Abstract

At low speed, the acoustic power radiated by electrical transport systems can be dominated by a special source of noise called audible magnetic noise. This noise comes from magnetic vibrations due to magnetic forces, which are especially strong in traction motors. Among them Maxwell forces excite the stator structure, creating harmful resonances during starting or braking. Firstly, this paper presents the phenomenon of magnetic resonances due to Maxwell forces in traction induction motors. Two principal noise sources are analytically identified, namely slotting harmonics and Pulse-Width Modulation harmonics. A fully analytical model of the electrical and vibro-acoustic behavior of a traction motor and its PWM supply is then described. This model is validated at different stages (electrical, mechanical and acoustic) using several vibro-acoustic tools (experimental modal analysis, operational modal analysis, operational deflection shapes, spectrograms) and numerical methods (electromagnetic and mechanical FEM, acoustic BEM). On the ground of this model, main magnetic lines are analytically characterized. Finally, this model is coupled with a multi-objective optimization method in order to design a new ALSTOM squirrel-cage induction motor by acting on the rotor geometry and the PWM strategy. The average and maximum magnetic noise levels during starting are minimized, while keeping the same traction characteristics. Two designs are chosen and manufactured. These prototypes are tested and successfully compared to simulations, and a reduction of 15 dB is observed when running the motors in real conditions. It is also shown that the choice of the switching strategy has a 15 dB influence on starting noise.

Preprint and full paper

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Reduction of the variable-speed audible magnetic noise radiated by railway traction motors

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