Motor Magnetic Center 1

[algonklin]

Am aligning a 2000 HP 200 RPM motor driving one of our small ball mills. What are the tolerances for setting mag center and will a tighter tolerance impact reliability or extend motor life? How are level tolerances?

 

[rasevskii]

algonklin-

A problem might be with the bearings. The motor will try to move into its magnetic center while energized, which will pull the bearings axially. That may destroy the cage on a roller bearing if that is what you have, or thrust a sleeve bearing too hard against a collar, both resulting in destruction. Had that once on a 1 MW motor...

regards, rasevskii

 

[algonklin]

The motor has only sleeve bearings and the OEM manual says that mechanical center is adequate, but am wondering if aligning within a few thou of mag center would improve reliablity or extend the motor life.

 

[rasevskii]

How do you know where the mag. center is? Possibly there is a ring mark on the shaft near one bearing that should be aligned with a pointer. That would be the mag. center. The bearings clearances axially should be symmetrical to that point. Can't really explain it better from here. A bit more axial slop, as it were, is probably much better than too tight. Also the driven machine may shift it axially.

Run it and inspect the bearings immediately for damage. A few minutes only the first time. Look at temperature rise of the bearing metals on the first run.

A good millwright knows all this by instinct I assume somebody like that is available...

regards, rasevskii

 

[algonklin]

We will run the motor to check rotation and establish mag center. What kind of tolerance is acceptable and how does this tolerance effect motor life and reliability.

 

[rasevskii]

I would not want to venture naming any tolerances, it is always safer to follow the OEM recommendations or better have a rep. for the OEM on site at the first start to give the go-ahead for the first turn. Something that big involves a lot of investment, and the client takes it on the hook if he starts up the unit without the OEM presence.

At least in power generation it is always that way.

Regarding life and reliability, that is strictly a mechanical issue, if the bearings are damaged or not. Electrically not an issue. At that size and rpm, I assume it is a straight induction motor arranged for heavy start duty.

If I was out in the bush alone with this machine, I would like to see 2mm or so of axial play to start with, but that is not an official recommendation...obviously...

Anyone else out there have a comment...?

regards, rasevskii

 

[electricpete]

NEMA gives tolerance for mechanical endplay. 1/4" up to a certain size and 1/2" for larger motors. I'm sure yours would be in the 1/2" range (assuming motor purchased to NEMA standards). Mechanical endplay also is typically listed on a nameplate on the motor.

There is a lot of discussion about trying to place the motor on magnetic center... which would involve running the motor uncoupled, marking the shaft with blue developer to identify that running position (presumably magnetic center). Then shutting down and verifying that if placed on magnetic center you are still a reasonably distance from the mechanical limits.

How far away from mechanical center does it need to be? The answer to that question depends on your coupling float and the thrust bearing clearance in the driven machine.

Without a careful study, putting it in center is the best bet in my opinion.

You can certainly try to put it off mechanical center to get on magnetic center, but only if you are sure you won’t have problems hitting mechanical limits.

Magnetic centering forces are very low. On the order of one or two hundred pounds. It will not be a problem for the thrust bearing of driven machine. There is stories of machines “hunting” from being off magnetic center... I have never seen it.

The bottom line imo:
mechanical centering is very important
magnetic centering is optional icing-on-the-cake stuff.
don’t lose track of the important stuff worrying about the optional stuff.
Put it on mechanical center or else study it carefully to know that where you are going to put it will be ok.


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[rasevskii]

Well, I would not be so sure that the mag. off-center force would be so low as to not damage a thrust collar or thrust bearing face over an extended time period. I am not an expert on ball mills but having seen one or two in operation I see that it is a very rough sort of cookie to deal with.
The better it is all lined up magnetically and mechanically the more reliable.

Possibly this is a synchronous/induction motor started up at no-load. In that case slip rings may be involved and care has to be taken to see that the brushes are not running close to the ring edges, also.

regards, rasevskii

 

[electricpete]

Personally I would have no reservations about the loading on the driven equipment bearings from the magnetic centering force. And I have never heard anyone express concern given the low magnitude of the centering force. Again one or two hundred pounds at the most... a drop in the bucket for bearings on a 2000hp driven machine.

The typical stated concern when off of magnetic center is "hunting" - some kind of dynamic phenomenon with a flexible coupling which may cause fatigue of disks on a shim pack type coupling.

Again in my opinion is the above concern is small compared to concern of allowing prolonged contact between motor bearing shaft shoulders and adjacent thrust bumpers which would be a much more severe condition. If you want to couple off of mechanical center, then you have to leave enough allowance for coupling float and thrust bearing clearnaces and thermal growth that you guarantee yourself you wont' do that. With 1/2" total endplay, I think you could go 1/8" on either side of mechanical center and still leave 1/8" margin against contact on each end - that should be enough assuming your machine has something like a Thomas Shimpack coupling and typical thrust bearings below clearance below 0.020", maybe 0.03" allowance for thermal growth

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[edison123]

For such a big motor with sleeve bearings, OEM would have marked the shaft with respect to one of the bearing pedestals for magnetic center. Involve OEM in this.

I normally try to set the magnetic center to match the mechanical center (which is the total play between two bearing shoulders) so that thermal expansion issues do not cause any shoulder rub.

Muthu

http://www.edison.co.in

 

[algonklin]

I am familiar at setting up these kinds of drives. The ball mill pinion bearing is held and we will run mag center, stop the motor and return to mag center and set the coupling gap to manufacture's specifications by axially moving the motor assembly. The coupling, by design will allow around .030" axially movement within the limited end float kit.
One of the issues I have is how accurately we can measure a rotating shaft's mag center due to in house safety and MSHA regulations.
We'll end up close and I'm not concerned with thrust or bearing issues, rather how the motor life, reliability, or performance will be impacted if we're off mag center by .020" or .030"

 

[Tmoose]

Hi rasevskii

you mentioned a thrust bearing failure "once on a 1 MW motor..."

I assume it was the driven equipment's bearing.
Did you attribute that to the motor running "off" magnetic center ? How far off?

thanks,

Dan T

 

[rasevskii]

for Tmoose-

Yes it was a roller bearing cage on the driven machine. The unit was a MG set of a 60hz motor driving a 180hz generator for transformer testing. Speed 900rpm minus the slip. The client had done the erection and the mag center of the motor was not respected although marked. The bearing was ruined after less than an hour or so operation. After replacing the bearing and realignment the unit was ok. It was a couple mm off alignment at least.

Obviously a roller bearing has no tolerance for axial misalignment. The motor itself had sleeve bearings.

Brass filings showed up in the bearing housing and that was the first clue to the failure. Had they continued to run it the damage would have been severe all around.

regards, rasevskii

 

[electricpete]

A problem might be with the bearings. The motor will try to move into its magnetic center while energized, which will pull the bearings axially. That may destroy the cage on a roller bearing if that is what you have, or thrust a sleeve bearing too hard against a collar, both resulting in destruction. Had that once on a 1 MW motor...

.........Yes it was a roller bearing cage on the driven machine. The unit was a MG set of a 60hz motor driving a 180hz generator for transformer testing. Speed 900rpm minus the slip. The client had done the erection and the mag center of the motor was not respected although marked. The bearing was ruined after less than an hour or so operation. After replacing the bearing and realignment the unit was ok. It was a couple mm off alignment at least.

Obviously a roller bearing has no tolerance for axial misalignment. The motor itself had sleeve bearings.

Brass filings showed up in the bearing housing and that was the first clue to the failure. Had they continued to run it the damage would have been severe all around.

1 – The destruction you refer to was due to sleeve against a collar (too far off mechanical center), or due to magnetic force (too far from mechanical center)?

2 – Does roller bearing mean “cylindrical roller bearing? Are you saying the driven machine had only cylindrical roller bearings and no bearing capable of handling continuous axial thrust?


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[rasevskii]

For Tmoose-

The roller bearing was on the driven end of the generator. The roller brass guide cage had been pulled toward the direction of the motor, resulting in the cage being ground away. There was as far as I remember no thrust bearing on the generator, if so it was on the NDE which was not inspected at the time. When the bearing was replaced by the client, I was not on site, as it took a couple weeks time.

The coupling was a solid bolted type, without any springing or dampers.

The mechanical work was the client's job, my job was the electrical. How they managed to get sold a sleeve bearing motor and a roller bearing generator I do not know. I returned to finish the electrical commissioning, how it fared afterward, I never knew.

regards, rasevskii

 

[electricpete]

If the bearing was ruined in an hour and axial alignment had anything to do with it, then it is safe to say it was a mechanical center problem (didn’t leave enough margin to limits of endplay) rather than a magnetic center probelm. That may be what you’re saying... not sure if I understood.

Miscellaneous comments - Typically every machine train has at least one bearing capable of taking thrust... probably on the NDE. Suprising to see a rigid coupling on a horizontal machine.


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[electricpete]

... the other alternative is the machine had only cylindrical roller bearings with no provision for thrust on any bearing. I have never seen that but I would certainly consider that a deficient design.

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[rasevskii]

Well that all happened more than 25 years ago, likely the factory is long closed and it has all landed in some junkyard. Don't buy any used 180hz generators without checking the bearings first...

rasevskii