National Instruments Order Analysis Toolset - Manual (Page 23)

Chapter 2 Order Analysis
LabVIEW Order Analysis Toolset User Manual 2-8 ni.com
harmonics overlap in the conventional power spectrum, you are unable to
identify different harmonics by using the conventional power spectrum.
Figure 2-6 shows the signal from the same PC fan as the one depicted in
Figure 2-3. However, in Figure 2-6, the rotational speed of the fan increases
with time.
Figure 2-6. Fan Run-Up
In Figure 2-6, as the rotational speed increases, both the fundamental
frequency bandwidth and the frequency bandwidths of related orders
widen, causing orders to overlap. Whether you can separate nearby orders
in a power spectrum depends on the rate of change in the rotational speed,
the window used, and the highest order of interest. In Figure 2-6, the power
spectrum is measured over the entire duration of the fan run-up and does
not really show any distinguishable orders. When the change of speed is
large enough, the spectra of orders eventually overlap. When the spectra of
STFT Spectrum
1500
1250
1000
750
250
0
F
requency (Hz)
31.9 31.9 31.9
500
Time (sec.)