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AbstractA compact and efficient flywheel energy storage system is proposed in this paper. The system is assisted by integrated mechanical and magnetic bearings, the flywheel acts as the rotor of the drive system and is sandwiched between two disk type stators to save space. The combined use of active magnetic bearings, mechanical bearings and axial flux PM synchronous machine assists the rotor-flywheel to spin and remain in magnetic levitation in the vertical orientation, while constrains the other four degrees of freedom in radial directions mechanically. The mathematical model of the proposed system has been derived. Three-dimensional finite element method is applied for studying the performances and verifying the mathematical model of the system. The analysis results support the feasibility of the system. I. INTRODUCTION N modern power industries, with the advances of high strength and light weight composite material, control technology and power electronics, the Flywheel Energy Storage System (FESS) is becoming a viable alternative to traditional chemical battery systems, with its advantages such as higher energy storage density, lower risk of overcharge and over-discharge, easier detection of the depth of discharge, operation over a wider temperature range, longer lifespan and environmental friendliness 1-4. As a result, FESS is now considered a promising technology for many applications including spaceflight, transportation, power industry, military, and building services. In general, a flywheel energy storage system is composed of a flywheel, magnetic or mechanical bearings that support the flywheel, a motor-generator to drive the flywheel and inter-convert the mechanical energy and electrical energy, control and power electronic devices, and touchdown bearings. This separate driving motor-generator in addition to magnetic bearings makes the rotor long and apt to produce bending vibrations. And the large motor-bearing system makes it difficult for miniaturization 5. To overcome these problems, a self-bearing permanent magnet motor is introduced. The motor combines magnetic bearing and motoring functionality into a single magnetic actuator. Such designs can reduce the overall length of a motor because less mechanical bearings are required, thus increasing power density, reducing weight, and lowering susceptibility to rotor dynamic vibrations 6. As shown in Fig. 1, there are three directions along x , y and z axes within the flywheel, such that six degrees-of-freedom (DOF) which are the displacement and rotation of every axis should be controlled with the help of mechanical or magnetic bearings. Mechanical bearings have the advantages of simple structure and easy operation, but the frictional loss and thereby, the use of lubrication should always be taken into consideration. Especially, the friction occurring on the bearing which is along the direction of the gravity, i.e., the direction along z axis in Fig. 1, is much greater than those in the other directions. For this reason, it is not practical to use mechanical bearings along this axis, while for the other axes, they can still be tolerated. Active magnetic bearings have many advantages over the conventional bearings. Such benefits include higher energy efficiency, lower wear, longer lifespan, absence of need of lubrication and mechanical maintenance, and wider range of operating temperatures. There are many studies concerning magnetic bearings, but most of them treat the bearing in which at least five DOF of the object are controlled. Since the control of each DOF requires a sensor, an actuator and a controller, the entire system becomes complex in terms of the design of its mechanical/ electrical part and the control system 7. Considering this, this paper presents a new concept of magnetic bearing, in which only 2 DOF of an axis, namely, the translation and rotation along and about axial directions respectively, are actively controlled. The motions in other directions are entirely restricted by mechanical bearings. The combined use of active magnetic bearings and mechanical bearings can cut down the complexity of control and make the system more stable, viable and cost-effective. Currently, axial flux permanent magnet motors (AFPM) used in many applications have become an appealing research field 8 9. They have several unique features such as high efficiency, high power and torque densities, low rotor losses and small magnetic thickness. However, the disadvantage is that the distributed windings have end-windings of significant length compared to the active part of the coil conductors. This FEM Analyses for the Design and Modeling of a Novel Flywheel Energy Storage System Assisted by Integrated Magnetic Bearing C. Zhang, Student Member, IEEE, P. Wu, Student Member, IEEE and K. J. Tseng, Senior Member, IEEE Centre for Advanced Power Electronics, Nanyang Technological University Blk S2, Nanyang Avenue, Singapore 639798, Republic of Singapore I Fig. 1. Three motion directions of flywheel. 0-7803-8987-5/05/$20.00 2005 IEEE. 1157 obviously results in poor machine performance, as a significant part of the machine copper (i.e., more than 50% of the total in most machine designs) is producing heat but no torque 10. Concentrated windings can solve this problem. Furthermore, they have simpler design, easier arrangement and higher efficiency. The finite element method (FEM) has proved to be particularly flexible, reliable and effective in the analysis and synthesis of power-frequency electromagnetic and electromechanical devices 11 12. The FEM can analyze PM circuits of any shape and material. A remarkable advantage of FEM analysis over other approaches to analysis of PM motor is the inherent ability to calculate accurately armature reaction effects, electromagnetic force and torque. In this paper, a novel flywheel energy storage system assisted by integrated magnetic bearing is proposed. The motor and generator are combined to be a single machine and the flywheel functions as the rotor in order to save space. Mechanical bearings are used to restrict the displacement and rotation along radial directions, and the displacement and rotation along axial direction are controlled by active magnetic bearings. The structure and electromagnetic design of the proposed system is presented along with the mathematical model. 3D FEM analyses are implemented to verify the mathematical model and support the feasibility of the system. Analysis results have been obtained and are presented in this paper. II. CONSTRUCTION AND GEOMETRY OF THE PROPOSED SYSTEM A. Configuration of the Entire System Fig.2 shows the cross-sectional diagram of the proposed flywheel energy storage system. Its components are listed in Table I. Items 1 and 8 are the upper and lower stators fixed on the system housing which is designed to dissipate radial kinetic energy from any rotor debris and ensure safety in the event of mechanical failure. Axial flux permanent magnet synchronous motor is implemented to drive the flywheel which is also functioning as a rotor. Mechanical rotational ball bearings are mounted on the outer rim of the rotor to constrain its radial motion and assist the rotation of the flywheel/rotor. This arrangement makes the structure very compact without using the shaft. But the large diameter of the bore of the mechanical bearing limits the maximum speed. By using fluid-film bearings, the DN value (bore diameter mm speed rpm) can reach 3,000,000 13. That means the maximum speed is less than 20,000 rpm when the bore diameter is 150 mm. In higher speed flywheel system, two mechanical bearings can be mounted at the ends of the shaft which is fixed in the middle of the rotor. With this arrangement, the speed may reach up to 60,000 rpm and above. The axial motion can be realized with the aid of 4 sliding ball bearings installed orthogonally on the rim of the rotational ball bearing. Non-contact eddy current displacement sensor and photo electrical sensor are set in the hollow center of the two stators to detect the displacement and angular position along z-axis when the rotor spins, (items #2 and #10 in Fig. 2). Touchdown bearings are necessary during starting operation or in the event of magnetic bearings failure. The touchdown bearings shall be mounted against the outer rim of the rotor. During normal operation, there is a less than 0.5 mm air-gap between all rotor surfaces and the touchdown bearings, thus achieving a mechanically contact-less environment. B. Basic Features of the Proposed System Fig.3 shows the basic features of the proposed system. The motor and generator with disk-type geometry are combined into a single electric machine as shown in Fig.3 (a). The rotor doubles as the flywheel and is sandwiched between two disk-type stators. This design maximizes the torque production area of the disk-type rotor. As shown in Fig.3 (b), each of the upper and lower stators carries a set of three-phase copper windings to be fed with sinusoidal currents; concentrated windings are implemented to reduce the power loss. If distributed windings are used, the winding-ends will span half the circumference of the rotor. The ends are much longer compared to the effective parts of coil conductors, and the copper loss of the windings will thus be larger. In this particular design, there are 6 coils, each of which surrounds a stator tooth. The distribution of three phases and directions of the three-phase currents at a particular instance are TABLE I COMPONENTS OF THE PROPOSED SYSTEM Item Number Item Name 1 Upper stator 2 Position sensor 3 Stator windings 4 Touchdown bearings 5 Rotational ball bearing 6 System housing 7 Rotor permanent magnets 8 Lower stator 9 Non-magnetic material guard ring 10 Rotation sensor 11 Flywheel-rotor 12 Fasteners 13 Sliding ball bearing Fig. 2. Cross-sectional diagram of the proposed system. 1158 shown in Fig. 4. Besides improved efficiency, simple structure and easy installation of the stator winding can also be realized in this design. Permanent magnets are mounted on both surfaces of the rotor, as shown in Fig.3 (c). The arrangement of these PMs and the magnetic flux flowing in the motor are depicted in Fig.4. PMs are settled in opposite directions in upper and lower rotor faces, so that they would attract each other and increase the total flux in the magnetic circuits. A guard ring made of high strength non-magnetic material is used to assist the PMs in resisting the centrifugal force, as shown in Fig.3 (d). Magnetic bearings can be realized by using attractive forces. The interaction between the stator and rotor fields produces an axial force that makes the rotor and stator attract each other. The currents of each stator can be independently adjusted to control the net forces on the rotor and keep it in the middle of the two stators. The net force along the axial axis can be obtained as 21 FF F= (1) Where 1 F is the force between the lower stator and rotor; 2 F is the force between the upper stator and rotor. The motor-generator is equivalent to two motors, the total torque T can be written as 12 TTT= + (2) Where 1 T and 2 T are torques generated by the upper and lower motor respectively. C. Dimensions of the Motor The size of axial flux motor can be transformed to that of an equivalent sized radial machine by the following formulas 2 oi DD D + = (3) 2 oi DD L = (4) where o D and i D are the outer and inner diameters of the axial flux disk-type motor, D and L are the inner diameter and length of the equivalent radial machine. Maximum torque is produced when /3 Roi KDD=. From output equation of the motor, we can get 2 0 s Q DL Cn = (5) and then, we can obtain 3 3 2 0 8 (1)(1) R o RRs QK D KKCn = + (6) where 0 C is the output coefficient, Q is the rating of machine in kVA, s n is the rated speed in r.p.s. 3 0 11 10 gav w CBAK =, cos EN N N KP Q = (7) where gav B represents the average flux density over the air-gap of the machine, also known as magnetic loading; A is the electric loading; w K is the winding factor; N P , N and cos N represent rated power, efficiency and power factor respectively; E K is the ratio between induced EMF and the voltage. In this design, 0.905 E K = . Minimum length of air-gap is set by mechanical constraints and is unlikely to be less than 0.3 mm. Magnets depth should generally be reduced to a minimal value so as to minimize the cost of the magnets. Manufacturing restrictions make it difficult to have magnets thinner than 2.0mm. In this design, g l is selected as 0.5 mm, and m l is set to be 2.5 mm. According to the design requirement data shown in Table II, the motor design results can be obtained as in Table III. This is just a test design to verify the feasibility of the system structure and the correctness of the mathematical model. So the rated Fig. 4. Motor development structure and 2D flux pattern. (a) (b) (c) (d) Fig. 3. Basic parts of the proposed flywheel system. (a) Stator-rotor Assembly. (b) Stator Windings. (c) Rotor-flywheel. (d) Non-magnetic guard ring. 1159 speed is only selected to be 1500 rpm. III. MATHEMATICAL MODEL As shown in Fig.3, the three-phase windings of the stator are denoted as a, b and c with the same winding number. Permanent magnets are mounted on the surface of the disk type rotor, a non-salient rotor is obtained as a result. The motor can be treated as a conventional synchronous motor, only if the field windings are replaced by permanent magnets. The PM motor can be readily analyzed by assuming that the permanent magnets of the rotor here have been replaced by an equivalent rotor current f i with the winding number f N . The waveform of the MMFs produced by the stator phase windings and the equivalent rotor current f i may be considered as coarse approximation of sinusoidal functions of s and r , the same as the distribution of the windings 14 15. Where s and r are the angles measured from the a phase stator winding axis and rotational d axis, respectively. Assuming the number of pole pairs is P , their functions are as follows cos 2 sin 2 s as as s s as s MMF N iP P N NP = = (8) ( ) () 2 cos 3 2 2 sin 3 2 s bs bs s s bs s MMF N iP P N NP = = (9) ( ) () 2 cos 3 2 2 sin 3 2 s cs cs s s as s MMF N iP P N NP = + =+ (10) cos 2 sin 2 f f fr f rf r MMF N iP P N NP = = (11) where s N is the number of turns of equivalent sinusoidally distributed winding in each phase of the stator. For the winding distribution depicted as Fig.3 (b), the pitch factor 1 p k = , the distribution factor d k = cos( / 6) = 3/2, so the winding factor 3/2 wpd kkk= . Then s N can be calculated as 4 s wph NkN = (12) where ph N is the actual number of turns in series per phase. The maximum value of the equivalent MMF produced by PMs , m MMF , is calculated to be 2 ff mmm Ni MMF H l P = (13) where, m l and m H denote the magnet length and the magnetic field intensity when the magnet is shorted by permeable iron. Then the value of f f N i can be achieved as 0 22 rm ff mm r B l Ni PHl P = (14) r B is the remanent flux density for the PMs, r is the relative permeability, and 0 is the magnetic permeability of the air with the value 7 410 . The effective air gap length between the surfaces of stator and rotor is defined as g , the magnetic flux density B and magnetic flux are shown as below: 0 s MMF B Bds g = (15) As an example, let us determine the total flux linkages of the winding due to current flowing only in a winding, leakage inductances are ignored here. / 0 22 / 0 () () sin 2 () . cos 4 s s P s as as s as s s s P so i as s N NdPP NR R iPdd Pg + = (16) 222 0 2 () 8 as o i s as s as R RN LL i Pg = (17) where o R and i R are the outer and inner radius of the stator. Similarly, we can get as bs cs s L LLL= = (18) TABLE III DESIGN GEOMETRICAL DATA No. of pole pairs 2 No. of slots 6 Outer diameter of stator 130 mm Inner diameter of stator 76 mm Permanent magnets length 2.5 mm Air gap length 0.5 mm Slot width 28 mm Slot depth 22 mm Stator yoke thickness 18 mm Rotor core thickness 60 mm Air gap flux density 0.805 T No. of turns per phase 416 TABLE II DESIGN REQUIREMENT DATA Rated power 1 kVA Phase current, rms 1.85 A Power factor 0.9 Efficiency 0.9 Rated speed 1500 rpm Frequency 50 Hz Slot fill factor 0.4 Remanent flux density 1.23 T Magnet recoil permeability 1.05 Carters factor 1.05 1160 222 0 2 () 8 f oif ff f as RRN LL i Pg = (19) The mutual inductance between the a and f windings is determined by / 0 22 / 0 ()() sin 2 () . cos() 4 s s P S asf as s f r s s P fo i f s N NdPP NR R iP dd Pg + = (20) In the same way as above, af L , bf L , cf L can be written as 22 0 2 () cos cos 8 oisf af m RRNN L PL P Pg = (21) 22 0 2 () cos( 2 / 3 ) 8 oisf bf RRNN LP Pg = (22) 22 0 2 () cos( 2 / 3 ) 8 oisf cf RRNN Pg =+ (23) Therefore, the other mutual inductances can be obtained as 1/2 ab ba ac ca bc cb s L LLLLL L= (24) Then, () = ff af bf cf f T af as ab ac a fabc bf bs ac b ba cb cs ccf ca L L L Li L L L Li L L Li L L LiLL = Li (25) where L is the inductance matrix of the motor, the inductances are determined by(18)(19) and (21)-(24). The inductance expression of (31) can be simplified when they are expressed in terms of 0dq variables 3/2 0 3/2 0 003/2 f fm f dm s d sqq L Li L L i = (26) The magnetic energy stored may be calculated as ()( ) 1 i 2 T fdq f d q Wii = (27) The attractive force s F can thus be obtained () 22 0 22 22 2 2 2 () 16 53 . 22 oi s ff sffd s d q RRW F g Pg Ni NNii N i i = = + (28) From the Flemings left-hand rule, the rotating torque s T can be expressed as 22 0 3( ) 3 () 216 oisf s dq qd f q RRNN TPii i Pg = = (29) Here, the air gap between the surfaces of the stator and PMs at the equilibrium point is defined as g l , so the effective air gap between the stator and rotor at the equilibrium point can be obtained as 0 (/) cg m r gKll= + (30) where c K is the Carters coefficient, which is approximately equal to 1. Then 1 F and 1 T can be calculated by substituting 0 g gz= + , 1dd ii= and 1qq ii= into (28)(29) whereas 2 F and 2 T can be calculated by substituting 0 g gz=, 2dd ii= and 2qq ii= into the same equations, where z is the displacement of the rotor in the vertical direction. The total force and torque are obtained by (1) and (2). The radial motions of the rotor are restricted by mechanical ball bearings. Therefore, the axial motion of the rotor is independent of radial motion. The dynamic equation of the axial motion of the rotor is z mz F f= + (31) where z f is the external force in the direction of z-axis, and the gravity is taken into consideration. The equation of total torque can be rewritten as Tq TJ Ki= (32) And , Tq J Ki= (33) where J is the moment of inertia, is the rotor angle and is the rotational speed. The voltage equations can be written as 1 () q a ddd rq ddd L Rd ivi pi dt L L L =+ (34) 1 () () ad m qqd rd r qqq q RLd ivi pi p dt L L L L = (35) IV. FEM ANALYSIS AND MODEL VERIFICATION A. Theory Magnetic fields in PM motors are always associated with transient excitations and nonlinear magnetic materials. The following three Maxwell equations are relevant to transient applications. HJ E = = G GG (36) /EBt= G G (37) 0B= G (38) where H G denotes the magnetic field density, J G is the electric current density, is the conductivity of the dielectric, and E G is the electric field intensity. From (36) and (37), we can obtain 1 0 B H t + = G G (39) The force and torque can be calculated as the derivative of the stored magnetic co-energy W with respect to a small displacement. The co-energy can be written as 1161 0 B V W H dB dV = GG (40) Then the component of instantaneous force s F in the direction of the displacement s is s dW F ds = (41) The instantaneous torque T with a small angular rotation displacement is represented by (, ) / i=const Wi T = (42) B. FEM Analysis The proposed system described in Section II was analyzed using a time-stepping three-dimensional finite element simulator 16. The mesh shape of analysis model is