Beating / modulating noise and vibration
Beating / modulating noise and vibration
(OP)
I have a drive train comprising of a motor (4 Pole 3 phase induction motor, 50Hz), resilient coupling and a compressor (screw compressor- 4 male lobes / 5 female lobe combination). These compressors make a strong modulating / beating noise, which gives a perceptional feel that something is wrong with the machine. I tried measuring noise as well as vibration data on the machine. Both the data indicate strong sidebands of 1.6Hz, which we hear as beating frequency. I have attached the measured vibration data for reference.
Since this machine is a direct drive machine (no gears), the rotational inertia on the driven side (compressor) is around 30% lesser compared to the regular machines. This is the only difference I could see it from the design point of view. Or is the absence of gears in this machine is by itself does not have the flywheel kind of effect, which I am not sure.
The sidebands are there in most of the response frequencies. The response frequencies are all multiples of the male lobe frequencies, as expected in a typical positive displacement compressor.
I also did a motor current signature analysis (MCSA) measurements and observed that the data is fine, with only a 30dB of 250Hz (5th harmonic) component in addition to the line frequency of 50Hz. I suspected whether I could get some side bands related to pole passing frequency, but I didn't get it.
Kindly looking for your valuable suggestions & views in this problem.
Regards & Thanks in Advance
Jeyaselvan
Since this machine is a direct drive machine (no gears), the rotational inertia on the driven side (compressor) is around 30% lesser compared to the regular machines. This is the only difference I could see it from the design point of view. Or is the absence of gears in this machine is by itself does not have the flywheel kind of effect, which I am not sure.
The sidebands are there in most of the response frequencies. The response frequencies are all multiples of the male lobe frequencies, as expected in a typical positive displacement compressor.
I also did a motor current signature analysis (MCSA) measurements and observed that the data is fine, with only a 30dB of 250Hz (5th harmonic) component in addition to the line frequency of 50Hz. I suspected whether I could get some side bands related to pole passing frequency, but I didn't get it.
Kindly looking for your valuable suggestions & views in this problem.
Regards & Thanks in Advance
Jeyaselvan





RE: Beating / modulating noise and vibration
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Beating / modulating noise and vibration
Is 1.6 Hz also the slip frequency?
RE: Beating / modulating noise and vibration
I was only trying to understand the problem as to what is causing this modulation.
My concern is whether this turns out to be a reliability problem, as my customers perceive. Is there any other measurements I can do to locate the problem?
I found very few literature / reference / case studies related to amplitude modulation (except those related to gears. I do not have gears in this machine)
I was thinking whether the torsional pulsation (being positive dispalcement type) is largely getting translated to speed pulsation, since the inertia on the driven side in this machine is lesser compared to gear driven compressors.
Any thoughts / directions ?
Tmoose.
I was looking for the slip frequency & polepass frequency in the motor current spectrum, but I couldn't find it. The sidebands around line frequencye are all 60dB less.
RE: Beating / modulating noise and vibration
For your query on slip frequency, this is 0.6333Hz. The speed is at 1462 rpm against synchronus speed of 1500rpm ( 50 Hz 4 Pole). I am not able to find where the 1.6Hz in coming from.
In the current signature, I could see only pole pass frequencies at 2.5Hz side bands at 50 Hz cenetr frequency with amplitiude of 35dB.
Thanks
Jeyaselvan
RE: Beating / modulating noise and vibration
On the other hand obviously there are electrical slip angles, but the numbers don't seem to add up.
yes the literature on this stuff is lacking.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Beating / modulating noise and vibration
It's been a while, so I forget long ago which parameters to use. Probably maybe no instrument power, velocity probe on an old Balmac swept filter (everything bur the fire alarm is down in the noise) or IRD DataPak. I'd have to play with with a signal generator for a while to get the set up even close. (Pure tone at 100 Hz loud enough to be 80 dBA on the meter, then play with the sensitivity so the spectrum overall was 80, etc, etc for a few higher frequencies. Sometimes just looking at it all linear made it clear what was really the bad guy.
RE: Beating / modulating noise and vibration
I suspect the problem is a kind of modulation (amplitude / speed I am not sure), since I see the sidebands of 1.6Hz in most of the system response frequencies (lobe meshing) . I understand beat as Greg said as sum of two closer sinusiods. The time data for beating and modulation looks the very same, I think.
Greg, the only joint in the system is the resilient jaw coupling which couples the motor and the compressor.
I am unable to find where the 1.6Hz sideband modulation is coming from. Could this be the load modulation itself (load being positive displacement, but the pulsation frequency quite high 4 times the motor speed).
Any possibilities with the lower load to motor inertia ratio?
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
Walt
RE: Beating / modulating noise and vibration
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Beating / modulating noise and vibration
Now correlation doesn't equal causation but they seemed pretty convinced it was a motor speed variation. I would have thought that the runout tolerances on a screw compressor (especially a contacting one) would be pretty tight and unlikely to cause serious out-of-balance so the motor seems the most likely to me.
M
--
Dr Michael F Platten
RE: Beating / modulating noise and vibration
Walt : This compressor does not have timing gears & the modulating valve. This is an oil injected compressor, in which the male rotor (4lobes)drives the female rotor (5 flutes). I will check for bearing frequencies.
Greg : Yes, the motor drives the male rotor, which in turn drives the female rotor. The female rotor runs at 4/5 times the speed of the male rotor / motor.
I have some new information from my latest test. When I couple the compressor to a slightly larger motor (larger GD^2, 30% higher), the modulation frequency gets dropped from 1.6Hz to 1.0Hz and audible irritation of modulating noise has also come down(not fully, but tolerable). I am not able to explain this.
Since this compressor is a direct drive machine without gears as stated in my first post, I was suspecting whether the inertia ratios on the drive and driven side has anything to do with this.
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
We have an interesting datapoint that the frequency of vibration went down upon changing the motor. We note there is a change in inertia, but we have to ask whether anything else changed.
Are you positive this is not pole pass frequency? How was running speed established (how good was the frequency resolution). I'm not sure why pole pass frequency modulation would show up on 2 different motors, but if it did the frequencies could be different even for same load level (since nameplate speed varies slightly).
In your original post you started asking about torsional oscillation. Certainly decrease in frequency with increase of inertia would be consistent with torsional resonance. And we know positive displacement type machines can excite torsional resonance.
BUT:
1 – the torsional excitation would typically be a much higher frequency.... what possible torsional excitation exists at 1.6hz on this machine?
2 – I would also expect to see sidebands around 1x in current signature.
If you look at this thread you will see example of a belt-driven recip pump that I believe experiences torsional oscillation:
thread237-249262: Low power factor, recip pump
When current and voltage are plotted together, there was evidence of change in power factor angle over time varying at the frequency that would be torsional oscillation frequency. In fact this particular torsional oscillation resulted in power reversals (points in the waveform where power factor dropped below zero and power was delievered from motor back to power system). Later at a LEMUG meeting I saw a very similar case study.
Some more comments about torsional resonance:
If we had 2 simple rigid masses (like motor and compressor) connected by flexible element (coupling), then the radian torsional resonant frequency is:
w = sqrt(Kcoupling/Jeq)
where Jeq = Jmotor*Jcomp/(Jmotor+Jcomp)
You might check the change in frequency against this formula for kicks.
BUT unfortunately, the dynamics of an induction motor are much more complicated than modelling it as a simple mass, even if we include torque associated with "torque speed curve" which suggests simple damping. The problem is that the torque speed curve is a steady state concept and does not apply to dynamic analysis. Krause lays out a very detailed model for induction motor dynamic analysis that is widely used. It results in 5x5 linearized state space matrix that gives 5 eigenvalues (2 complex pairs and a real value).
Attached are Krause's summary of predicted eigenvalues for 4 "typical" motors of speed 1800rpm. Horsepower ratings range from 3 hp to 2250 hp. There is assumed connected a load inertia equal to the motor inertia. The imaginary part of the complex eigenvalues would be the radian oscillation frequency... of course have to divide by 2*pi to get hz. The lowest of any of these motors in any of these conditions is in the neighborhood of 18 rad./sec (around 3hz) which is almost double or triple of your 1.6 and 1.2 hz. All of the others are quite a bit higher with next highest being around 40 rad/sec or 12hz.
Now it could be that your motor has higher inertia or significantly different parameters, but I sort of doubt it (for reasons discussed above torsional resonance seems not to fit). But if you care to post complete nameplate data and inertias I would be glad to compute the eigenvalues for you using Krause's method.
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RE: Beating / modulating noise and vibration
2 – I would also expect to see sidebands around line frequency in current signature.
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RE: Beating / modulating noise and vibration
Without knowing the torsional spring constant of coupling, we could still apply rigid-body model:
w = sqrt(Kcoupling/Jeq)
where Jeq = (Jmotor*Jcomp)/(Jmotor+Jcomp)
Let J1 and w1 correspond to motor 1 and J2 and w2 correspond to motor 2. Then
w1/w2 = Jeq2/Jeq1 = [J2/J1]*[(J1+Jcomp)/(J2+Jcomp)]
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RE: Beating / modulating noise and vibration
Regarding polepass frequency, I estimated that to be at 2.5Hz and was able to see that in the current signature with magnitude less than 60dB when compared to the 50Hz (line frequency). Typically we would expect to see that in case of motor rotor bar (and this is a new motor) related issues, I understand from your earlier posts in Engtips & maintenance forums. What I get in vibration (and noise) signature is a pretty clear 1.6Hz sidebands(measurements with 0.16Hz resolution). Meanwhile, I will collect information for Krause model for the detailed induction motor dynamic analysis and thanks again for offering me to carry out the analysis.
Make : Siemens
Power:15kW,Voltage:400+/-10%, Current:28A,Freq:50Hz
Eff:92%, Rated RPM:1478,pf:0.83 ,Class F
Brg - DE: 6310C3, Brg - NDE:6309C3
I have estimated the torsional natural frequency (from a 4DOF model) of the system as 65.3 Hz, which has good safety margin from the excitations. With the larger size motor, the couping torsional frequency is at 62.4Hz. If I need to change the torsional characteristics of the system significantly, I have to change the coupling stiffness. I am working on this as well.
My intention of changing the motor (in turn inertia) was to see whether this has a flywheel effect, since this compressor (direct drive) does not have gears compared to the conventional ones.
I do suspect misalignment as a concern, since the motor is coupled through an adapter ring (with spigots on either sides) to the compressor. This component has not undergone any dynamic designs and also has some history of previous manufacturing issues with even 500micron concentricity errors. The drive train is assumed to be self aligning (assumed?)with the housing on motor & compressor coupled through adapter.
I have a paper (attached),which cities experimental results of modulations arising out of misalignments. The paper says for his range of misalignments, the modulation frequency varies from 0.5Hz to 3Hz. This is new information to me, since I was of the view that misalignments induces 2X or higher excitation orders as well as phase vraitions depending on nature of mislaignments. I am not aware of any mathematical models predicting modulations arising out of misalignments. Kindly for your suggestions.
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
RE: Beating / modulating noise and vibration
That is good motor data. You don't happen to have 50% load data? (current, power factor, efficiency... any 2 of these 3)
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RE: Beating / modulating noise and vibration
The 50% data would be nice (to get better estimate) but not necessary.
Motor inertia would be nice (but not necessary...I have some thumbrules to estimate it).
What is necessary would be some idea of the inertia of the compressor or means to estimate it.
Also the Krause model focuses on electromagnetic side without much detail on mechanical side... therefore assumes rigid connection of the motor inertia and load inertia. I'll show you what I come up with (once compressor inertia is known) and you can decide if it is worth to try to modify the model for more detail on the mechanical side.
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RE: Beating / modulating noise and vibration
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RE: Beating / modulating noise and vibration
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RE: Beating / modulating noise and vibration
The torsional inertia of the complete drive train as below
J1=0.93/4; % Motor mass inertia in kgm^2
J2=0.012/2; % Coupling motor side(drive side)
J3=0.012/2; % Coupling airend side(driven side)
J4=3.69/9.81/4; % kgm^2, Airend mass inertia
k1=18240*9.81; % Nm/rad,MOTOR'S torsional STIFFNESS
k2=11913; % Nm/rad, Coupling torsional stiffness
k3=288245;% Nm/rad, Compressor shaft torsional stiffness
Thanks for your reference on the sidebands arising out of soft foot for the blower. I did carried out some bump tests, but could find the first natural frequency at 16Hz.
Apart from soft foot, " is there any other dynamics that could induce low modulation frequencies"?
Kindly for your suggestions.
Regards
Jey
RE: Beating / modulating noise and vibration
One other thing to check: is there evidence of oscillation at this frequency in the output pressure or flow indications?
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RE: Beating / modulating noise and vibration
My suspect is now on the rotor rub. Since when I try to freeze TWF for two revolutions, I could see four impacts per revolution corresponding to the lobe numbers ( 4 male lobes / 5 female flutes).
HOW DOES RUBBING RESULT IN LOW FREQUENCY MODULATION? This is what I am trying to answer this. I am looking for mathematical models / papers. Kindly for your suggestions and reference.
Thanks in advance
Jeyaselvan
RE: Beating / modulating noise and vibration
Induction motor noise in the form of beating, commonly arises from the interaction between mechanical vibration frequencies and line frequency. For a 4-pole motor, this interaction is between line frequency and twice actual running speed frequency.
4 pole 50Hz motor
Nominal Speed = 1500rpm = 25Hz
Actual speed is load dependant
Quote:For your query on slip frequency, this is 0.6333Hz. The speed is at 1462 rpm against synchronus speed of 1500rpm ( 50 Hz 4 Pole). I am not able to find where the 1.6Hz in coming from Unquote
Actual speed = 1462rpm – 2X = 48.73Hz - dS =1.3Hz
Where dS = difference between 2X running speed and line frequency
Quote:I have some new information from my latest test. When I couple the compressor to a slightly larger motor (larger GD^2, 30% higher), the modulation frequency gets dropped from 1.6Hz to 1.0Hz and audible irritation of modulating noise has also come down(not fully, but tolerable). I am not able to explain this. Unquote: –How about bigger motor, working less hard, higher actual running speed and lower slip, lower dS
I would suggest that before going off chasing esoteric hypotheses, you obtain some high resolution data (I further suggest a spectrum with a span of 200Hz and 3200 lines): note that the time taken to collect one spectrum of this nature will be quite long, so you need patience. It would be useful if you could collect several successive spectra and plot them in a waterfall format allowing you to see which components are present.
Beating is usually more of a nuisance than an actual problem, since operators complain about it. You need to quantify the vibration and compare to a standard in order to assess the severity of the problem in terms of potential to shorten machine life.
You can spend a lot of money and achieve little in these situations
RE: Beating / modulating noise and vibration
But TPL's calculation of the frequency of interest (1.3hz) does not seem relevant to me. 2x 1462rpm running speed would be 1.3hz away from 50hz line frequency.... but who cares.... line frequency is not present in vibration spectrum of an induction motor.
What is usually present in vibration is not 1*line frequency, but 2*line frequency. The interval of interest for adjacent peaks is pole pass frequency:
FP = (1500-1462)*4 = 152 cpm = 2.5hz.
This FP = 2.5hz would be the spacing between 2*LF vibration peak and 4*Running speed peak on a 4-pole motor.
Additionally, in presence of rotor electromagnetic assymetry (dynamic eccentricity or rotor bar defect0, the pole pass frequency 2.5hz is also the frequency of sidebands around running speed in vibration and around line frequency in current. The 1462 rpm speed measurement is to small extent corroborated by mentioned 2.5 hz sidebands in current (14 Jul 10 9:31) which is very normal at a level around 45 or more db below fundamental since there is always some assymetry.
However, I was just noticing you mentioned 35 db difference (14 Jul 10 9:31), which is very much on the high side for these current sidebands (high sideband magnitude, low db down from main peak). Other places you mentioned 60db. Can you clarify or post current spectrum?
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RE: Beating / modulating noise and vibration
A few more random thoughts:
1 – are there any of these 2.5 hz sideband around running speed? If so, it would be interesting to look at the shaft under a strobe tuned to running speed.
2 – Is this machine fed from vfd by any chance?
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RE: Beating / modulating noise and vibration
I will post the MCSA & vibration data & waterfall plots after making high resolution measurements by the end of this week.
TPL : I understand beating is more of a perception issue than the real issue, but this is critical for me, since if not addressed, this problem has a significant influence on the business and hence I am trying to address this. I want to make one point more clearer, the problem is MODULATION (with sidebands on either side of response frequencies) technically and not BEATING.(ofcourse, the perception & TWF for both the cases are the same, but not the spectrum. Kindly ignore this, if this is clarified apriori).
This modulation problem is there with most of our machines which have this particular type of compressor (am an OEM). Just to clarify that the problem is not for one particular machine, it is there for the whole family which uses this type of compressor.
However, on checking with our another family of compressors I do not see the kind the rubbing which I have posted on 3Aug 10 1:08. Hence my approach tends to converge more towards the compressor than the motor (Also, all the tests are done with new motors). The absolute vibration levels are 3 times higher than our regular machines (4.5g against 1.5g).
Thanks electripete : 1) I had plans to strobe the machine. Since I don't have one in hand, I will try this thorough external means. With 0.16Hz resolution for most of my data, I see the side band at 1.6Hz from vibn data. For MCSA, I used an Agilent scope in addition to recording through DAQ/Labview (I need to check the dynamic range of my DAQ card).
2) This machine does not have a VFD. I have plans to connect this through one to understand the sensitivity to operating speed (+/-20%).
Will post on the developments.
Thanks & Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
The waveform plot suggests that your quote of 4.5g is in peak terms - although you say this is higher than typical, 4.5g peak isn't really high - I wouldn't normally have any concerns until levels exceeded 6g.
The acceleration data has value, but can you collect the same data in velocity format?
Rubs generally don't last for very long - they either clear themselves by wearing between the contact surfaces or cause a bit of a bang. The nature of a rub means that its vibration characteristics are not very stable, showing significant changes with time. What are you basing the idea of a rub on? The waveform data doesn't really look like a rub, appearing to be more of an impulsive nature, such as that which might result from a pressure pulsation - can you get a pressure transducer into the suction and discharge line?. Any odd valves or piping arrangements that might explain your concerns? Is this compressor fitted with a slide valve that might be misbehaving?
I would suggest that if your concerns are related to your customer being bothered by noise, that you address this issue directly and follow the advice given by TMoose and carry out analysis of the noise.
You have lots of suspicions and concerns, all of which appear to be supported by assumptions rather than data. No harm in carrying out a precision alignment (and checking/eliminating softfoot) - check for piping strain as well.
Sorry to sound negative, but I do think you've missed something here. This type of compressor is inherently noisy in terms of vibration and sound.
RE: Beating / modulating noise and vibration
I will post the vibration velocity data after measurements. Will you kindly look into the attachment in my original post, where I have posted both the TWF and spectrum and some zoomed portions of spectrum with sidebands to make my point on modulation over beating. Please correct me, if my representation of the problem as modulation over beating is incorrect.
I was only comparing the acceleration values with machines, which do not have this modulation problem, since I suspect the modulation problem with all machines using this family of airends (terminology used to represent the compressor elements-for those not in compressor industry).
Wish to inform you kindly, all the assumptions (atleast most of them) are validated with measurements and the tests are all carried out to support my observations based on the attached PDPC chart, which I normally use for specific design & development related problems. I have made supporting measurements for nullifying each and every hypothesis in the chart that possibly could result in the problem I am working on.
I have also enclosed the pressure measurements made on the discharge of the compressor. I am aware that this sensor has a dynamic response only until 250Hz, however used to see whether any kindly of low freq modulation is induced by the downstream. This compressor does not have a slide valve.
Thanks again. I am determined to see to it, probably to the end of it.
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
You put a bigger motor in place and the problem reduces - do you have data similar to that shown to suggest that the slip changes?
Is it possible that the 1.6 Hz sidebands are irrelevant to this?
If it is the noise that is the concern, are you sure that the making vibration measurements is the right way to go? I would have thought that characterising the noise issues would have preceded, and driven the vibration measurements.
RE: Beating / modulating noise and vibration
Is it not the other way round for beat freq calculation?
1 cycle takes 0.64secs. Hence beating cycle frequency 1/0.64=1.6Hz.
You may kindly find attached the data with the larger size motor, with which the modulation changed to 1.1Hz (from 1.6Hz with orginal motor).
In fact all my analysis started with microphone measurements (because the complaint was primarily on noise & to ensure relaibility). The spectrum is a lot distorted, probably by the radiation effcienices of the housings.(Nevertheless the modulation is also observed from TWF of mic data as well, which i have attached in the last slide). When similar modulation is observed in vibration data, then all further analysis are carried out with vibration data. (I do have a mic meas at every test I carried out).
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
From your latest post QuoteNo modulation as seen in TWF, but spectrum shows 1.1Hz sideband ( may be low modulation index, hence not seen in TWF. Not audible as well)Unquote
Not audible as well????? doesn't all that just mean that the 'problem'just isn't there?
In the absence of anything other than a noise that your customer complains about, isn't the answer simply to use the larger motor?
What was the actual speed of the larger motor? Is it possible (with a slip if 38rpm) that the orginal smaller motor was simply a little undersized and that the noise is just its way of protesting?
RE: Beating / modulating noise and vibration
Since this is for a OEM product, I may not have the option to use a higher motor size, since these are manufactured in larger nos. Your point on undersized motor could be one possible cause, because quite often these motors (quite common to load upto 1.1SF on a 1.15SF motor)are loaded to their throat! The larger motor runs at 1483RPM (since we are not loading the motor to its rated load, being a larger size motor)against the nameplate rated speed of 1471rpm.
Having spent quite a good amount of time on this, I am more interested to know about the source of modulation. The areas I could think of with the larger motor could be
i) the effect of inertia ratios of the drive and driven side of a drive train: but why would this influence the modulation ?
ii) In the event of rub (usually these compressors contact at the pitch point, specifically pitch line, but if not? ), the larger motor has more accelerating torque and hence can quickly (1.1Hz) pass across the rub while with the normal sized motor this is quite hard for the motor to provide torque to overcome the rub (1.6Hz). I will look for this on tear down of this machine after my measurements.
Still, I will reserve the "inertia effect" for the last option, if I am not able to make progress in my diagnosis. Atleast, I will have a containment for the problem until I unserstand the cause.
Regards
Jeyaselvan
RE: Beating / modulating noise and vibration
a) Motor current spectrum analysis is a good method for detecting mechanical torsional vibrations
b) Have you tried to stop oil injection momemtarily to measure if modulation stops?
c) How about adding a trial flywheel mass to cooling fan end of motor to simulate the equivalant inertia of the larger motor that does not have modulation?
Walt
RE: Beating / modulating noise and vibration
Regards
Jeyaselvan