Please see the attached file which discusses the shape of stator static distortion in motors.

Opinions will be appreciated.

Dave

Stator_distortion_shape.doc (28 KB, 37 downloads)

I think that foot-sensitive vibration on 2-pole NEMA frame motors can involve more than change in airgap and associated magnetic forces acting on the core.

There is also the matter of how the core responds to that force (how dynamically stiff is the core to that rotating oval deformation) and to what extent is it transmitted to the stator frame and bearing housing.

I have related a story before and I will repeat it. We once purchased six 2-pole 100hp motors. We put into the specification:
1 - <0.08 ips on all points.
2 - Tested when bolted to a RIGID base.
3 - We want to witness the testing. We specified motor from an OEM whose NEMA motor factory happens to be less than 2 hours from our plant.

This specification was accepted by a distributor based on his communication with the factory.

We showed up at the factory to witness the test. They said they don't usually have customer witness points but they are happy to have us watch the test. They don't usually test on rigid test bed, but they were very confident (cocky even) that they could meet the spec based on the extra balancing they had done. So what do we see... 0.15 to 0.2 ips foot sensitive 2*LF vibration.

They took the machines apart, did some minor changes to improve concentricity of airgap, reassembled... exact same result.

Then they had phone consultation with some of their company gurus. They did some cutting and welding at strategic points near the ribs on the frame, reassembled, retested..... problem gone. Their changes were not intended to reduce the airgap eccentricity but to reduce the response of the stator frame to the magnetic force.

I have also heard from a motor OEM (different company) that one particular design of NEMA 2-pole motors from his own company that tends to experience foot sensitive 2-pole vibration because it efficiently couples the rotating oval deformation of the core onto the frame. This particular design involves core which is pressed into the frame with a very tight fit, we have very efficient coupling and very big problems with foot sensitivity.

Contrast that to a large (above NEMA frame) 2-pole sleeve bearing motor. Most likely the core is mounted to the bottom of the frame and not the sides. Side to side movement of the core is not going to transmit to the frame very well. In these large motors since there are larger overall dimensions involved but similar airgap distances, there is more potential for airgap distortion in these large machines, but less coupling of core motion onto the frame.

In summary

• for NEMA frame (<~250hp) 2-pole motors, I think foot sensitive vibration would much more likely be caused by change in coupling characteristics between magnetic force and frame response. Significant change in airgap dimension while adjusting feet is not likely for small machines.
  
• For above-NEMA (larger_ 2-pole motors it is more credible that frame distortion may alter airgap and cause increase in unbalanced magnetic pull which could show up as 2*LF at the bearings.

Last comment as I have expressed before: 2*LF vibration for magnetically-created vibration (especially on 2-pole) is like 1x vibration for mechanical. There are many different mechanisms. I have mentioned a few generalities from my limited understanding. The real story I'm sure can vary depending on the motor.

This message has been edited. Last edited by: electricpete,

Pete,

An interesting story... a few questions in this regard.

1. Why a rigid test bed was requested? Was it in order to provoke a possibility for soft foot ? I theory, with a rubber mat test bed all vibrations will be higher including 2xLF if there are air gap problems. Or was it in order to detect potential shaft mode shapes?

2. How did they finally eliminate 2xLF without actualy improving air gap eccentricity? Where 2xLF was measured: on the end bell or in the midspan of the frame?

3. In my attachment I actually discussed distortion of the stator core as oppose to stator frame, or in other words I considered them pretty well coupled together. So, what is your opinion on the distorted shape?

quote:

1. Why a rigid test bed was requested? Was it in order to provoke a possibility for soft foot ?

Yes. I always specify new NEMA-frame 2-pole motors be tested rigidly bolted down. NEMA specifications allow rigid or resilient mount. If they use resilient mount then problems with foot sensitivity will not be apparent until you rigidly mount it in your facility. I would rather give myself a good opportunity to find it before it gets to my facility and to make them fix it.

quote:

I theory, with a rubber mat test bed all vibrations will be higher including 2xLF if there are air gap problems. Or was it in order to detect potential shaft mode shapes?

Foot sensitive 2*LF to me means it gets worse when you bolt it down tight (as I have seen many NEMA frame 2-pole motors do). I am talking 2*LF, not the other frequencies. Why does bolting it tight change the vibration? Imo because the feet are not perfectly flat and small changes in frame stress which occur while tightening them down increase the coupling of magnetic forces to frame vibration.

quote:

2. How did they finally eliminate 2xLF without actualy improving air gap eccentricity? Where 2xLF was measured: on the end bell or in the midspan of the frame?

These happened to be open drip proof machines. Vibration was measured on the endbell, which is a solid beefy piece of steel rigidly attached to the center frame (unlike fan shroud outboard end of TEFC). They did not imjprove air gap eccentricity. They decreased the coupling of magnetic forces to frame vibration.

quote:

3. In my attachment I actually discussed distortion of the stator core as oppose to stator frame, or in other words I considered them pretty well coupled together. So, what is your opinion on the distorted shape?

If you consider them pretty well coupled together than I picture NEMA frame machine. Airgap eccentricity has very little to do with the 2*LF foot sensitive vibration in a NEMA frame machine IMO. Comparing a large machine with a 0.080" airgap to a small machine with 0.080" airgap, it should be obvious small strains (strain = fractional change in dimension) create less change in airgap for the small machine than th elarge machine. However even if the airgap is perfectly symmmetrical you can still have a rotating oval deformation of the core which can be coupled to frame vibration and that coupling imo can be foot sensitive.

I think the picture you draw might have relevance for large machines. They may have a long stator without full support on the bottom and there may be sagging of the frame in various ways similar to what you show and that saggin can be can be affected by feet tightening.

I have actually measured the deformation of the airgap due to the soft foot. The article appeared in Vibrations Volume 21, December 2005.
It was a case of a 200 hp, 1200 RPM, Frame 449TZ, TEFC motor. The motor was bolted by 3 feet to a 3” plate in vertical position. The profile of the airgap was measured on each of the 72 stator teeth. (Each measurement was rounded to the nearest thousands of an inch.) Then the 4th foot was tightened, so that the original gap between the foot and the steel plate decreased by 0.017”. Such a soft foot does not happen very often, I want to believe. I did not dare to tighten it any more, because I was afraid I would break the foot. The resulting deformation of the airgap was calculated to 0.00063” (a width of the human hair).
I believe, that a number of motors works happily with 30% eccentricity, particularly when they are squirrel cage motors and slower than 2-poles. One has to watch on the wound rotor motors, the effect of eccentricity is much more profound.
But generally I would say, that the eccentricity plays a less important role in 2xLF compared with the foot related resonance and other structural problems.
jank

Leave it to JanK to do a great experiment. Seems to support a conclusion that change in airgap during foot tightening is negligible even with large soft foot (at least for NEMA frame motors).

Kliman's "Electric Motor Handbook" section 11.2.3.2 states "The plane of the feet must be flat and they must be mounted on a flat surface. Unless this condition is met the motor frame may be stressed, causing abnormal resonances in the motor. This condition can be checked by selectively loosening the mounting bolts to see if there is a change in the overall vibration level."

He calls it a resonance as Jan call's it a resonance. I called it a change in mechanical coupling which is more general than resonance - means the force is the same, but the mechanical system changes such that resulting vibration is higher. I am not hung up on the terminology since the mechanism is a little bit unclear. But to me it doesn't exactly match characteristics of a resonance - I believe that 90% of 2-pole motors would exhibit foot-sensitive 2*LF vibration if you create a soft foot (by adding extram shims or removing shims) and then tightening down. And if you double the number of shims they will still have foot-sensitive 2*LF vibration. In constrast resonance is something that happens only at a very specific set of conditions... doesn't seem likely to me that it would happen for almost all NEMA frame 2-pole motors at almost all possible soft foot conditions.

Jan - Can you share a copy of your article with us?

Good post David!
Epete: Out of curiosity; how many (new) 2-pole motors do you find that fail your specs ? (percentage) I would guess that since the oem knew that you wanted the test performed on a rigid base, the feet were checked beforehand? Does your plant have tolerances for foot flatness?
Jank: Great info! I too would love to see a copy of the article.
Thanks gentlemen!
Jeff

Perhaps related...

Ralph Buscarello uses a term: "Foot Related Resonance" or 'FRR' [about which he can wax quite eloquently! ] There is an online book by him in which Chapter 21 is titled to that very subject!

Practical Solutions to Machinery and Maintenance Vibration Problems I attended a seminar of his on this subject at the 2006 SMRP Conference and he had a lot to say on it.

Might just relate!

I am posting the article that appeared in the Vibrations, the magazine of The Vibration Institute in December 2005, Volume 21, Number 4.
The profile of the motor airgap differed very little from the ideal circle. The “moves”, I am talking about in the article, are adjustments of the position of the rotor to bring the rotor to the best possible position in the airgap.
The shape of the stator iron is not necessarily an ideal circle. So the best position of the rotor in this complicated shape is not as straightforward as one would assume without a deeper analysis.
As you can see, the publisher did not agree with my conclusions
unconditionally.
The article is being posted with permission of The Vibration Institute.
jank

DEFORMATION_OF_AIR_GAP_DUE_TO_SOFT_FOOT.pdf (729 KB, 24 downloads)

That is just a great article. A very meticulous and scientific approach to studying a complex problem imo.

The editor seemed to have completely miss the point with his comment "The editor has had many reports of twisted stators that cause vibration at 120hz". Of course the observed correlation between soft foot and 120hz is clearly known and acknowledged in your article... what isn't known is the physical mechanism which leads to this correlationn. Observing 120hz "due to twisted stator" means nothing unless there was measurement of stator airgap (which I doubt).

I think what you are up against is the training phenomenon. Early on certain people like Tech Associates drew their own conclusions and put explanations into training material such as "the increase in 120hz vibration is due to change in the airgap". Then once this is widely taught in training materials, it takes on a life of its own. Sad to say training materials are not always right and this is another example imo.

In many cases that I've encountered high 2 x LF vibration, I found that wedges (& thus coils) were loose in the slots. In others, the magnetic putty in the slots had come out, thus we had both uneven air-gap (effectively) as well as loose coils. Replacing the putty or tightening the wedges generally removes the 2 x LF component.

I have attached data of an 8 MW DG Set that was rewedged with magnetic wedges. The wedges were not of uniform quality, which led to heavy vibration. The vibration would occur only on load, at no-load, vibration level was about 0.3 mm/sec, RMS (compared to about 14 mm/sec, RMS on load).

Not sure if this is relevant to the shape distortion issue, just some observations.

Regards,

Aditya

P. S. How does one attach a file nowadays? I am unable to do so. Will try again later.

Trying to attach the file once again.