## 3 Comparison of Different Surrogate Models and Motor Optimization Design
### The Influence of Design Parameters on Electromagnetic Performance  
The influence of stator tooth width on the electromagnetic performance of the motor is shown in Figure 8. The size of the stator tooth width determines the saturation degree of the tooth magnetic density and the utilization rate of the permanent magnet. As the tooth width increases from 7 mm to 9 mm, the no-load back EMF and power factor of the motor increase first and then decrease. The output torque increases continuously, and the torque ripple decreases first and then increases. In addition, with the increase in tooth width, the magnetic density of the tooth decreases, so the corresponding iron loss will continue to decrease, and the change in efficiency is small.  
Figure 9 shows the influence of the stator slot opening on the electromagnetic performance of the motor. As the slot opening increased from 2.5 mm to 3.5 mm, the no-load back EMF and efficiency of the motor decreased slightly, but the torque ripple decreased from 1.1% to 0.75%. In addition, the power factor increased from 0.85 to 0.89, the output torque increased from 2.16 kNm to 2.09 kNm, and the iron loss increased slightly.  
The pole arc coefficient of permanent magnet is a very important design parameter, which greatly affects the flux of each pole of the motor. The influence of the pole arc coefficient on the performance of the motor is shown in Figure 10. As the pole arc coefficient increased from 0.65 to 0.8, the no-load back EMF and efficiency of the motor continued to increase, and the increase was larger. The output torque increased from 2.15 kNm to 2.3 kNm, and the efficiency also increased from 95.2% to 96%. However, the torque ripple of the motor and the eddy current loss of the permanent magnet were greatly reduced.  
Figure 8: Influence of tooth width on electromagnetic performance. (**a**) No-load back EMF and power factor. (**b**) Torque and torque ripple. (**c**) Efficiency and core loss.  
Figure 7: The cross section of the stator and rotor unit motor of 70P168S motor.  
The thickness of the permanent magnet has little effect on the air gap flux density, but has a great influence on the working point of the permanent magnet. The influence of the thickness of the permanent magnet on the electromagnetic performance of the motor is shown in Figure 11. As the thickness of the permanent magnet increased from 5 mm to 10 mm, the no-load back EMF and power factor were increasing, and the output torque and efficiency were also improved. However, the influence of the thickness of the permanent magnet on the electromagnetic properties was smaller than that of the pole arc coefficient. Similarly, the torque ripple of the motor was reduced from 0.95% to 0.65%, and the eddy current loss of the permanent magnet was also reduced from 22 W to 18 W.  
As the most important parameter of motor design, the air gap length has a great influence on the performance of the motor. Figure 12 shows the influence of air gap length  
Figure 11: Influence of permanent magnet thickness on electromagnetic performance. (**a**) No-load back EMF and power factor. (**b**) Torque and torque ripple. (**c**) Efficiency and solid loss.  
Figure 9: Influence of slot opening on electromagnetic performance. (**a**) No-load back EMF and power factor. (**b**) Torque and torque ripple. (**c**) Efficiency and core loss.  
on the performance of the motor. When the air gap length of the motor increased from 1 mm to 3 mm, the torque ripple, iron loss and permanent magnet eddy current loss of the motor were greatly reduced. Because the air gap length increased, the air gap flux density harmonic content was greatly reduced.