Is critical speed the same as natural resonance?

[ScottI2R]

I was wondering, and explaining to the wife what critical speeds are. We run multiple stage compressors and one of them always faults on a vibration fault at shutdown. I figure it is passing thru a critical speed during wind down. We can stop this monitoring in the ladder of the control, no biggie. My question, as you see I am electronic, is critical speed the same as "natural resonance" with the associated harmonics?

Thank you,
Scott

I really am a good egg, I'm just a little scrambled!

 

[GregLocock]

Yes... but there are no harmonics.

That is if you look at a Campbell's diagram of a typical system, the resonances do not occur at any rational multiples of frequency.

However typically the excitation order lines are at neat multiples of rotor speed, if that's what you meant.

Cheers

Greg Locock


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[SomptingGuy]

Your mounting system will dictate the resonant frequency of your system (unlikely to be anything internal) and there is no way of not passing through it on a shutdown transient. Think washing machines walking around the kitchen during start-up/shut-down of their spin cycles.

Don't even think about the excursions made by your car engine!

- Steve

 

[GregLocock]

"Don't even think about the excursions made by your car engine!" To the seaside?

Cheers

Greg Locock


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[electricpete]

Roughly speaking, they are the same.

Actually the term "natural resonance" is probably not a good term.

A "natural frequency" is the frequency at which a system will vibrate after excitation is removed.

A "resonance" is the condition that exists when the exciting frequency matches the natural frequency.

I don't know an exact definition of critical speed, but it is similar to natural frequency in the sense that it tells us where the high vibration will occur.

BUT, if we are looking for a difference between critical speed and natural frequency, there is at least one. The natural frequency is a characteristic of the system, so you can find it by bump test of the system. However as we change speed, some characteristics of a machine can change (i.e. gyroscopic stiffening). So, you may not be able to determine a critical speed by bump test of the static rotor... you may have to spin it up or do some anlysis instead.

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

 

[GregLocock]

In a simple rotor the critical speed is the same frequency as the first bending mode, if that helps.



Cheers

Greg Locock


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

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[ScottI2R]

Thanks Guys.

However typically the excitation order lines are at neat multiples of rotor speed, if that's what you meant.

That is exactly what I meant. Poor choice of words on my part.

And Steve, that fault during shutdown really only causes an inconvenience the next time we go to start that compressor because the control stays in a faulted state. And we start these remotely. This causes us to have to go out to it, clear it, and request a restart from the control room again. All the while the header pressure is approaching the lab P4 setpoint of 285psig. I would go in the plc and change the ladder myself but the mfr is quite fussy about their proprietary controls. I've "heard" about doing that before.

Scott

I really am a good egg, I'm just a little scrambled!

 

[desertfox]

Hi Scott2R

Have a look at this article:-

http://books.google.co.uk/books?id=...=critical speed and natural frequency&f=false