads at 5hp.

    Mikie
    Glad its my shop I am responsible for - I only have to make me happy.

  7. #7
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    Feb 2017
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    5hp 18" bandsaw and 3hp 14" jointer/planer. I don't see a scenario where they will be started simultaneously

  8. #8
    Quote Originally Posted by dustin wassner View Post
    OK. thanks for the input. I just ordered bearings. I also noticed when it was disassembled that the rear bearing was replaced. Perhaps the one that got replaced was what caused the wear. I ordered cheap bearings off of ebay since I still don't even know if the motor will work. When bearings in these go bad, do they usually give a signal (screeching/ excess noise) or is it sudden? I ask because I would like to know what to keep an eye out for when using these cheaper bearings.

    A local motor shop told me the end bell journals are probably worn. I can't get a set of calipers in very well to measure, but the shaft has zero play. One journal has slight rub marks in it but it doesn't look like material is worn away...

    DW
    Sadly, I've found that some motor shops tend to exaggerate problems with induction motors. Maybe it's their way of getting more business. If you can't find a problem after they claimed some problem, I'd go with your measurements.

    Regarding the bearings, unless they are just junk, any decent bearing in a relatively clean, fairly light application (such as yours) should last many years. When bearings fail, they often make noise in operation and when you turn the shaft by hand you'll feel some "roughness" as you turn it. It just won't be smooth as you turn the shaft.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  9. #9
    For your use I would use a watch and time how long it takes the RPC to cost down to a stop. Wait a year or so so the seals are worn in. When the bearings start to wear they will not coast as long. At least I think that may tell you something.
    Bill

  10. #10
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    just fired up the motor and it is running great, but now I am experiencing another potential problem when starting the machines that it powers. Since it is unrelated, I will start another thread.

  11. #11
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    It definitely needs a good cleaning. The field and rotor marks are minimal and should not affect the motor. They are usually caused by worn bearings, so replace the bearings.

    I just answered your other post too. I would just clean up the motor, replace the bearings, and get used to the short buzz on startup of the tools.

    Charley

  12. #12
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    OK. so after putting the new, sealed, bearings in, it appears to be running fine, except I now notice that there is a very small amount of slop if I grip the shaft and push/pull up and down. It is definitely play between the rear bearing and rear journal. I am also, with a decent amount of force, able to slide the shaft back and forth by hand, and can feel the spring washer in the back pushing the shaft back in place. I went through a full cycle of pushing the shaft down and turning to see if I could feel the rotor rubbing the stator, and it feels like there is continuous clearance.

    Is this kind of play normal? I know the shaft will heat up and expand, I just wasn't expecting to be able to feel the play like I can

    thanks, DW

  13. #13
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    The magnetic field of the running motor should center the rotor while it's running, but sometimes they will move and rub when first starting or when slowing down. It doesn't matter, unless the noise bothers you. You can make, or buy, doughnut shaped shims of various thicknesses to put between the motor end cover and the bearing to reduce this movement, but you want to leave a little play to allow for metal expansion when the motor shaft heats up. An electric motor repair shop should have these shims in different thicknesses, but figure out how much gap you have, and in which direction with the rotor centered on the field, before installing them. Since you aren't actually using the motor shaft with a coupling to drive something when using it as a rotary phase converter, the in/out movement shouldn't affect anything, except maybe your peace of mind.

    Charley

  14. #14
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    the movement doesn't bother me, and neither does the noise, I am just failing to see how over several hundreds or thousands of start/stop cycles this will not eventually wear the journal. I have a .0015 feeler gauge I was going to try to wrap around the bottom third of the bearing od and see if it took out the slop and still allowed the shaft to move in and out.

  15. #15
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    Oh, your saying that the bearing hole in the end cover is too loose for the outer bearing race? I was thinking that you were referring to an in / out movement of the end shaft so I was providing information on fixing that.

    You could shim between the outer race diameter of the bearing and the bearing hole in the end cover, but be certain that you are centering the bearing in the hole and not forcing it to one side. A previous bearing probably locked up and spun the outer bearing race, wearing the bearing hole to a larger and usually out-of-round diameter. I have used blue Loctite in loose bearing holes of the end covers to keep the bearing race from spinning, if they were just loose, but still centered. You can get blue Loctite apart the next time that the motor needs repair, but not the orange or red. I have also shimmed between the inner race of the bearing and the shaft, but this was just a temporary repair until they could find a replacement motor. It could have been fixed by welding and then turning the weld down to fit the inner bearing race, but motors of less than about 10 hp usually aren't worth the cost of doing this unless the motor is a special frame design. The end cover bearing holes can also be fixed this way but the cost of doing it and then re-boring for the bearing to fit correctly is also high.

    If you aren't comfortable doing any of this, take the motor to an electric motor repair shop and let them repair it properly. When they give it back it will be better han new. My suggestions were based on industrial "fix it now" requirements rather than doing it the best possible way. The motor shops have a lot of experience rebuilding motors, especially large motors, and they do it right.

    Charley

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