### Principle

The ENS (**Electromagnetic Noise 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 is equivalent to the EVS (**Electromagnetic Vibration Synthesis**) for acoustic noise. It consists in **decoupling the acoustic characterization of the electric motor structure from the operational electromagnetic excitation**.

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 **exciting the structure with unit-magnitude rotating force patterns** at variable frequency in radial and tangential directions, on rotor and stator sides. One obtains some normalized acoustic Frequency Response Functions (FRF) that can be stored for later use. This ENS method can be called the "rotating wave excitation method".

Once the acoustic FRF are calculated, the electromagnetic loads are projected on the excitation basis and the acoustic 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 seperated from structure-borne noise due to rotor excitation, similarly to a **numerical transfer path analysis**.

The vibroacoustic FEA model must currently exist under Actran.

The ENS method is particularly useful when a high number of operational loads have to be calculated, for instance in variable speed calculations (e.g. NVH maps on torque speed plane) or in optimization mode (e.g. pole shaping).

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

### Limitations

ENS assumes linearity of the structural and acoustic models (no strong fluid to structure coupling). FRF must be calculated using an average damping independent of load case, structural and acoustic modes. 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

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