Trend of Permanent Magnet Synchronous Machines

Transcription

1 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 2007; 2: Published online in Wiley InterScience ( DOI: /tee Review Trend of Permanent Magnet Synchronous Machines Shigeo Morimoto a, Senior Member The performance of permanent magnet synchronous machines (PMSMs) has improved rapidly by the progress in elemental technologies such as electromagnetic material technology, computer-aided design technology, control technique, and drive circuit technology, and thus PMSMs are attractive as high-performance machines in various fields. This paper describes the recent technology and the trends in PMSMs. To begin with the PMSMs are classified by the ratio of magnet torque to the reluctance torque and their features described, then the trend of the motor design and the electromagnetic material for highly efficient PMSM is shown. The technologies that help to reduce the vibration and noise are also described, and the recently developed PMSMs for traction drive application are introduced Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. Keywords: permanent magnet synchronous machine, interior permanent magnet synchronous motor, reluctance torque, high efficient motor Received 2 October 2006; Revised 30 October Introduction The beginning of the permanent magnet synchronous machine (PMSM) goes back to 1930 in which the alnico magnet was discovered. At that time the performance of the magnetic material was not sufficient and a variable frequency power source such as the inverter was not utilized; thus the use of PMSM was limited. The performance of PMSM has improved rapidly by the progress of the elemental technologies such as electromagnetic material technology, computer-aided design technology, control technique, drive circuit technology andsoon. Now, the permanent magnet synchronous machine is widely used because it has many advantages, such as maintenance-free operation, high controllability, robustness against the environment, high efficiency and high power factor operation. Recently, there has been a great demand for energy saving because of environmental problems, and highly efficient motors are in demand in various fields. The development of high performance magnetic material and the cost reduction in addition to environmental problems have expanded the field of application of PMSM [1]. Figure 1 shows the characteristics and use of the recent neodymium-iron-boron (NdFeB) a Correspondence to: Shigeo Morimoto. morimoto@eis.osakafu-u.ac.jp Osaka Prefecture University 1-1, Gakuen-cho, Nakaku, Sakai , Japan Residual magnetic flux density Br [T] VCM CD Pick up OA/FA Motors Compressor Motors 35 HEV/Train Motors [koe] [MA/m] 3.0 Coercive force Hcj Fig. 1 Characteristics and use of the recent NdFeB magnet magnet [2]. The application of PMSM to the factory automation (FA), the compressor, and the vehicle has progressed by developing high coercivity magnetic material that can withstand high temperatures and an opposing magnetic field. This paper describes the recent technology and the trends of PMSMs. First, the PMSMs are classified by the ratio of magnet torque to reluctance torque, and their features are described. Next, the trend of the motor design and the electromagnetic material for highly efficient PMSM is shown. The technology reducing the vibration and noise is described, and the recently developed PMSMs for traction drive application are introduced Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

2 S. MORIMOTO 2. Classification and Features of PMSMs The torque T and the terminal voltage V a are given by (1) and (2) using the variables and parameters in the rotating d q coordinate. T = P n {ψ a i q + (L d L q )i d i q } (1) V a = (R a i d ωl q i q ) 2 + (R a i q + ωl d i d + ωψ a ) 2 (2) where, i d, i q are the d and q-axis armature currents, ω is the electrical angular velocity, R a, the armature resistance, ψ a, the magnet flux-linkage, L d, L q,thed and q-axis inductances, and P n is the number of pole pairs. The first term in (1) represents the magnet torque due to the permanent magnet and the second one represents the reluctance torque derived from the difference between the d-axis and q-axis inductances. The torque characteristic is discussed based on (1), and the maximum speed and the constant power speed range can be examined by (2). Figure 2 shows the ratio of magnet torque to reluctance torque for several synchronous machines. The synchronous machines can be classified into three kinds by the principle of torque production. The surface PMSM (SPMSM), in which the arc-shaped permanent magnets are mounted on the surface of a cylindrical rotor core as shown in Fig. 2 (a), is a pure PM machine, where only magnet torque is produced. The synchronous reluctance machine (SynRM) is a pure reluctance machine and it is not a PMSM (Fig. 2 (f)). The inset SPMSM (Fig. 2 (b)) belongs to the category of SPMSM in the classification by the magnet arrangement, but it is a PM/reluctance hybrid machine because it has magnetic saliency. The interior PMSM (IPMSM), in which the permanent magnets are buried into the rotor core as shown in Fig. 2 (c e), has magnetic saliency and is a PM/reluctance hybrid machine. The IPMSM can be classified into a reluctance torque assisted PMSM (region II) and a permanent magnet torque assisted SynRM (region III) by a torque generating mechanism. IPMSM has come to be used in various fields because it has the following advantages in comparison with SPMSM. 1. The stainless-steel-can, which is usually required in SPMSM, can be eliminated in the IPMSM and the rotor surface becomes the laminated core. Therefore, the eddy current loss on the surface of rotor can be greatly decreased. 2. The square-shaped permanent magnet can be used for IPMSM, and it brings about a decrease in magnet cost compared to the arc-shaped magnet. 3. IPMSM has saliency, where L q is larger than L d, and the reluctance torque can be effectively utilized in addition to the magnet torque. 4. The flux-weakening control acts effectively in IPMSM, and as a result the high-speed operation and the wide constant-power operation can be achieved. 5. IPMSM has many degrees of freedom in the design of both mechanical structure and torque-speed characteristics. 3. Design for Improving Efficiency 3.1. Rotor design Increasing the ratio of torque per current ampere improves efficiency by decreasing the copper loss. From (1), it is evident that such demand can (1) Pure PM Machine : Magnet torque : Reluctance torque (2) PM/Reluctance Hybrid Machine Increasing saliency (3) Pure Reluctance Machine I II III IV Increasing magnet flux Reluctance Torque Assisted PMSM PM Torque Assisted SynRM SPMSM Inset SPMSM IPMSM PMASynRM SynRM (a) (b) (c) (d) (e) (f) Fig. 2 Classification of synchronous machines by torque generating mechanism 102 IEEJ Trans 2: (2007)

3 TREND OF PERMANENT MAGNET SYNCHRONOUS MACHINES effective in reduction of power consumption and energy saving. Fig. 3 IPMSM with rare-earth permanent magnet. (Courtesy of Daikin Industries, Ltd) Efficiency h (%) Fig IPMSM SPMSM IM Speed (r/sec) Comparative efficiency performances of IM, SPMSM and IPMSM (torque: 1.5 Nm) be achieved by either increase of magnet flux-linkage ψ a or increase of difference between d-axis and q-axis inductances (L q L d ). The increase of the difference of inductances (L q L d ) can be achieved by optimal rotor design [3]. The use of rare-earth permanent magnet, of which magnetic energy density is about 10 times higher than that of ferrite permanent magnet, is effective for increasing magnet flux-linkage. The use of rare-earth permanent magnet in a PMSM has become common by improving performance and reducing the price of the rare-earth permanent magnet. Figure 3 shows the IPMSM with rare-earth permanent magnet produced in large quantities for air conditioners [4]. In this rotor design, the q-axis inductance becomes large and the reluctance torque increases because the magnets are placed inside the rotor deeply. The comparative efficiency performances of induction motor (IM), SPMSM and IPMSM are shown in Fig. 4 [5]. The maximum efficiency of IPMSM reaches 95%. The efficiency at low speed is more than 80% and it is about 20% and 10% higher than that of IM and SPMSM, respectively. The major operating region of air conditioners is low speeds; thus the efficiency improvement at low speed is 3.2. Stator design In order to improve efficiency more, the stator configurations are examined. The concentrated winding stator and the improvement of space factor by divided stator core are discussed. Figure 5 shows the IPMSM with the concentrated winding. Comparing Figs. 3 and 5, it is evident that the coil end of the concentrated winding becomes very short compared to that of distributed winding because the winding is directly around the stator tooth in the concentrated winding structure. As a result, the armature resistance of concentrated winding becomes smaller than that of the distributed winding, and thus copper loss decreases and high-efficiency drive can be achieved. The concentrated winding is effective to reduce the motor size and copper loss; however, it has some disadvantages caused by the harmonics of magnetic flux distribution. Such harmonics cause increase of core loss, and vibration and noise may increase, too. Moreover, the reluctance torque of IPMSM with concentrated winding will decrease compared to the distributed winding. To suppress the increase in core loss and vibration, the rotor design of the concentrated winding PMSM has been devised. For example, the difference in the design of flux barriers and magnet position can be seen from Figs. 3 and 5. The technique of reducing vibration and noise is discussed in Section 4. Figure 6 shows the comparative efficiency characteristics of IPMSMs for compressor motor with distributed winding and concentrated winding [5]. Using concentrated winding increases the efficiency, especially in low speed regions, in which copper loss is more predominant than core loss. In this case, the total loss decreases and the efficiency is improved by using concentrated winding; but the efficiency of concentrated winding IPMSM might become worse in high-speed regions, in which core loss is more predominant than copper loss. Concentrated winding is beginning to be used in place of the conventional distributed winding in many electric Fig. 5 IPMSM with concentrated winding. (Courtesy of Daikin Industries, Ltd) 103 IEEJ Trans 2: (2007)

4 S. MORIMOTO Efficiency η (%) IPMSM with concentrated winding IPMSM with distributed winding Speed (r/sec) Fig. 6 Comparative efficiency characteristics of IPMSMs with distributed winding and concentrated winding home appliances. However, excluding the situation where downsizing and thin motors are specially required, the use of the distributed winding is common in high power applications including electric vehicle (EV), hybrid electric vehicle (HEV) and traction drive Electromagnetic materials The motor performance is greatly improved by electromagnetic materials. Figure 7 shows one example of the transition of using silicon steels for compressor motors [6]. The demand for core loss decrease became a predominant factor when copper loss decreased by changing from IM to SPMSM to achieve more energy saving ((a) (b)). Copper loss has decreased further by applying IPMSM, but use of the reluctance torque caused the increase of core loss. In the early stage of the shift to IPMSM from SPMSM, the decrease of core loss was given priority, and the flux density was sacrificed ((b) (c)). In recent IPMSM, however, high saturation magnetization is demanded because Flux density B50 (T) Fig IPMSM (d) (c) 96 IPMSM (b) (a) 95 SPMSM Core loss W15/50 (W/kg) 94 IM Transition of the compressor motor for room airconditioner and the trend of using silicon steels 10 Residual magnetic flux density Br (T) Alnico Ferrite 95 SPMSM 96 IPMSM Bonded NdFeB 01 IPMSM Sintered NdFeB Coercive force Hcj (ka/m) Fig. 8 Trend of permanent magnet materials of the compressor motor the rotor design concentrating magnet flux and the concentrated stator winding are applied. Therefore the development of silicon steel that satisfies both a low core loss and a high flux density is the recent trend ((c) (d)). Figure 8 shows the trend in permanent magnet materials of the compressor motor [6]. The mass production of the PMSM for a compressor motor started in Since then a cheap ferrite magnet was applied to PMSM. IPMSM that had used the rare-earth permanent magnet (NdFeB) was mass-produced in The development of a permanent magnet material with high residual magnetic flux density and high coercivity is a recent trend. 4. Vibration and Noise Reduction The torque ripple and cogging torque of IPMSM become large compared with SPMSM because of the discontinuity in reluctance change between the rotor and stator. Moreover, the IPMSM with concentrated winding is noisier because of the stator frame deformation by the radial forces. Here, the design technique that decreases the torque ripple and reduces the noise is shown. Figure 9 shows the cross-section view of two-layer IPMSM with distributed winding. In the general design of PMSM, the magnets and the flux barriers are symmetrically arranged as shown by the broken lines in Fig. 9. In this conventional design, the relative positions between the outer edges of the flux barriers and the teeth are the same in all poles, as a result they meet and part at the same time. This situation causes a change of torque by rotor position and the torque vibration is generated. In order to reduce the torque vibration, the flux barriers are asymmetrically designed so that the relative positions between the outer edges of the flux barriers and the teeth do not correspond as shown by the solid lines in Fig. 9 [7]. Figure 10 shows the instantaneous torque of IPMSM with the symmetrical and asymmetrical rotors, where 104 IEEJ Trans 2: (2007)

5 TREND OF PERMANENT MAGNET SYNCHRONOUS MACHINES (a) Stator for concentrated winging. Flux Barrier : Symmetric Permanent Magnet : Asymmetric (NdFeB) Fig. 9 Cross-section of IPMSM with asymmetric flux barriers Flux Barrier Permanent Magnet (NdFeB) Holes Asymmetric (b) Rotor type A (c) Rtor type B (with holes) Fig. 11 Structure of stator and rotors of IPMSM Torque (Nm) Torque (Nm) Symmetric Rotor position in electrical angle (deg.) (a) Results of finite element analysis. Asymmetric Symmetric Time (s) (b) Experimental results at 6.5 min 1. Fig. 10 Instantaneous torque of IPMSM with symmetrical and asymmetrical rotors the IPMSM is controlled by the maximum torque per ampere control at the rated current. The asymmetrical flux barrier design greatly improves the torque ripple, and such effects are confirmed by the experimental results as shown in Fig. 10 (b). In this design, the torque ripple can be greatly decreased without sacrificing the average torque and without decreasing efficiency. As mentioned in Section 3.2, the vibration and noise increase by using the concentrated winding. In the concentrated winging IPMSM (Fig. 11), the radial force that intensively acts on stator causes the acoustic noise and the vibration. In order to reduce the radial force and the torque ripple, the rotor design with some holes inside the rotor is proposed [8]. Figure 11 shows the stator structure and two kinds of rotors. The holes inside the rotor (Fig. 11 (c)) decrease the equivalent opposed area of the stator and the rotor, and the magnetic flux density is reduced. Figure 12 shows the comparative characteristics of three kinds of prototype IPMSMs. The efficiency increases by using a concentrated winding, which is the same as shown in Fig. 6. The efficiency of the concentrated winding IPMSM with holes decreases at low speed region, but it is improved at high speed region as shown in Fig. 12 (a). The noise and vibration can be evaluated by vibration velocity, which is given by integrating the vibration signal of an acceleration sensor attached to the stator frame. The vibration velocity of the concentrated winding IPMSM is larger compared to the distributed winding IPMSM, but it can be reduced by making holes in the rotor (Fig. 12 (c)). Therefore, this design is effective for reducing the vibration without decrease in efficiency. 5. Traction Drive Application The motor for traction drive application has many requirements such as high torque capability up to base speed for accelerating and wider operating speed range for achieving sufficient output at the maximum speed. 105 IEEJ Trans 2: (2007)

6 S. MORIMOTO Efficiency h (%) 100 Overall values of vibration velocity (m/s) Rotor: Type A Stator: Concentrated winding Rotor: Type B Rotor: Type A Stator: Distributed winding (24 slots) Speed N (min 1 ) (a) Efficiency characteristics. Stator: Concentrated winding Rotor: Type A Stator: Distributed winding Speed N (min 1 ) (b) Vibration velocity characteristics. Rotor: TypeA Rotor: Type B Fig. 12 Experimental characteristics of three kinds of prototype IPMSMs Power P y dmin = y a - L d I am Speed N y dmin = 0 (ii) y dmin > 0 (i) y dmin < 0 Fig. 13 Typical profiles of speed versus power characteristic as a function of minimum d-axis flux-linkage The output capability of PMSM depends on the machine parameters and the control scheme [9]. The profile of the speed versus torque and power characteristics as well as the constant-power speed range depend on the minimum d-axis flux-linkage ψ dmin, which is defined by ψ a L d I am where I am is a ceiling current ampere. Figure 13 shows the typical profile of speed versus power characteristic as a function of the minimum d-axis flux linkage ψ dmin. This profile can be classified into two patterns by ψ dmin. In the case of ψ dmin < 0, the maximum operating speed is theoretically infinity but the maximum power decreases as ψ dmin decreases. In the case of ψ dmin > 0, on the other hand, a limitation of the operating speed exists but the maximum power is greater than that in the case of ψ dmin < 0. The condition of ψ dmin = 0 is optimum for an ideal constant-power operation implying that the theoretical constant-power speed range becomes infinity and the output power is enlarged as much as possible. For the application demanding a wide constant-power speed range such as a traction drive, the specific parameter ψ dmin of PMSM has to be designed close to zero. In such a design, the magnet flux linkage ψ a may become comparatively small. The high-speed constant-power operation is usually achieved by the flux-weakening control, where the terminal voltage is maintained within the ceiling voltage by using the negative d-axis armature reaction L d i d (see (2)) [9]. In this control, the danger of falling into the uncontrolled generator mode when the inverter unexpectedly stops has to be considered. The magnet flux linkage has to be designed so that the open-circuit induced voltage at the maximum speed is less than the maximum voltage rating of switching devices. Therefore, the magnet torque is limited by such constraints of the maximum open-circuit voltage, and thus an effective use of the reluctance torque by the optimal rotor design is important from a standpoint of supplementing the limited magnet torque with the reluctance torque. This is a design approach from the region II toward the region III in Fig. 2. This design is suitable for meeting the condition of ψ d min = 0, and is effective for reducing the iron loss under the light-load at high speed. Figure 14 shows the IPMSMs for the hybrid electric vehicles such as the 2000 Prius and the 2005 SUV [10]. The major points of consideration for IPMSM in the 2005 SUV are rotor mechanical strength reinforcement, iron loss reduction and effective use of reluctance torque. In the final design, two flat-shape magnets are arranged in a V-shape, and the rotor surface is dimpled to further reduce the iron loss and to minimize torque ripple as shown in Fig. 14. The ratio of reluctance torque to total torque under the maximum torque condition is significantly increased from 53% of Prius 00 to 63% of SUV 05, and the SUV 05 motor realizes efficiency above 95% over most of the operating region. Figure 15 shows the cross-section of the permanentmagnet reluctance motor (PRM) [11] developed for HEVs, which is a kind of IPMSM. Figure 16 shows the efficiency map of the PRM, ant it shows that the PRM can operate over a wide variable-speed range (1 : 5) with high efficiency (95 97%). The ratio of reluctance torque to total torque under the maximum torque per ampere condition is about 60% in the PRM. 106 IEEJ Trans 2: (2007)

7 TREND OF PERMANENT MAGNET SYNCHRONOUS MACHINES 1.0 Prius Stator Dinameter : 269mm Stack Length : 88mm Torque (p.u.) ~ 97 % 94 ~ 96 % 92 ~ 94 % 90 ~ 92 % SUV- 05 coil slot dimple air gap air magnet rotor core Stator Dinameter : 264mm Stack Length : 70mm stator core ~ 90 % Base speed Speed (p.u.) Fig. 16 Efficiency map of PRM Wedge Axis Vent hole Fig. 14 IPMSMs for the 2000 Prius and the 2005 SUV Stator Stator core Permanent magnet Permanent magnet Core Rotor core Fig. 17 Rotor cross-section of traction motor for a high speed train Fig. 15 Rotor Cross-section of permanent-magnet reluctance motor (PRM) any rotor copper loss is not generated and the mechanical loss and iron loss are reduced in PMSM. 6. Conclusions The reluctance torque ratio of the above-mentioned IPMSMs for HEVs is more than 50%, and such machines are located in region III in Fig. 2. The IPMSM of the traction motor for a high speed train is examined, and compared with the conventional induction motor. Figure 17 shows the rotor cross section of the developed IPMSM [12], where the rated rotational speed is 4140 min 1, the rated output power is 300 kw, the maximum voltage is 2300 V, the constant power speed range is 2. It is reported that the PMSM is lighter, efficient and more silent than the IM. The calculated total loss of PMSM was about the half of that of IM, because This paper described recent technology and the trend of PMSMs. In particular the optimal design of PMSM for efficiency improvement and vibration reduction and the traction drive application were explained. The application of PMSM has expanded to various fields. The application range of PMSM is expanding to much higher power, for example the PMSM of about 3000 kw for electric propulsion of a ship and the PM synchronous generator of about 2000 kw for gearless wind turbine generator have been developed. The performance of PMSM is greatly improved by further progress in elemental technologies such as electromagnetic material technology. Expansion of further applications of PMSMs is expected. 107 IEEJ Trans 2: (2007)

8 S. MORIMOTO References (1) Morimoto S, Takeda Y, Murakami H. Electric motors for home applications development of environment-friendly electric motors. EPE Journal 2004; 14(1): (2) Sadahiro K, Mogi H, Kaneko Y, Kakuno K. The state of the art of reluctance Torque assisted motors-recent trends in electromagnetic materials and their applications and evaluation methods. Proceedings of 2004 IEEJ Industry Applications Society Conference, 2004; III-7 III-12(in Japanese). (3) Honda Y, Higaki T, Morimoto S, Takeda Y. Rotor design optimization of a multi-layer interior permanent-magnet synchronous motor. IEE Proceedings-Electric Applications 1998; 145(2): (4) Ohyama K, Kosaka M. High efficiency control for interior permanent magnet synchronous motor. Proceedings of IPEC- Tokyo 2000, 2000; (5) Ohyama K. Idea of the reluctance torque assisted motors application. Proceedings of 2006 IEEJ Industry Applications Society Conference, 2006; III-7 III-12 (in Japanese). (6) Ohyama K, Morimoto S, Takeda Y, Dohmeki H. The state of the art of reluctance torque assisted motors. Proceedings of 2004 IEEJ Industry Applications Society Conference, 2004; III-3 III-6 (in Japanese). (7) Sanada M, Hiramoto K, Morimoto S, Takeda Y. Torque ripple improvement for synchronous reluctance motor using an asymmetric flux barrier arrangement. IEEE Transactions on Industry Applications 2004; 40(4): (8) Kabayashi T, Takeda Y, Sanada M, Morimoto S. Vibration reduction of IPMSM with concentrated winding by making holes. IEEJ-Transactions on Industry Applications 2004; 124(2): (in Japanese). (9) Morimoto S, Takeda Y, Hirasa T, Taniguchi K. Expansion of operating limits for permanent magnet motor by current vector control considering inverter capacity. IEEE Transactions on Industry Applications 1990; IA-26(5): (10) Kamiya M. Development of traction drive motors for the toyota hybrid system. IEEJ Transactions on Industry Applications 2006; 126(4): (11) Sakai K, Hagiwara K, Hirano Y. High-power and high-efficiency permanent-magnet reluctance motor for hybrid electric vehicles. Toshiba Review 2005; 60(11):41 44 (in Japanese). (12) Kondou M, Kawamura J, Terauchi N. Performance comparison between a permanent magnet synchronous motor and an induction motor as a traction motor for high speed train. IEEJ Transactions on Industry Applications 2006; 126(2): (in Japanese). Shigeo Morimoto (Senior Member) Shigeo Morimoto received the B.E., M.E. and Ph.D degrees from Osaka Prefecture University, Sakai, Japan, in 1982, 1984, and 1990, respectively. He joined Mitsubishi Electric Corporation, Tokyo, Japan, in Since 1988, he has been with the College of Engineering at Osaka Prefecture University, where he is currently a Professor. His main areas of research interest are permanent magnet synchronous machines, reluctance machines and their control systems. Dr. Morimoto is a member of the IEEE, the Society of Instrumental and Control Engineers of Japan, the Institute of Systems, Control and Information Engineers, and the Japan Institute of Power Electronics. 108 IEEJ Trans 2: (2007)