Maglev by a passive HT C superconducting short runner above the magnetic guideway

Transcription

1 Maglev by a passive HT C superconducting short runner above the magnetic guideway G. D Ovidio 1, F. Crisi 2, A. Navarra 2 & G. Lanzara 1 1 Transportation Department, University of L Aquila, Italy 2 Science and Technology Park of Abruzzo, L Aquila, Italy Abstract An experimental ring model device employing a high critical temperature superconductor (HT C S) short runner above the circular iron-homopolar magnetic guideway was designed and manufactured. The device simulates, by analogy, the electromagnetic behaviour of a magnetically levitated vehicle, with passive short superconductor runners on board, riding along a magnetic track. In the wide range of work conditions magnetic levitation experiments were conducted and lift and drag forces were measured. Numerical analyses by twodimensional finite element parametric models were carried out; the results are presented in terms of forces and flux density distribution. Keywords: high temperature superconductor, permanent magnet, levitation, modelling. 1 Introduction The aim of this application falls in a preliminary step of research program on use of HT C S material for magnetically levitated (Maglev) vehicle. The research activity is developing in the Innovative Technologies for Transport laboratory realised thanks to the co-operation between the University of L Aquila and the Science and Technology Park of Abruzzo. The magnetic levitation system is one of the most possible applications of bulk HT C S; the key to the application of these materials lies in the reason that they can be firmly levitated without any control. In the previous papers the authors presented preliminary numerical analyses [1], on use of HT C S sheets for magnetic levitation in transportation field; these

2 964 Computers in Railways IX studies verified that the entity of pressures, both levitation and traction, are widely compatible with transportation applications. In order to validate the numerical model results the authors have designed and built an experimental ring model device reproducing, by analogy, the electromagnetic behaviour of Maglev vehicle riding along a magnetic track [2], [3]. Instead to realise a linear machine of finite dimension, an experimental circular track (primary) interacting with a HT C S short type secondary has been manufactured. The device has already allowed testing the interaction between YBCO plate short secondary and same different configurations of track as following steps: 1. Active way with translating field producing by windings three-phase fed [4]; 2. Magnetic way with static field produced by permanent magnets (PM) in Halbach array [5]; This paper presents the results of tests realized by a novel configuration of track that, respect to the above mentioned, has been realised with homopolar permanent magnets arranged on iron plate. The aim of this phase of study is to evaluate the performance of system in term of lift and drag forces. Moreover numerical models were carried out in order to investigate deeply the phenomena in terms of magnet forces and flux density distribution. 2 Experimental ring model device An experimental ring model device reproducing, in wide range of work conditions, the interaction between HT C S runner and magnetic field of ironmagnetic track were designed and realised. The device simulates, by analogy, the behaviour of levitated vehicle with HT C S monoliths runner on board riding along a magnetic guideway. Particularly the experimental device (Fig.3) is constituted of 2 main components: a) Magnetic guideway (primary): circular inductor composed by a set of 10 homopolar circle sectors of Nd-Fe-B permanent magnets positioned on middle crown of iron ring U shaped; this structure can mechanically rotate around its vertical axis. Table 1 contains the main data of iron-magnetic guideway. Table 1: Main characteristics of circular guideway. Outer diameter of iron ring mm Inner diameter of iron ring mm Average radius mm PM Ring Outer radius mm PM Ring Inner radius mm PM Ring Thickness 25.0 mm

3 Computers in Railways IX 965 b) Superconducting runner (secondary): sector of circle constituted by close array of 4 YBCO (Yttrium-Barium-Copper-Oxide) monoliths trapezoidal shaped; the sector is properly fitted into the older filled with liquid nitrogen. Table 2 contains the main characteristics of YBCO circle sector. Figure 1 and 2 respectively show the view of HT C S circle sector and the drawing of each YBCO monolith. Table 2: Main characteristics of YBCO circle sector. Inner radius mm Outer radius mm Thickness 11.0 mm Angle of sector 42.3 degrees Trapped field > 1 T (77K) Critical current density > A/cm 2 (77K, self field) Critical temperature 91 K 10,59 40mm 40,17 40mm 3,71 32,59 3,71 Figure 1: View of YBCO sector. Figure 2: Drawing of single YBCO monolith. The shaft of inductor structure is coupled to a.c. machine fed by three-phase inverter in such way the mechanical speed of primary can be controlled in order to reproduce a wide range of work conditions. Figure 3: Outside view of experimental circular model device.

4 966 Computers in Railways IX A mechanical device with precision instruments allows varying the air-gap between primary and secondary. All tests have been carried out by locking the secondary and by controlling the mechanical rotational speed of primary. The interaction forces (levitation and drag) are evaluated on the secondary by the measurement system consisting of a load cell and torquemeter. Figure 3 shows an outside view of experimental model device. Respect to the vehicle riding along the magnetic way this experimental device realises the following analogies: - Circular inductor simulates the iron-magnetic guideway with infinite length; - HT C S sector simulates the short magnetic runner of vehicle; - Relative motion between vehicle and guideway is obtained by imposing mechanical rotation of circular way around its vertical shaft. Since the module and versus of inductor mechanical rotational speed are controlled by the coupled a.c. machine a wide range of work conditions are reproduced. 3 The experimental tests Significant experimental tests have been carried out in order to characterise the equipment and evaluate the effect of air-gap length and speed, on the levitation and drag forces. In the first phase a Hall probe positioned with different heights (H) from the surface of magnetic guideway has allowed to test the normal (B Z ) component of flux density distribution (Fig. 4). This preliminary work has allowed describing the flux density configuration on Y-Z plane with locked inductor and without secondary. When mounted on guideway, it generates a magnetic field that induces currents in the HT C S monoliths. Figure 5 shows the levitation pressure-rotational speed curves for different air-gap lengths (H T ); the levitation pressure is intended as ratio between levitation thrust entity and YBCO sector surface (58,1 cm 2 ). The experiments were conducted until a rotational speed of 500 rpm that correspond to 10,3 m/s. The levitation force keeps constant and not depends on the speed value; obviously it decreases at air-gap lengths rises. The results show that a large levitation force and stable equilibrium are obtained; the levitation force based on flux pinning for type-ii superconductors in mixed state is large enough. The system operates with a large net gap (suspension height) between guideway and magnetic runner and no feedback control for stable levitation is required. Moreover the measurements realised by the torquemeter show that the drag force is absent in correspondence of all slip values.

5 Computers in Railways IX 967 Figure 4: Normal component (B z ) of flux density distribution Vs. radial abscissa of circular inductor at different height (H). 3.5 Levitation pressure [N/cm 2 ] H T =5mm H T =10mm H T =15mm H T =20mm HT=30mm H T =40mm Rotational speed [rpm] Figure 5: Levitation pressure Vs. rotational speed at different air-gap lengths (H T ). These behaviours are very interesting for transportation application since the system operates with levitation forces in every phase of motion without drag forces opposing to motion of vehicle.

6 968 Computers in Railways IX 4 Analysis by finite element method Accurate two-dimensional (2D) finite element parametric model of experimental model device has been carried out in order to investigate deeply the interaction between the circular inductor and the HT C S secondary. The modelled equipment simulates the performance taking into account the non-linear iron behaviour, the saturation and end effects. The finite element parametric model has been approached by using several variables making it possible to analyse deeply the role that each variable assumes within the economy of the system in terms of performance. The mesh of the model uses elements with 8 nodes and it has been refined in most critical area like air-gap and monoliths. The model has been tuned starting from the experimental results of the previous tests; consequently the HT C S material has been modeled by taking into account a low electrical resistivity (10-10 Ωm), a relative permeability value equal to 1 and meshed as an homogeneous materials. These assumptions, even though limited and not detailed enough to describe the exact behaviour of type II superconductor materials, simulates with an acceptable degree of approximation the complexity of the phenomena and allows the convergence of numerical method. The prefixed objective to simulate the levitation forces in engineering terms has induced the authors to approach the problem excluding the use of models usually utilised to describe the pinning force of type II superconductors. The possibility of tuning the model with detailed data starting from the experimental results has persuaded the authors to use a numerical calculation based on above mentioned assumptions. Comparison between numerical and experimental results is shown in figure 6; it confirms that the measured and calculated forces are in good agreement. More in detail figure 6 shows levitation pressure vs. air-gap lengths in correspondence of the follow relative speed values: V Z = m/s and V Y =0. In order to increase the field strength on the top side of guideway, a U shaped ferromagnetic core has been analysed and its geometrical dimensions have been optimised respect to the characteristics of permanent magnets. The figures 7 and 8 show respectively the cross section on Z-Y plane of flux line distribution without and with the presence of HT C S monolith. Particularly figure 8 shows the flux line distribution relatives to system configuration in which the HT C S runner is perfectly centred (off-set δ=0) respect to the inductor axis of symmetry with air-gap length (H T ) of 10mm. It is evident how the field configuration generates the guidance effect. It is important underline that in the scheme of figure 8 the HT C S monolith can moves on Y-Z pane and rides above guideway along the perpendicular X direction. Figure 9 and 10 show respectively the flux density and flux line distribution in correspondence of system configuration in which YBCO monolith is not centred (δ=10mm and H T =10mm).

7 Computers in Railways IX 969 Levitation pressure [N/cm 2 ] Experimental data Numerical data Air-gap length [mm] Figure 6: Levitation pressure Vs. air gap-lengths (H T ): comparison between numerical and experimental results. Figure 7: Magnetic flux line distribution of guideway. Figure 8: Magnetic flux line distribution of guideway with presence of YBCO monolith (air- gap=1cm).

8 970 Computers in Railways IX Figure 9: Flux density distribution at δ=h T =10mm. Figure 10: Flux line distribution at δ=h T =10mm. Pressure [N/cm 2 ] Levitation Guidance Air-gap length [mm] Figure 11: Levitation and guidance pressure vs. air-gap lengths (H T ) at off-set (δ) =10mm.

9 Computers in Railways IX 971 Also in this case the system produces at the same time levitation and guidance forces as it is pointed out in figure 11; it shows the levitation and guidance force Vs. air gap lengths (H T ) in correspondence of δ=10mm. The values of guidance force are lower than levitation ones; moreover both levitation and guidance forces increase as H T decrease. The results show that there is a significant guidance force between HT C S runner and applied magnetic field of guideway. Both the large levitation force and the stable equilibrium are obtained by the proposed system. 5 Conclusions The interaction between field of iron-magnetic guideway and HT C S short type secondary has been investigated. The authors have designed and built experimental equipment consisting of a circular iron-magnetic way interacting with HT C S short type secondary constituted by 4 YBCO monoliths. The results of experimental tests have pointed out that the electromagnetic interaction produces stable levitation for all slip values; moreover the drag force opposing to the motion is completely absent. The results carried out by 2-D finite element model of experimental equipment have pointed the good agreement with experimental data. The numerical analyses have allowed to optimise the system configuration and significant guidance forces have been evaluated. The experimental and numerical results have provided the actual advantages on use of bulk HT C S for Maglev applications. References [1] Lanzara G. - D'Ovidio G.- Masciovecchio C.- Villani M. Superconducting Sheets for Train Support and Traction: Finite Element Analysis, Proceedings of 2 International Symposium on Linear Drives for Industry Applications (LDIA), Tokyo, Japan - April 8-10, [2] D Ovidio G. - Villani M. - Crisi F. - Monaco S. - Navarra A. - Lanzara G. Experimental Analysis of Interaction Between Circular Inductor Way and Inclined Sheet-Type Secondary. Proceedings of 8 International Conference on Computer Aided Design, Manufacture and Operation in the Railway and Other Advanced Mass Transit Systems (COMPRAIL), Lemnos, Greece, June 12-14, [3] D Ovidio G. - Crisi F. - Navarra A. - Villani M. - Lanzara G. LIM: Optimization of Secondary Width Respect to the Thrust, Proceedings of 17 International Conference on Magnetically Levitated Systems and Linear Drives (MAGLEV), Lausanne, Switzerland - September 3-5, [4] D Ovidio G. - Crisi F. - Navarra A. - Lanzara G. Magnetic Levitation by Superconductor Plates Short Secondary Interacting with Traslating Field: Experiences, Measuraments and Modelling Proceedings of JAPMED 03

10 972 Computers in Railways IX Third Japanese-Mediterranean Workshop on Applied Electromagnetic Engineering May 2003, Athens, Greece. [5] D Ovidio G. - Lanzara G. - Crisi F. - Navarra A. Electrodynamic Interaction Between HTSC Plate Short Secondary and Way with Permanent Magnet Arranged in Halbach Arrays. Proceedings 3 International Symposium on Linear Drives for Industry Applications, LDIA Birmingham, UK - September 8-10, 2003.