Golden silence

How can you hope to sell an electric car supposedly totally “silent” when it makes an audible whistling sound? Renault encountered this problem and approached the UTC Electro-Mechanical Engineering Laboratory (LEC) to help them solve the issue. This became the core business of the AVELEC Commission (Electric Vehicle Acoustics), which started in 2010. Here are some of the initial findings.

Golden silence

The whistling noises originate in power electronics, where 'rogue' frequencies can randomly generate vibrations and hence become a noise source. "The very low noise level of an electric motor, compared with an internal combustion engine, in fact allows the driver to hear just about everything! To solve these phenomena from design stage, we had to bring together various skills that were somewhat removed from the core business of our Renault Divisions: vibrations, acoustics, magnetism and electronics. Renault did not have this combination of skills in its in-house teams," explains Vincent LANFRANCHI, senior lecturer at UTC who had some previous experience in the area when he worked with ALSTOM on a similar set of problems.

It was doubtless for this reason that the LEC laboratriory was chosen to design and develop vibro-acoustic engineering solutions for Renault's electric motors and to pass on this knowledge and new skills to the Renault teams. According to the specifications of the study, LEC was invited to study one electric motor from almost every possible angle and to ensure reproducibility of the modelling and experimental protocols, given that each motor has its own specific parameters.

"We adopted a novel scientific approach to study motor-generated harmonics, coupling digital and analytic models" adds Vincent. "We had in essence to break some barriers between various skills and tools used to draft the design specification recommendations. I was personally impressed by the way Renault implemented the means needed to do this. It now remains for us to synthesise our findings, which we feel are quite satisfactory, to define the case limits and to draw up some prospects for future developments."

With a budget of 2.6 Meuros, AVELEC combines contributions from three innovative SMEs - Vibratec, Adetel Equipement and Cedrat Technologies. The contract will end in 2013. A thesis by Pierre PELLEREY on "Noise and vibrations in electric motors designed for use in automobiles" was also successfully defended during the same time; Pierre was indeed recruited by the British Dyson company, founded by inventor James Dyson. "AVELEC will be a clear benchmark for UTC's LEC notoriety in Europe", concludes Vincent LANFRANCHI.