Experimental Investigations Of Electromagnetic Forces In High-Temperature Superconductor Levitation Systems Under Different Conditions

According to the novel passive stable characteristic, high temperature superconducting levitation has become one of important applications of high temperature superconductors (HTSs), and the electromagnetic force characteristic researches on the typical superconducting levitation style-a superconductor and a permanent magnet (PM) levitation system, have become a basis of the superconducting levitating system operation design, and a subject of considerable interest, which have been extensively investigated. This Ph. D. thesis presents the experimental investigations of the static and dynamic electromagnetic force characteristics in the high temperature superconductor levitation system under different conditions. A few new phenomena and characteristics are found, using the macroscopic superconducting theory, and the results are explained.Firstly, the influences of the aging process of superconductors on the levitation force are measured with zero-field cooling (ZFC) and field cooling (FC). It is found that the ZFC levitation force of superconductors oscillates as a function of waiting time, and while for FC superconductors, with waiting time adding, the absolute value of the maximum attraction force increases. Three-dimensional levitation force and lateral force in the levitation system are investigated and the optimization levitation region is found under ZFC process. Meanwhile, a crossing in the force-displacement hysteretic curve of a melt processed HTS is found, and influences of the moving speed of permanent magnet on the levitation force and lateral force are also gained.Next, the effects of the initial cooling height (CH) on the levitation force in the symmetrical levitation system are discussed. After the frozen-image model is modified, the saturated characteristic of maximum levitation force is interpreted theoretically. On this basis, the levitation force relaxation is measured under different cooling heights, and a method to reduce the levitation force is presented and recognized by experimental measurements, which effectively reduces the relaxation with time and while the effective levitation force is not depressed. In the case of unsymmetrical levitation form, the experimental results show that the relaxations of the vertical and horizontal forces are strongly dependent on the initial CH. With CH decreasing, the transition of the lateral force from repulsion to attraction is found as well as the changing characteristics with time from decrease to increase. Additionally, when levitation height (LH) is fixed at the CH, the transition phenomenon is also observed in the levitation force behavior and their relaxation under different lateral displacements (LD). Subsequently, based on the Bean model and the Anderson-Kim thermal activated flux creep theory, relaxation transition in both levitation force and lateral force is qualitative predicted and a prediction criterion is developed in this thesis.Finally, the levitation drift phenomenon in the superconducting levitation system under magnet vibration is measured. In the levitation force-displacement hysteretic curve, when the measurement point is located on the departing process, the levitation drift occurs with decreasing levitation force subject to magnet vibration. After that, the influences of the critical current density, initial CH, LH, weight of levitated body and the amplitude of vibration, etc. on the levitation drift are analyzed. A suggestion that is to weaken or avoid the drift of the levitation system is presented.