energies Article Design and Analysis of Rotor Shapes for IPM Motors in EV Power Traction Platforms Myeong-Hwan Hwang 1,2, Jong-Ho Han 1, Dong-Hyun Kim 1 and Hyun-Rok Cha 1,* 1 EV Components & Materials R&D Group, Korea Institute of Industrial Technology, 6 Cheomdan-gwagiro 208 beon-gil, Buk-gu, Gwangju 61012, Korea; [email protected] (M.-H.H.); [email protected] (J.-H.H.); [email protected] (D.-H.K.) 2 Department of Electrical Engineering, Chonnam National University, 77 Youngbong-ro, Buk-gu, Gwangju 61186, Korea * Correspondence: [email protected]; Tel.: +82-62-600-6212 Received: 7 September 2018; Accepted: 27 September 2018; Published: 29 September 2018 Abstract: The recent increase in the use of permanent magnet rotor motors underlines the importance of designing a rotor with an interior permanent magnet (IPM) structure, high power, and high efficiency. This study analyzed the rotor shapes of IPM motors for electric vehicles. Five types of motor rotors for automobiles were analyzed, including two hybrid vehicles. In order to minimize the number of variables in the analysis, the size of the motor stators was fixed and only the rotor shapes were modified to compare torque, torque ripple, efficiency and back-electromotive voltage. When the motor properties were compared as a function of rotor shape, the rotor shape with the smallest magnet volume exhibited excellent results for torque, efficiency and torque ripple. Keywords: traction motor; electric vehicle; interior permanent magnet; vehicle motor; electromagnetic field analysis; cogging torque 1. Introduction Various regulations in the car industry are being introduced to respond to environmental changes caused by global warming [1]; for example, electrically powered powertrains have been developed to reduce exhausted gas and improve the power efficiency of vehicles. As a result, the demand for electrical vehicles, which are recognized as environmentally friendly, continues to surge [2]. Electric traction motors, which represent the most essential of technologies among this changeover, are needed owing to their wide velocity, high power density, and high efficiency [3–5]. With advances in electrical machines and recent control technologies, AC machines have become mainstream (as opposed to DC machines) and now dominated the traction machine market. Both synchronous and asynchronous AC machines are used in commercially available electric-powered vehicles. Induction machines contain the most mature manufacturing technologies, those that that have been developed over a number of decades [5,6]. Induction machine technologies are also relatively lower in cost and offer easier control. However, the conductors of the rotor increase the rotor copper losses and also the cooling requirement. The conductor usually applies to a lower operating efficiency as compared with permanent magnet synchronous machines (PMSM); therefore, the majority of automotive manufacturers are choosing appropriate PMSM for the traction motor in the vehicle [7]. Among the electric traction motors, interior permanent magnet (IPM) motors, which include rotors with embedded magnets, are increasingly being used as the driving motors for electric vehicles. The advantages of an IPM motor include its wide velocity and torque variation, high power, light weight, and energy efficiency. The most influential factor affecting the performance of IPM motors is Energies 2018, 11, 2601; doi:10.3390/en11102601 www.mdpi.com/journal/energies Energies 2018, 11, x 2 of 12 Energies 2018, 11, 2601 2 of 12 power, light weight, and energy efficiency. The most influential factor affecting the performance of IPM motors is the rotor shape. Therefore, determining an appropriate IPM rotor shape is essential forthe designing rotor shape. a highly Therefore, functional determining driving anmoto appropriater for electrical IPM vehicles rotor shape (EVs) is essential[8–11]. for designing a highlyAs functional the importance driving of motor IPM motors for electrical for EVs vehicles continues (EVs) to [ 8increase,–11]. various rotor structures have been Asproposed. the importance Only a few of IPMstudies motors havefor comprehensively EVs continues compare to increase,d the various rotor shapes rotor structures of IPM motors have inbeen order proposed. to establish Only a adesign few studies standard. have Accordingly, comprehensively this study compared attempted the rotor to identify shapes an of appropriate IPM motors shapein order for to an establish IPM rotor a design for standard.EVs by analyzing Accordingly, three this models study attemptedof major EV to identify motor anmanufacturers appropriate currentlyshape for anin mass IPM rotorproduction, for EVs as by well analyzing as two three hybrid models models. of major Based EV on motor the results manufacturers of this study, currently we planin mass to production,present a design as well standard as two hybrid for rotor models. shapes Based that on appropriately the results of thisreflects study, the we requirements plan to present of EVsa design [12]. standard for rotor shapes that appropriately reflects the requirements of EVs [12]. HoHowever,wever, thethe criticalcritical problemproblem is is that that analysis analysis and and improvement improvement could could waste waste both both time time and cost.and costTherefore,. Therefore, the magnet the magnet arrangement arrangement of high-power of high IPM-power rotor IPM shape rotor should shap bee should analyzed be andanalyzed developed and developedto reduce time to reduce and cost. time The and torque cost. T andhe torque rotation and speed rotation were speed analyzed were according analyzed to according rotor shape. to rotor Each shape.motor statorEach motor had the stator same had size the and same winding, size and with winding, a motor rotorwith ofa motor the same rotor size, of butthe withsame a size, different but witharrangement a different of magnetsarrangement [13]. of magnets [13]. MManyany studies of rotor shape have aim aimeded to reduce design and analysis time [1 [144]].. However, However, t thishis study focusedfocused onon timetime reduction reduction when when selecting selecting the the design design concept. concept. We believeWe believe that thethat results the results of study of studywill improve will improve design efficiencydesign efficiency by decreasing by decreasing the initial the design initial time design and enhancingtime and torque,enhancing power, torque, and power,efficiency, and depending efficiency, on depending the shape on of the IPM shape rotor. of IPM rotor. The remainder of this paper is organized as follows. T Thehe m mainain parameter parameterss of of the the IPM motor rotor andand fivefive analysisanalysis rotor rotor models models are are introduced introduced in Sectionin Section2, followed 2, followed by a by comparison a comparison of the of basic the basiccharacteristics characteristics of each of rotor each inrotor Section in Section3. The torque 3. The andtorque efficiency and efficiency of rotors areof rotors analyzed, are analyzed, in Section 4in. SectionFinally, 4. conclusions Finally, conclusions are given inare Section given5 in Section 5 22.. Analysis Analysis Model Modelss of IPM Motor Motorss To analyze the the characteristics characteristics of of IPM IPM m motorsotors as as a afunction function of ofrotor rotor shape, shape, the the outer outer diameter diameter of eachof each rotor rotor was was fixed fixed to ensure to ensure the thesame same conditions conditions and and to minimize to minimize the thenumber number of variables of variables.. In addition,In addition, the the magnet magnet arrangement arrangement of ofthe the rotor rotor was was adjusted adjusted to to standardize standardize the the size size and and shape shape of of the the stator and the windings windings so that that the the properties properties were were analyzed analyzed under under similar similar conditions. conditions. To To en ensuresure the accuracy of the analysis using different magnet arrangements and rotor shapes, the rotor shapes and ratios typical of actual cars were maintained as much as possible. Rotor shapes,shapes, shown shown in Figurein Fig1ure, are 1, general are general shapes, andshapes, this studyand analyzedthis study each analyzed characteristic each characteristicdepending on depending shape. Additionally, on shape. Figure Additionally,1d,e was Figure included 1d,e and w analyzed,as included because and analyzed, the V-shape because has a thesmaller V-shape magnet has volumea smaller than magnet other volume general than shapes other [15 general]. shapes [15]. (a) (b) (c) (d) (e) Figure 1. InteriorInterior Permanent Permanent Magnet Magnet Synchronous Synchronous Motor Motor Design Design Trend: Trend: ( (aa)) V V-shaped-shaped rotor from manufacturer T; ((bb)) doubledouble magnetmagnet shapeshape from from manufacturer manufacturer V; V; (c ()c delta) delta shape shape from from manufacturer manufacturer V; V;(d) ( hybridd) hybrid delta delta shape shape based based on the on Vthe shape V shape from from both manufacturerboth manufacturer T and T manufacturer and manufacturer N; (e) hybridN; (e) hybriddouble double V shape V from shape both from manufacturer both manufacturer T and manufacturer T and manufacturer V. These fiveV. These shapes five have shapes been previouslyhave been previouslyanalyzed [16 analyzed]. [16]. 22.1..1. S Statortator M Modelodel for A Analyzingnalyzing Rotor P Propertiesroperties Table 11 presents presents thethe specificationsspecifications