Hydrogenerator Vibration & Noise
Hydrogenerator Vibration & Noise
(OP)
I've got a unique situation with a recently re-stacked hydrogenerator. Vitals are:
# poles = 56 ; # stator slots = 360 ; line freq = 60 Hz ; rpm = 128.57
Old design had 4 stator core splits ; New design uses a continuous core stack
Vibration taken directly from the back of the core iron shows a 4.3 g-pk component at 720 Hz, with the next largest response at its first harmonic of 1440 Hz at 0.45 g. The adjacent keybar showed similar but slightly higher radial responses of 4.8 and 0.9 g-pk, respectively. Tangential responses on the keybar were 0.9 and 2.4 g, respectively.
Through each spectrum 120 Hz harmonics are present, but they are comparatively low level, typically 0.05 to 0.09 g.
The 720 Hz vibration creates significant audible noise, with levels that increase directly with load. At FSNL and with excitation on, we have about 75 dbA at 20' outside the generator enclosure. At 35MW load, we have 90+dB, with the predominant 720 Hz component.
Sister units to this problem unit operate at full load with noise levels in the 75 dBA range; no access for vibration measurements at this point.
Our client is not accepting the unit for operation based upon the noise being generated. We have had several theories regarding the source of the noise, with possible solution. But unfortunately, none of them has been very viable from an operational perspective.
I would appreciate any commentary on the issue.
Thanks,
Stan
# poles = 56 ; # stator slots = 360 ; line freq = 60 Hz ; rpm = 128.57
Old design had 4 stator core splits ; New design uses a continuous core stack
Vibration taken directly from the back of the core iron shows a 4.3 g-pk component at 720 Hz, with the next largest response at its first harmonic of 1440 Hz at 0.45 g. The adjacent keybar showed similar but slightly higher radial responses of 4.8 and 0.9 g-pk, respectively. Tangential responses on the keybar were 0.9 and 2.4 g, respectively.
Through each spectrum 120 Hz harmonics are present, but they are comparatively low level, typically 0.05 to 0.09 g.
The 720 Hz vibration creates significant audible noise, with levels that increase directly with load. At FSNL and with excitation on, we have about 75 dbA at 20' outside the generator enclosure. At 35MW load, we have 90+dB, with the predominant 720 Hz component.
Sister units to this problem unit operate at full load with noise levels in the 75 dBA range; no access for vibration measurements at this point.
Our client is not accepting the unit for operation based upon the noise being generated. We have had several theories regarding the source of the noise, with possible solution. But unfortunately, none of them has been very viable from an operational perspective.
I would appreciate any commentary on the issue.
Thanks,
Stan





RE: Hydrogenerator Vibration & Noise
720 Hz is 6x 120 Hz. My guess is that there is looseness that causes the high harmonic, and there must be a resonant structure near 720 Hz. I use a pressure gradient microphone to scan all radiating surfaces and any air vents. You can try a conventional pressure mircophone, but it is not as directional. You could use a light weight accelerometer to measure where the vibration "feels" high. If you find and correct the resonant structure and the sound and vibrations become normal levels, then you are done, otherwise you may have to track down where the looseness (rattle) is occuring and correct that. An ultrasound meter may also be helpful to detect location of the "rattle" noise.
Walt
RE: Hydrogenerator Vibration & Noise
I've been suspicious of looseness based upon the 120 Hz harmonics, but the group seems to feel that with the brand new re-stacked core that things are 'tight'. One area that is in question is the fit between the keybars (vertically mounted between the stator frame and core iron around the periphery of the core), and whether there should be any clearance there or not. Do you have any experience there?
RE: Hydrogenerator Vibration & Noise
I don't design generators, but based on designs that I have seen, I think all joints should be tight. Several years ago I worked on a project that had two new identical generators side-by-side. One had a loud noise, similar to your description, and the other did not. The manufacturer said all was OK. My recommendation to owner was to demand an extended warranty covering this issue.
Another project involved a large (450 MW) generator with very high vibration and sound at 120 Hz. The technical description (mostly spin control) was unbelieveable, because they were trying to avoid retrofit of a fix on several machines in service or on order. Good luck dealing with the Big Boys on this problem!
Walt
RE: Hydrogenerator Vibration & Noise
I will have to think awhile about possible electromagnetic sources.
If tighteness of the wedges were a concern (not in my opinion...I would expect to see 120hz + harmonics in that case... in fact have seen it before), then a tap test could be done if the machine were disassembled.
What's a keybar?
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RE: Hydrogenerator Vibration & Noise
RE: Hydrogenerator Vibration & Noise
300 slots sounds awfully big (how many horsepower and what voltage?). I don’t have experience with as big as machine as I think yours, but that sounds pretty high for long-term reliability of a big machine. Over time the vibration affects the coils in the slots, their semi-con coating (assuming they are above 7kv or so), the strand insulation (I don’t think those big guys have turn insulation), the core insulation etc.
You have 56 poles for each of three phases, so a total of 168 pole phase groups. To put 168 groups into 300 slots, you need a fractional slot winding (probably 132 groups of 2 plus 36 groups of 1. In the motor world, it’s not uncommon for fractional slot windings to have much much more electromagnetic noise than other motors. We have two families of large fractional slot winding motors at our plant and they are by far our noisiest motors at the plant.
I am pretty sure what you are seeing is electromagnetic related and likely inherent in the design of the machine.
Some info that may or may not help to pin it down:
1 – Does vib change with change in excitation.
2 – Does vib change with change in load.
3 – Is there a regular pattern of rise and fall of vib around periphery of the backiron. That may tell you the spatial harmonic of offending field and may provide a clue.
4 – Check for current unbalance in stator. That can increase electromagnetic vibration.
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RE: Hydrogenerator Vibration & Noise
I would get the OEM involved in analysing the electromagnetic harmonic performance.
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RE: Hydrogenerator Vibration & Noise
There is indeed a fractional slot winding (6 3/7 I believe).
We feel it is likely electromagnetic as well, and inherent in the design. But we've got a high noise (95 dB) situation, and a customer unwilling to accept the unit. So, we are trying to determine what exactly is the mechanism that is generating the 720 Hz. That's where we're hung up.
The vibration and noise increase directly with load and field current, but not with excitation (as reported to me).
We are currently outlining a battery of tests for the next site visit, and current balances are in there, as is consideration of using an external DC excitation to see if ripple effects are contributing to the 720 Hz activity.
Any other thoughts from an electrical testing perspective?
Thanks for the help.
Stan
RE: Hydrogenerator Vibration & Noise
The 4 pieces I assume were 4 90 degree quadrants? If so that seems like it could be due to:
A - ease of manufacture:
B - intentional part of the electromagnetic design.
If A, I would assume the 4 quadrants would be fitted together tightly as tightly as possible. If B I would assume maybe a small gap.
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RE: Hydrogenerator Vibration & Noise
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RE: Hydrogenerator Vibration & Noise
RE: Hydrogenerator Vibration & Noise
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RE: Hydrogenerator Vibration & Noise
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RE: Hydrogenerator Vibration & Noise
-keybars wedged in tight
-keybars loose or removed.
-install another temporary support between keybars for extra stiffness
Also thinking some more about determining the spatial harmonic, assuming it's a rotating wave rather than a standing wave, you couldn't determine it from vibration magnitude, but you could determine it from phase measurements around the backiron.
If noise is the issue more than vibration, is it possible that some kind of treatment to absorb/damp the sound would help? (I suspect this might be a last-resort option if you can't limit it at the source).
There are a number of other checks you might consider if they're easily achievable, but don't have any apparent tie to the problem: airgap check, winding resistance test, pole drop test.
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RE: Hydrogenerator Vibration & Noise
http://ho
Let me know after you download it so I can delete it and free my website space back up. Thx.
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RE: Hydrogenerator Vibration & Noise
We are looking at re-tuning the core by linking mechanically to the frame, but IMHO it is way too trial and error. I doubt if anyone will be able to model and predict the effects.
The rewind contractor is standing firm that the vibration levels are acceptable, and I essentially agree except for the noise being generated. I'm personally leaning toward damping materials on the frame sections and sound absorbtive materials on the interior of the overall generator enclosure.
RE: Hydrogenerator Vibration & Noise
Regards,
Tom Moritz
Mechanical Engineer
US Bureau of Reclamation
RE: Hydrogenerator Vibration & Noise
Thanks,
Stan
RE: Hydrogenerator Vibration & Noise
Can't help you out with a point of contact on this subject. I'm relatively new to Reclamation and most of the people I've met are mechanical engineers by trade.
Just a few more questions, you obviously used a FFT spectrum analyzer to measure the vibration spectrum. Can you tell me what frequency span and how many lines of resolution you used? The slot pass frequency for this unit is 771Hz, not to far off from the 720Hz you stated in the original posting. From what I've read (I've got limited knowledge in electromagnetics) slot passing excitation is a result of differences in reluctance of the stator iron and the winding. Doesn't seem like this would be an easy problem to fix.
Good luck,
Tom Moritz
Mechanical Engineer
US Bureau of Reclamation
RE: Hydrogenerator Vibration & Noise
Data was taken out to 2 kHz with 1600 lines resolution.
RE: Hydrogenerator Vibration & Noise
Take a look at the following thread on the Vibration Institute's forum page: http://www.vibinst.org/forum/topic.asp?TOPIC_ID=78
Regards,
Tom Moritz
Mechanical Engineer
US Bureau of Reclamation
RE: Hydrogenerator Vibration & Noise
I estimated slot pass frequency: 2*slots*line freq/poles. This produced the 771Hz.
More accurate is ((2*slots/poles)+-1)*line freq. Using this calc I come out with 831Hz and 711Hz. I don't know the significance of the +-1 component in the equation.
Your FFT resolution was definately high enough. How broad was the peak? What window were you using?
Tom Moritz
Mechanical Engineer
US Bureau of Reclamation
RE: Hydrogenerator Vibration & Noise
RE: Hydrogenerator Vibration & Noise
Tom Moritz had a very good idea about stator slot pass frequency. Multiply rotor speed x #Slots and get 771.4 Hz. A 2kHz spectrum with 1600 lines only has 1.25 Hz resolution, so stator slot frequency is a good possibility. The excitation mechanism can be aerodynamic (siren effect) due to high spot (one rotor bar) on rotor or rotor eccentricity. Look for sidebands around stator slot pass frequency spaced at rotor speed indicating amplitude modulation.
Walt
RE: Hydrogenerator Vibration & Noise
1-The vib depends on load and excitation current. 2-The backiron is vibrating like crazy.
Btw this is hydrogenrator. I assume it would have a salient pole rotor with 56 pole pieces.
Out of curiosity Sam, can you clarify this:
"The vibration and noise increase directly with load and field current, but not with excitation (as reported to me)."
I'm pretty sure that field current and excitation are the same thing. The rotor is a dc circuit and rotor field current varies with excitation voltage. Right?
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RE: Hydrogenerator Vibration & Noise
In my post on Aug. 17 I stated some confusion regarding the +-1 component in the formula for calculating slot pass frequency. Am I correct to say the +-1 components are the sidebands you refer to in your posting on Nov. 15?
Regards,
Tom Moritz
Mechanical Engineer
US Bureau of Reclamation