National Instruments Order Analysis Toolset - Manual (Page 19)

Chapter 2 Order Analysis
LabVIEW Order Analysis Toolset User Manual 2-4 ni.com
Like classical harmonic analysis, order analysis is a powerful tool for
gaining a better understanding of the condition of rotating machinery.
However, compared to harmonic analysis, order analysis is more effective
for the analysis of rotating machinery because you can use order analysis
when a machine runs at a constant speed and when the rotational speed
varies. As described in the Effect of Rotational Speed on Order
Identification section, harmonic analysis is effective only when the
rotational speed remains constant.
Effect of Rotational Speed on Order Identification
The ability to make a reliable identification of individual orders from the
conventional power spectrum depends on whether rotational speed remains
constant or varies. This section discusses the effect rotational speed has on
the conventional power spectrum and discusses classical harmonic analysis
and order analysis in relation to rotational speed.
Constant Rotational Speed
At a constant rotational speed, you can identify orders from both the
conventional power spectrum and the frequency-time spectral map.
Figure 2-3 illustrates the analysis of a vibration signal acquired from
a PC fan running at a constant speed.