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Author Topic:   MOTOR MAGNETIC CENTER
aliabrar
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posted 05-08-2002 01:40 PM     Click Here to See the Profile for aliabrar   Click Here to Email aliabrar     Edit/Delete Message   Reply w/Quote
We have an issue of magnetic centering of a motor of following specification

1100 KW
6.3 KV
744 rpm
ABB make

The motor has two sleeve bearings and has a total float of 10 mm. The rotor locating collars are not supposed to sustain continuous axial loads.

During solo run the rotor moves to its extreme non driving end againt the expected middle of the total float.

This motor drives a Cooling Water Pump and is coupled through a flexible shim packed coupling. During coupled operation it stretches the coupling by more than 3 mm when DBSE is set with motor mechanical centering (i.e., mid of 10 mm float)

Any Comments or Remarks?

[This message has been edited by aliabrar (edited 05-08-2002).]

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W Ron Brook
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posted 05-08-2002 02:22 PM     Click Here to See the Profile for W Ron Brook   Click Here to Email W Ron Brook     Edit/Delete Message   Reply w/Quote
Sounds like this motor has had either the rotor iron or the stator iron moved. Check to make sure the stator hasn't been moved in the housing (axially), or that the rotor hasn't been restacked, or in some other way rebuild/reworked. Then, see if you can look at the relative positioning of the rotor iron to the stator iron. This, after all, is what the motor is trying to accomplish when hunting for magnetic center. Once you have been able to "center" the rotor iron in the stator iron, the rotor should be close to where it will run. Now see if the motor bearings have shims that can be shifted front to back, or back to front, whichever is called for the make certain the the shaft shoulders stay off of the bearings.
Good Luck!

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OLI
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posted 05-08-2002 02:23 PM     Click Here to See the Profile for OLI   Click Here to Email OLI     Edit/Delete Message   Reply w/Quote
Is it rebuilt?
Rebuilding and or rewinding is known to
offset electrical center.
Any other unintended electrical/mechanical "abuse" that may have moved internal windings
should give similar results like sudden stops
for mechanical reason or similar.
Have seen rebuilding giving result as an axial oscillator where the mechanical limits and the changed electrical center was fighting and neither was winning. Some axial amplitude. When resetting with the new electrical center as is, it stopped.
Didn┤t see the above, it was not there before I wrote, same thinking, 1 minute later. Agree 100% to the above.

[This message has been edited by OLI (edited 05-08-2002).]

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Jan Krepela
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posted 05-08-2002 08:32 PM     Click Here to See the Profile for Jan Krepela   Click Here to Email Jan Krepela     Edit/Delete Message   Reply w/Quote
Look if the cooling fan(s) pull the rotor in one direction.
Jan

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electricpete
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posted 05-09-2002 06:48 AM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Good comments already. One other item I learned recently is that magnetic centering tendency can also be impacted by the location of the radial vent paths on the stator and the rotor. If properly designed those sets of vents are aligned when rotor rests within center of stator. But if improperly designed or changed during restack of either core they may not align... rotor will try to move to align the vents.

I am curious how you measured the stretch on your coupling?

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Mike Drake
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posted 05-10-2002 07:48 AM     Click Here to See the Profile for Mike Drake   Click Here to Email Mike Drake     Edit/Delete Message   Reply w/Quote
All good input, but I go with Ron Brook's take. Typically an 8 pole motor would produce dense enough flux to prevent the vent slots from moving the rotor much, though it could make it noisy. The cooling fan, if shaft mounted, would have to be designed to move a lot of air to thrust the rotor that much, again on the 8 pole. I have seen both of the above conditions occur.
However, I have seen a similarly sized ABB Boiler feed pump motor in Southern Illinois that had a rotor restack in which the stack was not properly positon on the shaft and what is described was exactly what happened.

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electricpete
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posted 05-10-2002 08:35 AM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Mike - with due respect, please don't dismiss the effect of the vent ducts. I attended a course on motors with Bob Wall (ex GE, motor engineer for 50+ years). He said he has seen vent duct problems cause high axial thrust on numerous occasions.

The difference in flux between stator iron and adjacent (axially) air space is the almost same in the vent duct as it is at the ends of the core. If rotor vent is offset axially so that rotor iron is sitting in the low flux by the stator vent and rotor vent is sitting in the high flux adjacent to stator iron, then there will be a force trying to align the iron and align the vents, similar to what occurs if the entire rotor iron is not centered in the stator iron axially.

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aliabrar
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posted 05-10-2002 08:48 AM     Click Here to See the Profile for aliabrar   Click Here to Email aliabrar     Edit/Delete Message   Reply w/Quote
Thanks for the valuable comments.

The motor was run solo again today, this time the fan was removed to see effect of fan thrust on rotor positioning. The result was same as that previously experienced i.e., the rotor again moved towards outboard end extreme.

The motor was removed during plant shut down for internal inspection, how ever no winding or stacking job was done.

I think the possible solution seems to axially realign the bearings of motor towards its electric center.

(Regarding one question related to pre-stretch measurement, the procedure is simple, a circumferential line on the motor shaft is scribed when in operation and its realtive position then can be compared with standstill condition of zero coupling pre-strech to find out actual running pre-stretch.)


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Mike Drake
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posted 05-15-2002 12:47 PM     Click Here to See the Profile for Mike Drake   Click Here to Email Mike Drake     Edit/Delete Message   Reply w/Quote
Hey, Pete -
Wasn't trying to dicount the effect of offset vents, just felt the likelyhood of occurance is lower. With that perspective I suppose a vented design with the rotor offset will also offset the vent slot alignement!
As mentioned in the previous post, the scenario reminded me of a particular ABB motor from the past. With that rotor in mind (design wise) I would not have expected vent slot alignment issues as it was a long iron stack with widely dispersed vents. ALso, in a two pole motor, as mentioned, vent slot issues have been more likley, in my experience, for the very reasons you mentioned.
In an 8 pole motor the size mentioned, unless wound with a short, very tall iron stack (if possible) or possibly a more practical stack with many vent slots I would still suspect vent slot alignment after the other considerations. All from the armchair quarterback positon we play on the web. My intent was not to discount, rather to prioritize by probability based my experience and the experience of those I have worked with.

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