Design of a high-speed superconducting bearingless machine for flywheel energy storage systems. Li, WL; Chau, KT; Ching, TW; Wang, Y; CHEN, M

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1 Title Design of a high-speed superconducting bearingless machine for flywheel energy storage systems Author(s) Li, WL; Chau, KT; Ching, TW; Wang, Y; CHEN, M Citation IEEE Transactions on Applied Superconductivity, 2015, v. 25 n. 3, article no Issued Date 2015 URL Rights This work is licensed under a Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International License.; 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

2 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE Design of a High-Speed Superconducting Bearingless Machine for Flywheel Energy Storage Systems Wenlong Li, Member, IEEE, K.T.Chau,Fellow, IEEE, T.W.Ching,Senior Member, IEEE, Yubin Wang, and Mu Chen Abstract In this paper, an 8-pole/12-slot high-speed superconducting bearingless machine is proposed for flywheel energy storage systems. The proposed machine adopts a homopolar configuration: the rotor only consists of iron lamination with eight salient iron poles and the 12-slot stator accommodates all three groups of windings: the high temperature superconducting (HTS) field winding; the armature winding; and the suspension winding. With the HTS field winding, eight iron poles in the rotor are magnetized as 4-pole-pair electromagnets. Also, by adjusting the dc current in the HTS field winding, the air-gap flux can be controlled flexibly. By deploying the suspension winding which shares the same slot with the armature winding, a two-degree-of-freedom suspension force can be generated for bearingless operation. Index Terms Bearingless machine, energy storage, flywheel, homopolar machine, superconducting machine. I. INTRODUCTION RENEWABLE POWER generation has been developed on an accelerated pace in recent years. Total renewable power capacity worldwide exceeded 1,560 gigawatts in 2013 and is still expanding [1]. However, the intermittent and stochastic features of renewable sources degrade the power system performances such as system stability, reliability, and power quality. For solving the above problems, one possible and effective way is to deploy energy storage systems to decouple power generation from the demand side. With energy storage systems, the so-called peak-load shifting technology can be realized and the power system stability can be improved [2]. The flywheel energy storage system (FESS) stores energy via a rotating mass driven by an electric machine in the form of kinetic energy [3]. The stored energy is proportional to the weight of the rotating mass and the square of the rotating speed. Therefore, the high speed it rotates, the more energy can be stored. Compared with batteries, FESSs have many distinct advantages such as high power density, high energy density, and very long service time. As a core component in a FESS, the electrical machine realizes energy inter-conversion between Manuscript received August 13, 2014; accepted October 30, Date of publication November 4, 2014; date of current version January 19, This work was supported in part by a grant (Project No ) from The University of Hong Kong, Hong Kong, China and a grant (Project No. MYRG067(Y1-L2)-FST13-CTW) from the Research Council of the University of Macau, Macao Special Administrative Region, China. W. Li, K. T. Chau, Y. Wang, and M. Chen are with Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong ( wlli@eee.hku.hk; ktchau@eee.hku.hk; ybwang@ eee.hku.hk; muchen@eee.hku.hk). T. W. Ching is with the Faculty of Science and Technology, University of Macau, Macau, China ( twching@umac.mo). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TASC Fig. 1. Proposed superconducting bearingless machine. (a) Front view. (b) Side view. the kinetic energy and the electric energy. Therefore, a suitable candidate is the machine which can offer high power density, high efficiency, high reliability and very wide speed range. The purpose of this paper is to propose a superconducting bearingless machine for flywheel energy storage systems. The rotor consists of iron core only. The stator accommodates the field winding, the armature winding and the suspension winding respectively. Due to the passive rotor, the reliability and robustness can be achieved. By utilizing the field winding, the power density can be improved and the speed operation range can be extended. Moreover, due to the bearingless structure, the mechanical friction caused by mechanical bearing is dramatically reduced. Therefore, the self-discharging rate of FESS can be improved. II. MACHINE DESIGN A. Machine Topology Fig. 1 illustrates the proposed superconducting bearingless machine which adopts a homopolar configuration [3] [5]. From the front view, the proposed machine has a similar structure as IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

3 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 2. Winding configuration. (a) Armature winding. (b) Suspension winding. that of an 8-pole/12-slot surface-mounted permanent magnet (PM) synchronous machine. From the side view, it can be observed that the proposed machine consists of two pairs of the stator segment and the rotor segment which artfully share the same stator yoke and rotor yoke for integrating them into one machine. The iron poles of the two segments are deployed in an interleaved manner which is a half pole-pitch away from each other in the circumferential direction. It should be emphasized that since the flux path of the proposed machine is inherently 3-D, for achieving a low magnetic reluctance, only stator teeth and iron poles on the rotor are iron laminated and the stator back iron and rotor back iron consist of solid iron cores. The stator houses the field winding and the armature winding and the suspension winding. Fig. 2 shows the winding configuration of the armature winding and the suspension winding which share the same slot space. The 3-phase 8-pole armature winding adopts the concentrated manner and the 2-phase 2-pole suspension winding adopts the distributed manner. The armature winding spans the total stack length and the suspension winding only spans one stator segment length. The field winding is deployed between the two segments. In this design, the field winding adopts high temperature superconducting (HTS) coils with injecting dc current for field excitation. Since the HTS field winding locates at the stationary part, the cooling facility can be easily implemented [6]. Therefore, the power density is improved due to the dual-excitation sources. In addition, by adjusting the dc current flowing in the HTS field winding, the magnetic field is flexibly controlled. The amplitude and polarity of magnetization in the air-gap can be easily adjusted. With this merits, flux weakening can be realized for motoring operation especially for the high-speed operation, and flux control for voltage regulation when operating as a generator. For an initial design, the air-gap flux density of the proposed machine is limited within 1 T and the current densities of the HTS coil and armature winding are limited to 100 A/mm 2 and 5 A/mm 2 respectively. Therefore, the iron core adopts conventional magnetic steel and the HTS field winding uses the popular BSCCO-2223 which can operate up to 100 K and be cooled with off-the-shelf GM cryocooler [7]. B. Operating Principle Due to 3 groups of windings in the proposed machine, the air-gap flux is more complicated than the conventional Fig. 3. Suspension force generation. (a) X-component of suspension force. (b) Y-component of suspension force. synchronous machines. In this section, the air-gap flux density is calculated to analyze the proposed machine operating principle, especially the suspension force generation by the 2-phase 2-pole suspension winding. By ignoring the magnetic saturation effect, the flux fringing effect and the magnetic reluctance of the iron core, the air-gap permeance can be expressed as: { 2μ0 πlr 0 P ag (θ m )= k 0 g 0, for facing iron poles (1) 0, otherwise where μ 0 is the permeability of the free space, l is the length of one stator segment, r 0 is the mean radius of air-gap under the salient poles, k 0 is the Carter s coefficient, and g 0 is the air-gap length. Therefore, the resultant air-gap flux density can be expressed as: B ag (θ m )= (F f + F a + F s )P ag (θ m ) (2) 2πlr 0 where F f, F a, and F s are the magnetomotive forces (MMFs) due to the field winding, the armature winding, and the suspension winding, respectively. With the expression of the air-gap flux density, the machine performances, such as the induced voltage and the developed torque can be deduced which is similar to the conventional machine calculations. In this paper, only the suspension force calculation is elaborated. Fig. 3 depicts the suspension force generation principle where the solid line with arrow represents the magnetic flux induced by the field winding and the armature winding, and the dash line with arrow represents the magnetic flux induced by the suspension winding. As shown in Fig. 3(a), the flux produced by the phase X of suspension winding strengthens the flux of armature winding at the right hand side and weakens at the left hand side. Therefore, a suspension force towards right can be generated. Similarly, as shown in Fig. 3(b), an upward suspension force is generated. By controlling the excitation in the suspension winding, the two force components can be adjusted. By using the square of the sum of the air-gap flux density [8], the two suspension force components can be expressed as: { Fx = lr 0 2μ 0 2π 0 B g (θ m ) 2 cos θ m dθ m F y = lr 0 2μ 0 2π 0 B g (θ m ) 2 sin θ m dθ m (3)

4 LI et al.: HIGH-SPEED SUPERCONDUCTING BEARINGLESS MACHINE FOR FLYWHEEL ENERGY STORAGE SYSTEMS TABLE I KEY DESIGN DATA Fig. 5. Air-gap flux density. (a) Excited by HTS field winding. (b) Excited by suspension winding and HTS field winding. Fig. 4. Magnetic flux density vector distribution. (a) Flux vectors produced by HTS field winding. (b) Resultant flux vectors produced by suspension winding and HTS field winding. It should be noted that when the pole number is greater than 8, a constant suspension force can be generated by injecting dc currents into the suspension winding without position sensors [5], [8]. III. ANALYSIS An 8-kW prototype is designed and the key design data are listed in Table I. To analyze and assess the performance of the proposed machine, 3-D finite element method (FEM) is applied for the magnetic field calculation and the circuit simulator is used for the dynamic analysis. A. Static Performances Since the suspension winding is incorporated into the proposed machine, the flux pattern and distribution are different from that of conventional machines. Two cases under different circumstances are studied: case 1 is the flux excited by the HTS field winding only and case 2 is the flux excited by the HTS field winding and the suspension winding. Fig. 4 illustrates the distributions of magnetic flux density vectors. It can be found that the flux path for case 1 is 3-dimenional which flows through the stator teeth, stator yoke, iron poles on the rotor and the rotor Fig. 6. Induced voltage waveforms. yoke. On the other hand, the flux generated by the suspension only circulates in the cross section of one machine segment. Fig. 5 shows the air-gap flux densities for the two cases. It shows that iron poles on the rotor are magnetized into the same polarity for the same machine segment but with opposite polarities for different machine segments. Fig. 6 exhibits the induced voltage waveforms for the two cases. It can be found that the voltage waveform for case 2 almost coincides with that of case 1 in terms of the amplitude and phase angle. It is due to the fact that the fluxes produced by suspension windings circulate in a 2-D plane and cancel off each other in two segments. Fig. 7 shows the consequent cogging torque waveforms of the above two cases. As shown

5 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 7. Cogging torque waveforms. Fig. 9. Flux controlling capability. excitation rather than permanent magnets, which enables the air-gap flux density to be adjusted continuously. Therefore, the EMF amplitude can be regulated when the rotor speed varies. Hence, a wide speed operation range can be achieved. When the proposed machine works as a motor for storing energy, the rotor speed can be improved further by using this flux control capability. Fig. 8. Suspension force characteristics. in Fig. 5, the amplitude of flux density for case 2 is larger than that of case 1, due to the superposition of two magnetic fields. So it causes almost two times the cogging torque as compared with that of using only the HTS field winding for excitation. B. Dynamic Performances Based on (3), when MMFs of the HTS field winding and the armature winding are kept constant, by injecting different dc currents into the suspension winding, different magnitudes of suspension forces are generated. Fig. 8 shows the suspension forces produced by the 2-phase suspension winding under different injecting currents. The suspension forces of 150 N and 200 N are produced when the MMFs of the suspension winding are 100 A-t and 200 A-t respectively. The nonlinearity is due to the magnetic saturation in iron cores. It should be noted that for an active magnetic bearing, the stiffness and damping coefficient can be changed based on the specific closed-loop control system. Thanks to the low external load of the flywheel system, an appropriate stiffness and damping coefficient can achieve a low vibration capability. Since the HTS field winding is also fed by the dc current to provide the main flux for machine operation, the air-gap flux can be regulated much easier as compared with that of those hybrid-excited machines [9], [10]. Fig. 9 shows the electromotive force (EMF) waveforms when the rotor operates at 40 krpm, 60 krpm and 80 krpm, respectively. The merit of the proposed machine is the use of HTS field winding for field IV. CONCLUSION A high-speed superconducting machine is proposed for FESSs in this paper. The proposed machine adopts the homopolar configuration. The rotor consists of iron only which can enhance the robustness. The stator has a similar structure as that of a synchronous machine. There are 3 groups of winding deployed in the stator: the armature winding for energy interconvention, the field winding for providing and regulating the main flux, and the suspension winding for levitating the rotor to reduce the mechanical friction. With these merits, the proposed machine can operate efficiently in both motoring and generation modes for the FESSs. REFERENCES [1] [Online]. Available: gsr/2014/gsr2014_full%20report_low%20res.pdf [2] R. Hebner, J. Beno, and A. Walls, Flywheel batteries come around again, IEEE Spectrum, vol. 39, no. 4, pp , Apr [3] E. Severson, R. Nilssen, T. Undeland, and N. Mohan, Outer-rotor ac homopolar motors for flywheel energy storage, in Proc. 7th IET Int. Conf. Power Electron., Mach. Drives, 2014, pp [4] O. Ichikawa, A. Chiba, and T. Fukao, Inherently decoupled magnetic suspension in homopolar-type bearingless motors, IEEE Trans. Ind. Appl., vol. 37, no. 6, pp , Nov./Dec [5] E. Severson, R. Nilssen, T. Undeland, and N. Mohan, Analysis of the bearingless AC homopolar motor, in Proc. 20th Int. Conf. Elect. Mach., 2012, pp [6] K. Sivasubramaniam et al., Development of a high speed HTS generator for airborne applications, IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp , Jun [7] S. S. Kalsi et al., Development status of rotating machines employing superconducting field windings, Proc. IEEE, vol. 92, no. 10, pp , Sep [8] A. Chiba et al., Magnetic Bearings and Bearingless Drives. Boston, MA, USA: Newnes, [9] W. Li, K. T. Chau, J. Z. Jiang, and F. Li, Design and analysis of a fluxmnemonic dual-magnet brushless machine, IEEE Trans. Magn., vol. 47, no. 10, pp , Oct [10] K. T. Chau, W. Li, and C. H. T. Lee, Challenges and opportunities of electric machines for renewable energy, Progr. Electromagn. Res. B, vol. 42, pp , Apr