Today, we're presenting a literature review on oil-cooled rotor motor designs for electric vehicles. The article details the optimization process for various variables in the oil circuit design and compares different solutions. This article interprets the design process, hoping to help you solve practical problems.
I. Oil circuit route
First, let's look at the overall solution for the motor oil cooling system we're going to discuss. The oil circuit layout is shown in the following diagram:
This solution differs from traditional solutions in that it adds a rotor cooling oil path to the standard stator water cooling system. Cooling oil flows from the front cover into the housing, forming a ring-shaped oil path in the stator core, then collects at the rear cover and flows into the rotor, exiting from the rotor to the front cover.
II. Motor oil cooling structure
To achieve the above oil circuit, the structure of the front and rear covers and the housing of the motor is shown in the following figure:
It is worth mentioning that the axial oil passage of the motor housing adopts multiple inlet and outlet methods, which reduces the flow resistance of the oil passage.
In addition, the rotor is manufactured in two stages and then welded together (this manufacturing process can be found in another foreign article, which describes the friction welding process for shafts; those interested can add my WeChat at the end of this article). The rotor structure is shown in the following figure:
III. Simulation Iteration Process
The basic simulation process is shown in the figure below:
The simulation process is based on a two-way coupled analysis of the temperature and electromagnetic fields. First, an initial temperature is given, then the losses at this temperature are calculated through electromagnetic simulation, and these losses are then transferred to the temperature field analysis. This process is iterated repeatedly until a steady state is reached. To shorten the simulation time, a 2D digital model is used for the electromagnetic field simulation , and a 3D digital model is used for the temperature field simulation . Empirical values are used as references for the heat transfer coefficients between the rotor and stator relative to the air gap.
IV. Actual Measurement Verification
The actual temperature values at different locations on the motor were measured and compared with the simulated values. Taking the operating state of 2300 rpm and 7.38 Nm as an example, the simulation error was found to be within 10% . Specific values are shown in the following figure:
V. Motor Optimization
1. Cooling oil passages for the casing
The following diagram shows three different types of oil passages:
The following figure shows the stator and rotor temperatures of the three structures under different flow rates:
As shown in the diagram, we can determine the casing oil passage structure by comprehensively considering the system flow rate and temperature requirements. It is evident that from a to b , when the cooling oil flow rate is low, the cooling effect of the windings is significantly improved, while the improvement in cooling effect is not significant for c compared to b . When the cooling oil flow rate is high, the cooling effect of c , whether for the windings or the rotor, is inferior to b , even though its structure is more complex. This indicates that when designing the casing oil passages, we need to consider the cooling oil flow rate to find the optimal cooling solution that matches the flow rate and passage design.
2. Rotor inlet and outlet oil ports
The angles of the rotor's oil inlet and outlet are optional variables, and these variables can be set to the angles shown in the figure below.
By simulating several specific angle values, the results shown in the figure below can be obtained.
The comparison shows that the third combination is the optimal solution.
VI. Testing Methods
The actual prototype has six oil cooling channels on the stator housing. See the diagram below:
To measure the temperature of the stator and rotor, thermistors are placed on the stator coil, core, and casing, respectively. Since direct measurement on the rotor is not possible, label paper is used for measurement. The measurement points are as follows:
Test system:
VII. Test Results
Three conditions: air cooling, single-shell oil cooling, and shell plus shaft oil cooling.
result:
After 80 minutes, the air-cooled motor temperature reached 130 °C and had not yet reached equilibrium.
After 80 minutes, the motor temperature reached 110 ℃ with single-casing oil cooling , achieving equilibrium.
After 30 minutes of using a housing with shaft oil cooling, the motor temperature reached 80 °C, indicating equilibrium.
In addition, comparing the timeline, the cooling effects of single-shell oil cooling and shell plus shaft oil cooling are roughly the same before 10 minutes. After 30 minutes, there is a significant difference in the cooling effects of the two, and this difference is widening.
The cooling performance of this solution is compared with that of common single-shell cooling and oil-injection solutions, as shown in the table below:
VIII. Summary
Compared with traditional air cooling, this solution reduces the coil temperature by 50% , and compared with single-shell oil cooling, it reduces the coil temperature by 38% . Therefore, it is an effective solution to improve the cooling capacity of the motor.
