Differences between the run up and run down in a rotor

[heitor]

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

 

[MachineryWatch]

It probably takes longer to coast down than it does to run up. This means that if the rate of acceleration and deceleration are relatively constant for the run-up and coast-down there is more "dwell" time in the region of influence of the shaft critical during the coast down. The rotor has longer to react to the forces created by the critical.

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

 

[bklauba]

I agree with Mr. H, in that the responses will be different, for the reasons sited. If plotting the acceleration vs. rotor speed, the "trailing edge" which will be higher RPM'd (and normally on the right) during the scan up, will have a gentler slope than the leading edge. Vica versa on the way down.

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

 

[heitor]

Thank you for the answers, but I was not able to understand.
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?

 

[sms]

Skip made some assumptions that may not be true. On a normal induction motor the run up time is typically about 6 to 10 seconds, while it may take up to 15 minutes to coast back down. Thus there is a significantly longer dwell time in the critical on the rundown

 

[sms]

If the run up and coast down are about the same amount of time, you might look at torque load on run up vs coast down. Also it is not clear if you are taking measurements on the motor, or on some driven piece of equipment. If the driven piece of equipment then the hydraulics, or gas dynamics on run up may be different than on coast down. If the motor, there is electromagnetic effects on run up that are not present on run down.

Please describe the configuration and what exactly you are gathering data on. Is this in the field or on a shop test?

 

[bklauba]

The reason that there is a difference between the run up and run down, more than likely, has to do with the fact that you are going thru a critical speed. You probably haven't noticed this on balancing jobs where you have not gone beyond a critical speed.

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

 

[MachineryWatch]

Good points BK. I need to clarify my earlier response. I didn't say that the rate of acceleration and deceleration were the same. I said that the rate of acceleration was constant from 0 to full speed and the rate of deceleration from full speed to 0 was constant. Except, I didn't say it that clearly. Why didn't you all know what I was thinking?!

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

 

[jds22]

What is the driving force during acceleration?

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