Source of Large Motor Vibration
Source of Large Motor Vibration
(OP)
My organization is working on a problem with a 15,000 HP motor which exhibits two to three times the level of vibration when voltage is applied than when not. The rotor and rotor bearings have been test throughly and they are balanced and true. The vibration levels are very low when the rotor is spinning without voltage applied to the stator. This phenomena occurs in the field and in tests at the factory. There are several of these motors in service and all exhibit the same phenomena but one has vibration levels that are exceptionally high when power is applied. The stator is the last component of the machine not yet subjected to intensive scrutiny. That is begining now. Suggestions for the direction of the investigation are greatly appreciated.





RE: Source of Large Motor Vibration
1. Stator clamp ring or stator through bolts loose.
2. A change in RPM was required so the stator was rewound without regard to the ratio of stator bars to rotor bars (in simple terms). This causes the air gap flux to (in some cases) rotate in the "wrong" direction while the rotor is going in the "correct" direction.
3. A bad guess on the fractional coil arrangement.
4. if there is a drive supplying this motor, certain harmonic frequencies will resonate with the L of the windings and the C of the insulation.
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
Many thanks!
RE: Source of Large Motor Vibration
If the airgap is not consistent (when hot) this would cause more rotor deflection.
A shorted or grounded (in 2 or more places) turns in the rotor windings which are a little more common in newer machines cause an uneven field distribution which twists the rotor while it is running. Shorted field windings are very difficult see from testing; they are usually noticed by the amount of vibration.
The field winding should be centered within the stator if it is not a vibration may result due to thrust along the shaft.
Best Regards
Ray Micallef, P. Eng.
Power Generation/Utility Industry
RE: Source of Large Motor Vibration
It is dificult to make any assumptions regarding vibration unless the Frequencies of the same vibration are known.
Most defects or abnormalities that may cause your high vibration have a distinct frequency.By analyszing these
frequencies one is able to find out what is happening with a particular machine.
You say that most motors of the same type, exhibit much the same vibration but this particular motor shows higher levels.
Is this vibration Freq related to 2xLine Freq (120hz,on 60 Hz power)?Or is it related to the running speed of the motor.I guess one can exclude Power problems as being the culprit,both field and Facory tests show the same problem.
I am curious to know if these higher Vib. levels are being measured with Online instrumentation such as Phasor Probes (Eg. B Nevada)or is it a Portable Vibration Analyzer?
Not knowing what kind of Freq. we looking at,I can only
offer some speculation on your problem .I presume these motors all have Journal Bearings and possibly Forced oil circulation through the sumps.I don't know if you have already tried a different grade of lubricating oil.At times a small change in oil grade can affect motor vibrations quite a lot.Another thing I would look for are loose slot wedges.In some cases the slot wedge (Fiber) material have shrunk and became loose causing a higher level of vibration.As the looseness increases so do the vibrations.This condition would cause levels to drop considerably when Power is turned off.
Good luck
GusD
RE: Source of Large Motor Vibration
2) Who is manufacturer?
3) What is cooling method?
RE: Source of Large Motor Vibration
This appears to rule out mechanical imbalance.
The frequency 1x appears to rule out static air gap problem.
You have checked rotor trueness which rules out dynamic air gap problem.
Try applying voltage with motor uncoupled... if vibration remains you can rule out misalignment.
Also check for distortion of stator frame due to soft foot.
* Check for shorts in rotor which can cause 1x running speed vibration.
RE: Source of Large Motor Vibration
Do you know the no. of stator and rotor slots ? Since you mention this problem exists on all the similar motors, I think that there could be design problems. Some combinations of stator slots / rotor slots tend to produce electro magnetic vibrations and such combinations should be avoided.
You said
"The vibration levels are very low when the rotor is spinning without voltage applied to the stator."
How could the rotor spin when there is no power supply ?
Do you mean coasting down phenomenon ?
RE: Source of Large Motor Vibration
1. Have you check the power supply quality?
2. How old is the machine?
3. What kind of load is driven?
4. What is the starting arrangement?
5. Is there a speed control?
RE: Source of Large Motor Vibration
A shorted turn on the rotor will cause an imbalance in the magnetic circuit of the machine, and may also cause uneven heating of the rotor. If the rotor experiences uneven heating, the hotter side will expand and introduce a slight bend into the rotor. Either cause could explain the increse in vibration.
Do you know if the vibration is worse at high field excitation? If you want to test this, remember to leave the field at the test level long enough for the rotor to heat up (or cool down if testing at low field) - these big machines have long thermal time constants.
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
Pls verify what are the dominant vibration frequencies (measured on supplied motor).
You mentioned that “The freq of the vibration was primarily at 1x of running speed 515 RPM, hence, 515 Hz” – but for the 12/14-pole motor at 60 Hz the rotation frequency is respectively 10 Hz (speed 600 rpm) and 8.6 Hz (speed 515 rpm).
RE: Source of Large Motor Vibration
The comments apply only to a synchronous machine, and obviously not to a PAM motor.
RE: Source of Large Motor Vibration
Ray Micallef, P. Eng.
Power Generation/Utility Industry
RE: Source of Large Motor Vibration
Q: How many "arms" or "legs" are used in the rotor?
A: There are not discrete "arms" or "legs". There are two 1/4" steel rings around the stator iron which are welded to a support box housing with a continuous steel support rails running down each side of and the length of the box parallel to the length of the stator iron.
Q: Who is manufacturer?
A: Sorry but I am not at liberty to yield that information.
Q: What is cooling method?
A: Fan cooled.
Q: I am curious to know if these higher Vib. levels are being measured with Online instrumentation such as Phasor Probes (Eg. B Nevada)or is it a Portable Vibration Analyzer?
A: There are two measurement systems that have provided the vibration data and each agrees with the other. One system includes an X direction prox probe mounted to look at the shaft vibration at both the outboard and inboard sides of the motor. Also included is a key phasor on the motor shaft. These probes feed a Bently-Nevada 3500 rack which in turn feeds the information to the facility's DCS. The other system used the aforementioned probes but with Y direction prox probes added at the outboard and inboard sides. All these probes provided input to a Bently-Nevada "Audre" analyzer.
Q: Also check for distortion of stator frame due to soft foot.
A: This test was performed as a running soft foot test and it was determined that soft foot was not an issue.
Q: Try applying voltage with motor uncoupled... if vibration remains you can rule out misalignment.
A: The motor was uncoupled and alignment was not an issue. Note, however, that whenever we do couple the motor to the load, the motor vibration usually drops by 50% or so.
Q: How could the rotor spin when there is no power supply ?
Do you mean coasting down phenomenon ?
A: Yes, I was referring to the difference in vibration between the powered spinning motor and the coast down. The vibration levels reduce by about a factor of 5 once the power is removed and the motor is allowed to coast.
Q: Have you check the power supply quality?
A: Yes, it is very good.
Q: How old is the machine?
A: The machine is brand new.
Q: What kind of load is driven?
A: The load is a fan. But note that the worst vibration occurs in the uncoupled state.
Q: What is the starting arrangement?
A: Starting is a full voltage, across the line start.
Q: Is there a speed control?
A: The only speed control is done through the use of the PAM motor windings. A speed switch is used to switch from low to high speed. This switch merely reconnects the motor's single winding to achieve the speed change.
Q: Pls verify what are the dominant vibration frequencies (measured on supplied motor).
You mentioned that “The freq of the vibration was primarily at 1x of running speed 515 RPM, hence, 515 Hz” – but for the 12/14-pole motor at 60 Hz the rotation frequency is respectively 10 Hz (speed 600 rpm) and 8.6 Hz (speed 515 rpm).
A: At low speed the dominant vibration frequency is 515Hz which equals the low speed running speed (515 RPM). At high speed the dominant vibration frequency is 600Hz which equals the high speed running speed (600RPM). There is very little 2x or above components.
RE: Source of Large Motor Vibration
Suggested stator checks - winding resistance, surge test (phase comparison can find difference between phases due to miswinding). Air gap check (normally would cause 2*LF but still should check).
Seems like two more possible problems might be dynamic eccentricity due to excessive clearances or cracked/broken rotor bar. Can you do a high-resolution vib spectrum and look for pole pass frequency sidebands around 1x, 2x etc which is one indication of these problems. Also for rotor bar check - look at current spectrum ... look for pole pass sidebands around 60hz... or do a "single-phase test".
Cracked/open rotor bar problems are of course more common in high inertia load (like fans), particularly if there is a long start time, although it depends on motor design compared to application and it tends to develop after repeated starting, not new.
As for dynamic eccentricity ... were bearing clearanced checked (usually plastigage)?
Seems like another thing to ask... if horizontal readings are perhaps more than 4x vertical that would seem to suggest resonance. If vertical readings are perhaps 2 times horizontal that might suggest vertical resonance or base looseness.
RE: Source of Large Motor Vibration
The motor may have experienced some adverse conditions during transportation and installation, perhaps an excessive mechanical shock.
The power supply quality should be checked.
Other causes addressed in above postings may apply.gh cracked or open rotor bar(s) can be the
RE: Source of Large Motor Vibration
Suggestion: Although cracked or open rotor bar(s) can be the case, it is less likely that the new motor would experience these, since it might have gone through the QA/QC. Some QA motor test report or certification should be sought from the manufacturer.
The motor may have experienced some adverse conditions during transportation and installation, perhaps an excessive mechanical shock.
The power supply quality should be checked.
Other causes addressed in above postings may apply.
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
Sorry for my confusion. The primary frequency of vibration at low speed (515RPM) is 8.58 Hz and at high speed (600RPM)is 10.0Hz.
RE: Source of Large Motor Vibration
did you get the no. of stator & rotor slots ?
What does the motor(s) manufacturer have to say for these vibrations ?
RE: Source of Large Motor Vibration
I'll get the stator/rotor slots .... I'll need to lookup the drawings once I'm back in my office.
The manufacturer doesn't really have a clue thus far but they claim that the electrical design has been thoroughly reviewed an should be fine. They are reluctant to share the details of their review with us.
Early on in the issue we discovered that the rotor was designed with insufficient material in the "spider" arms that hold the rotor lamination core to the shaft. The rotor was re-designed and new rotors were produced. We had assumed at that time that the silver bullet had been found and indeed it helped the problem (as we assume it stopped some of the dynamic air gap variation while the machine was running) but the vibration issue did not entirely go away. The manufacturer has tried to balance the "electrical" vibration out with a comprimise balance (comprimise between the mechanical balance and reducing the electrical vibration) but with marginal success.
They are now looking at performing performing a surge test on the coils and if nothing is found there then they are thinking of discecting the thing. Ultimately they are looking at rewinding the stator.
Thanks for your interest!
RE: Source of Large Motor Vibration
I was referring to what you called the "spider arms!" Were there three or four? We had a such a machine vibrate intermittently with the 3-arm spider.
The root cause was unexpected "weakness" of rotor's 3-armed spider (wye-configuration). The initiating phenomenon was shaft-rub where the shaft exited thru a wooden (hard-to-believe) bushing in the surrounding enclosure that contained the forced-air cooling system. The rotor was also replaced with one having a 4-armed spider (cross-configuation)
RE: Source of Large Motor Vibration
I did not hear any indication that you had positively ruled out open rotor bars. It's unusual in a new motor but you never know.
RE: Source of Large Motor Vibration
Another thing that comes to my mind about these vibrations - stator coils being loose in the slots.
RE: Source of Large Motor Vibration
BUT, seems possible we might also see vibration at syncronous speed. Boscorio - have you done a high-resolution spectrum to confirm whether this is in fact the vibration is at 1*running speed or syncronous speed?
RE: Source of Large Motor Vibration
The 1x vibration frequency is at running speed, not synchronous speed. The peak of the spectrum occurs at that frequency and not at the synchronous frequency.
thanks for your interest!
RE: Source of Large Motor Vibration
RE: Source of Large Motor Vibration
another thought. This being a PAM motor, the speed change is obtained by manipulating the winding connections. If the winding connections do not ensure pole-phase group symmetry, then uneven magnetic pull across airgap will result leading to 1 X RPM electrical vibrations.
Could you mail me the stator winding connection diagrams for both the speeds to eewemf@myrealbox.com ?
RE: Source of Large Motor Vibration
it looks like syncronous motor,
So stator supplied with 3 phase AC voltage
(hopefully symmetric - better to check).
If vibration appears during load - reason
in one coil pair on the stator provides
different level of magnetic flux.
Assuming coils itself are OK,
I would check connection pole to core - this is real resistance to flux or
check voltage between artificial star center and real star center in winding - may be difference.
Thanks,
Vladimir.
RE: Source of Large Motor Vibration
I have tried to get my hands on those drawings but the manufacturer won't give them up. They're claiming the drawings are proprietary and they don't want to give away their designs. Hmmmm.....
I understand that PAM motors have an inherient imbalance in their flux patterns due to the nature of the method that generates the poles for a given speed. Despite the best of designs, it appears that such an imbalance may be something one must live with.
If I do come up with any drawings then I will certainly keep your offer in mind. Thank you.
RE: Source of Large Motor Vibration
You mention in your original post that there are a number of motors exhibiting the same behavior, but one is worse than the others. You also mentioned that these motors have been rewound because of an original problem. (I have to admit that my head is spinning with the amount of quality information you are getting from this site)
Especially if the "other" motors display unacceptable performance, this sounds an awful lot like a motor design flaw that is exacerbated in one particular motor by an exceptional electrical or mechanical defect. My quess is one of the mentioned possibilities upsetting the electrical "balance" of the motor.
Given the size of these motors and the fact that there is more than one motor involved, it sounds like there could be some sizeable monetary consequences. This could degenerate into what we politely call a "Urinating Contest" here in the states. If you don't find an obvious answer/solution shortly, I would get a highly reputable motor test lab on my side. Have them wring that motor out both electrically and mechanically. If they can't find it, you can at least claim to have done your due diligence on this problem.
Best of Luck