Is critical speed the same as natural resonance?
Is critical speed the same as natural resonance?
(OP)
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
Thank you,
Scott
I really am a good egg, I'm just a little scrambled!





RE: Is critical speed the same as natural resonance?
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Is critical speed the same as natural resonance?
Don't even think about the excursions made by your car engine!
- Steve
RE: Is critical speed the same as natural resonance?
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: Is critical speed the same as natural resonance?
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.
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(2B)+(2B)' ?
RE: Is critical speed the same as natural resonance?
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: Is critical speed the same as natural resonance?
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!
RE: Is critical speed the same as natural resonance?
Have a look at this article:-
http://