Differences between the run up and run down in a rotor
Differences between the run up and run down in a rotor
(OP)
Hello.
I'm working with a rotor and after balancing I am able to go beyond the first critical speed.
I noted that the recorded response (order tracking) of the displacement probes are different for the run up and the run down tests. The run down response gives me lower levels of vibration than the run up.
What do you think? What could explain this phenomena?
Thank you very much!
heitor
I'm working with a rotor and after balancing I am able to go beyond the first critical speed.
I noted that the recorded response (order tracking) of the displacement probes are different for the run up and the run down tests. The run down response gives me lower levels of vibration than the run up.
What do you think? What could explain this phenomena?
Thank you very much!
heitor





RE: Differences between the run up and run down in a rotor
The dynamics of the shaft are also different. There is torque being applied to the shaft during the runup that is not there during the coast down. This could increase damping of the shaft critical response.
Skip Hartman
http://www.machinerywatch.com
RE: Differences between the run up and run down in a rotor
This phenomenom is also seen during swept-sine modal analysis, or similar situations, like VSR Process scans.
I sometimes describe it as "ring time" vs. "reverse ring time".
BK
RE: Differences between the run up and run down in a rotor
It is true that the machine has an electrical motor and the rate of change of the speed (RPM/s) is almost the same for the run up and the run down. Then the time for the run up is almost the same for the run down. Why there's more dwell time in the run down?
RE: Differences between the run up and run down in a rotor
RE: Differences between the run up and run down in a rotor
Please describe the configuration and what exactly you are gathering data on. Is this in the field or on a shop test?
RE: Differences between the run up and run down in a rotor
Resonance displays several characteristics, including:
- local max of amplitude (peak)
- standing wave (nodal and anti-nodal pattern)
- esp. in high inertia situations, a finite time period 'tween driving at the exact resonance freq., and full amplitude response.
Think of a large resonating body, being externally excited and in a steady state. Turn the excitation off, and the waveform will decay in an expotential fashion (assuming nominal damping). The time it takes to decay by a certain amount (defined in terms of dB or % of reduced amplitude) is often referred to as "ring time".
If, instead, the excitation process has just begun, there will be a build-up period which will eventually plateau in the steady state. Since the effect is somewhat (but not exactly) in reverse, I refer to this as "reverse ring time".
During the run up, this steady state is not quite achieved; there is not enough time. Depending upon the rate of excitation freq. increase rate, the resonance peak will be not quite achieved; it could even be "jumped over", i.e., truncated by the quick motor acceleration.
On the way down, in a coast mode, with the over-critical speed somewhat exciting the resonance, at least more than the sub-critical, the rotor will "ring" a bit more. The resonance represents a peak in load on the drive, so there might even be a slowing down of the coast-down rate thru the critical speed.
This phenomenon is often seen when doing the VSR Process on large, massive (like 15 - 150 ton) fabrications, which certainly do take their time, both at the beginning of resonating, and, once you get them going, as they decay. Sometimes it takes more than a minute after excitation is zero before parts like a transfer line press frame, which is about the size of a three-story apartment building, stop trembling.
Hope this makes sense.
BK
RE: Differences between the run up and run down in a rotor
I was actually trying to explain what sms said. He just did a better job. I also have seen what BK describes, as well, during a coast-down where the rotor actually hangs in the area of resonant amplification longer where the rate of deceleration actually seems to diminish while the rotor is in this speed region.
Skip Hartman
http://www.machinerywatch.com
RE: Differences between the run up and run down in a rotor
Is this completely smooth?
Or will it be a certain number of pulses per rev?
Even an electric motor may not be completely smooth poles etc and therefore there may be impacts from the the driving force during acceleration...
If these force impacts are sharp in nature they will excite the rotor structure (ie a delta function)... during decel I assume there are no forces and just frictional forces which will not be impacting in nature and therefore not excite the rotor...
Thoughts?
John