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.
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Source of Large Motor Vibration
- Thread starter boscorio
- Start date
Since you said that the vibration is the highest when the electrical power is turned on, the problem is in the "electric" side of the circuit. I have seen this problem before several times and there are several causes.
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.
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.
electricpete
Electrical
What is the speed of the generator and frequency of the vibration?
- Thread starter
- #4
The freq of the vibration was primarily at 1x of running speed 515 RPM, hence, 515 Hz. This is a 60 Hz machine. The vibration of the machine during coast down was primarily at 1x of running speed but when power was applied the 1x vibration was 2 to 3 times that during the unpowered state. Note that the machine is a two speed PAM motor and has a second speed, 600 RPM. At the high speed, the vibration is 4 to 5 times lower than at low speed. However, even at high speed the machine displayed a higher vibration powered than unpowered.
Many thanks!
Many thanks!
Vibrations could be caused from electrical issues.
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
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
Hello boscorio
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
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
electricpete
Electrical
I assume it is a syncronous machine since you say the vib appears when you energize [the field].
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.
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.
boscorio,
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 ?
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 ?
Something we experience on generating plant, which at 365MVA is a little larger than the machine you are discussing, may be applicable if the drive is a synchronous machine:
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.
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.
The original post mentioned that the subject motor is a two speed PAM (Pole-Amplitude-Modulation)motor, which is a single winding two speed squirrel cage induction motor. Then, why there are posts here about "field current / Shorted turns in the rotor winding" ?
boscorio,
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).
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).
Sorry Gentlemen, I have tunnel vision at times, Squirrel Cage. The cage alignment within the rotor should be verified. It may be slightly misaligned in relation to the rotor and there may be adjustments.
Ray Micallef, P. Eng.
Power Generation/Utility Industry
Ray Micallef, P. Eng.
Power Generation/Utility Industry
- Thread starter
- #16
This post is to answer many of the questions that have been asked since the original post. Thank you very much for your interest and suggestions in this matter. I apologize for being so long in answering the questions. Further insights, questions, and comments are welcome.
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.
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.
electricpete
Electrical
OK, new DOL-start 2-speed squirrel cage induction motor driving fan. 1x vibration on either speed higher when uncoupled (rules out misalignemtn). Vib 2-3x higher when energized than coastdown - suggests electrical related problem although not positively. Soft foot checked. Rotor and bearings are balanced and true.
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.
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.
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Althoucase, 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.gh cracked or open rotor bar(s) can be the
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
jbartos
Electrical
- Jan 15, 2000
- 6,440
Correction, I beg your pardon
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.
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.
boscorio, I don't understand your answer to rotman's comments about the frequency. Do you really mean that at 515rpm the vibration frequency is 515Hz, that is 60xrotational frequency (rpm being 'per minute', Hz being 'per second'). Or am I missing something really obvious.
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