How to simulate an uneven airgap?


This article shows how to use MANATEE software specialized in noise calculation of electric motors to calculate the effect of uneven airgap on noise and vibrations due to electromagnetic excitations.


The non-circularity of stator and rotor laminations can be due to segmentation effects, welding effects, manufacturing tolerances, deflections due to weight, thermal expansion and centrifugal forces, etc. The non-circularity of the laminations modulate magnetic forces and therefore significantly change the vibration and acoustic behaviour of the machine under Maxwell forces. Stator non circular bore radius only introduces new magnetic force wavenumbers (it represents a generalization of static eccentricity) while rotor non circular bore radius introduces both new magnetic force wavenumbers and frequencies (it represents a generalization of dynamic eccentricity). Rotor asymmetry can change the frequency content of vibration and sound spectra, contrary to stator asymmetry.

Both effects can be studied in MANATEE using the permeance / mmf electromagnetic model. Two types of uneven airgap shapes can be defined in MANATEE:

  • periodic change of the lamination shape (“flower-shape”)
  • user-defined shape

GUI implementation

In the GUI, all eccentricities are defined in the Fault panel.

Scripting implementation

To study the effect of a periodic change of the lamination shape (“flower-shape”), you have to set the following Input :

Input.Geometry.Nsegs = 6;
Input.Simu.type_stator_shape = 1;

This indicates that a 6 segment or 6 weldings stator is considered with +/- 20% deflection from the nominal airgap. The last parameter ensure that the stator periodic deflections due to segmentations and weldings are taken into account in the model.

Plot commands

The shape of the airgap can be checked with the command plot_airgap_shape :

Airgap shape
Airgap shape

A display amplification factor is used to plot the non-circularities in the structure Plot, you can change it by directly specifying


Validation cases


See also

Static and dynamic, radial and axial eccentricities can also be simulated in MANATEE.

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