The driving conditions of vehicles, such as rapid acceleration, lasting downhill and so on, which demand for drive power, are rapidly increasing. For an electric vehicle driven by in-wheel motor (IWM), the IWM needs to provide greater power to maintain the normal operation of the vehicle. At present, the pursuit of high power density for IWM reduces the volume of motor under the same power. But the loss density is increased correspondingly, which will lead to overheating of IWM. In this paper, a 15-kW IWM drive system is taken as the research object. Based on the establishment of the temperature analysis model, instantaneous thermal characteristics of the IWM drive system are analyzed under the long climbing condition with low-speed and rapid acceleration driving cycles. The results show that the temperature of the stator winding and stator core is relatively high under the two driving cycles. The temperature of the stator winding is always in the highest level, and the maximum all appear on the end winding. The highest temperature of the end winding can reach 229.49 • C under the rapid acceleration driving cycles, which significantly exceeds the limitation for the insulation (155 • C) and will seriously affect the normal operation of the IWM and the vehicle. Therefore, in the follow-up design of cooling system, spray cooling with good local cooling effect can be adopted to focus on the cooling for the end winding. This paper can provide theory for the design of the feasibility thermal cooling solution.INDEX TERMS In-wheel motor drive system, thermal characteristics, driving cycles.

In this paper, a 15 KW in-wheel motor (IWM) is taken as the research object, and the coupling factors among the electromagnetic field, temperature field and flow field are analyzed, and the strong and weak coupling factors between the three fields are clarified, and by identifying the strong and weak coupling factors between the three fields, a three-field coupling analysis model for IWM with appropriate complexity is established, and the validity of the model is verified. In a certain driving condition, the electromagnetic field, temperature field and flow field characteristics of IWM are analyzed with the multi-field coupling model. The result shows that, after the IWM runs 8440 s under driving conditions, in this paper, the IWM electromagnetic torque of the rated working condition is 134.2 Nm, and IWM the electromagnetic torque of the peak working condition is 451.36 Nm, and the power requirement of the motor can be guaranteed. The highest temperature of the IWM is 150 °C, which does not exceed the insulation grade requirements of the motor (155 °C), the highest temperature of the permanent magnet (PM) is 65.6 °C, and it does not exceed the highest operating temperature of the PM, and ensures the accurate calculation of components loss and the temperature of the motor. It can be found, through research, that the electromagnetic torque difference between unidirectional coupling and bidirectional coupling is 3.2%, the maximum temperature difference is 7.98% in the three-field coupling analysis of IWM under rated working conditions. Therefore, it is necessary to consider the influence of coupling factors on the properties of motor materials when analyzing the electromagnetic field, temperature field and flow field of IWM; it also provides some reference value for the simulation analysis of IWM in the future.