**4. Conclusions**

This paper established an electromechanical coupling model combined with the traction control and vehicle models. The co-simulation is under traction, constant speed, and braking conditions. Additionally, this research reveals the dynamic characteristics of rotor and stator current during vehicle operation through numerical research. Overall, the conclusions are given as follows.

When the vehicle starts, the current of traction motor stator and rotor have a significant mutation, so measures should be taken to protect the motor. The stator current amplitude reaches 1400 A, and the amplitude of the rotor current reaches 1250 A. The stator current and rotor current gradually decrease with the vehicle speed increase in the traction process. Similarly, the stator current and rotor current decrease gradually with the vehicle speed decrease in the braking process. Both the stator current and rotor current maintain a stable amplitude in the constant speed process.

In order to analyze the change trend of current amplitude of traction motor under the transient condition of variable speed and load, the RMS value is used to describe the change of current amplitude. Similarly, short-time Fourier transform (STFT) is used to analyze the

time-frequency characteristics of the current. In the traction and braking process, the RMS value of the stator three-phase current is smaller than that of the braking process. Moreover, the RMS value tends to decrease with the increase or decrease of the vehicle speed during traction and braking. In the traction process, the RMS value of stator current decreases gradually from about 200 A, and while braking, the RMS value of stator current decreases gradually from about 206 A. Moreover, in the constant speed process of 100 km/h and 40 km/h, the RMS value varies between 71 A and 73 A, respectively. The high harmonic of driving shaft rotation frequency and the frequency of gear meshing can be seen in the stator current spectrum. At the vehicle speed of 100 km/h, the stator current frequency (about 49 Hz) is twice as high as the pinion shaft rotation frequency (about 24.5 Hz). The same pattern is observed at a speed of 40 km/h. Except for the rotational frequencies at two constant speeds, these higher harmonics are evident and obscure during the traction and braking process.

In the traction process, the RMS value of the rotor three-phase current is smaller than that of braking. Therefore, during the traction process and braking process, the RMS value of the current all tends to decrease gradually. In the traction process, the RMS value of the rotor current decreases gradually from about 185 A, and while braking, the RMS value of rotor current decreases gradually from about 190 A. The RMS value of the rotor current fluctuates stably in the range of 6 A to 13 A during constant speed. However, it tends to decrease during the traction and braking process.

**Author Contributions:** Conceptualization, K.Z., J.Y., C.L., J.W. and D.Y.; methodology, K.Z., J.Y. and J.W.; software, K.Z. and J.W.; validation, K.Z., J.Y., C.L., J.W. and D.Y.; formal analysis, K.Z., J.Y. and J.W.; investigation, K.Z. and J.Y.; resources, J.Y., C.L., J.W. and D.Y.; data curation, K.Z., J.Y. and J.W.; writing—original draft preparation, K.Z.; writing—review and editing, K.Z.; visualization, K.Z.; supervision, J.Y., C.L., J.W. and D.Y.; project administration, J.Y., C.L., J.W. and D.Y.; funding acquisition, J.Y., C.L., J.W. and D.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper was supported by the National Natural Science Foundation of China (grant number 51975038), the Beijing Natural Science Foundation (grant number 3214042, KZ202010016025, L191005), the Beijing Postdoctoral Research Foundation (grant number 2021-zz-114), the Open Research Fund Program of Beijing Key Laboratory of Performance Guarantee on Urban Rail Transit Vehicles (grant number PGU2020K001), the Fundamental Research Funds for the Beijing Universities (grant number X21049), and the Development of High-Level Teachers in Beijing Municipal Universities (grant number CIT&TCD201904062).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
