Investigation on the causes of abnormal vertical shaking of high-speed EMU carbody

• Field tests revealed abnormal carbody vertical shaking in a high-speed EMU at 38 Hz. • Traction motor excitations triggered the bogie torsional mode during acceleration. • Coupling with carbody local bending modes amplified the vertical vibration response. • A rigid-flexible coupled model successfully reproduced the dual-resonance mechanism. • Results provide insights for vibration mitigation and structural optimization in EMUs. In response to the vertical abnormal shaking phenomenon observed in a certain type of high-speed electric multiple unit (EMU) during operation, this study aims to reveal its vibration mechanism and propose improvement measures to enhance train running quality. Dynamic on-track tests were conducted under four operating conditions, with acceleration sensors installed on the carbody, bogie frame, traction motor, and gearbox. Vibration signals were analyzed using time–frequency methods, and a rigid-flexible coupled dynamic model incorporating finite element–based flexibility of bogie and carbody was developed using condensation and Lanczos techniques. Results show that shaking occurs only during acceleration or deceleration when the traction motor is operating, with a dominant frequency consistently around 38 Hz and independent of blower status. Near 50 km/h, motor excitation triggers the bogie’s first torsional mode, amplifying vibrations at the frame ends and transmitting them to the carbody roof through the secondary suspension. This frequency coincides with local carbody bending modes, forming a dual resonance effect. The findings, validated by simulations and experiments, reveal the amplification path of traction-induced vibration and provide guidance for vibration isolation, modal optimization, and resonance-avoidance strategies in high-speed trains.