BK -- Order Tracking of a Coast-down of a Large Turbogenerat

http://www.cadfamily.com/a/CAE_FEA_CFD/BK/BK----Order-Tracking-of-a-Coast-down-of-a-Large-Turbogenerat_6556.html

Analysis of vibration or acoustic signals from rotating machines is often preferred in

terms of order spectra rather than frequency spectra. An order spectrum gives the

amplitude and/or the phase of the signal as a function of harmonic order of the rotation

frequency. This means that a harmonic or sub-harmonic order component remains in

the same analysis line independent of the speed of the machine. The technique is called

tracking, as the rotation frequency is being tracked and used for analysis. Most of the

dynamic forces exciting a machine are related to the rotation frequency so interpretation

and diagnosis can thus be greatly simplified by use of order analysis.

The classical problem of smearing of the frequency components caused by speed

variations of the machine is solved by using order analysis. In situations where the

frequency components from a normal frequency analysis are smeared together, proper

diagnosis will only be facilitated via order analysis.

Of particular interest is the analysis of the vibrations during a run-up or a coast-down

of a machine in which case the structural resonances are excited by the fundamental

or the harmonics of the rotational frequencies in the mechanical system. Determination

of the critical speeds, where the normal modes of the rotating shaft are excited, is very

important on large machines such as turbines and generators.