The nondestructive testing of agricultural products has always been a key technology for the modernization of agriculture and food. By applying a sinusoidal voltage (current) excitation signal of variable frequency, the relationship between the amplitude, frequency and phase of the response signal is obtained, and the measured response function in a certain frequency range is obtained, constructing the correlation between impedance spectroscopy and matter properties. Electrical impedance spectroscopy (EIS) is a widely used method for the nondestructive characterization of agricultural products, and its applications in the agricultural field has attracted increasing attention. This paper summarizes the research of electrical impedance spectroscopy (EIS) in the detection of grain quality, fruit and vegetable quality, meat quality and food quality from 2005 to 2022. The potential and development direction of electrical impedance spectroscopy in the nondestructive testing of agricultural product quality are prospected, which provides a reference for scientific researchers who applied electrical impedance spectroscopy in agricultural product quality detection.

Aiming at the influence on the actual speed caused by the suspension winding′s rotor induced current of a squirrel-cage bearingless induction motor (SBIM), a novel three-speed wound bearingless induction motor (WBIM) is designed based on establishing the stator–rotor equivalent circuit, deriving the exciting current-suspension slip ratio equation and analyzing the magnetic field distribution with odd pole pairs and dipole pairs. The stator is embedded with a fixed four pole winding and a 2/6 pole-changing winding, while the rotor is wound with series–parallel windings connecting at a slip ratio ring. By changing the connection method of the stator coils, adjusting the three-phase voltage source, and controlling the short circuit of the rotor winding central point, it can realize that the rotor only induces the torque winding′s magnetic field under three speeds. Combining with the finite-element analysis, the induced current, air gap magnetic density, suspension force, and electromagnetic torque of the traditional SBIM and the new WBIM are quantitatively compared. Finally, an experimental platform is built for further verification. The results show that the designed three-speed WBIM can not only effectively suppress the induced current of the suspension winding and reduce its influence on the motor torque and speed, but also makes the motor has a better start-up and steady-state performance.

To enhance the working efficiency of a bearingless induction motor with the specific pole, an accurate modeling method considering the iron loss is proposed, and the optimal rotor flux-oriented control strategy is studied in this article. First, the mutual inductance expressions with rotor eccentricity of the torque winding and suspension winding are deduced and verified theoretically. In addition, the two windings are converted into the same circuit and the composed mutual inductance is discussed. Second, the accurate equivalent circuit model on the d -, q -axis considering the iron loss is established, and the optimal rotor flux is figured out. Third, by theoretical deduction, the rotor flux expression is directly turned into the function of the equivalent composed winding current, and the parameter identification method based on the simplified flux-current model is proposed to enhance the control accuracy. Further, the delayed field weakening strategy is introduced in the control system to suppress the rotor vibration. Finally, an experimental platform is built to validate the practicality. Theoretical analysis and experimental results indicate that the adopted optimal rotor flux control both improves the motor efficiency and achieves an excellent levitation performance.

To improve the quality of starting torque and suspension force, a composite cage rotor bearingless induction motor (CCR-BIM) is designed, which adopts a composite cage rotor structure that combines an inner rotor and an outer rotor. First, the overall structure of the CCR-BIM is designed, the composite cage rotor of the CCR-BIM is specially designed and analyzed for the induction principle, and the mathematical model of suspension force and torque is deduced. Second, the initial structural parameters are determined, and motor qualities, such as starting torque quality and suspension force quality, are compared and analyzed between the proposed motor and BIM using a finite element model (FEM). Third, based on the response surface model (RSM), a multi-objective improved NSGA-II is constructed, and the three optimization objectives of starting torque, suspension force, and suspension force pulsation are optimized. Finally, the results of the experimental setup prove that the starting torque increases by 6.98%, the suspension force increases by 5.45%, and the suspension force pulsation decreases by 18.54%. The effectiveness of the proposed motor and the correctness of the multi-objective optimization strategy are verified.

To reduce the harmonics impact of a bearingless induction motor (BIM), an improved modeling method considering the quadratic terms of the rotor eccentricity and a new stator structure of half-slot are proposed. Firstly, the accurate Maxwell force model consisting of the suspension force and unbalanced magnetic pull is established. The theoretical results are expanded to multi-pole situations, and the controllable suspension range is discussed. Secondly, the influence on the Maxwell force amplitude caused by harmonics is analyzed, and the force model is revised combing with the limit formula solution method for the BIM with concentrated winding. For the BIM with distributed winding, the winding distribution of the rotation system and suspension system are studied, the optimal slot number is found and proved by theoretical derivations, and finally a novel half-slot structure of the suspension winding is proposed. Theoretical analysis, simulation results as well as experimental results all indicate that the proposed method can effectively reduce the influence of the harmonics and further make the Maxwell force and rotor position has lower maximum fluctuation compared with the initial machine.