Design and Flux-Weakening Control of an Interior Permanent Magnet Synchronous Motor for Electric Vehicles

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1 Ths artcle has been accepted for publcaton n a future ssue of ths journal, but has not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 1.119/TASC , IEEE > Paper ID-56< 1 Desgn and Flux-Weakenng Control of an Interor Permanent Magnet Synchronous Motor for Electrc Vehcles Yue Zhang, Student Member, IEEE, Wenpng Cao, Senor Member, IEEE, Sean McLoone, Senor Member, IEEE and John Morrow, Member, IEEE Abstract Permanent magnet synchronous motors (PMSMs) provde a compettve technology for EV tracton drves owng to ther hgh power densty and hgh effcency. In ths paper, three types of nteror PMSMs wth dfferent PM arrangements are modeled by the fnte element method (FEM). For a gven amount of permanent magnet materals, the V shape nteror PMSM s found better than the U-shape and the conventonal rotor topologes for EV tracton dr ves. Then the V shape nteror PMSM s further analyzed wth the effects of stator slot openng and the permanent magnet pole chamferng on coggng torque and output torque performance. A vector-controlled fluxweakenng method s developed and smulated n matlab to expand the motor speed range for EV drve system. The results show good dynamc and steady-state performance wth a capablty of expandng speed up to tmes of the rated. A prototype of the V shape nteror PMSM s also manufactured and tested to valdate the numercal models bult by the fnte element method. Index Terms Electrcal vehcles (EVs), fnte element method (FEM), permanent magnet synchronous motors (PMSMs), torque rpple, flux weakenng. E I. INTRODUCTION LECTRIFIED VEHICLES (EVs) have been consdered as a major techncal advancement over conventonal fossl fuel based vehcles, because of ther low emssons and hgh effcency [1]-[]. At the heart of EV drvetran s the electrcal motor, whch converts electrcty (generally from a battery) nto mechancal energy to drve the wheels. The motor drves can be based on drect-current (DC) motors, nducton motors (IMs), swtch reluctance motors (SRMs), permanent magnet synchronous motors (PMSMs), swtched flux permanent magnet motors (SFPMMs) []. In the lterature, dfferent motor topologes for EVs are compared [5]-[8] and results show that PMSMs have the best performance n terms of system effcency, power densty and power-speed range whle ther drawbacks are assocated wth lmted reserve and Automatcally generated dates of recept and acceptance wll be placed here; authors do not produce these dates. Ths work was supported by Royal Socety, UK and Natonal Natural Scence Foundaton, Chna under Grant (Correspondng author: Yue Zhang.) Yue Zhang, Sean McLoone and John Morrow are wth the School of Electroncs, Electrcal engneerng and Computer Scence, Queens Unversty Belfast, Belfast, BT9 5AH, UK (e-mal: yzhang5@qub.ac.uk). Wenpng Cao s now wth the Power Electroncs, Machnes, and Power System group, Aston Unversty, Brmngham, B 7ET, UK supply stablty of the rare-earth materals they rely on. Although both nteror PMSMs and surface mounted PMSMs can be utlzed for EV tracton drves, the former are more sutable for EV tracton drves [9] as they can offer hgher relablty and overload ablty than the latter. Wthn nteror PMSMs, There are varous rotor topologes avalable, whch offer some flexblty and desgn freedom wth regard to the placement of permanent magnets. Rotor topologes and parameters can lead to dfferng performance of EV motor drves. A mult-objectve optmzaton approach for the nteror PMSM desgn s proposed n [1], wth the response surface methodology ntroduced to dramatcally save the CPU-tme wthout de-gradng accuracy. Moreover, torque rpple reducton of nteror PMSMs has also attracted much attenton as nose and vbraton can be ssues that affect drve comfort. An optmzaton of rotor slot geometry s proposed n [11]. Rotor pole skewng s adopted n [1] and the current harmonc njecton s employed n [1]. Alternatvely, a genetc algorthm approach wth effcent FEM s proposed to mnmze coggng torque for PMSMs [1]. For EV tracton applcatons, a wde speed range s hghly desrable. Interor PMSMs can extend ther operatonal speed range by weakenng ther magnetc flux (constant power operaton). Moreover, other methods proposed n the lterature nclude magnet segmentaton and rotor ron brdge dmenson optmzaton. Both are proved to be effectve n mprovng the motor s flux weakenng capablty [15]. In ths paper, three types of nteror PM topologes are analyzed and compared. Based on the same amount of permanent magnet materal used, the s altered to fnd the most sutable nteror PMSM topology for EV tracton drves. The magnetc cross- couplng effect has been taken nto consderaton to ncrease the calculaton precson. The PMSM wth a V-shape rotor magnet s chosen and researched further (e.g. nductance, slot openng, permanent magnet chamferng) to compare the effectveness of torque rpple reducton. Then a flux-weakenng vector-control drve system s bult n Matlab to provde a control scheme for hgh-speed operaton. A V-shape rotor PMSM s fnally prototyped and expermentally tested to verfy the developed numercal models. II. ROTOR TOPOLOGIES For PMSMs, the permanent magnet can be postoned n the rotor lamnatons as U shape, V shape and conventonal (c) 16 IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.

2 Ths artcle has been accepted for publcaton n a future ssue of ths journal, but has not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 1.119/TASC , IEEE > Paper ID-56< shape. Three typcal motor topologes are shown n Fg.1, whle the motor specfcatons are lsted n Table I. The followng assumptons are made n order to compare dfferent motor topologes: ) the stator, shaft, ar gap and wndngs are dentcal; ) the same amount of permanent magnet materal s utlzed; ) the dstance between the rotor slot (whch contanng the permanent magnet) outer edge and the shaft center s dentcal (namely, same magnetc nsulaton brdge sze). performance, especally at hgh-speed flux-weakenng condtons [17]. For the three nteror PMSMs, a current source s employed as power supply and the under dfferent d axs and q axs current are obtaned, as shown n Fg. to Fg.. U shape rotor motor -1 q(a) (c) Fg. 1. Magnetc feld dstrbuton at no-load condton U shape rotor Vshape rotor (c) Conventonal shape rotor. TABLE I M OTOR SPECIFICATIONS Parameter Value Power Rated Voltage (ampltude) Rated speed No. of pole pars Stator outer dameter Stator nner dameter Rotor outer dameter Stack length kw 85 V rpm 1 mm 18 mm 16.6 mm 88 mm Fg.. V shape rotor motor Fg.. conventonal shape rotor motor q(a) 1 q(a) In general, nteror PMSMs can be analyzed under the rotatng synchronous frame, as the stator wndngs are energzed by d axs current d and q axs current q respectvely, The motor performance can be descrbed by the nductances L d and L q. Due to the magnetc cross- couplng effect between the magnetc flux path n the d axs and q axs, accurate calculaton of nductance parameters should take t nto consderaton. Therefore the FEM can be utlzed as below to obtan nductance parameters: ( ) (, ) (1) PM q d d q d ( d, q ) PM ( q ) Ld( d, q) () d q( d, q ) Lq ( d, q ) () where Ψ d and Ψ q are the flux lnkage n d axs and q axs, respectvely, and Ψ PM s the flux lnkage excted by the permanent magnet alone [16]. The salency of the nteror PMSM can be descrbed by the, whch s the rato of the q axs nductance L q over the d axs nductance L d. The salency of the rotor can drastcally change the motor q It can be observed that the s affected by both d and q wth smlar trends but at a dfferent level due to the dfference n magnetc path n the three motors. The salent rate decreases as q axs current ncreases, however, such trend s not obvously observed when a hgh d-axs current s appled. Ths s owng to the magnetc couple effect between the d-axs and q-axs magnetc feld. At almost all the workng ponts, the V-shape rotor motor has the hghest, ndcatng the largest salency torque when the same power supply s appled. The V-shape nteror PMSM can be controlled wth a wde constant-power regon, whch s hghly valued n EV tracton drves. III. TORQUE RIPPLE REDUCTION It s deal that EV tracton motors have low coggng torque, torque rpple and hgh power densty. The coggng torque n PMSMs s formulated as [18]: zl ef Tcog ( ) ( R R1) ngnb nz snnz () r n1 p Where αs the rotaton angle, z s the slot number, L ef s the axal length of armature, R 1 s the outer radus of rotor, R s the nner radus of armature, G n s the n th Fourer transform (c) 16 IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.

3 Ths artcle has been accepted for publcaton n a future ssue of ths journal, but has not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 1.119/TASC , IEEE > Paper ID-56< Lad (mh) coeffcent of the permanent magnet geometry parameter, B r s the ar gap flux densty n an equvalent slotless motor. An analytcal method based on a vrtual PM concept and the superposton prncple for calculaton of the coggng torque of nteror PMSMs wth mult flux barrers n the rotor s llustrated n [19]. Whle the PM motor torque rpple can be expressed as []: P T r l * ( hf f sn(( h 1) t )) (5) rpple g stk s, h r, h e d g h6m 1 m1,,... Where P s the number of poles, r g s the ar gap radus, l stk s the stack length, f s,h, f r,h are the h th stator and rotor MMF harmoncs, respectvely. It can be observed that the motor torque rpple s relatve to the hgh order harmonc components n the MMF. The V-shape rotor nteror PMSM s analyzed by FEM and the motor wndng d axs and q axs nductances at rated load n one electrcal perod are shown n Fg Laq (mh) Fg. 5. The d- and q- axs nductances. d-axs nductance. q-axs nductance As the exstence of harmonc components n nductances, the torque rpple s always present. In order to decrease the coggng torque and torque rpple for the V-shape nteror PMSM, the stator and rotor parameter confguratons can be adjusted accordngly. A. Effect of Stator Slot Openng The coggng torque (peak to peak), torque rpple (peak to peak) and average torque of the V-shape nteror PMSM at rated voltage as a functon of slot openng wdth are presented n Fg.6. Torque (N*m) T(coggng)P-P T(rpple)P-P T(avg) Slot openng wdth (mm) Fg. 6. Varaton of coggng torque, torque rpple and average torque As the slot openng wdth decreases, both the coggng torque and torque rpple n nteror PMSM are reduced. Clearly, the slot openng wdth mpacts manly on coggng torque. For nstance, the coggng torque reduces by 9% (1.5Nm at.6 mm slot open wdth and 15.6Nm at the.mm s lot open wdth) whle the torque rpple reduces by 7% (.8Nm at.6mm slot open wdth and.9nm at the.mm slot open wdth). The average torque s slghtly affected by the ncrease n the slot openng wdth and t gves rse to the magnetc path reluctance. B. Magnetc Pole Chamferng The pole chamferng method for each chunk of permanent magnets for the V-shape nteror PMSM s shown nfg.7, where a denotes the pole chamferng wdth: Fg. 7. Magnetc pole chamferng By changng the chamferng wdth, the average torque, torque rpple (peak to peak) and coggng torque (peak to peak) can be altered, as shown n Fg.8. T orque (N*m) a (mm) T(coggng)P-P T(rpple)P -P T(avg) Fg. 8. Varaton of coggng torque, torque rpple and average torque From Fg.8 t can be seen that the pole chamferng can also mpacts on the coggng torque and especally the torque rpple. By changng a from to 5mm, the torque rpple can be reduced by.n*m. As the average torque s also affected, t s necessary to strke a balance between the output torque and torque rpple when adoptng the pole chamferng method for nteror PMSMs. IV. FLUX-WEAKENING CONTROL Due to the lmtaton of the converter capablty, flux weakenng s a commonly-accepted scheme to expand the speed range beyond rated for nteror PMSMs. Ths s acheved by adjustng the d current to cancel out some magnetc feld produced by permanent magnets. In ths paper, a vector control drve system s desgned and smulated n Matlab. It conssts of two operatonal modes: constant torque mode and constant power mode. The constant torque mode s for the motor runnng at a speed lower than the rated speed ( rpm n ths case). The motor s controlled to generate the maxmum torque wth the least current below ths speed whle the constant power mode s for the motor runnng at speeds hgher than the rated speed (flux weakenng operaton). The developed control drve system s schematcally shown n Fg. 9: (c) 16 IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.

4 Ths artcle has been accepted for publcaton n a future ssue of ths journal, but has not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 1.119/TASC , IEEE > Paper ID-56< 1 PI - u S * * d u d S - - PI * * q u q 6 PI - + d 5 u d u q e q 7 e SVPWM 8 a b Inverter c IPMSM Fg. 9. The PMSM flux weakenng control. 1) Reference speed. ) Flux weakenng block. ) Voltage calculaton. ) Decouplng block. 5) Integraton. 6) Inverse Park transform. 7) Clark transform. 8) Park transform..6s, the load returns to 1Nm and the reference speed s back to rpm. The correspondng motor responses are shown n Fgs It can be seen that f the motor speed s below the flux weakenng speed rpm, the offset flux weakenng current s zero. As the motor speed s hgher than rpm, the flux weakenng block acts dynamcally and produces the flux weakenng current through the PI controller based on actual motor speed. The drve system operates n flux weakenng mode to expand the speed range. The whole control drve system has shown good dynamc and steady performance n response to speed and load varatons wth capablty of expandng the speed range up to tmes, whch s suted for EV tracton drves. Torque (N*m) Fg. 1. Torque response V. EXPERIMENT The back electromotve force (EMF) s a key parameter to evaluate the motor effcency and torque performance. So the dstance between the two magnets of neghborng poles (denoted L n Fg. 1) can be adjusted. Table Ⅱshows the EMF fundamental component ampltude and ts proporton n the whole spectrum as a functon of L. Speed (rpm) Fg. 11. Speed response. Offset flux weaken current (A) Fg. 1. Offset flux weakenng current. The PMSM drve s controlled va two closed loops: a speed loop and a current loop; a specal PI controller s utlzed n the flux weakenng block: f the motor reference speed s lower than the flux weakenng speed (rated speed), the PI controller saturates wth no offset flux weakenng current produced. Once the actual motor reference speed s hgher than the flux weakenng speed, a negatve flux weakenng current s produced by the PI controller to reduce the flux as per the speed extenson requrements. The offset flux weakenng current s a current added to the d axs current to decrease the flux densty for the motor under flux-weakenng operaton. At s, the load s 1Nm and the reference speed s set at rpm. At.1s the motor load decreases to 5Nm and the reference speed s up to rpm. After.s, the reference speed s set at 8rpm and the load s decreased to 5Nm. At Fg. 1. Dstance between a neghborng par of poles TABLE Ⅱ M OTOR EMF FUNDAMENTAL COMPONENT PROPORTION L(mm) 5 6 Fundamental 66.V 69.9V 71.97V 7.9V 77.1V ampltude (V) Proporton 97.81% 97.% 96.6% 95.85% 95.% It can be seen that, as L ncreases, the fundamental ampltude ncreases but ts proporton n the whole spectrum decreases. So a compromse should be acheved. Moreover, L cannot be set too large as t may decreases the length of magnetc nsulaton brdge and reduces the rotor core mechancal strength n hgh speed operaton. Followng a numercal study, a kw V-shape nteror PMSM for EVs s prototype. The photographs of ts stator and rotor are shown n Fg. 1. Fg. 1. Photographs of the prototype machne. The stator. The rotor. Fg. 15 presents the measured phase back EMF over one electrcal cycle under rated speed under no load, as compared to numercal results by FEM. It s observed that the two results agree well wth each other; whle Fg. 15 shows (c) 16 IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.

5 Ths artcle has been accepted for publcaton n a future ssue of ths journal, but has not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 1.119/TASC , IEEE > Paper ID-56< 5 the fundamental ampltudes of measured EMFs at dfferent motor speeds. Clearly, expermental tests have valdated numercal models used for motor performance analyss. Voltage (V) Experment result Smulaton result Fg.15. Expermental results Comparson of expermental and numercal results for EMF EMF fundamental ampltude under dfferent speed VI. CONCLUSION In ths paper, the salency of three dfferent nteror PMSM topologes consderng the magnetc cross-couplng effect s nvestgated. Smulaton results suggest that V-shape nteror PMSMs have a hgher than other types for EVs. The torque rpple s next nvestgated by FEM and t can be mnmzed by shortenng the slot openng wdth and magnetc pole chamferng. Smlarly, the coggng torque can be dramatcally suppressed by reducng the slot openng. Moreover, the motor capablty to expand the speed range up to tmes has been acheved by the desgned flux weakenng drve system. The effectveness of the smulaton models has been also expermentally valdated by testng a prototype motor. tran, IEEE Trans. Appl. Supercond., vol. 1, no., Jun., pp [11] U. Seo, Y. Chun and J. Cho, A technque of torque rpple reducton n nteror permanent magnet synchronous motor, IEEE Trans. Mag, vol. 7, no.1, Oct. 11, pp. -. [1] W. Q. Chu and Z. Q. Zhu, Reducton of on load torque rpples n permanent magnet synchronous machne by mproved skewng, IEEE Trans. Mag, vol. 9, no.7, Jul. 1, pp [1] G. H. Lee, S. I. Km and J. P. Hong, Torque rpple reducton of nteror permanent magnet synchronous motor usng harmonc njected current, IEEE Trans. Mag, vol., no.6, June. 8, pp [1] S. L. Ho, N. Chen and W. Fu, An optmal desgn method for the mnmzaton of coggng torques of a permanent magnet motor usng FEM and Genetc algorthm, IEEE Trans. Appl. Supercond., vol., no., Jun. 1, pp [15] S. Duan, L. Zhou and J. Wang, Flux weakenng mechansm of nteror permanent magnet synchronous machnes wth segmented permanent magnets, IEEE Trans. Appl. Supercond., vol., no., Jun. 1, Art. ID [16] G. Q, J. T. Chen and Z. Q. Zhu, Influence of skew and cross couplng on flux weakenng performance of permanent magnet brushless AC machnes, IEEE Trans. Mag, vol. 5, no.5, May. 9, pp [17] K. Km, J. Lee and H.Km Mult-objectve optmal desgn for nteror permanent magnet synchronous motor, IEEE Trans. Mag, vol. 5, no., Mar. 9, pp [18] D. Wang, X. Wang and Y. Y, Optmzaton of magnetc pole shftng to reduce coggng torque n sold rotor permanent magnet synchronous motors, IEEE Trans. Mag, vol. 6, no.5, May. 1, pp [19] J.Seo and H. Cho, Analytcal modelng for calculatng coggng torque n nteror permanent magnet machne wth mult flux-barrers, IEEE Trans. Appl. Supercond., vol., no., Jun. 1, Art. ID. 59. [] S. Han, T. Johns, Torque rpples reducton n nteror permanent magnet synchronous machnes usng stators wth odd number of slots per pole par, IEEE Trans. Ene Cov, vol. 5, no.1, Jan. 1, pp REFERENCES [1] C. C. Chan, The state of the art of electrc, hybrd and fuel cell vehcles, Proc. IEEE, vol. 95, no., Apr. 7, pp [] C. C. Chan, A. Bouscayrol, and K.Chen, Electrc, Hybrd, and fuel cell vehcles: Archtectures and modelng, IEEE Trans. Veh. Technol. vol. 59, no., Feb. 1, pp [] V.T. Buyukdegrmenc, A. M. Bazz, and P.T. Kren, Evaluaton of nducton and permanent magnet synchronous machne usng drve-cycle energy and loss mnmzaton n tracton applcatons, IEEE Trans. Ind. Appl, vol. 5, no.1, Feb. 1, pp. 95. [] Z. Q. Zhu and D. Howe, Electrcal machnes and drves for EV electrc, hybrd and fuel cell vehcles Proc. IEEE, vol. 95, no., Apr. 7, pp [5] X. Lu, Z. Q. Zhu, and D.Wu, Evaluaton of effcency optmzed varable flux reluctance machne for EVs/ HEVs by comparng wth nteror PM Machne, n Proc. IEEE ICEMS, Hangzhou, Chna, 1, pp [6] J. Joss, M. Popesuc, and D.Staton, A Comparson of an nteror permanent magnet and copper rotor nducton motor n a hybrd electrc vehcle applcaton, n Proc. IEEE IEMDC, Chcago, USA, 1, pp. 5. [7] G. Pellegrno, A. Vagat, B. Boazzo and P. Guglelm, Comparson of nducton and PM synchronous motor drves for EV applcaton ncludng desgn examples, IEEE Trans. Ind. Appl, vol. 8, no.6, Nov. 1, pp.. [8] D. G. Dorrell, A. M. Knght and M. Popescu Comparson of dfferent motor desgn drves for hybrd electrc vehcles, n Proc. IEEE ECCE, Atlanta, USA, 1, pp [9] J. H. Seo and H. S. Cho, Coggng torque calculaton for IPM havng sngle layer based on magnetc crcut model, IEEE Trans. Mag, vol. 5, no.1, Oct. 1, Art. ID 811. [1] Y. Fujshma, S. Wakao and M. Kondo, An optmal desgn of nteror permanent magnet synchronous motor for the next generaton commuter (c) 16 IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.