Can beating frequency in a srew compressor be at 22 Hz? 2

[mechanicaljw]

Hello All,

We are having a screw compressor that has 3 male lobes and 5 female flutes that runs on a 90kW, 4 pole, 50 Hz motor with synchronization speed of 1480 rpm. However, at a speed of 1350 rpm, we are experiencing a resonance frequency of 22 Hz. We are unable to pin point the source of this frequency and to tell whether it is torsional or structural? Because the driver is the female i computed the difference between 2 times line frequency, which is 45Hz and the excitation frequency and got 22.5 Hz. In our measurements we are getting 22 Hz and error of about +1 as the resonance freuqncy. So at times we are close to 23Hz. Since i know the beating frequency to be result of two frequencies interacting, i am woundering whether this is the cause of our problem? Or am I doing something wrong? Because for beating to occur from what i have read the two interacting frequencies are usually quite close to one another? What is also happening is that at this 1350 rpm where we have this resoance the torque suddenly increases. So we to figure out whether the this is as a result of the resonance?

Any thoughts would be appreciated.

Thanks!

Jimmy

 

[Strong]

1350 rpm = 22.5 Hz!! Am I misunderstanding your stated problem?
If this is the excitation frequency, then resonance could be lateral and excited by misalignment or unabalance or it could be torsional. I would conduct impact-response test on motor-compressor-piping and also measure torsional vibrations. I use a stain-telemetry system for both static and dynamic torque (torsional vibrations).

Walt

 

[mechanicaljw]

Hi Walt,

Thanks for your thoughts. I probably was not too clear in my problem statement. The resonance frequency we are getting at 1350 rpm is 22.5 Hz. So the question now is where is it coming from? So i was thinking that it could be due to the difference between two times the line frequency and excitation frequency (ie 2*45-1350*3/60=22.5Hz). The reason being when i compute this I am getting 22.5Hz, which is the resonance frequency. This difference i am thinking could be interpreted as the beating frequency, since it is as a result of the interaction of two close frequencies. This is where i am not sure and would love to know whether having 22.5 Hz as beating frequency is feasible?

Thanks for your suggestions.

Jimmy

 

[electricpete]

What Walt strong said makes sense to me.
Isn't 22.5 hz the 1x running speed vibration?

1350/60=22.5

=====================================
(2B)+(2B)' ?

 

[mechanicaljw]

Hi Electricpete,
You are right that it is the 1x running speed vibration. So does this mean that this is the source of the problem? And if yes how could this be possibly resolved? I am new and so forgive me if my questions are basic.

Thanks!
Jimmy

 

[Strong]

Jimmy,

2x line frequency is 100-Hz (electrical from Magnetostriction with 50-Hz power)
22.5-Hz x 5-lobes is 112.5-Hz (Pulsation frequency, assuming the female-rotor is direct-driven without gears)
The possible Beat Frequency is 112.5 - 100 = 12.5-Hz (Beat period is 1/12.5 = 0.08-seconds and probabaly not audible.

Have you actually performed vibration analysis on this machine?

Walt

 

[mechanicaljw]

Hi Walt,

Thanks again for your input and your suggestions. We have done and still doing vibration analysis on the machine. To your comment. The line frequency at where we are having the the resonance is 45 Hz at the speed of 1350 rpm. The motor has a VFD attached and so at this speed the line frquency is 45 Hz. Inserting that into what you did would give us a possible beat frequency of 22.5 HZ? Or are you suggesting that the line frequency for a motor is independent of the rotational speed?

I would be grateful if you can refer me to any citation that best describes how the vibration analysis should be performed.

Another critical point (question): We did monitor the input torque and did an FFT of this input torque and clearly we have 22.5 Hz as a resonance frequency in this input torque signal to the compressor. But how do find the component(s) that this input signal is setting into resonance using vibration analysis?

Your help is appreciated and I'm grateful.

Jimmy

 

[Strong]

Jimmy,

I forgot about the VFD!
2x line frequency (from VFD) is 90-Hz
This is not likely to Beat with Pulsation frequency at 112.5-Hz (yes, the difference is 22.5-Hz, but no Beat here)

What is the dominant vibration frequency when motor speed is 1350 rpm? Why do you say a Beat is present; audible or in measurements?

As stated before, if dominant vibration frequency is at motor speed, then excitation source is probably unbalance or misalignment. A resonant sructure or torsional may provide amplification. A Beat is not a source of excitation, but it can be the result!

Walt

 

[GregLocock]

A beat frequency cannot cause a strucural problem, as it doesn't actually exist.

That is, a signal at 100 Hz, and another at 105 Hz will beat at 5 Hz, but there is no physical energy at 5 Hz.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mechanicaljw]

Another problem or question about why are we having an increase in the torque at this resonance frequency? Any idea or theory behind this behavior? We have done modal analysis (FEA) of our compressor and gear housing and we are getting low frequencies that we suspect might be the source of our excitation. Can this be the case?

@Walt: Thanks so much for your input and insight. Well appreciated. To your question about the dominant vibration frequency when our motor speed is 1350 rpm. I am actually not on site where we have our machine. So i have asked the guy in charge to feed me this information. I would let you know when he writes back.

Thanks!
Jimmy

 

[BrianE22]

Your force (or torque) can go up or down at resonance depending on whethor you are applying it on the spring end of the spring/mass system or on the mass end of the spring/mass system.

 

[GregLocock]

Sorry I should add "in a linear system" to my previous post. If you add ratchets or stiction or any number of complex systems then you can generate power at the beat frequency

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mechanicaljw]

Hi Greg,

Thanks for the response to my question. I wanted to know whether you have any citation (paper or a book, etc) that you can refer me to that discusses the subject of my qeustion is some details? If I understood you correctly, you are suggesting we may be applying the torque at the spring end, in this case our prop-shaft or what? This is because we are seeing an increase in our torque at resonance.

Thanks!
Jimmy

 

[amanuensis]

The behavior described by Greg is hard to understand (for me).
Linear system : No energy at the beat frequency
Non-linerar system : Energy at the beat frequency

Is it due to the fact that modes are totally uncoupled in a linear system, while it is not the case in a non-linear system ?

Thanks,
Amanuensis

 

[mechanicaljw]

Hi Amanuensis;

Thanks for the contribution. I would like to say that from previous discussions it was said that the 22.5 Hz resonant frequency is not a beat frequency (please see the posts by Walt & Greg above). This particular machine had an original housing (old design) that did not show resonance at this frequency of 22.5 Hz. The current one is a modification of the old one where the gear housing and drive prop-shaft connection have changed. So like Greg said it is likely due to misalignment or balancing. But this is only one side of the question. The other one is the reason we are seeing the torque increase when we run at the 1350 rpm motor speed? I wanted Greg to provide me with explanation or better with some citations that i can read to try to understand where he is coming from. In case you have any reference(s) I would be glad to have them as well.

Thanks!
Jimmy

 

[GregLocock]

There's no cites, it is maths. If you have a 22 hz sine and a 23 hz sine driving a system, then absent non linearities, there is only energy at 22 and 23 hz.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mechanicaljw]

Hi Walt,

To your last question about the dominant vibration frequency when the motor speed is 1350 rpm in our machine. The value is at the 1st harmonic of the first stage gear frequency (2880 Hz).

Thanks!
Jimmy

 

[mechanicaljw]

Hello Greg,

I have a follow up question about why the torque is increasing at resonance. Can we say that because we are seeing a torque increase at resonance that the resonance is torsional?

Thanks again and forgive me if my questions are basic. I am new in this area and still learning.

Regards,
Jimmy

 

[GregLocock]

Sorry I look at data and plots, working from verbal descriptions usually drives me bananas.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[amanuensis]

Yes, but non-linearity does not necessarily lead to energy at the beat frequency. The main effect of non-linearity is certainly harmonic distorsion.