## 2 Comprehensive Comparison of Four Pole-Slot Ratio Schemes
### Comparison of Electromagnetic Performance of Four Kinds of Pole-Slot Motors  
In order to compare the fairness and rigor, the current density, permanent magnet dosage, stator and rotor size and no-load back EMF of the four pole-slot ratio motors were kept similar or equal in this paper. By changing the winding setting, the four motors achieved rated performance. As shown in Figure 2, the no-load back-EMFs of the four pole-slot ratio motors were basically the same, among which the no-load back-EMF of the 70P84S motor was higher, and the no-load back-EMF of the 70P168S was slightly lower. In addition, it can be seen from the harmonic distribution map that the harmonic content of the 70P84S motor was the most abundant, mainly including 5 th, 7 th, 9 th, 11 th, 13 th and 15 th harmonics. The 9 th harmonic of 70P168S motor was larger. The high-order harmonics of the 80P90S motor accounted for \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter Name** & **Value** \\ \hline Rated power (kW) & 37 \\ Rated speed (rpm) & 160 \\ Rated voltage (V) & 350 \\ Rated torque (Nm) & 2208 \\ Stator outer diameter (mm) & 960 \\ Rotor outer diameter (mm) & 837 \\ Stack length (mm) & 125 \\ Airgap length (mm) & 1.5 \\ Power factor & \textgreater{}0.9 \\ Efficiency (\%) & \textgreater{}94 \\ Maximum torque (Nm) & \textgreater{}8800 \\ Torque ripple (\%) & \textless{}3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main data and performance requirements of low-speed high-torque motor. Figure 1: Cross-sectional view of the unit motor with four pole–slot ratios. (**a**) 70P84S (10P12S). (**b**) 70P168S (10P24S). (**c**) 80P90S (8P9S). (**d**) 80P180S (8P18S). a relatively small proportion. The high-order harmonics of 80P180S accounted for a large proportion. Figure 3 describes the no-load radial air gap flux density and harmonic distribution of four pole-slot ratio motors. It can be seen that the air gap flux density of the 70P168S and 80P180S pole-slot ratio motors was higher. Therefore, the distributed winding can improve the air gap flux density of the motor. In addition, the air gap flux density mainly contained 3 rd, 5 th, 7 th, 9 th, 11 th and 13 th harmonics. Among them, the motor of 70P84S had a greater high-order harmonic content. The harmonic content of 70P168S motor was small. The no-load magnetic density and magnetic line cloud diagram of the four pole-slot ratio motors are shown in Figure 4. It can be seen from the figure that the tooth magnetic density of the 70P168S motor was close to saturation, and the magnetic flux leakage are more. The 80P180S motor had the largest number of slots, but the saturation of the teeth was not serious, and the magnetic flux leakage was the least. The teeth of the 70P84S motor and the 80P90S motor were basically not saturated. The load simulation was carried out by using the voltage source. The output torque and maximum torque curves of the four pole-slot ratio motors are shown in Figure 5. When the four motors all reached the rated performance, the torque ripple of the 70P84S motor was the largest, which was 7.9%, while the torque ripple of the 70P168S motor was 1.4%, which is the smallest. In addition, the maximum output torque of 80P168S was 9.9 kNm, and the overload multiple was 4.48 times. The maximum output torques of the 70P84S motor and the 80P90S motor were similar, no more than 8 kNm, and the overload multiple was far less than four times. It can be seen that the distributed winding motor can put out greater torque. Figure 3: The no-load radial air gap flux density curves and harmonic distribution of four pole-slot ratio motors. (**a**) No-load radial air gap flux density curves. (**b**) Harmonic distribution. Figure 2: The no-load back EMF curves and harmonic distribution diagrams of four pole-slot ratio motors. (**a**) No-load back EMF curves. (**b**) Harmonic distribution. Figure 6 shows the power factor, efficiency and loss of the four pole-slot ratio motors.