FFT2 contour plot of radial magnetic force (plot_Fr_fft2)

Post-processing documentation
Matlab commandplot_Fr_fft2
Module EM.pp
Description PLOT_FR_FFT2 plots the 2D Fourier transform of airgap magnetic radial force on outer structure (rotor or stator) per unit area including natural frequencies
See also plot_Ft_fft2, plot_Fr_fft2_stem, plot_Fr_cfft2


This MANATEE software post processing gives the 2D Fourier Transform of radial magnetic forces per unit area:

Stator radial pressure FFT2 and natural frequencies
Stator radial pressure FFT2 and natural frequencies

The top part of the FFT shows the magnitude of the harmonic force wave while the bottom part shows the phase angle of the harmonic force waves. The phase angle is useful to identify if a pulsating force occurs (e.g. two harmonics with opposite wavenumbers and same phase angle). Negative wavenumbers account for the different propagation direction of force waves.

The largest force harmonics should occur at r=2p (twice the number of pole pairs), f=2fs (twice the fundamental electrical frequency), and at r=0 f=0 (static force). Note that only the lowest force wavenumbers are important in magnetic noise and vibration process.

When spectral leakage occurs (e.g. due to slip in induction machines with a too low number of revolutions, or when importing flux or current data that is not exactlty periodic) one can obtain

Airgap radial pressure FFT2 and nature frequencies (+,x)
Airgap radial pressure FFT2 and nature frequencies (+,x)

The reference level of the pressure dB scale can be tuned in


For further plot options (e.g. change axis) see How to change plot options.

This type of graph is very useful to identify which are the main harmonic forces responsible for magnetic noise and vibration of electrical machines. When a large force harmonic is close from a natural frequency, large vibration and noise can be expected.

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