Electric motor armature/rotor balancing

[apprentice87]

Hi all!!

Anyone know anything about balancing of electric motor rotors??

I have a compressor i am rebuilding. It has a 16HP three phase motor. I separated the motor from the compressor and ran it up, the bearings were noisey and one end let out a horrendous shrieking sound...Not nice...

Anyway, i rebuilt the thing with new bearings, good quality ones, of the proper "grade" too [C3]

Guess what....Problem was still there but not quite as bad..

Now, it is definitely not a "electrical" problem, as, if the motor is running, even once you shut the power off sometimes as it is running down, it will do the shriek thing..

It vibrates quite a lot when it is running anyway, but when you shut it off and let it run down, if you place your hand on it, it will get to certain revs where the vibration will get much worse before dying away again. It is definitely a balance problem...

I cannot see that the shaft is bent as it is about 2" diameter, BUT, there is a large fan mounted on one end, AND the fan is slightly distorted where some muppet has stripped the motor before, AND i think it is the fan end which does the vibrating....

Do i try and balance the thing myself, or are there proper firms that do this sort of thing?? Assuming there are, do they press the fan off and balance the rotor and then the fan, of do they do the lot in one??

Any idea who does this sort of thing [i am in south wales] but do not mind travelling, and any idea what it costs??

Thanks all!!

john..

 

[Sidewinder]

john,

Yes this can be done, and there are peeps around that can do it in SW too.

You need to be sure that it is the motor rotor first though to save you

 

[apprentice87]

Hi Paul, Yes, i have done this! the motor is quite separate from the compressor, in fact it is sitting on the floor here!!!

I cannot take the back end off and run it if i am understanding you correctly, as it is the back end bell that holds the bearing up.

Off power and running down it still vibrates, and as i say goes thorough a "bad patch" at about half speed. [bit like your car wheels might go through a bad patch at 40, but by 60 all is smooth!!]

I suppose i could stick it in the lathe and DTI it, but it is rather large so i am not quite sure if you could bend it if you wanted to, the shafts are at least 2 inch diameter! Here is a photo of the motor!!

View attachment 2434

and here is the pump it powers..

View attachment 2435

john..

 

[Zeespark]

Canoeboy, the address for that company doesnt actually exist. They say Empsom Street, but its actually Empson Street

AndyGuinness

 

[Zeespark]

LOL I am just saying mate, its doesnt bode well for them if its not sorted.

AndyGuinness

 

[apprentice87]

Hi Canoe,

Yes, i have seen the balancing weights, i think it is the fan rather than the rotor though, as it [the fan] has been bent a little in the past and runs out of true.

here is a picture of the rotor as i was stripping the motor;

View attachment 2436

and here is one as it was going back together, just before i discovered it still vibrated, [Not a happy bunny...!!]

View attachment 2437

There is no way i could afford a new motor, they would be thousands i expect...

Anyone any idea what the proper balancing people charge??

john..

 

[apprentice87]

Hi Canoe,

Yep, they certainly were big!!

1, The old ones were C3 too. I was thinking of getting a "tighter" grade, but as it is a motor, and "warms up" i thought that this was unwise..

2, They were good bearings, ball races they are, SKF ones i think.

3, I fitted them with a press, but i made sure that i only pressed the centre ring so as not to damage them.

4, The bearings are a lovely fit in the housings..

I think it is the fan that is distorted, it is certainly that end of that motor that makes the nasty noises.. I think it might help to have "tighter" bearings, as i think it is some sort ot "resonant" effect causing the shaft to bounce around in the bearings, as, when it decides to make the shrieking sound, it will often stop if you kick it!!!!!

john....

 

[apprentice87]

Hi Canoe,

Attachments open ok at my end!!! you must have a popup blocker or something..

No idea as to motor frame size, the motor was made in about 1976 and has a big long part number that means absolutely nothing to me!!

It is 12Kw and 1450 RPM

john...

 

[apprentice87]

Hi again Canoe,

It was never put back into service, put it back together, ran it un coupled from the compressor for about an hour and discovered it was still the same, and it has sat untouched for about a year now.

It is an old motor made by someone called "newman Industries" wish i could get a fan!!

The rotor itself it balanced by riveted on washers, there are some on the fan too, but i am sure some must have fell off or something. It is difficult to tell with large bearings full of grease, but i am sure that it likes stopping in roughly the same place a lot so i think the balancing might be miles out....

It does not make a lot of difference if i run it with the key in or not, but i did think half a key might be the way to go...

By the way, when it is on the compressor, it does not have any belts to pull the thing and help the rotor not to bounce about, as it is a direct drive thing.

I have never actually run it "in one lump" as it were, someone had disconnected all the electrics from the rather [very!!] complicated "dual control" star/delta starter, so i removed the motor just to make sure i had the wiring right when i sorted the starter, as i was a bit worried that it might go the one way in star and then try to go the other when it switched over to delta or something else silly, so even though i checked and double checked, i thought it safer to remove the motor. This is when i discovered the bearing problem....

john...

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Hi Canoe,

I stripped the motor when i discovered the terrible noises the bearings made...

I will try to upload some photos to my forum album, give me two minutes....

john....