### Principle

The EVS (**Electromagnetic Vibration Synthesis**) is an algorithm designed to speed up electromagnetically-excited noise and vibration calculations and get more physical insights on the e-NVH behaviour of the machine.

It consists in **decoupling the vibrational characterization of the electric motor structure from the operational electromagnetic excitation**, as one would do experimentally using an impact hammer or an electrodynamic shaker to measure Frequency Response Functions.

As electromagnetic forces due to Maxwell stress can be decomposed in a **discrete series of rotating force waves** with specific wavenumbers which depends on slot / pole combination and winding pattern, this characterization can be carried by **artificially exciting the structure with some specific rotating force patterns** at variable frequency in radial and tangential directions. In some cases, one might also have to calculate the effect of variable frequency moments at tooth tips or one magnet poles.

One obtains some normalized Frequency Response Functions (FRF) that can be stored for later use. This EVS method can be called the "rotating wave excitation method". Another EVS method consists in exciting each tooth of the stator with variable frequency radial, tangential and bending moment excitations. This method is called "lumped excitation method".

Once the FRF are calculated, the electromagnetic loads are projected on the excitation basis and the vibration response is synthesized without additional calculations. Besides, **torque ripple, Unbalanced Magnetic Pull, and radial ripple excitations can be separated in the e-NVH response**. Air-borne noise due to stator excitation can also be separated from structure-borne noise due to rotor excitation, similarly to a **numerical transfer path analysis**.

The EVS method is particularly computationally efficient at variable speed (when combined with Load Extrapolation Algorithm), or during electromagnetic optimization (current shaping, pole shaping, notching).

An example of rotating wave Electromagnetic Vibration Synthesis is illustrated here:

### Limitations

EVS assumes linearity of the structural model. FRF must be calculated using an average damping independent of load case and structural mode. Operational damping under a specific load case can only be taken into account by FRF modification during synthesis process. FRF calculation do not include gyroscopic effects or strong magneto-mechanical coupling.

### Application to MANATEE software

EVS is a key algorithm of MANATEE to perform more efficiently e-NVH calculations. See the article how to perform EVS in MANATEE.