Numerical investigation of the noise generation of electric motors in urban air mobility vehiclesa).

Current concepts of urban air mobility vehicles usually rely on a set of individual electrically driven propulsion systems. Although aerodynamically generated noise from the rotors or fans remains the main contribution to the overall noise, noise generated by electric machines may well add to the total noise. This is especially true for fast-spinning electric motors that cause high-frequency noise contributions, which may notably increase the total noise and lead to unwanted psychoacoustic effects. In the current paper, the noise generation of two versions of permanent magnet synchronous machines in a conventional inrunner configuration, intended for use in a concept vehicle propelled by eight tiltable rotors, is investigated through numerical simulations and analytical models. One version is a direct drive, designed to deliver the torque and rotational speed required for the rotor directly. The other is a geared version, delivering a higher rotational speed and lower torque, thus requiring an additional gear box to drive the rotor. The results show that the geared motor version leads to notable noise contributions at frequencies of 2 kHz and 9 kHz. Coupled with a high radiation efficiency in this frequency range, the findings imply that noise from electric machines contributes to the total noise from the vehicle.