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