## 3 Simulation Results and Analysis
### Results and Discussion: Torque Performance  
After the no-load performance analysis, the average output performance of the two structures with different driving currents is simulated by adopting the control strategy of the maximum torque per ampere (MTPA) method. At the same time, the original motor is  
Figure 6: Simulation results of no-load air-gap flux density: (**a**) air-gap flux density distribution waveform; (**b**) air-gap flux density harmonic distribution. Figure 7: Simulation results of no-load EMF: (**a**) phase-EMF waveforms; and (**b**) phase-EMFs harmonic distribution. tested under the same current condition, and the results and the test data are compared in Figure 8.  
And the test data curve trend is similar to the simulation curve trend, but slightly lower, which validates the rationality of the simulation model. There are some mismatches between the simulation results and experiments in Figure 8. The average output torque of the simulation data and the test at the rated condition is around 89 N\(\cdot\)m and 99 N\(\cdot\)m, respectively. And the average output torque of the simulation data and the test with the limited condition is 196 N\(\cdot\)m and 209 N\(\cdot\)m, respectively. The difference is caused by the actual value and simulation results of the iron loss at high frequency. In the simulation, the power supply is the ideal sinusoidal wave. However, in the real test, the power supply comes from the converter, which brings higher harmonics and more iron loss. Further, due to the increase in the iron loss, the average output torque of the test is lower than the simulation data. The torque-current phase angle characteristic curves of the two structures with the rated current (115 A) fed are shown in Figure 9. The motor of the optimized structure achieved maximum torque at 34\({}^{\circ}\), while the motor of the original structure achieved maximum torque at 35\({}^{\circ}\). It should be noted that the simulation results are slightly higher than the actual value because the mechanical loss is not completely considered. Focusing on the no-load condition, the cogging torques of the two structures are compared. The waveforms of the cogging torque in the no-load condition at a speed of 3000 rpm show that the peak-peak torque of the optimized structure is 2.9 N\(\cdot\)m, and the original is 3.1 N\(\cdot\)m. Figure 10 and Table 5 indicate that the cogging torques are greatly  
Figure 8: Average output torque performance. Figure 9: Torque-current phase angle characteristic curves of two structures of motors under the rated current (115 A) excited. reduced in both structures if the stator skewed-slot angle is changed from 0\({}^{\circ}\) to 7.5\({}^{\circ}\). The peak-peak cogging torque value of the optimized structure is nearly similar to the original one. The torque waveforms of the two structures under the rated operating condition are shown in Figure 11 and Table 6. The torque ripple percentage of the optimized structure is reduced from the original 14.9% to 7.1%. The output torque comprises many harmonic torques of different frequencies, such as the 6 th and 12 th, which are shown in Figure 11b,d. The largest harmonic percent of the torque for the original structure is 4.6% and 7.2% corresponding to the 6 th and 12 th, respectively, and the largest harmonic percent of the torque for the optimized structure is 3.3% and 5.5% corresponding to the 6 th and 12 th, respectively, under the no skewed-slot condition, and the largest harmonic percent of the torque for the original structure is 2.5% and 3.2% corresponding to the 6 th and 12 th, respectively, and the largest harmonic percent of the torque for optimized structure is 1.0% and 2.1% corresponding to the 6 th and 12 th, respectively, under the skewed-slot condition, from which it can be clearly seen that the harmonic order of the output torque is 6k (k = 1, 2, 3...). It is shown that the lowest harmonic order of the output torque is 6 and the other harmonic orders of the output torque are the multiples of the lowest harmonic order.