Some Hints to Identify Vibration Problems
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This is a list of practical hints in order to identify vibration problems. They can direct the solution process into the right direction and could be the basis of the problem elimination.
Be aware of the dominating vibration frequency whether it is rotational 1X or its harmonics 2X, 3X or subharmonic below 1X like 0.45X or a critical speed frequency.
In case of 1X (rotational or unbalance frequency), look at the relationship between shaft and pedestal vibrations: high shaft vibrations can mean any kind of unbalance.
High pedestal vibration can mean structural resonance problems. Another feature of those resonances is the fact that there is a pronounced direction of dominant vibrations (axial, horizontal or vertical).
In case of structural resonances find the spot of the biggest relative motion between elements (pedestal-foundation, etc.), this is usually the weak spot.
- 2X vibrations are mainly created by the generators, either by:
Sag excitation by the rotor: the vibration is not dependent on operation parameters.
Magnetic excitation by the stator: the vibration will appear as soon as the field current is engaged.
Newly appearing 2X vibrations, especially at run-down and half speed of the 1st critical, might indicate a lateral rotor crack.
An increase in vibration at a warm run-down compared to a cold run-up as well as a higher pronounced 1st critical usually indicates a thermal unbalance of the rotor.
Vibrations often react, when the load changes. This can be due to temperature changes or the torque changes, when load changes. Vibrations lag behind the temperature changes, but they react immediate together with the torque change.