J. Zheng

Method to reduce levitation force decay of the bulk HTSC above the NdFeB guideway due to lateral movement

A magnetic levitation vehicle using bulk high-Tc superconductors (HTSC) is considered as a promising transportation type thanks to its lateral inherent stability, but previous studies have found that the levitation force (LF) decays due to lateral movement. In this paper, a pre-load method is presented to reduce the LF decay, and the experimental results indicate that this method is very applicable in supressing this decay in spite of the applied field and material property of the bulk HTSC, and this effect can be ascribed to the reduction of the hysteresis loss in the bulk HTSC, i.e. more trapped magnetic flux after the pre-load case. In the end, experimental results indicate that the Halbach PMG has an advantage to reduce the cost of the PMG, but its rate of LF decay is also larger due to lateral movement

The present status of the high temperature superconducting Maglev vehicle in China

Since the first successful running of the people-carrying high temperature superconducting (HTS) Maglev test vehicle on 31 December 2000, about 27 000 people have taken it, and the accumulated running distance is about 400 km. The levitation force of the onboard HTS equipment is measured periodically, and new experimental results measured on 5 March 2003 show that the performance of the onboard HTS Maglev equipment is almost the same as that of two years ago. Experimental results indicate that the long-term stability of the HTS Maglev vehicle is good. This further proves the feasibility of the HTS Maglev vehicle for practical transportation. It is worth mentioning that all the results are measured at a low speed; however, investigations of the dynamic performance of the HTS Maglev vehicle at high speed are necessary for practical application. Research on the dynamic performance of the HTS Maglev vehicle is ongoing.

Maglev performance of a double-layer bulk high temperature superconductor above a permanent magnet guideway

In order to improve the performance of the present high temperature superconducting (HTS) maglev vehicle system, the maglev performance of single- and double-layer bulk high temperature superconductors (HTSC) was investigated above a permanent magnet guideway (PMG). It is found that the maglev performance of a double-layer bulk HTSC is not a simple addition of each layers levitation and guidance force. Moreover, the applied magnetic field at the position of the upper layer bulk HTSC is not completely shielded by the lower layer bulk HTSC either. 53.5% of the levitation force and 27.5% of the guidance force of the upper layer bulk HTSC are excited in the double-layer bulk HTSC arrangement in the applied field-cooling condition and working gap, bringing a corresponding improvement of 16.9% and 8.8% to the conventional single-layer bulk HTSC. The present research implies that the cost performance of upper layer bulk HTSC is a little low for the whole HTS maglev system.

Performance of a Small-Scale High Temperature Superconducting Linear Synchronous Motor Prototype

Four coils made of YBCO-coated conductor wire were fabricated and connected in series to make up the excitation system of a linear synchronous motor system with a stator made of ordinary copper wire. The electromagnetic forces experienced by the superconducting coils with respect to the stator were studied in the static case. We began with the study of one single coil, followed by two coils connected in series, and finally, four coils in series from which the largest force obtained was of 53.9 N at a gap of 10 mm at 77 K. The critical current, n-value, and inductance were also measured for the coils so that the power dissipation of the field windings can be calculated. This paper also helps us understand whether linear motors with superconducting components are currently economically feasible with present commercially available superconducting wire.

The relationship between levitation force and stiffness in symmetrical and unsymmetrical applied fields

The two key parameters for levitation performance over a permanent magnet (PM) guideway are the levitation force and the stiffness of the high temperature superconductor (HTS) YBaCuO bulk. Their distributions along a lateral displacement (LD) were measured in zero-field-cooling (ZFC) and in field-cooling (FC) conditions. There are two cases of applied magnetic field for a single HTS bulk: symmetrical or unsymmetrical about the bulk axis. Although the levitation force has a linear relationship with the applied field in the symmetrical case, such a linear relationship disappears once the applied field becomes unsymmetrical. Furthermore, the levitation stiffness is measured in FC, and they still change more drastically than the levitation force in ZFC. In particular, the levitation stiffness has a linear relationship with the associated levitation force, whether the applied field is symmetrical or unsymmetrical. Its lateral distribution at a gap of 20 mm in FC agrees with the levitation force distribution at a gap of 10 mm in ZFC.

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.

A Double-Superconducting Axial Bearing System for an Energy Storage Flywheel Model

The bulk high temperature superconductors (HTSCs) with unique flux-pinning property have been applied to fabricate two superconducting axial bearings for an energy storage flywheel model. The two superconducting axial bearings are respectively fixed at two ends of the vertical rotational shaft, whose stator is composed of seven melt-textured YBa2Cu3O7-x (YBCO) bulks with diameter of 30 mm, height of 18 mm and rotor is made of three cylindrical axial-magnetized NdFeB permanent magnets (PM) by superposition with diameter of 63 mm, height of 27 mm. The experimental results show the total levitation and lateral force produced by the two superconducting bearings are enough to levitate and stabilize the 2.4 kg rotational shaft. When the two YBCO stators were both field cooled to the liquid nitrogen temperature at respective axial distances above or below the PM rotor, the shaft could be automatically levitated between the two stators without any contact. In the case of a driving motor, it can be stably rotated along the central axis besides the resonance frequency. This double-superconducting axial bearing system can be used to demonstrate the flux-pinning property of bulk HTSC for stable levitation and suspension and the principle of superconducting flywheel energy storage system to visitors.

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.