A Permanent Magnet Linear Synchronous Motor Drive for HTS Maglev Transportation Systems

JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO., JUNE 008 5 A Permanent Magnet Linear Synchronous Motor Drive for HTS Maglev Transportation Systems You-Guang Guo, Jian-Xun Jin, Lu-Hai Zheng, Jian-Guo Zhu, and Hai-Yan Lu Abstract A permanent magnet linear synchronous motor (LSM) for a high temperature superconducting (HTS) maglev system has been studied, including the motor structure, control strategy, and analysis techniques. Finite element analysis (FEA) of magnetic field is conducted to accurately calculate major motor parameters. Equivalent electrical circuit is used to predict the drive s steady-state characteristics, and a phase variable model is applied to predict the dynamic performance. Preliminary experiment with a prototype has been made to verify the theoretical analysis and the HTS- synchronous driving technology. Index Terms High temperature superconducting (HTS), HTS magnetic levitation, maglev transportation system, permanent magnet linear synchronous motor.. Introduction For the capability of generating high levitation force density with passive and self-stabilizing features [], high temperature superconducting (HTS) materials have attracted strong interest of research in many applications, such as frictionless bearings for flywheel energy storage systems []-[4] and maglev devices for transportation vehicles [5]-[9]. For driving the transportation systems, the linear motor drive is more advantageous than its rotary counterpart due to the lack of the mechanic transform and has advantages such as simple structure, no abrasion, low noise, high accuracy, and easy to maintain. Therefore, the linear motor integrated with HTS magnetic levitation system can be applied in transportation (maglev) domestic appliances and many industrial automation control fields Manuscript received March 30, 008; revised May 4, 008. Y.-G. Guo is with the Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin, 30030, China; and the Faculty of Engineering and Information Technology, University of Technology, Sydney, NSW 007, Australia (e-mail: youguang@eng.uts.edu.au). J.-X. Jin and L.-H. Zheng are with Center of Applied Superconductivity and Electrical Engineering, University of Electronic Science and Technology of China, Chengdu, 60054, China (e-mail: jxjin@uestc. edu.cn, zluhai@ uestc.edu.cn). J.-G. Zhu and H.-Y. Lu are with the Faculty of Engineering and Information Technology, University of Technology, Sydney, NSW 007, Australia (e-mail: joe@eng.uts.edu.au, helenlu@it.uts.edu.au). such as computer-controlled machining tools, semiconductor manufacturing equipments and so on. There are various types of linear motors such as DC, induction, and synchronous motors. Among those, the permanent magnet linear synchronous motor (LSM) with power electronic drive circuits seems very promising. This paper investigates the application of a LSM for driving a small scale prototype HTS maglev vehicle [5]-[7]. The motor structure, control strategy, analysis technique and the preliminary experiment results are presented.. Prototype HTS Maglev Vehicle Fig. illustrates the small scale prototype HTS maglev train with dimension of 000 mm 70 mm 80 mm and an unloaded mass of 5 kg. The vehicle is magnetically levitated by the interaction of the HTS bulk mounted on the vehicle bottom and the s placed on the ground along the driveway. Because the interaction force between the s and the maglev bulks is passive and self-stabilizing, it does not need any comprehensive control and at the same time it can act as the guiding force. By contrast, the conventional transportation system relies on wheels and rails for supporting and guiding. The contact between the moving wheels and the stationary rails will cause friction, energy loss, mechanical wear and noise. It also limits the train s maximum speed. As a result, maglev vehicles have now become very attractive due to their non-contact operational mode, particularly for high speed transportation. In the prototype maglev train, the driving force is provided by a linear drive in both sides of the vehicle. On each side is a LSM: the moving part includes the s mounted on back steel; the stationary part includes the coils placed in the slots of the side steel track. In a motor, the interaction between the mover s and the stator currents (and also the stator teeth) generates large attraction force besides the driving force. Such a lateral attraction force is unwanted. In the double-side design, the attraction forces produced by the two motors can be mostly cancelled [0]. 3. Motor Structure and Dimensions As shown in Fig., on each side of the vehicle is a LSM for generating the driving force. Fig. shows the LSM structure viewed from the top. On the back iron of

6 the vehicle are mounted 4 s, each of which has a length (along the movement direction) of 60 mm, a thickness (magnetization direction) of 0 mm, and a width (transverse) of 30 mm. The thickness of back iron is 5 mm. In the stationary side track, coils wound around the teeth are connected to form 3 phase windings, any two of which are separated by 0 electrical. The tooth has a length (along the movement direction) of 0 mm, a depth of 8 mm, and a width (transverse) of 30 mm. The slot width (along the movement direction) is chosen as 0 mm. The main air gap length is 8 mm. The 3 phase windings are supplied via a single-phase to 3-phase frequency-variable power source. It can be seen that only the coils facing the mover s will generate the driving force. The rest of the coils would produce extra resistance, copper loss and leakage flux. To overcome these problems, the stator winding can be divided into sections. In practice, the inverter can be connected to only the windings coupled to the mover s at each moment [0]. Vehicle HTS bulk JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO., JUNE 008 Fig.. A small scale prototype HTS maglev vehicle. Steel back Tooth Steel back 4. Motor Parameter Computation For calculating the motor parameters, the conventional equivalent magnetic circuit method can be used. However, the magnetic circuit method can only provide approximate results as it has difficulty in including the detailed structure and the non-linearity of magnetic materials. On the other hand, numerical analysis like finite element method (FEM) can overcome this difficulty. In this paper, the major motor parameters such as winding flux and inductance are calculated based on the magnetic field distributions by using finite element analysis (FEA). The magnetically relevant parts of the linear motor (one side) are shown in Fig., and the non-linear magnetic property, i.e., the B-H curve of silicon steel used as the magnetic material of the side tracks and back irons, is given in Fig. 3. Back iron Stator teeth S N Moving direction N S S N Permanent magnets Side track Fig.. -D view of the linear synchronous motor. N S B (T) B (T).4.0.6. 0.8 0.4 0 0 0000 0000 30000 40000 H (A/m) Fig. 3. B-H curve of side track and back iron. Two-dimensional field computation in one pole-pair region is carried out, which can be sufficient for theoretical analysis. It should be noted that three-dimensional analysis within the whole motor could be conducted for considering the fringing effect and asymmetry among three phases at the expense of longer computational time. The flux, defined as the flux of a phase winding produced by the mover s, can be determined from the magnetic field distribution at no-load. The flux waveform can be calculated by moving the mover for one pole-pair pitch in several steps. To increase the computational accuracy and efficiency, the stator and rotor are separately meshed and kept unchanged when the mover moves. After the meshed mover moves to a new position, the meshes are stitched with those of the stator along the middle air gap. For the time stepping analysis, the meshing of middle air gap line is controlled such that the nodes of the stator and mover sides coincide at each step. Fig. 4 shows an example when the mover has moved one pitch τ from the position x=0. The materials are numbered as: ) air, 3) South (with the magnetization direction along the y-axis direction), 4) North (with the magnetization direction along the negative y-axis direction), 5) back iron, and 6) side track. The copper coils, number as ) and placed on the stator slots, are not shown for clarity. Instead, the copper coils are replaced by air for no-load analysis as they have the almost same magnetic permeability as air. 5 3 Fig. 4. Finite element meshes when Δx=τ. On the y-direction boundary lines of the stator and mover, and on the open middle air gap line, the magnetic vector potentials obey the periodical boundary conditions as (), (), and (3), respectively, 6 4 3

GUO et al.: A Permanent Magnet Linear Synchronous Motor Drive for HTS Maglev Transportation Systems 7 AZ( τ, y) = AZ( τ, y) () AZ( τ +Δ x, y) = AZ( τ +Δ x, y) () AZ( xg, yg) = AZ( xg + τ, y g ) (3) where the origin is located at the center of the bottom line, Δx is the moved distance of the mover (e.g. τ in Fig. 4), y g is the y-coordinate of the middle air gap line, x g is the x-coordinate of the node on the left open air gap line, and 0 x g Δx. The top and bottom boundary lines can be assumed to follow the flux-parallel condition although air layers may be added for somewhat more accurate solutions. Based on the no-load magnetic field solutions at various mover positions, the flux, defined as the flux of the phase winding produced by the mover s, can be obtained as in Fig. 5. It is observed that the flux waveform is an almost perfect sinusoid. The flux waveforms are three phase symmetrical, i.e., they have the same magnitude but are shifted by 0 electrical to each other. This is a fundamental requirement for three phase drive. permeability of the s is almost the same as that of air). Based on the magnetic field solutions, some other parameters, such as cogging force, electromagnetic force and core loss, can also be worked out. 5. Performance Prediction Considering that the back EMF waveform is very close to sinusoidal, the steady-state performance of the motor can be predicted by the conventional equivalent electrical circuit as shown in Fig. 6, where E is the induced phase winding back emf due to the mover s, R is the phase winding resistance, and X =πf L is the synchronous reactance. L is the synchronous inductance, which equals the self inductance plus half the mutual inductance for the case of symmetrical 3-phase windings. JX R I E VV Flux (mwb) Fig. 5. flux linking one coil. Mover position (ele. deg.) From Fig. 5, one can work out the maximum flux linking one coil as 0.57 mwb. Considering that a phase winding consists of active coils at any instant, the flux is calculated as φ=.4 mwb. When the vehicle moves, an electromotive force (EMF) is induced in the stator windings. By differentiating the flux of phase winding against time, the back EMF is determined with an rms value as E = π fn k φ (4) c N where f is the frequency, N c is the number of turns of phase winding, and k N is the winding factor. Winding inductance is another important parameter for the motor performance. As the behavior of an electrical circuit is determined by the incremental inductances instead of the commonly defined apparent inductance, the winding inductances of the LSM at different mover positions are computed by using a modified incremental energy method []. The calculations reveal that the winding inductances are almost constant at different mover positions, which may be due to the large effective air gap (e.g. the Fig. 6. Equivalent circuit model of synchronous motors. If the motor is controlled by the brushless DC scheme, under the optimum condition, i.e., the winding current and back emf are in phase, then the electromagnetic driving force can be calculated by Ei a a+ Ei b b+ Ei c c 3 πφ Fem = = NI c = KFNI c (5) v τ where K F is the force constant and N c I is the phase winding current in ampere-turns. For a given terminal voltage V, the relationship between the mover speed and electromagnetic force at the steady-state can be determined by v = RF LF RF RF em π em em em KE V m + + τkf KF m πlf em + τk F where m=3 is the number of phases, K E =K T /m is the back EMF constant. For predicting the dynamic performance of the motor, the phase variable model can be applied [],[3], which consists of several electrical and motion equations as K E (6) V = r i + dλ / dt+ e, k=a, b, c (7) k k k k k k c λ = L i (8) q F q= a kq ei + ei + ei a a b b c c em = + Fcog (9) v

8 dv m F Bv F JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO., JUNE 008 em L dt = (0) All above variables are used as their conventional meanings. The profiles of L, e and F cog can be obtained from a series of nonlinear FEA solutions, in which the mover position dependence and saturation effect are considered. 6. Experimental Validation To validate the above idea, a preliminary experiment has been conducted. Fig. 7 shows the HTS magnetic levitator and its positioning track. Fig. 8 (a) shows the experimental LSM, including the stator composed of 3φ copper windings on the bottom, the mover constituted of four s on the top, and a varying frequency power source. The stator structure is shown in Fig. 8 (b). coils forming 3φ windings are connected as Phase A: -4-7-0; Phase B: -5-8-; Phase C: 3-6-9-. Fig. 7. HTS magnetic levitator tested on its track in the Center of Applied Superconductivity and Electrical Engineering, University of Electronic Science and Technology of China. The mover is composed of four NdFeB magnets which are mounted on a steel back. The magnet is 6 cm long along the movement direction, 3 cm width in the transversal direction, and cm height in magnetization direction. Four magnets are arranged as N-S-N-S. The LSM is supplied by a BT40 frequency variable power source with a rated power of 750 W, rated current of 5 A, output voltage range of 0 V to 0 V (3 phase AC) and output frequency range of 0. Hz to 400 Hz. The experiments conducted under different frequencies show that the mover can move forward when the phase sequence is changed the mover move backward. This preliminarily validates the theoretical analysis. Based on this preliminary work, an optimized LSM will be developed for driving the prototype HTS maglev vehicle. 7. Conclusions A permanent magnet linear synchronous motor prototype for driving a small scale HTS levitator, which is levitated by -HTS maglev materials, has been studied. The design and analysis results of the linear driving system are presented, including magnetic field FEA for computing major parameters, equivalent electrical circuit for predicting steady-state performance, and phase variable model for simulating dynamic performance. Preliminary experiment has been conducted for validating the theoretical analysis with the prototype built. This work forms the base for further development of practical HTS- linear synchronous driving systems. (a) (b) Fig. 8. The experimental LSM driving system: (a) the system and (b) stator coils, built by the Center of Applied Superconductivity and Electrical Engineering, University of Electronic Science and Technology of China. The major stator dimensions include cm tooth length (along the movement direction), 3 cm tooth width, and.8 cm tooth height. The slot width (along the movement direction) is also cm, so the pole pitch is 6 cm. When the power supply is set to be 5 Hz, the rated velocity can be calculated as 0.6 m/s by the following equation: v = τ f () References [] J. X. Jin, High T c superconductor theoretical models and electromagnetic flux characteristics, Journal of Electronic Science and Technology of China, vol. 4, no. 3, pp. 0-08, Sep. 006. [] J. X. Jin, L. H. Zheng, R. P. Zhao, J. Zhang, L. Jiang, J. H. Chen, Y. L. Jiang, and H. Y. Zhang, Practical applications of high temperature superconductors, Nature Sciences, vol., no., pp. 48-79, Dec. 006. [3] S. Nagaya, N. Kashima, M. Minami, H. Kawashima, and S. Unisuga, Study on high temperature superconducting magnetic bearing for 0kWh flywheel energy storage system, IEEE Trans. on Applied Superconductivity, vol., no., pp. 649-65, Mar. 00. [4] T. Matsumura, S. Hanany, J. R. Hull, B. Johnson, and T. Jones, Magnetic field inhomogeneity and torque in high temperature superconducting magnetic bearings, IEEE Trans. on Applied Supercond., vol. 5, no., pp. 36-39, Jun. 005. [5] J. X. Jin, L. H. Zheng, J. Wen, Y. G. Guo, J. X. Chen, and J. G. Zhu, HTS levitated mobile technology and prototype, in Proc. 007 IEEE International Conference on Integration Technology, Shenzhen, China, 007, pp. 47-5.

GUO et al.: A Permanent Magnet Linear Synchronous Motor Drive for HTS Maglev Transportation Systems 9 [6] J. X. Jin, Y. G. Guo, J. X. Chen, L. H. Zheng, and J. G. Zhu, HTS levitation and transportation with linear motor control, in Proc. 6th Chinese Control Conference, Zhangjiajie, China, 007, vol. 6, pp.0-4. [7] Y. G. Guo, J. X. Jin, J. G. Zhu, and H. Y. Lu, Design and analysis of a prototype linear motor driving system for HTS maglev transportation, IEEE Trans. on App. Super., vol. 7, no., pp. 087-090, Jun. 007. [8] J. X. Jin and L. H. Zheng, Technology and development of high temperature superconducting linear motors, Small & Special Electrical Machines, vol. 36, no. 3, pp. 58-6, Mar. 008 (in Chinese). [9] J. X. Jin, L. H Zheng, Y. G. Guo, and J. G Zhu, Development of high temperature superconducting machines, Journal of Japan Society of Applied Electromagnetics and Mechanics, vol. 5, Suppl., pp. S88-S9, Sep. 007. [0] G. W. Mclean, Review of recent progress of linear motors, IEE Proc. Part B, vol. 35, no. 6, pp. 380-46, Nov. 988. [] Y. G. Guo, J. G. Zhu, and H. Y. Lu, Accurate determination of parameters of a claw pole motor with SMC stator core by finite element magnetic field analysis, IEE Proceedings Electrical Power Applications, vol. 53, no. 4, pp. 568-574, Jul. 006. [] O. A. Mohammed, S. Liu, and Z. Liu, A phase variable model of brushless dc motors based on finite element analysis and its coupling with external circuits, IEEE Trans. on Magnetics, vol. 4, no. 5, pp. 576-579, May 005. [3] Y. G. Guo, J. G. Zhu, J. X. Chen, and J. X. Jin, Performance analysis of a permanent magnet claw pole SMC motor with brushless dc control scheme, in Proc. International Power Electronics and Motion Control Conference, Shanghai, China, 006, pp. 3-6. You-Guang Guo was born in Hubei, China, in 965. He received the B.S. degree in 985 from Huazhong University of Science and Technology, China, the M.S. degree in 988 from Zhejiang University, China, and the Ph.D. degree in 004 from University of Technology, Sydney (UTS), Australia, all in electrical engineering. He is currently an ARC (Australia Research Council) research fellow with UTS and a professor with the School of Electrical Engineering and Automation, Hebei University of Technology, China. His research fields include measurement and modeling of magnetic properties of magnetic materials, motor design and optimization, and numerical analysis of electromagnetic field. He has published over 70 refereed technical papers including 86 journal articles in these fields. Jian-Xun Jin was born in Beijing, in 96. He received B.S. degree from Beijing University of Science and Technology in 985, M.S. degree from University of New South Wales, Australia in 994, and Ph.D. degree from University of Wollongong, Australia in 997. He was a research fellow and Australian ARC project chief investigator and senior research fellow with Australian University of Wollongong from 997 to 003. He is currently a professor and the Director of the Center of Applied Superconductivity and Electrical Engineering, UESTC. His research interests include applied high temperature superconductivity, measurement, control and energy efficiency technology. Lu-Hai Zheng was born in Zhejiang, China, in 980. He received B.S. degree in 005 from Huaiyin Teachers College, China. He is currently pursuing the Ph.D. degree with UESTC. His research interests include high temperature superconducting machine technology. Jian-Guo Zhu received his B.S. degree in 98 from Jiangsu Institute of Technology, China, M.S. degree in 987 from Shanghai University of Technology, China, and Ph.D. degree in 995 from UTS, Australia. He is currently a professor and the Director of the Center for Electrical Machines and Power Electronics at UTS, Australia. His research interests include electromagnetics, magnetic properties of materials, electrical machines and drives, power electronics, and renewable energy systems. Hai-Yan Lu received her Bachelor and Master degrees in electrical engineering from Harbin Institute of Technology, China, in 985 and 988, respectively, and Ph.D. degree from Faculty of Engineering, UTS, Australia, in 00. She is currently a lecturer with Faculty of Engineering and Information Technology, UTS. Her current research interests include optimal design of electromagnetic devices, modeling and numerical simulation of magnetic materials, and soft computing techniques and applications in power systems.