View Full Version : Motor repair

06-05-2006, 06:12 AM
Hi Guys,
I wonder if any of you have come across and repaired this problem before. I was building a RPC so I purchased a 4kw motor at auction it was chinese and I thought the fan cover was bent onto the fan blades easy fix, I got it for $20. Wrong see pictures. Someone had dropped it onto its shaft breaking the casting on the fan end. I welded the motor fan cap end up OK, well cocky **** welding we call it in Australia, looks terrible but sticks:) The next problem was, I found the motor ran OK but was getting hot. The welding had distaughted the end cap of the motor. I took it to pieces yet another time and tried to true it as best i could on the lathe. It is running a lot better but still getting hot 40-50 deg c, How much clearence does the motor have to have between the motor rotor and the stator? It has worn a small groove on one side of the stator, but appart from the heat build up it doesn't appear to bother it? Can I machine a little off the top of the motor stator where the rotor has made the groove in the stator to hopefully, stop the friction heat?
Thanks in advance.

Alan in Oz
06-05-2006, 07:20 AM
Fairly good smash job there and good bush repair. Have you tried rotating the end housings to the other bolt holes just on the off chance that they are enough out of whack to move the rotor a bit and stop the rubbing.

John Stevenson
06-05-2006, 08:24 AM
Not much time right now but I'll reply later toning [ UK time ]
I do these sort of repairs for a living and it's not that hard a job.

A.K. Boomer
06-05-2006, 09:08 AM
Im no expert on this but im suspect of chineese motors in the first place,
#1, after veiwing your photo's to me its highly unlikely that all your heat is due to the slight contact thats going on, maybe at first but clearance will be rapidly gained between the two surfaces and things should get progressivly better in a short while, so if you let it run then let it cool and run it again you should see no where near the heat as the first time, that motor is allot of mass to heat up and i think its posible you have bigger "issues"... to create the heat needed to cook a motor of that size that little area would have to be purple all around its parimeters and i really dont see that...

2, as fairly neat as most of those field windings look dont be fooled, allot of them can be hand wound, this can be a problem if somebody gets distracted while winding (say they have some rice in the kettle boing over) and skips a loop or two hear and there, This eqautes to the motor fighting itself and can be a HUGE heat problem, sometimes you can get an idea by ohms testing the individual fields if you can segregate them but every motor is different and I dont know the layout...

3, you bought this thing at an auction and its very posible that it gave the previous owner hell (possibly because of #2??) , the break in the case was not by accident but was actually a method allot of people use to stop an item from being used but not throw it away because of the scrap value (esp. with the price of copper nowadays)

last but not least if its an induction motor then it most prob. has a start capacitor and a run capacitor, a faulty run capacitor could really mess things up and would need to be checked into....

Also keep in mind that im a hack when it comes to electrical but thought id give this a run:p good luck

A.K. Boomer
06-05-2006, 09:26 AM
Oooops --- clearance question, you can take a little off for clearance but keep in mind now your running the equilelent of a mild "electrical eccentric" in wich your ever so slightly jacking with the critical magnetic wave timeing and strength,,, I would look into oversize boring the end cap and retrueing it all in before i went the other route,,,,,,, you can bore the end cap and you really dont even need to be right on with your diameter, bore it to what you think is where you want to be (no more than .010" over) and then install the perfect size feeler gage all around the bore so the bearing fits in snug, assemble with some locktight stud and bearing mount, have fun getting it back apart! cheers

John Stevenson
06-05-2006, 01:21 PM
Your problem is that the welded bearing housing isn't central / square to the rotor axis.
Not a lot you can do about that as welding distorts anyway and you have no control over it.

What you have to do now is correct the bearing housing from a given point.
The only given point on that housing is the flange diameter in pic 4080011 that mates up with the housing.
You need to grip this in the outside jaws on a lathe large enough for this housing, a length of all thread and a couple of nuts and big washer thru the spindle are also advisable to stop it whanging down the shop [ don't ask !! ]

Now go to the first pic, 4080001 and you will see three cast pads between the three lugs.
You need to take a light pass on the ribbed face and a couple of light passes along these pads to for a small step.
These will now be running true with the register.

Spin round in the chuck, grip on the pads and re-insert the all thread with a washer right at the bottom of the bearing housing but not large enough to foul a boring tool.

Next job is to bore the housing out about 100 thou a side [ plus 0.200" ] to accept a sleeve.
In fact it's best to make this sleeve first if you only have one lathe.
Allow a thou to thou and a half interference fit and a dab of loctite and press in.
Now you need to bore out to fit the bearing, bore about 1/2 thou under and polish to fit with emery cloth.

This will cure your catching and over heating problem.


06-05-2006, 03:48 PM
{Spin round in the chuck, grip on the pads and re-insert the all thread with a washer right at the bottom of the bearing housing but not large enough to foul a boring tool.} WHAT?

06-05-2006, 04:32 PM
Perzactly what John said. Mount with outside facing out first, make the cut on the pads. Spin round ( face the inside of the end bell out) so you can bore the bearing recess.

John Stevenson
06-05-2006, 04:35 PM
Sorry what I should have said was:-
"Spin round in the chuck, grip on the pads and re-insert the all thread with a washer right at the bottom of the bearing housing but not large enough to foul a boring tool." :D

You need to have a washer at the bottom of the bearing housing hole to keep it back against the chuck jaws but you don't need a big washer that's the full diameter or else the boring tool will foul it when you bore out for the sleeve.


06-06-2006, 03:53 AM
Thanks very much everyone, some very good ideas there. I have to go bush to help a mate, but look forward to getting back and I'm sure with the advise you have given me, get the motor running cool. Its going on rotary inverter so won't have much load on it anyway.
All the best Gunna

06-06-2006, 05:18 PM

I've seen an induction motor overheat after the rotor rubbed the stator because the stator laminations are burred over so there is metal to metal contact allowing eddy currents to flow. The laminations are made with a thin layer of enamel, paint, lacquer etc. to prevent metal to metal contact between them and so to hold eddy currents in the steel down. The one case where I met up with this was a much larger motor (800hp or so) so it was possible to get inside it and clean it up. If this interests you I can write up a procedure for testing and cleanup but I don't know how you will get space to work inside the motor.


A.K. Boomer
06-06-2006, 06:06 PM
Good info, cant imagine 800hp's and what kind of volts and amps it takes, crazy power...

John Stevenson
06-06-2006, 06:31 PM
If you look at the third picture that Gunna has posted you can see where the rotor has rubbed the stator.
From that picture the 'damage' looks to be no more than a rub mark and once the rotor is central again then it shouldn't have any bearing on the matter.

Frighten thing now is the cost of doing these repairs. At one time it didn't pay to repair motors under 5 HP against new.
Talking to one of the rewinders last week and he had just bought a brand new 160Kw, about 200 HP motor for £1600 UKP.
To repair one end case and a rewind with new bearings would have cost nearly £3,000 UKP.

A.K. Boomer
06-07-2006, 11:15 AM
From the pictures its to hard to tell, maybe gunna already checked into this but Ken has a valid point, there is a reason why they segregate the individual fields and its really not for reasons of electricity but more so the magnetic flux transferance that occures when there is more contact in the pieces in wich they were disigned for (i.e. ferreous material bridging the gap between two larger peices of ferreous material)

think of it like an I.C. engine with fuqed up ignition timing.... Cheers

06-07-2006, 11:48 PM
I built a motor once pretty much from scratch, but using existing laminations. I glued the stack of lams, then turned it to clean up the outer dimension. Doing that pretty much shorted out all the lams at the surface. The motor runs, but it gets hot. It also stops very quickly once power is removed, and not because of friction anywhere. Now it sits on a shelf as a reminder of what not to do to a stack of laminations.
Maybe one day I'll play with it some more.

John Stevenson
06-08-2006, 03:40 AM
Whist I can accept what AKB says that serious rubbing can roll over the laminations in this case it is clear that it's not the case.
The picture is quite clear in that it shows about 7 poles with rub marks.
To go deeper and roll these over with a rotor that size would show on more than 7 poles.
You can even see the insulation strip between the poles.

Usually damage of this kind is caused when a bearing gives way whist running and kept running. In Gunna's case he's got an end shield that slightly out and rubbing, but he's not kept running long enought to cause any damage.

I often get motors in where they have let go and damaged the laminations, usually a bit of dressing cures these problems but it takes a lot to get a stator this far.

Due to the low cost of motors most of my repairs are on specialised motors, inbuilt's, DC's and special cases.
I often have to do exactly the same repair as Gunna but I know straight away that the housing is going to be out of line so I sleeve the housing without even checking.
Often I don't have the other motor parts anyway as they are rewinding the stator.

It's not an uncommon job and is even catered for by specialised companies. There is a company in Canada called Shaver-Kudell that make bearing sleeves, shaft sleeves, new fans and other parts for motor repairers.