# A Novel Design of Permanent Magnets for the Air Gap Magnetic Field of Hollow-Cup Motor
## 4 Experimental Verification  
In order to clarify the effect of improvement the AMF waveform based on the eccentric design of the PMs and to improve the magnetic properties of the PMs of the motor, two sets of PMs were sintered with high-performance NdFeB permanent magnet materials, tile PMs, and PMs with an eccentricity value of 3.5 mm.  
The test prototype was manufactured according to the parameters given in Table 1. The material of the inner and outer rotors was electrically pure iron with a rustproof surface treatment, and the top surface of the inner rotor was scored to ensure that the scores corresponded to the magnetic joints when the prototype was installed to ensure test accuracy. The PMs were made of NdFeB and were designed with different eccentric dimensions according to the simulation results. The inner and outer rotor models and the PM model are shown in Figure 11.  
Figure 10: AMF optimisation results.  
Figure 9: Schematic diagram of the AMF as the eccentricity changes.  
The experimental platform, shown in Figure 12, was mainly composed of the following parts: clamping mechanism, rotary motor, Hall magnetometer, mechanical arm, clamping mechanism consisting of a three-jaw chuck on the centring mechanism, three jaws along the radial movement, connected by thread on the chuck, and adjustable radial position clamping motor model. The rotary motor was firmly connected to the three-jaw chuck and drove the rotation of the three-jaw chuck. The Hall magnetometer measured the strength and direction of the magnetic field at the position of the probe with an accuracy of 0.001 T. The mechanical arm clamped the magnetometer and could adjust its position along the radial direction. After the prototype was assembled and installed on the three-jaw chuck of the experimental platform, the Hall magnetometer was adjusted after clamping and placed at the midpoint of the HCM air gap and the Z- phase of the PMs. The rotary motor drove the prototype to rotate 360\({}^{\circ}\) to measure the AMF of two sets of PMs.  
The FEA result and the experimental result of the PMs under the eccentricity values of 0 mm and 3.5 mm are shown in Figure 13 and Table 2. For the HCM prototype composed of tile-shaped PMs (with an eccentricity value of 0 mm), the values of \(B_{max}\) and \(\tau\) were 0.3649 T and 32.5%, respectively, in the simulation results and 0.3631 T and 29.4%, respectively, in the experimental results, with errors between the experimental and simulation results of 0.48% and 9.54%, respectively. For the HCM prototype composed of the PMs with an eccentricity value of 3.5 mm, the values of \(B_{max}\) and \(\tau\) were 0.3377 T and 57.8%, respectively, in the simulation result and 0.3362 T and 56.3%, respectively, in the experimental result, with errors between the experimental and simulation results of 0.44% and 2.60%, respectively. The experimental result was in general agreement with the simulation result, further confirming the correctness of the theory. This further illustrates that, in the case of parallel magnetisation of the PMs, the AMF of the motor could be changed to the required waveform by changing the shape of the PMs.  
Figure 11: HCM and PM model.  
Figure 12: AMF test experimental platform.  
Analysis of the experimental results showed that the value of \(\tau\) was 29.4% when the HCM used tile PMs and increased from 29.4% to 56.3% when the HCM used PMs with an eccentricity value of 3.5 mm. The experimental results were in good agreement with the simulation results, proving that the eccentric PM design could effectively improve the proportion of the flat section of the motor AMF and further improve motor performance.