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

[dustin wassner]

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

[Darcy Warner]

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.

[Mike Henderson]

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

[Dan Friedrichs]

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.

[dustin wassner]

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

[Mike Heidrick]

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

[dustin wassner]

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

[Mike Henderson]

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

[Bill Dufour]

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

[dustin wassner]

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.

[Charles Lent]

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

[dustin wassner]

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

[Charles Lent]

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

[dustin wassner]

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.

[Charles Lent]

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

[dustin wassner]

OK so after taking it apart again I measured the od of the new bearings and old ones with a micrometer. They are the same size. I had no noise/play with the old bearings. I also set the old bearings in the journals and checked play and tried to slide a .0015 feeler gauge between the bearing o.d and journal. As far as I can tell, the journal has zero wear. When I lay the endbell flat and set the new bearing in it (the rotor and shaft are pointing straight up), I can feel the play I was feeling before. So, I think it might actually be coming from within the bearing itself. Not sure if this is normal/OK but like I said, I bought cheap sealed bearings from ebay. They were $13 for both. There is actually grease leaking around one of the seals already. I am now wishing I had just left the shielded Japan bearings in it that did not give me issues...

[Charles Lent]

If it was mine, I would clean and de-grease the bearing hole and the outside of the outer race of the bearing and then put some blue Loctite on the outer race of the bearing, then re-assemble the motor. The blue Loctite will keep the outer race of the bearing from spinning until the bearing goes bad. In use as a rotary phase converter, the motor is just coasting most of the time. There is a brief load on the windings whenever you are starting up another 3 phase machine, but then this motor is just coasting again. I doubt that you will ever need to take this motor apart again when using it this way in your home shop. It's good enough for your needs. Get on with your life.


Charley

[Jerry Bruette]

According to the data plate in your other post you need a 6206 and a 6205 bearing. You can buy them with shields or seals, won't make a difference if your motor is totally enclosed, data plate says uniclosed but I don't know what that is. I would get the bearings with seals myself and you take out any doubt about the frame.

I would go to a bearing supplier...maybe Motion Ind. and buy SKF bearings, ignore the country of manufacture they're good bearings, and they shouldn't cost a whole bunch. The SKF numbers would be 6206 2RS C3 and 6205 2RS C3. 2RS means a rubber seal on each side and C3 is the internal clearance of the bearing.

If you think there is excessive clearance in the end bells use GREEN sleeve retainer to hold the bearing in the end bell. Green is designed for this application, Blue, Red, and Purple Loctite is designed as a thread locker not a sleeve retainer. To be exact I would use Loctite 641, it will fill a .008 diametral gap and can be disassembled without heating should you need to replace bearings in the future.

I also wouldn't worry about greasing the bearings in the future, they'll last a good long time especially if there's no radial load on them. Just be sure to pay attention to the way they sound while the motor is running and change them out right away when you hear them start to fail.

We have 20 HP motors at work on a shot blaster that run sixteen hours a day with about 50 pounds of weight on the shaft at 3600 RPM and they haven't been greased in three years and they are running strong yet.