# A Novel Design of Permanent Magnets for the Air Gap Magnetic Field of Hollow-Cup Motor
## 1 Introduction
Through further simulation and experimental analysis of the effect of unequal thickness PMs on the AMF, [PERSON] et al. [13] revised this theory. [PERSON] et al. [14] presented a novel design method for built-in motors. The slot design is performed for the rotor face. The slot positions and the shape parameters of the slots under the optimal solution are obtained using the simulation method, which can effectively reduce the harmonic component of the AMF and increase the flat section width of the AMF. Due to the presence of cogging slots in conventional BLDC motors, it is not possible to eliminate the fluctuations in the AMF caused by the cogging slots, which in turn leads to the generation of cogging torque. For surface-mounted motors, optimising the AMF waveform often starts with designing the PM structure. For built-in PM motors, optimising the AMF waveform often starts with designing the outer diameter of the rotor core to achieve a nonuniform radial air gap thickness; thus, rational optimisation of the PMs structure or the rotor core structure can effectively bring the AMF waveform close to a trapezoidal waveform, which in turn brings the counter-electromotive force waveform close to the ITW, contributing to a suppression of torque pulsation and reduction in motor vibration and noise. Different structures of motors have different magnetic paths, but the essence of their magnetic paths is the same. In the magnetic circuit, the magnetic lines of force start from the N pole of the PMs, pass through the permeable material and the working air gap, and converge at the S pole of the PMs. In the magnetic circuit, the PMs provide the magnetic momentum, and the magnetic flux passing through the magnetically conductive material generates a magnetic pressure drop. Due to the development of materials science, the relative permeability of various types of magnetically conductive materials is very low, and the magnetic pressure drop generated by the magnetic flux passing through the magnetically conductive material is often very small. The AMF generated by the PMs interacts with the current-carrying coils energised on the cup stator to make the motor work properly. The properties of the permanent magnet material determine the performance of the PMs. Due to the wild temperature variations in the spacecraft operating environment, samarium cobalt alloy materials are often used for PMs in spacecraft in order to avoid high-temperature demagnetisation. Samarium cobalt alloy material has good magnetic properties with a coercivity of up tp 25 MGOe, and a temperature coefficient of less than \(-0.037\%\)/K. However, it is not easy to sinter into an eccentric structure, which is difficult to process and costly. Therefore, this paper selects NdFeB materials with high performance and excellent temperature stability. NdFeB materials are widely used in the field of motors due to their excellent electromagnetic properties [15]. The NdFeB permanent magnet easily loses excitation when the working environment of the motor is poor and the iron core loss is large [16; 17]. In order to solve this problem, there are two research directions: doping of NdFeB with other rare Earth elements or metal oxides during the sintering and forming process [18; 19; 20; 21]. Studies have shown that doping with Ho [18], co-doping with Al/Cu [19], or co-doping with Ga/Cu [20; 21] during the sintering and formation of NdFeB PMs can effectively improve the microstructure of the PMs and further enhance their magnetic properties. The magnetic properties of NdFeB materials can also be improved by optimising the NdFeB sintering manufacturing technology [22; 23], then changing the bottleneck particle size of the permanent magnet material powder [24]. The improved neodymium iron boron material has a higher service temperature (about 470 K), an extremely high coercivity (35 MGOe), and a relatively excellent temperature coefficient (\(-0.049\%/\)K). This paper deals with the HCM using a large air gap.