## 1 Introduction  
The low-speed high-torque permanent magnet motor has the advantages of high efficiency, a high power factor, high safety and stable operation [1, 2, 3]. At present, low-speed high-torque permanent magnet motors are widely used in various industrial production situations such as mining, manufacturing and transportation, and have a very large application prospect [4, 5]. However, due to the unique application and performance requirements of low-speed high-torque permanent magnet motors, there are few studies on low-speed high-torque permanent magnet motors at present, and there is a lack of a prototype with which to measure and verify the theory, so the research is still not comprehensive and in-depth. At present, the research on low-speed and high-torque permanent magnet motors is mainly about the exploration of new structures, the improvement in motor performance and the optimization of motors. In [6, 7, 8], a double-stator structure is proposed to improve the torque density of low-speed high-torque permanent magnet motors. The kriging response surface model coupled with MOGA is used to optimize the torque ripple [6]. However, the design of the double-stator structure is more complex, and the initial design is a more difficult task. In [9], a novel composite rotor dual-stator low-speed high-torque motor was proposed. The rotor consists of a synchronous reluctance rotor and a surface-mounted permanent magnet rotor. This structure makes full use of the internal space of the low-speed high-torque permanent magnet motor, but the structure is still too complex; design and control are a problem to be considered. There are many studies on the performance improvement in low-speed high-torque permanent magnet motors. The influence of rotor dynamic eccentricity on the performance of a low-speed high-torque permanent magnet motor has been studied [10]. It was found that an increase in rotor eccentricity will lead to an increase in the harmonic distortion rate of air gap flux density and stator copper loss, and the other performance of the motor does not change much. Refs. [11; 12] studied the new structure optimization of a low-speed high-torque motor with the concentrated winding of the true fractional slot and the influence of the circumferential segmentation of the permanent magnet on the rotor loss, which reflects the advantages of the concentrated winding of the true fractional slot, which is easy to be embedded, and the number of spans at the end of the winding is small. The comparison of different pole-slot ratio schemes has also been studied. Ref. [13] comprehensively compared three kinds of pole slot ratio schemes, including the integer slot and fractional slot. The results show that the overall performance of the 32P132S true fractional slot is better. The harmonic distortion rate of the no-load back EMF of the motor under the three pole-slot ratios of 32P48S, 40P48S and 42P54S was compared, and it was found that the waveform of 40P48S was more sinusoidal [14]. However, it is not enough to compare the no-load back EMF only. The pole-slot ratio has a great influence on the electromagnetic performance of the motor, such as torque ripple, maximum output torque and loss, which should be comprehensively compared. The optimization research on low-speed high-torque permanent magnet motors mainly focuses on the exploration of different optimization algorithms. The differential evolution algorithm is used to optimize the multi-objective optimization of the motor, so that the output torque ripple of the fan is smaller and the operation is more stable [15]. In [16; 17], the combination of fuzzy theory and the Taguchi method was introduced to transform the multi-objective optimization problem into a single-objective problem, and the range of design variables was continuously updated during the optimization process. Then, the motor was optimized according to the new range of values, which effectively improved the optimization accuracy of the Taguchi method. [PERSON].