# Influence of Interference Fit and Temperature on High-Speed Permanent Magnet Motor
## 4 Results and Discussion
### Temperature  
When exploring the factors of rotor strength, temperature is also one of the factors that cannot be ignored. In the simulation, the temperature and stress field changes in the rotor system are mainly considered. The rotor system is regarded as an isothermal body, and the temperature load is applied to it to analyze the stress changes in the interference fit structure at different temperatures. The temperature range selected in the simulation corresponds to the actual working environment. The actual minimum operating temperature is 20 \({}^{\circ}\)C. On this basis, the temperature is increased to find the highest range that the system can withstand. The unit for the temperature values mentioned is Celsius. When exploring the correlativity between temperature and stress, the coefficient of thermal expansion is an important parameter to be considered. The coefficient of the thermal expansion of the magnet, sheath, and end covers do not change obviously when the temperature ranges from 0 to 400 \({}^{\circ}\)C. Their coefficients of thermal expansion are approximately linear with temperature. Therefore, it is determined that the average coefficient of the thermal expansion of the magnet is 9 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C, and that of the sheath and end covers is 1.7 \(\times\) 10\({}^{-5}\)/\({}^{\circ}\)C. The metal strength decreases when the temperature is increased. At about 250 \({}^{\circ}\)C, the strength of the metal increases slightly. However, the reduction in the metal strength is not obvious below 200 \({}^{\circ}\)C [28]. In this case, the change in the strength can be ignored. As shown in Figure 14, the sheath is a thin-walled part where stress is affected weakly by the change in temperature. However, with the increase in temperature, the stress of the magnet and end covers increases gradually, and strength failure does not occur until up to 183.11 \({}^{\circ}\)C.  
According to Figure 15, it can be determined where the failure occurs. The failure position of the magnet is on both end faces, while both end covers fail at the top of the small diameter sides. From the simulation results, it can be obtained that the operating temperature of the rotor system should not be too high, and it should be kept below  
Figure 14: The stress distribution of the magnet, sheath, and end covers at different temperatures. Figure 13: The stress distribution at different rotor speeds. 183.11 \({}^{\circ}\)C to avoid a high temperature, leading to the strength failure of the magnets and end covers. Compared with ordinary motors, high-speed motors have a high speed and large vibration, so the heating problem is more serious when working, which leads to the change in interference fit contact stress. In order to further understand the influence of temperature on the interference fit structure, from the point of view of strength failure, simulation is used to analyze the temperature, under which a failure problem will occur within the range of effective interference. According to the above results, when the temperature gradient rises, the contact stress of the interference fit structure also increases. Therefore, the failure temperature point of the interference fit structure can be determined at a high working temperature. With the standard interference quantity of 60 \(\upmu\)m and failure temperature of 183.11 \({}^{\circ}\)C as the reference, the temperature of the interference fit structure strength failure within the range of the effective interference quantity is further explored. A summary of the final results is shown in Figure 16. When the interference volume ranges from 20 to 70 \(\upmu\)m, the temperature reaching the failure condition decreases step by step and reaches the inflection point, at which time the temperature is 182.3 \({}^{\circ}\)C. From 80 to 90 \(\upmu\)m, the failure temperature gradually increases, and it rises to 190 \({}^{\circ}\)C at 90 \(\upmu\)m. Excluding other possible influencing factors, the results are still consistent after multiple simulation verification.