Methods for motor noise evaluation and control in electric vehicles

Abstract: Complete vehicle, system and component target setting for noise, vibration and harshness (NVH) are important activities within automotive product development. New challenges arise when electric motors are introduced into cars traditionally powered by internal combustion engines (ICE). Although the electric powertrain is generally quieter than ICEs, the sound character however is completely different and necessarily not more preferable. The noise signature from an electric traction motor is characterized by speed-dependent high frequency tonal components from the dominating electro-magnetic harmonics, covering a wide rpm-range. With relatively low levels of broad band masking noise from tires and wind, the tonal components can be accented in a large frequency range and contribute to perceived annoyance for the occupants inside the car. Methods for good practice of NVH development work for traditional vehicles have been established and refined during the last decades. With the rapid increase of various types ofelectric powertrain-equipped low emission vehicles, new methodology is required in order to understand and fulfill expectations on these unaccustomed vehicle-acoustic phenomena. The objective with this thesis was to investigate the audible perception of the electric powertrain and develop and validate methods for assessment of noise transmission. The findings aim to support the complete vehicle, system and component target setting processes to secure a desired sound quality in future electric vehicle launches. The thesis is based upon four papers. The first two papers address the human perception of the sound from the electric powertrain during driving. In Paper 1, the evaluation methodology of electric powertrain sound quality was investigated. It was concluded that the mean ratings of a specific acceleration sound stimulus was judged similarly in two laboratory test environments (listening room and “sound car” respectively) as during on-road driving. Further, with lower level of high frequency (>1 kHz) tonal components, the overall satisfaction and annoyance ratings were improved. The annoyance due to tonal components, stationary as well as speed-dependent, was further studied in Paper 2. The psycho-acoustic metric prominence ratio (PR) was used to quantify the tone levels relative to the adjacent background noise. It was found that for PR-levels below 3 dB, the perceived annoyance was similar independent of frequency range. Increased PR-level provided higher probability of detecting the tones and the perceived annoyance was significantly increased compared to PR<3 dB for tones above 2.5 kHz. The findings from Paper 1 and 2 provide foundation for complete vehicle interior acoustic target specification. The following two papers (Paper 3 and 4) focus on the main mechanical system properties that are responsible for the airborne radiation and transfer of electric powertrain induced noise. Paper 3 has emphasis on the experimental determination of acoustic transfer functions (ATFs) between the car’s interior to multiple positions located on the stator shell surface of an electric rear axle drive. It addresses the ATFs’ spatial resolution with respect to estimated interior noise for theoretically derived and measured surface vibrations. A reciprocal relation was assumed; therefore the measured ATFs could be used to inversely reproduce the sound transmission from the stator shell to the interior. ATFs and surface velocities are also central in Paper 4, where a method for in-situ determination of radiated sound power from a stator shell was presented and validated. By acquiring the operational deflection shape due to an electro-magnetic radial force wave, a simplified expression for the radiation efficiency was estimated. Paper 3 and 4 highlights the advantages of the rotational symmetric force excitation acting on the stator housing which enables sound power as an acoustic source characterization which perhaps is the most adequate measure for system target setting directed towards external suppliers.