The nonlinearity index of signals was defined based on intra-wave modulation and the nonlinear characteristics for hunting instability of high-speed EMUs with a drive system controlled with a direct torque were studied. Firstly,an electromechanical coupling co-simulation model including a vehicle dynamic Simpack submodel of high speed EMUs and a Matlab/Simulink submodel of drive system controlled with a direct torque was built.Then the EMU's dynamic responses were calculated.Finally,the nonlinearity index of the dynamic responses was calculated based on Huang transformation.The simulation results indicated that when the EMU's hunting motion instability happens,the nonlinearity indexes for lateral vibrations of bogie and car body increase dramatically,the nonlinearity indexes for car body lateral acceleration and bogie lateral displacement are larger than 0.1 and 0.3,respectively under the full unstable condition.%基于信号的波内调制现象定义信号的非线性指标，并研究高速动车组在驱动工况下（驱动系统在直接转矩控制下）蛇行失稳的非线性特征。首先建立包括高速动车组的车辆动力学Simpack子模型和牵引传动系统的直接转矩控制Matlab／Simulink子模型在内的机械电气耦合联合仿真模型，然后计算动力学响应，最后基于黄变换计算列车在蛇行失稳状态下动力响应的非线性指标变化。仿真结果表明：若列车出现蛇行失稳，转向架和车体横向振动非线性指标会急剧增大；完全失稳情况下，车体横向加速度非线性度大于0．1，转向架横向位移非线性度大于0．3。
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