## 5 Compilation of Accelerated Test Load Spectrum
### Selection of Reliability Test Conditions  
The experimental conditions for the rotor fatigue aging coupled failure mode can be selected based on the overall distribution of temperature and speed. By jointly counting the temperature and speed distribution of all user data, the frequency under different load levels is calculated and shown in Figure 26a. Using Equations (9) and (10), the damage distribution under different load levels is calculated and shown in Figure 26b. Three reliable test conditions are selected: low speed/low temperature, medium speed/medium temperature, and high speed/high temperature, each lasting 100 s. The specific test condition points are shown in Table 5. It can be seen from the frequency distribution and damage distribution curves that the proportion of low-speed working conditions and the corresponding damage ratio are relatively high. At this working condition, the rotor temperature is mainly distributed between 45 \({}^{\circ}\)C and 69 \({}^{\circ}\)C; therefore, the first working condition point selected is 640 rpm and 60 \({}^{\circ}\)C. When the speed is in the range of 4072 rpm to 8357 rpm, the motor is in the medium-speed working condition. The overall damage distribution of the rotor under this condition is not only high but also relatively uniform. At this point, the rotor temperature is mainly distributed between 69 \({}^{\circ}\)C and 112 \({}^{\circ}\)C; therefore, the second working condition point selected is 6640 rpm and 90 \({}^{\circ}\)C. The core of preparing the acceleration test load spectrum is to transfer the damage from small loads to large loads based on the principle of damage equivalence in order to shorten the test time. Therefore, although the frequency and damage ratio of the high-speed working condition are not high, the third working condition point selected is 10,070 rpm and 110 \({}^{\circ}\)C, which belongs to a relatively harsh working condition.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Failure Mode** & **Damage Targets** \\ \hline Fatigue aging coupling & 0.24 \\ Alternating loads fatigue & 0.013 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Damage targets under different failure modes.  
Figure 26: Speed–temperature frequency and damage distribution.  
The typical operating conditions for the rotor under alternating cyclic fatigue failure mode can be selected based on the overall distribution of the mean and range of the rotor speed. By performing rainflow counting on the rotor speed data for all users, the frequency distribution under different means and ranges of rotor speeds is obtained, as shown in Figure 27. The damage distribution under different means and ranges of rotor speeds can be calculated using Formula (11), as shown in Figure 27. Based on the distribution of the load, the range of the rotational speed was roughly divided into three zones: (0, 3500) for the low-rotational-speed range, (3500, 7000) for the medium-rotational-speed range, and (7000, 10,000) for the high-rotational-speed range. One larger and more frequently occurring operating condition was selected from each range, and the final operating condition points were selected as shown in Table 6.  
As shown in Figure 26, 60 \({}^{\circ}\)C is the most frequent temperature point, and this temperature point is also a relatively balanced result. Therefore, setting the temperature of all three operating conditions to 60 \({}^{\circ}\)C is reasonable. Each operating condition lasts for 100 s, and the rotational speed rises from 0 to the maximum range and then drops to 0 for one cycle, which lasts for 4 s. One complete operating condition lasts for 25 cycles, as shown in Figure 28.