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ADVANCED ENGINEERING 2(2008)1, ISSN 1846-5900

OVERVIEW OF MAGNETIC LEVITATION PRINCIPLES AND THEIR APPLICATION IN MAGLEV TRAINS1

Čermák, R.; Bartoň, L.; Spal, P.; Barták, J. & Vavřík, J.

Abstract: This contribution deals with magnetic levitation. An overview of principles of magnetic levitation is given and some examples are presented. Two principles were selected for a laboratory model of magnetic train and two models are being designed and built. Both models are described in the paper. First model is based on HTS and permanent magnet levitation. The second model is based on electromagnetic suspension. Both models will be used for education and research. Keywords: actuator, magnetic levitation, HTS, MAGLEV

1 INTRODUCTION

High speed railways is topic very often discussed in context of efficiency of air transportation on shorter distances (less than 1000km) – see Fig.1, ecological problems and global warming connected with air transportation, etc. Magnetic levitation opens new way of development of this type of railway transport.

Fig. 1. Efficiency of various types of transportation [11]

Magnetic trains have many advantages comparing to conventional railways.

Several studies proved, that magnetic railways [11], [12], [22] 1 This work was supported by the project IGFST-09/2007 from University of West Bohemia. We would like to thank to Ing. Pavel Karban and Assoc. Prof. Bohuš Ulrych from the Faculty of Electrical Engineering, for their help with the Comsol Multiphysics software.

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• generate less noise at the same speed than conventional high speed railways like ICE 3 or TGV (from 85 to 90%) ,

• can reach higher slope angle than conventional high speed railways (up to 10%), • have shorter time for reaching high speeds than conventional high speed railways

(distance required for reaching speed 300km/h is app.5km for Transrapid and app.30km for ICE),

• have higher safety than conventional high speed railways. Magnetic railways have also lower operational costs – about 60% of ICE 3

(because of lower energy consumption, low wear, etc.), but initial costs are app. 60% higher than ICE 3 [12]. 2 PRICIPLES OF MAGNETIC LEVITATION

Magnetic levitation can be based on several principles (a detail explanation can be found in many publications, e.g. [1], [2], [3], [4], etc.). In the following text some of them will be mentioned. 2.1 Direct diamagnetic levitation and levitation of superconducting materials It is well known for several years, that diamagnetic materials have an ability to partially screen out external magnetic field from the volume. This effect can be also used for levitation at room temperature. But there are two significant limitations: /1/ there is not too many strongly diamagnetic material in the nature, /2/ high magnetic field is required ([5], [6]). Very strong magnetic field (about 16 Tesla) is necessary to levitate for example water drops or some living creatures [6]. Using these very strong magnetic fields allow to levitate permanent magnets using e.g. water (diamagnetic material) at human fingertips [5]. This effect is sometimes called diamagnetically-stabilized levitation.

a) b)

Fig. 2. a) Screening out of magnetic field from the volume of the Type I superconductor [4], b) Simple superconducting bearing [1], [11]

Meissner and Ochsenfeld found, that superconducting material is an ideal

diamagnetic material, therefore it can levitate in external magnetic field. Several types of magnetic bearings based on this principle were proposed (e.g. [7]). This approach has one important limitation – very low temperatures are required.

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Very fast development of new superconducting materials allows using of cheaper media for cooling – liquid nitrogen. Nowadays, a critical temperature of so called high temperature superconductors is around 130K.

For levitation purposes the most often used material is YBCO (a material based on Ytrium-Barium-Copper-Oxygen - melt textured YBa2Cu3O7-x with Y2BaCuO5 excess) with a critical temperature about 90K. As a cooling media the liquid nitrogen is used.

Fig. 2. Levitation of a YBCO pellet above a set of permanent magnets 2.2 Permanent magnets Permanent magnets generate high forces (either repulsive or attractive) at room temperature. Earnshaw proved in [8], that it is not possible to reach permanent magnets for stable levitation in all 6DOF. Therefore another device (or principle [9]) must be used for stable levitation – mechanical constraints, diamagnetic stabilization [5] or gyroscopic stabilization (e.g. levitrons [9]).

Very interesting technical applications have so called Hallbach's arrays – special arrangement of permanent magnets, generating high magnetic field on one side of the array and almost zero on the other. Recent research [10] shows, that it is possible to generate very high forces by stacking of thin layers of permanent magnets with various orientation of the magnetic field.

a) b) c)

Fig. 3. Stacking of permanent magnets [10] a) by Fremerey, radial stiffness app.220 N/mm,

b) by Yonnet , Hallbach array, radial stiffness app.420 N/mm, c) by Lang [10], radial stiffness app.613 N/mm,

2.3 Electrodynamic suspension (EDS) Electrodynamic levitation is based on Lorentz's repulsive forces generated between moving superconducting coil and aluminium guideway (for principle in details see [4]).

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This principle is used in Japanese Maglev vehicles MLU, MLx at Miyazaki a Yamanashi test line (Japanese Railways).

High magnetic field on board is required (usually superconducting coils). It can have negative influence to passengers with pacemakers, magnetic data storage, etc. Therefore magnetic shielding is required.

This type of levitation is able to levitate the train, only when moving above some critical speed. Therefore, it must be equipped with some devices for supporting the train when slowing down, standstill or starts moving (typically wheels).

2.4 Electromagnetic suspension (EMS) Electromagnetic suspension works like an active magnetic bearing. This principle is sometimes called a servo-stabilization. Sensors measure the air-gap between an electromagnet and guideway. Control system tries to keep it constant. This principle is very often used in commercial applications like Transrapid [12]

Fig. 4. EMS levitation principle

Servo-stabilisation is able to hold the body in the required position, even if the train is standstill. Therefore no wheels are required for assuring of the main levitation function. However some retainer wheels for safety purposes are usually employed.

Typical for this levitation principle is lower air-gap between the magnets and the guideway (in comparison to EDS) and existence of a feedback control system.

2.5 Inductrack Inductrack system is sometimes called "EDS with permanent magnets" [14]. Instead of using superconducting coils on board, strong permanent magnets are employed. Similarly to the EDS levitation concept, Inductrack also requires devices for support, when the speed of the train is around zero. This concept is still in the laboratory testing phase, and there is no commercial application in this time. 3 MODEL OF SUPERCONDUCTIVE MAGLEV VEHICLE

The principle selected for the laboratory experiment is levitation of a superconductive material (ideal diamagnetic material) above a guideway with permanent magnets.

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3.1 Design of the guideway The guideway of the laboratory model was designed as a linear, 130mm wide and 2.5 meters long track, made out of rare-earth NbFeB permanent magnets (PM).

PMs are arranged in rows with the same orientation of magnetic field – see figure 5. The guideway is linear with no turns, therefore we assume, that there will be no

significant lateral forces. Lateral motion of the train model is prevented only by the shape of the magnetic field (see next paragraph).

Fig. 5. Orientation of PM at the magnetic guideway

Several configurations, with different number of magnets and different orientations

were tested numerically (see next paragraph) and experimentally. Finally an optimal configuration was selected. The selected configuration consists of five rows of PMs placed on ferromagnetic plate. The whole guideway is fixed on a frame, which allows simple movements.

Fig. 6. Levitation of YBCO pellet above a magnetic guideway – FEM simulation results

The test rig is designed for demonstration of the diamagnetic levitation, rather than

for reaching of high speeds. Therefore, there is no drive in the test rig. Movements of

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the vehicle are done only using of gravity (one side of the track moves up and down and causes slow movements of the vehicle model).

3.2 Modeling and simulation using FEM Several computations were done to be able to compare different variants of the PM arrangement. All the computations were done numerically using Comsol Multiphysics software (an AC/DC Module). Magnetostatic analysis was done in all models. Magnetic properties of the ferromagnetic plate with permanent magnets were set according to catalog values – see table 1 for more details. Properties of levitating body (a superconducting pellet from YBCO) were set as nearly ideal diamagnetic material (relative permeability almost zero), because the software doesn't allow zero input.

Relative permeability of YBCO pellets 0.005 Relative permeability of the guideway (linear material model was used) 1000

Remanent flux density for PM 1.2T

Tab. 1. Material properties of the model

Several results can be seen on the figure 6. The magnetic field surrounding the guideway consisting of four-rows, resp. five-rows of PMs without inserted superconductor pellet and the pellet inserted not exactly in the central position, is displayed in the left pictures. The right pictures show the same guideway with inserted superconductive pellet centred by the magnetic field. A stable lobe in the middle of the five-row track and very strong centring effect can be observed. 3.3 Design of the vehicle model The vehicle model was designed to enable keeping of YBCO pellets in the considerable amount of liquid nitrogen. Two pellets with diameter 28mm and thickness 14mm were used as the main levitators, which lift the vehicle above the guide way. Four pellets with diameter 14mm and thickness 8mm were used as auxiliary levitators, which should stabilize lateral swinging of the vehicle. The arrangement of the pellets can be seen on the figure 7.

Fig. 7. Top-view of the vehicle cross-section

The vehicle is made out of polyurethane foam with very good thermal properties (thermal insulation). The outer shape and internal liquid-nitrogen-container were done using hot-wire-cutter. It was not possible to make the whole shape at once. Therefore the vehicle is glued from several slices, using UHU-Por and Purex glue.

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4 MODEL OF EMS-TYPE MAGLEV VEHICLE

Electromagnetic suspension works like an active magnetic bearing. This principle is sometimes called a servo-stabilization. Sensors measure the air-gap between an electromagnet and guideway. Control system tries to keep it constant. 4.1 Design of the guideway The guideway is 2.5m long linear track, consisting of several parts fixed on a frame.

The most important is ferromagnetic guideway, made out of a steel bar, closing the magnetic circuit with the levitation coil. Cross-section of the bar is selected according to dimensions of the coils to keep the cross-sectional area constant.

An aluminium stripe on the top of the bar is intended for measurement of the distance between the vehicle and the guideway. A eddy-current displacement sensors BALLUFF with the measurement range from 0.5 to 2 mm are used.

There is a linear drive stator for driving the vehicle in the middle of the guideway. However, in the real EMS vehicles (e.g. Transrapid) the linear drive is integrated with the levitation coils, we decided to separate these two components to eliminate coupling between them.

Fig. 8. Guideway cross-section for model of EMS vehicle

We also suppose that lateral forces in the model are negligible. Therefore, there are no lateral stabilising coils like in the real EMS applications. 4.2 Design of levitation coils The core of the electromagnet consists of lamination, thin sheets of soft-magnetic iron, to minimize eddy currents in the core. A C-shape core was selected for our experiment. Thickness of the lamination sheets is 0.5mm.

Fig. 9. C-shape of the levitation electromagnet

ferromagnetic guideway

levitation coil

displacement sensor linear drive

aluminium stripe

frame

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Parameters of a PWM power amplifier, used for driving of the current, are for determination of wire diameter. Mean value of power amplifier output parameters are Us = 25V and Is = 5A. It is obvious, that the number of turns of the coil is limited by the geometrical shape of the coil core and it depends on the wire diameter. Therefore, the number of turns was computed in iterations.

length of the wire 43.2m coil cross-sectional area 87.8mm2 diameter of the wire 0.4mm core cross-sectional area 145mm2 A dimension (see Fig.9) 13.5mm coil ohmic resistance 6.188Ω B dimension (see Fig.9) 6.5mm coil current 4A L dimension (see Fig.9) 14.5mm number of turns 700 H dimension (see Fig.9) 10mm

Tab. 2. Coil parameters

The coil was wound according to above mentioned computations. (see paragraph

2). Number of turns was 700, copper wire with diameter 0,4 mm. We used a CNC lathe SUF16 with revolution control as a winder. A simple counter of revolutions, based on microcontroller PIC with LCD display, was build to be able to make exact number of turns.

Attractive force of the electromagnet depends on number of turns, coil current, core dimensions, permeability of vacuum and size of the air-gap as follows (1).

2

022

2

02

44 sSμIN

sSμF

F jjm == (1)

Maximum weight of the MAGLEV train model, including levitation coils, is app.

2.5kg. 4.3 Design of the vehicle Vehicle model is based on simple aluminium frame with fixed levitation coils, eddy-current displacement sensors and moving part of the linear drive. The chassis of the vehicle is made out of polyurethane foam.

Fig. 10. Design of the overall vehicle model

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4.4 Control system and power electronics Vehicle is hovering above the guideway due to control loop, which keeps the air-gap constant. The hardware of the loop consists of eddy-current sensors BALLUFF with the range 0.5-2mm; power amplifiers MSA-240KC from APEX; and a computer control system based on AD/DA card MF614. Software solution is based on MATLAB platform. A RealTime Windows Target and Extended RealTime Toolbox are used for reading/sending data from/to the process. The controller is a simple PD controller developed in Simulink and communicating with the process using RT Input and RT Output blocks from real time toolboxes.

5 DISCUSSION

A laboratory model of the superconducting Maglev vehicle hovering above a guideway with permanent magnets was designed and built. Although the real commercial application of this type of Maglev vehicle will be very expensive and hardly feasible, as a laboratory model demonstrating basic principles of magnetic levitation is very useful.

The second experiment, the EMS MAGLEV model, is more useful for practical applications. However, this model is again only for education purposes, because it is very simple.

The main purpose of these experiments is to demonstrate to students principles of the EMS levitation and superconductive levitation. It also allows to demonstrate numerical computations of magnetic field, together with verification of the model, and design of simple control system (for the EMS experiment). 6 CONCLUSION

Magnetic levitation is very promising technology for the high speed transportation.

An overview of principles of magnetic levitation is given. Two of them are being implemented in building of laboratory models of MAGLEV trains at the UWB.

References: [1] Mayer, D., Magnetic levitation and it’s applications, in Czech, ELEKTRO 1/2003,

pp. 4-12 [2] Jayawant, B.V., Electromagnetic levitation and suspension techniques, Edward Arnold,

London, 1981. [3] Sinha, P.K., Electromagnetic suspension – Dynamics and control, Peter Peregrinus,

London, 1987. [4] Moon, F.C., Superconducting levitation – Applications to bearings and magnetic transpo-

rtation, John Wiley&Sons, New York, 1994. [5] Geim, A.K.; Simon, M.D.; Boamfa, M.I. & Hefflinger, L.O., Magnetic levitation at your

fingertips, Nature, Vol.400, 1999, pp.323-324 [6] Berry, M.V. & Geim, A.K., Of flying frogs and levitrons, Eur.J.Phys.18, 1997, pp.307-313 [7] Hull, J.R., Superconducting bearings, Superconducting Science and Technology 13, 2000,

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luminferous ether., Trans.Camb.Phil.Soc.,7, 1842, pp 97-112 [9] Genta, G.; Delprete, C. & Rondano, D., Gyroscopic Stabilization of Passive Magnetic

Levitation, Meccanica 34, Kluwer Academic Publ., 1999, p.411-424

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[10] Lang, M. & Lembke, T.A., Design of permanent magnet bearing with high stiffness, ISMB-10, Martigny, Switzerland, 2006, pp.221-224

[11] Vavřík J., Magnetická levitace v dopravní technice, bachelor thesis, UWB Plzeň, 2007. [12] Transrapid website – www.transrapid.de [13] AMLEV website - http://www.amlevtrans.com/ [14] Wikipedia - http://en.wikipedia.org/wiki/Maglev_train [15] Wei, R.; Sun, G. & Liu, Y., The development status and future prospects of Maglev

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Authors: Čermák Roman, Ing., Ph.D., University of West Bohemia, Department of Machine Design, www.cadam.riteh.hr/whoiswho/CVs/Cermak_Roman.htm Bartoň Lukáš, Ing.; Spal Petr, Ing.; Bartak Jiří, Ing.; Vavřík Jan, Bc. University of West Bohemia, Department of Machine Design, Univerzitni 8, CZ-30614, Plzeň, Czech Republic