Causes and measures of low-frequency swaying of linear induction motor metro vehicles

After more than 10 years of operation, a metro line in China experienced a low-frequency swaying of vehicles with linear induction motors (LIMs). Field tests and numerical simulations were carried out to investigate the cause and control measures of the vehicle swaying. Experimental studies were carried out to assess worn wheel-rail profiles, track irregularity, vehicle dynamics performance, and vibration characteristics. The relationships between wheel-rail contact equivalent conicity, track irregularity, and swaying features were analyzed. Vehicle dynamics simulation was conducted to uncover the mechanism of abnormal swaying and its key influencing factors. Effective measures against the swaying were proposed from three aspects: controlling the wheel-rail equivalent conicities, optimizing suspension parameters, and managing track irregularities, which were validated by field tests. Analysis results show that when vehicles with varying mileage operate at 70-90 km·h^-1 on straight tracks and curves with radii over 1 km experience lateral swaying at a low frequency of about 2 Hz. The maximum lateral ride index exceeds 4.0 during the swaying. The measured equivalent conicities of the wheel-rail contact are about 0.1-0.2. This swaying differs from the vehicle hunting instability caused by the low conicity of wheel-rail contact. The cause lies in the closeness of three frequencies: bogie hunting frequency, the natural frequency of the vehicle carbody upper swaying, and excitation frequency of periodic track irregularities with wavelengths of 11-13 mm. Suppressing swaying can be achieved by either reducing the bogie hunting frequency through the use of low-conicity wheel-rail profiles (less than 0.05) or eliminating 11-13 mm irregularities to remove the 2 Hz excitation source. By increasing the longitudinal stiffness of the primary suspension from 10 MN·m^-1 to 18 MN·m^-1, reducing the lateral stiffness of air spring from 0.183 MN·m^-1 to 0.120 MN·m^-1, lowering the lateral damping of the secondary suspension from 40 kN·s·m^-1 to 20 kN·s·m^-1, and employing a wheel-rail friction coefficients of 0.1-0.2, the swaying can be reduced to a certain extent. The results provide theoretical guidance for mitigating low-frequency swaying in linear induction motor metro vehicles, offering important engineering application value.