With the continuous deterioration of the environment, society is paying increasing attention to the development of new energy vehicle technology, and the technological development of key components of new energy vehicles (electric motors and transmissions) is also receiving more and more attention. The electric motor is the main drive source, and the transmission is the power transmission and distribution mechanism; the performance design and integrated research of the two largely determine the performance of the entire vehicle. If the electric motor temperature is too high and not cooled in time, its performance cannot be realized, and safety will be greatly reduced. The transmission contains a passive cooling system (such as ATF in automatic transmissions and Mobil in single-speed reducers of electric drive systems), which implements a thermal management solution based on its structural design. If the transmission's passive cooling system can be used to dissipate heat from the electric motor rotor or shaft within the assembly structure, this represents two different thermal management solutions for the electric motor. This paper will analyze the temperature field changes of the assembly structure (mainly focusing on the electric motor) and its future development trends using the equivalent thermal resistance network method for these two thermal management solutions.
Cooling system research and analysis
Typically, the main drive motor and transmission are installed as two separate components in new energy vehicles, and their cooling methods are also considered separately. The motor is mainly cooled by a water-cooled jacket, while the transmission relies on a passive cooling system within the transmission housing. However, if the motor and transmission are integrated into a single assembly, their cooling methods need to be considered holistically. This addresses many previous shortcomings, such as the need to separately consider the installation locations of the motor and transmission, and the sealing requirements at their connections. The integrated motor and transmission assembly structure also allows for better utilization of the motor's performance and a more efficient use of the transmission's internal passive cooling system, but it also has disadvantages. This paper will primarily focus on the thermal management analysis of the integrated motor and transmission assembly structure. Given the heat dissipation effect and structural complexity of the transmission's passive cooling system, the thermal management temperature analysis will mainly focus on the motor components.
In the assembly integrated structure, the main heat transfer path consists of two parts: heat conduction and heat convection. Since the heat conduction efficiency is highly correlated with the material properties and contact performance of the components themselves, and the material properties and structural thermal properties of the assembly structure are relative, the analysis of the temperature field distribution of the assembly structure mainly focuses on the thermal convection analysis of the structure (since the allowable temperature of the assembly structure is always kept below 200°C, the generation of thermal radiation is ignored).
1. Analysis of heat source and boundary conditions
With the development and reform of new energy vehicle technology, the requirements for the integration and power density of the powertrain structure are becoming increasingly stringent. During operation, the motor components of the powertrain generate rotor losses, stator losses, and winding losses, while the transmission components generate frictional losses. These losses are ultimately converted into heat energy, causing various components within the powertrain to heat up. The boundary conditions are primarily determined by the influence of heat source transmission. In the powertrain structure, the heat sources are mainly generated by friction between gears and between oil and the housing; their cooling is mainly achieved through passive cooling systems, resulting in good heat dissipation management. The heat sources in the electric motor are concentrated in the stator, rotor, and windings (including copper losses, iron losses, mechanical losses, and stray losses). Various cooling solutions can be considered. This paper mainly focuses on the temperature field thermal analysis of the electric motor components in the powertrain structure. A common cooling solution is to design a cooling water jacket around the stator, connecting it to the vehicle's thermal management system to actively cool the electric motor while meeting the vehicle's thermal management requirements. In an integrated structure, if the cooling effect of the oil inside the housing is fully considered and it is introduced to the motor rotor shaft for cooling, it will be a different mode of thermal management solution.
2. Electric Motor Structural Analysis
Motor housing - water cooling jacket
Transmission gear combination
electric motor drive shaft
Transmission housing
Before conducting thermal analysis, it is necessary to first observe and analyze its structural composition. The structure is shown in Figure 1. It consists of the motor body structure including the stator and rotor, the front and rear end covers of the water cooling jacket, the gearbox housing structure, and the internal gear assembly.
Figure 1 Overall structural appearance
Its internal structure, as shown in Figure 2, consists of a motor stator, rotor, and gearbox gears.
Figure 2 Overall internal structure
3. Analysis of Motor Cooling Schemes
Since the electric motor structure described in this paper is integrated with the transmission, in addition to the traditional method of adding a water-cooled jacket to the stator, the transmission's internal passive cooling system can also be used to cool the motor rotor shaft. However, this approach faces many challenges, such as whether the flow rate and pressure of the lubricating oil inside the transmission are sufficient to ensure its proper flow within the motor rotor, and whether oil cooling of the motor rotor is effective. The following sections will analyze the temperature field and feasibility of these two approaches.
Thermal Management Analysis of Electric Motors
As mentioned above, this assembly thermal analysis mainly focuses on two different thermal management schemes for the electric motor. By comparing and analyzing the temperature field distribution of various components inside the motor, the differences between the two schemes and the feasibility of future development trends are determined. Figure 3 below is a schematic diagram of the radial cross-section of the electric motor (including the heat transfer path).
Figure 3 Radial section of main components of the electric motor
When analyzing the temperature field distribution of the two thermal management schemes, it is assumed that the heat loss of the motor is absorbed by the motor water jacket and the internal oil cooling of the rotor through heat conduction and heat convection, and is converted into the temperature rise of the motor. The motor housing, end cover, and gearbox housing are all at room temperature under windless conditions, and a heat dissipation simulation analysis is conducted under these premises. The analysis is performed with the same material properties and boundary conditions for all components. To simplify the preprocessing and calculation process, the following assumptions are made regarding the temperature rise model and heat conduction/dissipation problem of the assembly structure:
(1) All losses in the assembly structure are converted into heat and transferred through the heat dissipation medium;
(2) The issue of heat dissipation on the surface of the assembly housing is not considered for the time being;
(3) The effect of material changes with temperature is not considered;
(4) The effects of thermal radiation are not considered;
(5) The skin effect is not considered.
1. Thermal Management Scheme 1 (Thermal management scheme for the water-cooled jacket of the motor stator only)
This section focuses on the temperature field analysis of various components of the electric motor when the motor cooling thermal management system only utilizes stator water cooling in the overall assembly structure. Heat transfer in the motor rotor primarily occurs through heat conduction from the rotor material itself to the rotor surface, followed by heat convection through the air gap between the rotor and stator to the stator components. Finally, heat is dissipated through heat conduction within the stator and heat convection from the water-cooled jacket. Based on the temperature field distribution of the equivalent thermal resistance network model, it is necessary to calculate the material thermal performance parameters of the motor stator, rotor, and air gap, as well as the heat transfer coefficient. Figure 4 shows a schematic diagram of the motor water-cooled jacket.
Figure 4. Water-cooled heat dissipation structure of electric motor
The thermal management analysis of the electric motor body utilizes the equivalent thermal resistance network method model. First, the material thermal performance parameters of each component of the motor need to be determined. Second, the equivalent heat transfer coefficients for heat convection between motor components need to be calculated, such as the equivalent heat transfer coefficient between the stator water jacket and water, and the equivalent heat dissipation coefficient of the air gap between the stator and rotor. Fluid dynamics simulation software is used to perform simulation calculations to obtain the necessary parameters for heat conduction and convection. Finally, these parameters are input into the mathematical matrix model of the equivalent thermal resistance network method, which easily yields the temperature field changes of each component.
By analyzing the thermal management input conditions of the motor water-cooled jacket and the initial conditions of the water-cooling simulation, the heat transfer path of the motor structure can be analyzed, and the required thermal performance parameters in the equivalent model can be obtained. Furthermore, the equivalent heat transfer coefficient h between the water-cooled model and the air gap between the stator and rotor in the motor can be obtained using simulation software.
Calculating the equivalent heat transfer coefficient of the air gap between the stator and rotor of an electric motor is slightly more complex. This paper uses equivalent static air to calculate its heat transfer coefficient. By using the equivalent air gap model between the stator and rotor, material properties such as Prandtl number, thermal conductivity, kinematic viscosity, and average specific heat capacity are obtained. Based on fluid theory calculation formulas, parameters such as Reynolds number and Nusselt number of the air gap can be obtained, and finally, the heat transfer coefficient of the equivalent gas can be derived.
The equivalent heat transfer coefficient is obtained through the above calculations. Based on the heat conduction and convection models in the structure, the thermal resistance, heat capacity, and other parameters of each component can be calculated. With the heat source, thermal resistance, and other parameters, an equivalent thermal resistance network model can be established based on the heat transfer path in the motor (see Figure 5). The basic mathematical matrix model can be derived from the model. Finally, the temperature field changes of each component can be obtained, which can be used as complete boundary conditions to calculate the effective solution. Finally, the temperature field changes of each component of the motor under rated and peak operating conditions can be obtained in the assembly structure when the motor thermal management scheme only uses the stator water cooling jacket.
Figure 5. Equivalent model of the overall heat transfer path of the electric motor
2. Thermal Management Solution Two (Stator Cooling and Rotor Oil Cooling for Motors)
As an integrated structure of the vehicle's drive unit and power transmission unit, effectively controlling the temperature changes and temperature differences within the assembly structure is one of the effective means to improve the overall vehicle performance. Therefore, it is necessary to make full use of the available cooling resources within the assembly structure. In the assembly structure, the transmission or differential operates in a self-cooling mode, i.e., a passive cooling system. In the integrated structure, the main interface between the electric motor and the transmission or differential is the rotating component. If the oil cooling system from the passive cooling system can be introduced into the rotating component of the electric motor, i.e., the rotor drive shaft, this will significantly reduce the temperature difference of the electric motor rotor shaft, thereby reducing the water cooling requirements of the electric motor stator. This effectively reduces the overall temperature rise of the electric motor, ultimately achieving performance improvements such as increased motor efficiency and power density. This section will focus on a thermal management cooling scheme that, based on the water cooling of the electric motor stator, introduces an oil cooling system from the transmission or differential into the electric motor rotor shaft. The equivalent model using the thermal resistance network method will be used to calculate and analyze the temperature field changes of each component of the electric motor.
Since the temperature field analysis is conducted for the same integrated structure, the thermal properties parameters required for the thermal analysis simulation calculations, such as heat source, thermal resistance, thermal conductivity, and heat transfer coefficient, can all be obtained from the previous section. The difference lies in the additional heat transfer path at the rotor shaft. The motor rotor incorporates the oil cooling system provided inside the transmission, and the flow rate and flow resistance of the oil in this system will vary with the differential speed. Therefore, the cooling effect of the motor rotor and even the entire motor will vary under different operating conditions. Given the complexity of the operating conditions, the temperature field analysis here only focuses on the cooling effect of the motor under rated and peak operating conditions. Because the motor rotor, i.e., the electric spindle, incorporates cooling channels, a heat convection transfer path is added at this node. That is, the rotor heat source transfer path is changed from only transferring to the stator side to transferring to both the stator side and through the rotor shaft oil channels for heat convection transfer. Given the complexity of the oil cooling system introduced into the transmission or differential (involving a series of structural optimization designs such as cooling oil channels and oil baffles), a detailed analysis of the complex structure of the oil cooling system will not be conducted here. Assuming the vehicle is operating under rated and peak conditions for the electric motor, and the equivalent passive cooling system of the transmission or differential is handled by an oil pump (with fixed structures affecting oil flow, such as oil passages and oil baffles), the pressure P and flow rate q introduced into the motor rotor shaft are obtained. The equivalent thermal resistance network model of the motor can then be derived (see Figure 6). The convective equivalent heat transfer coefficient and other parameters in the shaft oil passages are calculated using the method described above. Based on the equivalent mathematical matrix model, the temperature field changes of various motor components under rated and peak operating conditions in steady-state operation are obtained.
Figure 6. Equivalent circuit model of the electric motor after rotor cooling is introduced.
It can be seen that by reducing the thermal resistance between rotor components and evenly distributing the heat loss of the rotor shaft, the temperature rise of the rotor is obviously reduced, which also indirectly reduces the heat dissipation pressure of the stator water cooling jacket. As can be seen from the figure, the temperature rise of motor components is reduced in different operating conditions, but the overall temperature rise is reduced. That is, the cooling requirements of the motor stator water cooling jacket are reduced or the overall heat dissipation of the motor is made more uniform and effective, and the requirements for the thermal management of the whole vehicle are also reduced.
Feasibility analysis of thermal management schemes for electric motors
As described above, a comparison of the temperature field changes under two different thermal management schemes for the electric motor in the assembly structure (see Figures 7 and 8) shows that the oil cooling system, which introduces a passive cooling system at the rotor shaft, effectively reduces the internal temperature difference of the motor, especially for rotating components. However, if the motor's own heat dissipation requirements can be met, using only a stator water-cooled jacket thermal management scheme will significantly reduce a series of problems, such as reducing motor manufacturing costs and processing technology, and eliminating the need to consider issues related to rotor sealing and strength. Furthermore, if motor performance efficiency and power density are emphasized, introducing this thermal management scheme at the rotor shaft will be very effective, although it will also face a series of challenges. However, given the emphasis on integration and power density ratio, this will be a future development trend.
Figure 7 Comparison of rated operating temperature changes for two thermal management schemes of electric motors Figure 8 Comparison of peak operating temperature changes for two thermal management schemes of electric motors
Conclusion
The feasibility study and existing problems of adding rotor shaft oil cooling (from the passive cooling system of the transmission or differential) to the existing stator water-cooled jacket of the electric motor are as follows:
1. The motor rotor is cooled by the differential.
(1) Depending on the arrangement of the transmission, special oil baffles and cooling oil passages need to be designed in the transmission. The speed of the differential determines the oil splashing capability of the passive cooling system, whether the oil can splash into the oil passage of the motor rotor shaft and then return to the transmission oil passage;
(2) The flow rate of the cooling oil circulated by the transmission solenoid valve or release pump will be uncontrolled. The oil flow rate will be affected by the differential speed. Whether the cooling flow rate or cooling capacity is sufficient usually requires a series of tests to determine its cooling capacity, and then the oil passage or oil baffle ring will be optimized (the reasonable flow rate is calculated by CFD simulation).
(3) The oil pressure in the cooling oil circulation system is very low, close to atmospheric pressure, about 0.2 to 0.3 MPa . How to obtain the oil pressure difference between the inlet and outlet?
2. The coolant comes from the transmission (from the transmission cooling cycle), so a separate coolant pump or solenoid valve control needs to be considered.
(1) Is the cooling oil pressure sufficient (approximately 0.3–0.5 MPa , not exceeding 0.8 MPa);
(2) Cooling flow rate: depends on the motor rotor cooling requirements - inlet and outlet temperature difference and the cooling capacity of the coolant in the system, etc.
In addition, oil has a large heat capacity and good dielectric properties, making it a relatively ideal direct cooling medium. However, oil itself is viscous, and internal friction heat is generated during agitation due to the multi-layered flow. An improperly designed cooling oil passage for an oil-cooled motor can result in insufficient contact between the oil and the heat-generating components, failing to effectively remove heat, or exacerbating internal friction in the cooling oil. This leads to poor cooling performance and may even cause the temperature to rise instead of fall. Therefore, the proper design of the cooling oil passages in an oil-cooled motor is crucial for effective cooling.
In summary, temperature field analysis of different cooling thermal management schemes for electric motors from a steady-state perspective reveals that, considering factors such as manufacturing cost and operational safety risks of the entire assembly structure, using only a water-cooled stator jacket is the most appropriate solution while meeting thermal management requirements. However, when emphasizing performance parameters such as power density, effective output rate, and structural volume of the electric motor assembly, it is necessary to minimize its temperature rise, i.e., to reduce the node temperature of components as much as possible. Typically, the temperature rise of rotating parts in an electric motor is relatively high and difficult to dissipate during operation, making oil cooling of the rotor particularly necessary. Furthermore, the above analysis shows that its structural design is feasible. This is also an inevitable research trend given the increasing emphasis on integration and power density in the future.
