This is a new 75 HP 60 Hz 2-pole motor that drives a stator cooling pump for our main generator. After mounting this new motor on a very stout newly constructed pad, an uncoupled run produced 0.275 ips-p horizontal vibration. Offending frequencies were exclusively 7200 cpm. We have four of these units...each mounted and configured in a similar manner. The other three yield no vibration problems. The unique thing about this particular motor is that it is one of the new generation high efficiency motors.

I've taken several steps to diagnose high vibration...including:
1. Several iterations of asking craft to correct any softfoot condition. Residual softfoot measured with a dial indicator is always less than 0.002". One attempt at correcting softfoot was with the motor in service...continually checking vibration response. I can't ask them any more (they're kinda pissed).
2. Skim cut the motor feet and test run it on a machining table...with a completely different power supply. Same high vibration.
3. Buy a new motor (identical high efficiency model). Try all the diagnostic steps on it that we tried on the first...without success.
4. Perform an impact test. Closest response peak was at about 8100 cpm...or about 13% away from 7200 cpm.
5. Perform a motor coastdown test. The 7200 cpm fell out abruptly when power was cut. A much smaller 2X shaft speed peak followed the coastdown.

We get acceptable vibration numbers (<0.08 ips-p) when the motor is just sitting on the floor or pad. As soon as we torque hold down bolts, you hear a change in tone and horizontal vibration climbs to between 0.250 to 0.350 ips-p. I still hold some interest in the fact that there is a horiz resonance peak at about 8100 cpm. My intuition is that 7200 is far enough down the response curve to be of minimal influence. Also, it would surprise me that a commonly manufactured motor from a major manufacturer would have an inherant resonant response...on both of the ones we tried?

I have heard comments that some of the new high efficiency motors vibrate at higher amplitudes. I am thinking, now, that this high efficiency model is just more sensitive to very small changes in mounting distortion. It's a major manufacturer...with a model name that I hear often. We can't get it to run with acceptable vibration numbers. I would normally expect an uncoupled run to yield <0.1 ips-p. I can't think of another diagnostic step to take? We've already swapped to another motor of the same model.

Should I:
1. perform more diagnostics (please suggest what else we can do...cause I can't think of anything else)
2. accept higher vibe thresholds for high efficiency motors
3. give up on this high efficiency model

Thanks in advance,
George

In a perfect world, I wouldn´t accept higher vibration on "hi efficiency" it should not be any magic about them. My humble guess is that the rotor-stator gap is smaller and possibly it makes the motor smaller in design compared to classic motors. This would also make the magnetic forces stronger at 2xLF. Aluminium cast rotors are just as bad as on normal motors. I can´t help when looking at your description to suspect a resonance problem when the motor is tied down on your foundation, 13% in knock test may be close enough in operation to give problem/amplification. In newer motor versions I have found that the motor is very weak in design and barely holds together for transport, about the same from 2 major manufacturers here. I have also found it to be common to have very large feet. Are yours the size of Goofy´s feet? They need shim support along the length of the foot, 70-80% of the motor length, or the stator will get severe shakes as it can´t keep itself up. It seems you don´t run them speed controlled so you may have the option to detune by making foundation weaker/stiffer or add some mass to verify the problem. I try to put a large concrete block on the poor motor to make a big change to prove the point. Last resort is usually to put in another brand of motor, they may be funny in design but almost never the same way, so you may have a new problem as a change. Good luck. Olov

Certain some designs of 2-pole motors are more susceptible to 2*LF vibration than others, but I don't think it is necessarily linked to high efficiency designs. I do agree with Oli that the airgap on high-efficiency motors is generally smaller to improve power factor, which is one design factor that can make them more sensitive to small deviations. High efficiency designs also tend to have higher airgap flux density which is another factor that makes them sensitive. But again, the problem is not limited to high efficiency motors.

I'm guessing we're talking about foot-related vibrations. In that case the vibration goes away with some or all four feet loosened (should only be attempt loosening more than one foot with motor uncoupled and perhaps loosened only after motor is started).

I think the biggest features affecting 2*LF foot-related vibrations on NEMA frame motors (i.e. below ~ 400hp) have to do with how the core is attached to the frame and whether there is excessive interference between core and frame. These are features that unfortunately can't be repaired in the field. That's what engineers from two different OEM's have told me. Sometimes it can be accomodated by step-shimming or planing the feet and planing the area that the motor will be installed on, but sometimes you can't get it close enough to stop the vibration.

We bought some NEMA frame 2-pole motors for a historical 2*LF problem application. We ordered thru distributor from a manufacturer that happenst to be local. We specified that the replacement motors were to be < 0.06 ips when tested bolted to a rigid foundation. We also specified that we wanted to observe the testing. It was an unusual request since the manufacturer is a production facility typically not taking special requests, but they took ours because they were confident they could meet it. The motor manufacturer typically tested them on reslilient mount (allowed by NEMA), but was confident he could meet 0.06 ips bolted to rigid mount based on his balance specification (!). We observed the testing and guess what... 0.2 ips horizontal 2*LF. The motor manufacturer pulled the motors apart and did careful checks and machining to improve all tolerances related to concentricity. Still 0.2 ips 2*LF! Then after a series of phone calls to company gurus far away, they made some kind of modification of some welds on some ribs in the stator frame. Poof... the 2*LF vibration went away (or else we just couldn't see it on the bearing housings any more...I'm not sure which). A big lesson leanred: specify your 2-pole motors to be tested bolted to rigid support.

I'll bet the vibration would go away with plastic shims (see www.precisionbrand.com). Those you have to put in and give them two weeks or so to see the full effect. We have had very good luck making 2*LF go away on 2-pole NEMA frame motors by using that product. It remains a little bit of a hassle for the life of the machine. If you ever go back and even loosen the hold-down bolts (or certainly if you remove or realign the machine) you have to replace the plastic shims. We have had a number of problems where the 2*LF was solved and then years later it came back when the machine was maintained for other reasons and those plastic shims were not reinstalled or reinstalled correctly (need one on top of shim pack and one on bottom of the shimpack). The manufacturer of those shims calls it a "temporary" solution. We have been using them for 10 years and as far as I know, vibration hasn't come back on any of those machines other than when we messed around with the machine and didn't reinstall correctly.

Our stator cooling water pumps/motors are skid mounted with a complex support structure. I think in the past we had reduced vib on these machines by putting wedges into gaps below the motor support.

Then again, most motor repair shops and OEM's will tell you that two-pole 2*LF (especially the foot-related variety on NEMA frame motors) is a relatively benign vibration. The vibration starts with rotating oval deformation of the stator core, comes through the stator frame to the bearing housings, rather than originating from force between rotor and stator. So the force transmitted through the bearings is not as large as it would be if you had an off-center rotor creating 2*LF unbalanced magnetic pull with restraining counter-force applied by the bearings.

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

Here is a similar case. By coincidence it was also 75 hp, 3600 rpm motor. If I am not mistaken it may also have been the high efficiency motor.
In the attached ppt file you can see, that the resonant frequency was almost exactly 120 Hz, causing a no load vibration over 0.26 in/sec, almost everything at 2x.
The fix was a partial separation of the foot from the body of the motor, similarly as is shown on the third slide. (The picture does not show the 75hp motor).
In my case I believe that the vibration did not have anything to do with the high efficiency. Just the structure was resonant at very wrong frequency.
I also think that the 13% separation (8100 cpm)is not sufficient.
jank

RESONANT_FEET_ON_75_HP.ppt (457 Kb, 40 downloads)

quote:

Originally posted by George D'Entremont:
It's a major manufacturer...

Siemens?

Michael
No...email me.
George
gxd1 at pge dot com

George,

I have done like Oli suggested but with sandbags. It worked as a temporary fix while we constructed a "tuning fork" which worked fine for the remaining 15-20 years before the building was demolished.

This was before PE motors, but it was a 2-pole with a close resonance on a CT pump. Being on the roof of the building, there was little change of damage or tampering and it was lots cheaper than altering the pipes.

To answer your question-no, I don't think higher vibration should be acceptable simply because it is a PE motor. It is more common in PE motor because of the smaller air gap, imo, but not acceptable.

PE = Premium Efficiency?

Hey Danny Harvey, You related to Ross and Sheridan...right? This is JTCC instructor of MEC-154, MEC-254, and IND-125. Small web, eh!

Ross and Sheridan are better known around here by their professional names, Tote and Climb.

They both speak highly of you. Do they stay awake in class?

Pete,

PE=Premium Efficiency

This message has been edited. Last edited by: Danny Harvey,

I'll guess which is which or who is who. Lot of potential in both young lads. Good students!

Enough potential to support me in my retirement?

George:

We had the same problem here with high efficiency motors. Ended up calling the OEM and it was indicated that the 7200 CPM frequency was the 'running design', in essence, the method in which they were manufactured equaled the product of the 7200 CPM component and was deemed normal to occur and at elevated amplitudes.

You might want to contact the specific OEM and put the question.

Pete: a portion of your reply has me wondering; the part about the plastic shims diminishing the 7200 CPM (2 times AC line frequency) amplitude. I don't know much about electricity other than I am afraid of and addicted to it! Am I to assume the plastic shims act as insulators? If so, might they also be expected to work on motors driven by VFD's?

Forgive me if I have greedily misinterpreted where you were taking this...as always, looking for the easy way out! All the VFD’s here are driving me nuts! I would love to have something of a ‘filter’ to get rid of all the induced electrical noise within a typical spectrum caused by the VFD's.

Gary

You can get some more info on the plastic shims here (you have to register first):
http://www.precisionbrand.com/DownloadFile.aspx?dFileId=135

It doesn't act as an insulator. It is an initially soft shim material that hardens after installation. This accomodates imperfections in the foot and/or base and prevents bending the motor feet/frame during holddown bolt tightening. At least that's the way I look at it.

Maybe others would say it is detuning a 2*LF resonance. Jan's case was interesting in that respect since the 2*lf was proven by bump test. I remember Arnie used to talk about adding belleville washers below the hold-down bolts in attempt to detune 2*LF resonance (where is Arnie these days, anyway!). If resonance is the cause of foot-sensitive 2*LF in general, it seems a little strange to me that so many machines could happen to be resonant at 2*LF, but you never know. Perhaps each case is a little different and there is not one explanation for all.

Either way, the plastic shims have always worked for us in eliminating foot-sensitive 2*LF vibration on horizontal 2-pole NEMA frame motors. We have applied it around 7 or 8 machines.

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

The Sof' Shoe shims Pete is talking about are not your conventional 'plastic' shims which I don't think would do anything to address a soft foot problem.

I've never used the Sof' Shoe shims but I think I will try them. I have a number of 2-pole motors that have what I'd call excessive vibration, but most have been running for years that way.

Is excessive vibration a "problem" if nothing bad ever happens as a result of it?

Rusty,

Your question: Is excessive vibration a "problem" if nothing bad ever happens as a result of it?

You may had said it lightheartedly but what is the answer? We all have experienced that situation at one time or another. Yea, I'm sure this isn't a 'one size fits all' but it is an interesting question and even more so, answer only if it 'is' an opinion.

George has spent a fair amount of time performing tests to determine cause and hopefully find solutions. As Danny has offered, higher vibration is not acceptable.

If it 'is what it is', what should be done? What can be done? What are we all doing about it?

Gary

epete,
Anything that changes the stiffness of a foot connection to it's base (bellville washers, plastic shims, etc.) will change the rigid body modes of the motor. If you think about how quickly the two ridgid body modes shift down in frequency for a small change in stiffness, that answers the questions. Also, many 2 pole motors that I have modeled do indeed have a second ridgid body mode near 7200 cpm.