This paper presents a comprehensive overview of the latest studies and analyses of the cooling technologies and computation methods for the automotive traction motors. Various cooling methods, including the natural, forced air, forced liquid and phase change types, are discussed with the pros and cons of each method being compared. The key factors for optimizing the heat transfer efficiency of each cooling system are highlighted here. Furthermore, the real life examples of these methods, applied in the latest automotive traction motor prototypes and products, have been set out and evaluated. Finally, the analytical and numerical techniques describing the nature and performance of different cooling schemes have been explained and addressed. This paper provides guidelines for selecting the appropriate cooling methods and estimating the performance of them in the early stages of their design.

In recent years, the rapid development of the transportation industry has led more and more scholars to devote themselves to the research of its drive motor. Among all relevant aspects of research, thermal management is absolutely the one that has been neglected in the past but is now getting more and more attention. This paper presents a comprehensive and detailed summary of motor thermal management for transportation, including thermal analysis methods, key factors in thermal analysis, and various cooling methods. Firstly, the three mainstream thermal analysis methods are introduced, and the improvement of each method in recent years is summarized. The feasibility and advantages of multi-method united simulation are discussed, and some novel thermal analysis methods are also listed. Then, key factors in thermal analysis are analyzed in depth and the determination methods are given. In the fourth section, the research related to motor cooling methods in recent years is reviewed, including the improvement of traditional methods and novel methods. Finally, suggestions for the future development of thermal management are given. The key is not only to reveal novel analysis methods, tools, methodologies, cooling structures, strategies, pros and cons, and foresight for the advanced drive motor but also to provide guidelines for predicting the temperature distribution and selecting the appropriate cooling methods in the motor design process.

This paper addresses the heat transfer coefficient (HTC) associated with the hollow-shaft rotor cooling of a traction motor. In such a hollow-shaft cooling system, the coolant is made to flow through the shaft hole in order to cool the rotor. The HTC is estimated using computational fluid dynamics (CFD), where the effect of rotational velocity as well as the coolant flow rate have been accounted for. Experiments are then used to validate the accuracy of CFD models with the assistance of an analytical lumped-parameter thermal network (LPTN) approach. On the basis of CFD simulations and the experimental prototype testing, it is concluded that the rotational speed can significantly increase the convective heat transfer of the shaft hole about 3.8 times at 4500 rpm above the level of the stationary condition. As a result of such analysis, a new dimensionless correlation of the Nusselt number with the Reynolds number is derived.

The convective heat transfer coefficient (CHTC) is a critical parameter that is required for developing an accurate and efficient thermal design of electrical machines. However, the existing empirical CHTC correlations are invalid for an oil cooled hollow-shaft rotor. On this basis, a simplified numerical model based on computational fluid dynamics (CFD) methods was developed to provide a qualitative understanding of the rotational effects on the convective heat transfer across a range of operation speeds. Then experiments were undertaken to validate the data obtained from numerical models and to estimate the impact parameters on the CHTC, such as the rotational velocity, coolant flow rate and coolant temperature. On the basis of the numerical and the experimental results, it is concluded that the rotation can significantly increase the CHTC of the shaft inner wall surface above the level of the stationary condition. However, the axial flow rate and the viscosity of the coolant have less influenced on convective heat transfer for the high rotational speeds. As a result of such analysis, a general correlation is defined by using Nusselt numbers as a function of rotational Reynolds numbers and Prandtl numbers.

A correct understanding of fluid flow and heat transfer characteristics in oil injection cooling for hairpin end winding is crucial for accurate thermal analysis. This article utilizes a meshless computational fluid dynamics (CFD) approach, based on moving particle simulation (MPS), to develop a simplified numerical model. The model provides qualitative insights into fluid dynamics and heat transfer mechanism in hairpin end winding. Experimental validation is conducted to compare the numerical data and identify key factors influencing oil coverage rate (OCR) and heat transfer coefficient (HTC), including the number and placement of inlets, coolant flow rate and temperature, and rotation speed. The results indicate that increasing the number of inlets and optimizing their placement enhances cooling uniformity. Furthermore, the HTC is strongly influenced by the coolant flow rate and temperature, with an average HTC of 130 W/m²/K achieved at an inlet temperature of $40~^{\circ }$ C and a flow rate of 8 L/min. The highest HTC values are found in the lower winding layers due to the downward flow of oil. Rotation also improves the OCR and HTC by approximately 20% and 37%, respectively, at 1000 r/min, as oil droplets are sprayed onto the winding surfaces, particularly near the rotor. However, at higher rotational speeds, the cooling efficiency stabilizes, and the effect of rotation becomes less significant.

This paper investigates an oil-based shaft cooling system as applied to a 30,000 rpm automotive traction motor. Firstly, iron and air friction losses are identified over a range of operating speeds. Analytical and numerical approaches are then proposed to estimate thermal performance of steady and dynamic states, respectively. Finally, experiments are conducted to evaluate the impact characteristics of this cooling system, such as the rotational speed, the inlet coolant velocity and the amount of heat loss. Based on simulation and testing of prototype, the shaft rotational velocity can significantly enhance the heat exchange between coolant and internal surface of the hollow-shaft, across whole length of the shaft. However, when the rotational velocity reaches 30,000 rpm, the effect of high rotational velocities on heat exchange weakens due to the flow becoming saturated.