Flux Concentrator Optimization of PMG for High-Temperature Superconducting Maglev Vehicle System

The permanent magnetic guideway (PMG) composed of permanent magnet (PM) and steel is developed under flux concentration principle, which is the crucial component of high-temperature superconducting (HTS) maglev vehicle system. Optimum PMG design is an effective way to increase levitation force and associated stiffness for improving the load capability of HTS maglev vehicle. In order to realize higher vertical field component B _z in upper surface, three PMG demonstrators with three different forms of flux concentrator are fabricated with same volume of magnet. The levitation performances of onboard HTS bulks array over them are studied. The experimental results indicate that the PMG with a permanent magnet as the flux concentrator would produce biggest levitation force, levitation stiffness and trapped flux when interacting with HTS superconductor.     

Superconducting bulk magnet for maglev vehicle: Stable levitation performance above permanent magnet guideway

High-temperature superconducting (HTS) maglev vehicle is well known as one of the most potential applications of bulk high-temperature superconductors (HTSCs) in transported levitation system. Many efforts have promoted the practice of the HTS maglev vehicle in people's life by enhancing the load capability and stability. Besides improving the material performance of bulk HTSC and optimizing permanent magnet guideway (PMG), magnetization method of bulk HTSC is also very effective for more stable levitation. Up to now, applied onboard bulk HTSCs are directly magnetized by field cooling above the PMG for the present HTS maglev test vehicles or prototypes in China, Germany, Russia, Brazil, and Japan. By the direct-field-cooling-magnetization (DFCM) over PMG, maglev performances of the bulk HTSCs are mainly depended on the PMG's magnetic field. However, introducing HTS bulk magnet into the HTS maglev system breaks this dependence, which is magnetized by other non-PMG magnetic field. The feasibility of this HTS bulk magnet for maglev vehicle is investigated in the paper. The HTS bulk magnet is field-cooling magnetized by a Field Control Electromagnets Workbench (FCEW), which produces a constant magnetic field up to 1 T. The levitation and guidance forces of the HTS bulk magnet over PMG with different trapped flux at 15 mm working height (WH) were measured and compared with that by DFCM in the same applied PMG magnetic field at optimal field-cooling height (FCH) 30 mm, WH 15 mm. It is found that HTS bulk magnet can also realize a stable levitation above PMG. The trapped flux of HTS bulk magnet is easily controllable by the charging current of FCEW, which implies the maglev performances of HTS bulk magnet above PMG will be adjustable according to the practical requirement. The more trapped flux HTS bulk magnet will lead to bigger guidance force and smaller repulsion levitation force above PMG. In the case of saturated trapped flux for experimental HTS bulk magnet, it is not effective to improve its maglev performances by increasing of charging magnetic field, when the guidance force at WH 15 mm is 5.7 times larger than that by DFCM of FCH 30 mm. So introducing HTS bulk magnet into the present maglev system is feasible and more controllable to realize stable levitation above applied PMG, which is an important alternative for the present HTS maglev vehicle.     

Correlations Between Magnetic Flux and Levitation Force of HTS Bulk Above a Permanent Magnet Guideway

In order to clarify the correlations between magnetic flux and levitation force of the high-temperature superconducting (HTS) bulk, we measured the magnetic flux density on bottom and top surfaces of a bulk superconductor while vertically moving above a permanent magnet guideway (PMG). The levitation force of the bulk superconductor was measured simultaneously. In this study, the HTS bulk was moved down and up for three times between field-cooling position and working position above the PMG, followed by a relaxation measurement of 300 s at the minimum height position. During the whole processes, the magnetic flux density and levitation force of the bulk superconductor were recorded and collected by a multipoint magnetic field measurement platform and a self-developed maglev measurement system, respectively. The magnetic flux density on the bottom surface reflected the induced field in the superconductor bulk, while on the top, it reveals the penetrated magnetic flux. The results show that the magnetic flux density and levitation force of the bulk superconductor are in direct correlation from the viewpoint of inner supercurrent. In general, this work is instructive for understanding the connection of the magnetic flux density, the inner current density and the levitation behavior of HTS bulk employed in a maglev system. Meanwhile, this magnetic flux density measurement method has enriched present experimental evaluation methods of maglev system.     

Influence of the Air Gap between Adjacent Permanent Magnets on the Performance of NdFeB Guideway for HTS Maglev System

For the permanent magnet guideway (PMG) of the high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system, there should be many air gaps between two adjacent permanent magnets by connecting, which may fluctuate magnetic field in the propulsion direction of the vehicle. A three-dimensional (3D) model of a PMG is built up using FLUX 3D software. The magnetic field density of an NdFeB PMG is measured and simulated by 3D and 2D models. Comparison among their results indicate that the simulation results of the 3D model agree better with the measuring results than that of the 2D model, and thus the 3D model is more suitable to describe the PMG. By the model, the influence of the air gap on the uneven of the magnetic field density in the propulsion direction is studied. It is found that the magnetic field 15 mm above the PMG is roughly even in the propulsion direction, although the magnetic field at 2 mm is not even. Since the working height 15 mm is a sensitivity parameter for the onboard high temperature superconductor (HTSC), the levitation force at working height 15 mm above PMG is measured, which indicates that the influence of the air gap on the levitation force is very small so as to be ignored in the quasi-static state.     

Observation of the Field,Current and Force Distributions in an Optimized Superconducting Levitation with Translational Symmetry

The superconducting levitation realized by immersing the high-temperature superconductors (HTSs) into nonuniform magnetic field is deemed promising in a wide range of industrial applications such as maglev transportation and kinetic energy storage. Using a well-established electromagnetic model to mathematically describe the HTS, we have developed an efficient scheme that is capable of intelligently and globally optimizing the permanent magnet guideway (PMG) with single or multiple HTSs levitated above for the maglev transportation applications. With maximizing the levitation force as the principal objective, we optimized the dimensions of a Halbach-derived PMG to observe how the field, current and force distribute inside the HTSs when the optimized situation is achieved. Using a pristine PMG as a reference, we have analyzed the critical issues for enhancing the levitation force through comparing the field, current and force distributions between the optimized and pristine PMGs. It was also found that the optimized dimensions of the PMG are highly dependent upon the levitated HTS. Moreover, the guidance force is not always contradictory to the levitation force and may also be enhanced when the levitation force is prescribed to be the principle objective, depending on the configuration of levitation system and lateral displacement.     

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

A magnetized bulk high-T _c superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-T _c superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.     

Influence of Vertical Vibrations on an Array of Bulk HTSC Above the Permanent Magnet Guideway

The dynamic response of high-temperature superconducting (HTS) maglev vehicle system, which consists of a high-temperature superconductor (HTSC) array of seven YBa₂Cu₃O_7−x bulks above permanent magnet guideway (PMG), was investigated experimentally. The dynamic stiffness and displacements and the levitation force of the bulk HTSCs were investigated as function of the external excitation amplitudes in the range of 1–4 mm and frequencies in the range of 1–400 Hz. The levitation force and the resonance frequency and the dynamic stiffness of the bulk HTSCs are correlative to reduce with the excitation frequency lower than 30 Hz and the applied excitation amplitude higher than 3 mm due to the energy loss. However, the experimental results show the reduction occurred in the first several cycles, which proves that the array of the bulk HTSCs can sustain the external excitation vibration in the vertical direction due to its strong dynamic stiffness. The experimental results are helpful in the application of the HTS maglev vehicle in the future.     

Influence of Re-magnetized HTS Bulk Samples on the Levitation Performances of a Maglev System

High-T _c superconducting (HTS) bulk samples used for a maglev system can be re-magnetized after being subjected to another applied magnetic field which may influence the levitation performance of a HTS bulk over the permanent magnetic guideway (PMG). In order to clarify the relationship between the HTS bulk’s re-magnetization and its levitation performance, a YBCO bulk was re-magnetized at different values and its levitation performance studied experimentally above a permanent guideway (PMG) system. The results show that the increase of trapped magnetic flux inside the HTS bulk is closely related to the magnitude and direction of the applied field during the re-magnetization process. Furthermore, the levitation force changes on the HTS-PMG system affected by the re-magnetized HTS bulk are discussed allowing for optimization improvements to the maglev system load capabilities.     

3D-Modeling Numerical Solution of Guidance Force of HTSC Bulk Above Permanent Magnetic Guideway

This paper presents a 3D-modeling numerical method to simulate the electromagnetic behavior of high-temperature superconductors (HTSC) while the bulk departs from the center line of the guideway. In this paper, for a typical superconducting levitation style with a bulk superconductor suspended above a permanent magnet guideway (PMG), the guidance characteristics of the system were investigated computationally followed by experimental work. Based on the proposed 3D-modeling numerical solutions of electromagnetic behavior of a HTS bulk sample, we used the grid multiplexing technology in which the meshing nodes inside the HTS bulk represent the centers of pinning to simulate flux pinning of HTS bulk. The resolved code of the 3D-modeling of high-temperature maglev was developed with the FORTRAN language. Comparisons between the experimental and numerical results indicated the effectiveness of the method utilized in the paper.     

Enhanced Maglev Performance by Field Cooling for HTS Maglev System in Curve Negotiation

In the practical operation of high-temperature superconducting (HTS) maglev system, the problem of curve negotiation cannot be neglected. During the process of going through curve path, the maglev vehicle would laterally deviate from the center of the permanent magnet guideway (PMG) because of centrifugal forces. To explore the performance variation of the maglev system in this process, the electromagnetic forces (levitation force and guidance force) of a HTS bulk levitation unit were experimentally investigated by different eccentric distances (EDs). The ED is emulated by laterally moving the levitation unit relative to the PMG. Experimental result shows that in field-cooling (FC) conditions, the levitation force generally shows a increase tendency with the growing EDs, while it keeps decreasing in zero-field-cooling (ZFC) conditions. And, the levitation force with large EDs in the FC condition is larger than that in the ZFC condition. The guidance force is enhanced in both conditions within ED of 15 mm, and in FC condition, the force is larger than the case of ZFC. Comparing the above two important magnetic force results, the FC condition is recommended for the safe operation of the HTSn system in curve negotiation.     

Remagnetization effects due to lateral displacement above a PMG on bulk HTS magnet

For a high-T_c superconducting (HTS) maglev system with large force requirements, the use of magnetized bulk high-T_c superconductor magnets (MBSCMs) is a good candidate because of its strong flux pinning ability and corresponding high trapped flux. Different from the rare-earth permanent magnet (PM), the trapped flux of a MBSCM is sustained by the supercurrent produced by a magnetizing process, so the trapped flux is sensitive to variations of the supercurrent. The lateral displacement of a MBSCM above a PM guideway (PMG) will provide disturbance of the applied field and then alter the supercurrent as a process of remagnetization. Different magnetization histories will bring different remagnetization characteristics and consequently diverse levitation performances for a MBSCM during the lateral displacements. When the MBSCMs are applied into the HTS maglev system, the influence of lateral displacements on levitation performance should be taken into consideration. This article investigates the remagnetization characteristics of a MBSCM when it is subject to the lateral displacements above a PMG with different trapped magnetic flux and opposite magnetization polarities. Relevant analyses about the internal supercurrent configuration based on the critical state model are also included to better understand the remagnetization characteristic of a MBSCM.     

Numerical Evaluation of Levitation Force Oscillation of HTS Bulk Exposed to AC Magnetic Field over NdFeB Guideway

The levitation force of the YBCO bulk over an NdFeB guideway used in the high-temperature superconducting (HTS) maglev vehicle system is oscillated by the application of the AC external magnetic field. In our previous work, we interpreted that the oscillation is due to the shielding current fluctuation caused by fluctuant external magnetic field. In this paper, based on the Bean model, an analytical model is adopted to evaluate the levitation force. Comparing with the experimental results, the calculated results show good matching. The model can reveal the oscillation characteristics of the levitation force of HTS bulk which is being exposed to AC external magnetic field. Therefore, the levitation force oscillation of the HTS bulk in the maglev vehicle system can be evaluated by this numerical method.     

Dynamic Simulation of High Temperature Superconductors Above a Spinning Circular Permanent Magnetic Guideway

Before a high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system can be fully applied and operational, the study of its dynamic characteristics is necessary. With the developed HTS Maglev dynamic measurement system (SCML-03), with a circular permanent magnet guideway (PMG) of 1.5 m in diameter, the vehicle’s translational motion above a PMG can be effectively simulated with the PMG allowed to rotate freely. Levitation force measurements of a high temperature superconductor (HTSC) array of seven YBa₂Cu₃O_7−x bulks were carried out above regular (linear) and a simulated (circular) PMG. The levitation force above a linear PMG segment and a circular PMG segment in the static state is found to be in good agreement with each other. The levitation force in the dynamic state is found to slowly attenuate since the presence of a rotating circular PMG below the HTS array is found to be analogous to the application of an AC external magnetic field.     

Study of Magnetic Field Inhomogeneity Due to Different Positional Deviations of a Permanent Magnet Guideway

This paper focuses on the magnetic field inhomogeneity at the rail gap due to five kinds of position deviations between two segments of permanent magnet guideway (PMG). PMG model is built by the finite element analysis software, and effectiveness of the model is proven by comparing the actual magnetic field with the calculated value. The influence of the magnetic field by the gap, horizontal displacement and vertical displacement was investigated. Kinetic characteristics of HTS maglev when it passes the rail gap are studied. The change of magnetic field at the rail gap is also studied in these cases: one PMG rotates relative to another PMG and the gap is not perpendicular to the direction of PMG. It is found that magnetic force between two PMGs will enlarge the position deviations. Finally, the methods are introduced for improving the homogeneity of magnetic field of PMG and then reducing the magnetic loss due to magnetic field inhomogeneity.     

Levitation Force Relaxation of HTS YBCO Bulk under Load

The loading capacity of the high temperature superconducting (HTS) maglev vehicle is closely relative to the maglev characteristics of the HTS bulk under the load. The investigation of the characteristics is necessary for the application of the HTS maglev vehicle. In our previous work, it was found that the levitation force relaxation increased with the increase of the reloading. In this paper, in terms of a HTS maglev system composed of the permanent magnetic guideway (PMG) and a HTS YBCO bulk, the levitation force relaxation under different loads is investigated by the numerical calculation and the experiment. It is found that the normalized relaxation rate of the levitation force increased with the increase of the load. The influence of the load on the current density distribution is also discussed in this paper. This work might be helpful for the research of the loading capacity of the HTS maglev vehicle.      

Levitation-Force Investigation of High-Temperature Superconducting Maglev System Under Different Variations of the Vertical Displacement

In the practical application of high-temperature superconducting (HTS) maglev vehicle, the vehicle body ascends and descends in the vertical direction frequently with the passengers on and off boarding, such vertical movement may change the levitation performance but this influence have not been researched. In this article, the influence of the vertical movement on the levitation force in two different types of permanent magnet guideway (PMG) and different movement displacements was experimentally analyzed. We found that the levitation force drastically decays first, and then decays slightly, which is similar to the relaxation phenomena. Meanwhile, the experimental results indicate that the levitation force decay trends to enlargement with the decrease of the working height (WH) and with the increase of the vertical displacements. These phenomena can be ascribed to the magnetic hysteresis loss in the bulk high-temperature superconductor (HTSC).     

Influence of the Vertical Inclination of Permanent Magnet Guideway on Levitation Characteristics of Hts Maglev System

In the application of high temperature superconducting (HTS) magnetic levitation (maglev) system under vertical inclination of permanent magnetic guideway (PMG), the component of the total weight of levitation body above the PMG will be changed. Therefore, the influence of the vertical inclination of PMG on levitation characteristics of HTS maglev system cannot be ignored, such as the levitation gap, the levitation force, guidance force and driving force of the linear motor. In order to investigate the influence of the vertical inclination angle on levitation characteristics of the HTS maglev system, a HTS maglev launch platform has been designed and fabricated for the investigation the influence of vertical inclination angle between the range of 0° and 18° on the levitation and guidance and driving force parameters of the HTS maglev launch platform. Experimental results show that the levitation gap was the main levitation characteristic for HTS maglev system under vertical inclination of PMG, which increased with increment of the vertical inclination angle. However, the levitation force, and the driving force of the linear motor decreased. The guidance force could not be influenced by the increment of levitation gap. The experimental results are helpful toward improving the running performance of the HTS Maglev launch system.     

Dynamic response characteristics of the high-temperature superconducting maglev system under lateral eccentric distance

Off-centre operation of high-temperature superconducting (HTS) maglev systems caused by inevitable conditions such as the misregistration of vehicle, crosswind and curve negotiation, may change the distribution of the trapped flux in the HTS bulks and the magnetic interaction between HTS bulks and the PMG. It impacts on the performance of HTS maglev, and more seriously makes the maglev vehicle overturned. Therefore, understanding the performance of the HTS maglev in off-center operation is very important. In this paper, the dynamic response characteristics of a cryostat with twenty-four onboard YBaCuO superconductor bulks were experimentally investigated at different eccentric distances under loads before the initial FC process. Parameters such as vibration accelerations, displacement, natural frequency and dynamic stiffness were acquired and analyzed via the B&K vibration analyzer and laser displacement sensors. Results suggest that the natural frequency and dynamic stiffness of the maglev vehicle would be obviously reduced with the eccentric distance, posing negative effects on the stability of HTS maglev.     

An Asymmetrical Permanent Magnet Guideway Design for the High Temperature Superconducting Maglev Curve

An asymmetrical permanent magnet guideway (PMG) design is considered for the high temperature superconducting (HTS) Maglev during traveling on a curved path. This translationally asymmetrical PMG design can produce a better guidance performance, which cannot be provided by the common translationally symmetrical PMG design. The additional guidance improvement is attributed to its asymmetrical magnetic field distribution. The outside part of the asymmetrical PMG has more magnetic material than the inside part, so that the outside magnetic field density is enhanced or becomes a multi-pole distribution. These two effects result in a larger guidance force and a better curve negotiation ability. Above the asymmetrical curved PMG, the HTS Maglev vehicle system can overcome larger centrifugal forces and run with a smaller or even zero lateral displacement. Moreover, this asymmetrical PMG design is helpful to realize the “straight line to curve to straight line” running environment. The improvement effect and running feasibility of the asymmetrical PMG is calculated and proven for future use in HTS Maglev curves.     

High-Performance Permanent Magnet Railway Design Consideration of Magnetic Stiffness Parameters of YBCO Bulk Arrays

The monopole permanent magnet railway (PMR) is used in the present high-T _c superconducting (HTS) maglev vehicle system. In order to improve the performance of the present maglev vehicle system, many studies have been done to optimize the maglev system. Stiffness is one of the important parameters for maglev vehicle system design. In this article, measurement of the vertical and lateral magnetic force stiffness of HTS bulk arrays above monopole PMR and double-pole PMR are measured. Comparing the experimental results, it is found that the magnetic force stiffness of bulk arrays above double-pole PMR is better than that of monopole PMR. The experimental results show that the running stability of HTS maglev vehicle system using double-pole PMR is much better than that of using monopole PMR.