Botian Zheng

Magnetic and levitation characteristics of bulk high-temperature superconducting magnets above a permanent magnet guideway

Due to the large levitation force or the large guidance force of bulk high-temperature superconducting magnets (BHTSMs) above a permanent magnet guideway (PMG), it is reasonable to employ pre-magnetized BHTSMs to replace applied-magnetic-field-cooled superconductors in a maglev system. There are two combination modes between the BHTSM and the PMG, distinguished by the different directions of the magnetization. One is the S-S pole mode, and the other is the S-N pole mode combined with a unimodal PMG segment. A multi-point magnetic field measurement platform was employed to acquire the magnetic field signals of the BHTSM surface in real time during the pre-magnetization process and the re-magnetization process. Subsequently, three experimental aspects of levitation, including the vertical movement due to the levitation force, the lateral movement due to the guidance force, and the force relaxation with time, were explored above the PMG segment. Moreover, finite element modeling by COMSOL Multiphysics has been performed to simulate the different induced currents and the potentially different temperature rises with different modes inside the BHTSM. It was found that the S-S pole mode produced higher induced current density and a higher temperature rise inside the BHTSM, which might escalate its lateral instability above the PMG. The S-N pole mode exhibits the opposite characteristics. In general, this work is instructive for understanding and connecting the magnetic flux, the inner current density, the levitation behavior, and the temperature rise of BHTSMs employed in a maglev system.

Study on the Magnetic Field Inhomogeneity of a Halbach Permanent-Magnet Guideway Due to Different Defects

The track regularity of a permanent-magnet guideway (PMG) is an important issue to ensure the riding comfort and running stability of high-temperature superconducting (HTS) maglev vehicles. Compared with defects of typical railway tracks, similar defects on PMG could affect the magnetic field at the levitation height and then influence the running performance and the riding comfort of the maglev system. In this paper, the influence of four kinds of Halbach PMG defects on magnetic field distribution was investigated by a finite-element simulation method. According to the calculation results, it is found that the sensitivity of the magnetic field inhomogeneity varies with each kind of defect. The vibration experiments found that the magnetic field inhomogeneity does bring vibration to HTS maglev vehicles. The results could provide significant references for future PMG design in the HTS maglev system.

Levitation Performance of YBCO Bulks in Supercooling Condition Under a Low-Pressure Environment

Taking the influence of low pressure on levitation performance of high temperature superconducting (HTS) maglev into account, we established a simple super-cooling platform based on pumping method to study the levitation performance of YBCO bulks above the Halbach permanent magnet guideway (PMG) under different pressure conditions. Through measuring the temperature of liquid nitrogen (LN2) during the decreasing process of pressure, it is known that the LN 2 would turn into a super-cooling state as the pressure reduction. Measurements of the levitation force versus vertical motion and the force relaxation were performed both in case of zero-field-cooling (ZFC) and field-cooling (FC). The experimental result showing that the decreasing pressure is beneficial to improve the levitation performance of HTS bulks, and the maximum force increase up to 23.5% and 22.1% were realized in ZFC and FC compared with the atmospheric pressure, respectively. Moreover, the low pressure could also reduce the hysteresis loop area and restrain the relaxation decay of levitation force. These results imply that, the low pressure environment in evacuated tube will be beneficial to the levitation performance of HTS maglev system

Nonlinear vibration behaviors of high-Tc superconducting bulks in an applied permanent magnetic array field

The nonlinear vibration of high temperature superconducting (HTS) bulks in an applied permanent magnetic array (Halbach array) field, as a precondition for commercial application to HTS maglev train and HTS bearing, is systematically investigated. This article reports the actual vibration rules of HTS bulks from three aspects. First, we propose a new numerical model to simplify the calculation of levitation force. This model could provide precise simulations, especially the estimation of eigenfrequency. Second, an approximate analytic solution of the vibration of the HTS bulks is obtained by using the method of harmonic balance. Finally, to verify the results mentioned above, we measure the vertical vibration acceleration signals of an HTS maglev model, consisting of eight YBaCuO bulks, oscillating freely above a Halbach array with large displacement excitation. Higher order harmonic components, which indicate the nonlinear vibration phenomenon, are detected in the responses. All the three results are com...

Magnetic Characteristics of Single-Block and Multi-Block Nd-Fe-B Permanent Magnets at Low Temperature

Based on the intrinsic temperature dependence of permanent magnet (PM) materials, the changing magnetic characteristics of three Nd-Fe-B magnet configurations with temperature were studied. A single PM, a traditional permanent magnet guideway (PMG) with steel as the flux collector in the center of the guideway (PMG A), and a Halbach-type PMG (PMG B) were investigated in cooling experiments. Liquid nitrogen was used to cool the PM and the PMGs from room temperature to the low temperature of 77 K, while the temperature and magnetic field signals were collected simultaneously. The results show that, with the temperature decrease, both the single PM and PMGs have a significant increase in their magnetic fields, which reach peak values. The magnetic flux density of the traditional PMG A shows a more significant increase of 20.3%, while the growth rates of the single cylinder PM and PMG B are 11.1% and 8.4%, respectively. As the temperature drops below a certain value, a spin-reorientation effect takes place inside the PM and causes a rapid decline of the magnetic flux density. Finally, compared with the magnetic field value at room temperature, the magnetic field variation of the single PM, PMG A, and PMG B were 5.7% decrease, 4.3% increase, and 0.7% decrease, respectively. The results provide basic data on the variation of magnetic characteristics of PM and PMG with temperature and contribute to research on superconducting levitation systems.