Comparative Study on Novel Dual Stator Radial Flux and Axial Flux Permanent Magnet Motors

Comparative Study on Novel Dual Stator Radial Flux and Axial Flux Permanent Magnet Motors WithFerrite Magnets for Traction ApplicationWenliang Zhao1,Thomas A.Lipo2,Life Fellow,IEEE,and Byung-Il Kwon11Department of Electronic Systems Engineering,Hanyang University,Ansan426-791,Korea2Department of Electrical and Computer Engineering,University of Wisconsin-Madison,Madison,WI53706-1691USA This paper proposes two novel traction motors with ferrite magnets for hybrid electric vehicles(HEVs),which have competitive torque density and efficiency as well as operating range with respect to a referenced rare earth magnet motor employed in the third-generation Toyota Prius,a commercialized HEV.The two proposed traction motors,named as dual stator radialflux permanent magnet motor(DSRFPMM)and dual stator axialflux permanent magnet motor(DSAFPMM),adopt the same design concept, which incorporates the unaligned arrangement of two stators together with the use of spoke-type magnet array and phase-group concentrated-coil windings for the purpose of increasing torque density and reducing torque ripple.Afinite element method is utilized for predicting the main characteristics,such as back electromotive force,cogging torque,electromagnetic torque,iron loss, and efficiency in both of the proposed motors.Moreover,a comparative study between the proposed DSRFPMM and DSAFPMM is performed under the same operating condition.As a result,it is demonstrated that both of the proposed ferrite permanent magnet motors could be good alternatives for traction application,replacing the rare earth magnet motors.Index Terms—Axialflux permanent magnet motor(AFPMM),dual stator,ferrite permanent magnet motors,finite element method(FEM),hybrid electric vehicles(HEVs),radialflux permanent magnet motor(RFPMM),torque density,traction motors.I.I NTRODUCTIONR ECENTLY,the demand for electrical propulsion systems with high torque density,high efficiency,and low cost for hybrid electric vehicles(HEVs)is becoming more and more urgent due to environmental and energy considerations. In order to satisfy the demand,permanent magnet synchronous motors(PMSMs)with rare earth magnets have been primarily employed as the main traction motors,benefitting from the excellent properties of rare earth magnets.However,the rare earth materials,such as neodymium and dysprosium,in rare earth magnets exhibit the problem of high cost and limited supply,which introduces a serious challenge for the mass production of HEVs.Therefore,continuation of research for development of high-performance synchronous motors with less or no rare earth magnets is of importance. Accordingly,several substitute motors with a reduced amount or free of rare earth magnets have been investigated [1]–[3].Compared with induction motors and switched reluc-tance motors,PMSMs with ferrite magnets are thought to be better candidates,considering their efficiency and vibration properties,and especially their feature of making possible use of both reaction torque and reluctance torque.However, conventional ferrite magnet motors generally have much poorer torque/power density than rare earth magnet motors due to the lesser properties of ferrite PMs.Although some improved ferrite PM motors,such as a motor with a segmented rotor structure in[3],spoke-type motor with segmented wing-shaped PMs in[4],as well as spoke-type motor with two-layer PMs per pole in[5],have been proposed for replacing rare earth magnet motors,their performance in terms of torque/power density,efficiency,andfield-weakening capa-bility,as well as irreversible demagnetization durability,are restricted when compared with the target performance requiredManuscript received March7,2014;revised April26,2014;accepted May30,2014.Date of current version November18,2014.Corresponding author:B.-I.Kwon(e-mail:bikwon@hanyang.ac.kr).Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/TMAG.2014.2329506Fig.1.Design concept.to replace the rare earth magnet motors under the same operation condition.In[6],a novel structure for an axialflux PM motor(AFPMM)incorporating the unaligned arrangement of two stators together with the use of a spoke-type permanent magnet array and phase-group concentrated-coil windings was explored for the purpose of increasing torque density and reducing torque ripple.In this paper,the idea is extended to radialflux PM motors(RFPMMs)and a comparative study is performed between a dual stator RFPMM(DSRFPMM)and dual stator AFPMM(DSAFPMM).As traction motors using ferrite permanent magnets,the proposed two motors are designed to be competitive with the target performance values from the rare earth magnet motor mounted in a commercialized HEV of the third-generation Toyota Prius[1].The investigation of motor performance is based on the same outer diameter and axial length,as well as the same ratings such as rated speed and power,as those of the referenced rare earth magnet motor.The main characteristics, such as back electromotive force(EMF),cogging torque,elec-tromagnetic torque,iron loss,and efficiency,of both proposed motors are predicted by thefinite element method(FEM).II.M OTOR T OPOLOGIES AND O PERATING P RINCIPLE A.Basic Design RulesBased on the design concept as shown in Fig.1,a specific combination of stator slots and magnet poles depending on the stator winding construction was deduced as expressed in[6].0018-9464©2014IEEE.Personal use is permitted,but republication/redistribution requires IEEE permission.See /publications_standards/publications/rights/index.html for more information.Fig.2.Winding configuration and resultant EMFvector.Fig.3.Topologies of DSRFPMM and DSAFPMM.The number of slots in one stator for three phases isQ =3n 1n 2.(1)The number of magnet poles is2p =3n 1n 2+n 2(2)where n 1is defined as the number of coils for one phase group,and n 2is the number of groups for one phase.It is noted here that the slot and teeth width within one phase group are designed to be π/2(elec.),whereas the slot width between two different phases is 5π/6(elec.),for producing a three-phase balanced back EMF.Assuming that the combination of n 1=4and n 2=2is selected,the resultant EMF vector is shown in Fig.2.Due to the opposite winding direction of the adjacent coils and the alternating magnetization directions in permanent magnets,the induced EMF of every coil will follow the same direction resulting in the largest EMF vector.B.Motor TopologiesThe proposed DSRFPMM and DSAFPMM are shown in Fig.3.Their specifications with respect to the target values in [1]are illustrated in Table I.Both motors feature the same outer diameter and axial length as the target motor as well as considering the dimension of winding end turns.Ferrite magnets are circumferentially magnetized with alternately reversing magnetization direction in polarity and arranged as a spoke-type array for air gap flux magnification.Phase-group concentrated-coil stator windings are wound around each stator tooth and the two stators adopt axially unaligned configurations displaced by one tooth width.The DSRFPMM adopts a smaller inner air gap than the outer air gap,aiming at obtaining similar magnetic flux density in each air gap for similar output performance in both air gaps.C.Operating PrincipleThe operating principle of two designed traction motors is illustrated in Fig.4.Since three-phase balanced back EMF is produced,the condition within one phase group is displayed.When the rotor pole rotates to become aligned with the teeth ofTABLE IS PECIFICATIONS OF D ESIGNED DSRFPMM ANDDSAFPMMFig.4.Operating principle.upper stator and slot of lower stator,almost the total PM flux will flow into the upper air gap,reaching the maximum of flux as illustrated in case 1.As the rotor rotates,the PM flux flows into two air gaps simultaneously as shown in case 2.After 90o (elec.)rotation from case 1,the same effects as case 1will occur in the lower air gap as shown in case 3.Assuming that sinusoidal currents are injected into the coils,a higher resultant torque can be obtained than the conventional motors in which two air gaps work independently as is verified in [6].III.FEM R ESULTS AND D ISCUSSIONA.Magnetic Flux Density,Back EMF ,and Cogging Torque The magnetic flux density distribution for the no load case with the DSRFPMM and DSAFPMM was analyzed at the rated speed as shown in Fig.5(a)and (b),respectively.Three positions are displayed corresponding to the cases in Fig.4.In case 1,almost the total PM flux of the proposed motors flows into the outer/upper air gap corresponding to phase A of the outer/upper stator winding (the red dashed box area).In case 2,the PM flux flows into two air gaps simultane-ously.In case 3,the same effects as case 1will occur in the inner/lower air bining with the same principle in other two phases,a three-phase balanced back EMF is produced,as shown in Fig.6.Fig.7demonstrates the advantage of unaligned arrangement of two stators for cogging torque minimization.The peak-to-peak values of cogging torque in the proposed DSRFPMM and DSAFPMM are 1.826and 1.135Nm,respectively,which are 0.9%and 0.5%with respect to their rated torque.The DSRFPMM contains higher cogging torque than the DSAFPMM due to the adoption of a smaller air gap.B.Torque CharacteristicsThe electromagnetic torque for both of the proposed motors is obtained by feeding them with a three-phase balanced sinu-soidal current source for the sake of performance estimation.ZHAO et al.:COMPARATIVE STUDY ON NOVEL DSRFPMM AND DSAFPMM8104404Fig.5.(a)Magnetic flux density distribution in DSRFPMM.(b)Magnetic flux density distribution in DSAFPMM.Magnetic flux density distribution at no loadcase.Fig.6.Three-phase backEMF.Fig.7.Coggingtorque.Fig.8.Average torque with respect to current phase angle.Fig.8shows the relationship between the average torque and current phase angle.The maximum average torque for the DSRFPMM and DSAFPMM is the same (208.8Nm)at the current phase angle of 20°and 15°,respectively.Thus,the output power for both motors is the same (60.5kW),whichFig.9.Electromagnetic torque with respect to rotational angle.TABLE IIP ERFORMANCE E STIMATION FOR DSRFPMM AND DSAFPMM BY FEMslightly exceeds the target value.The instantaneous torque with respect to the rotational angle for both of the proposed motors is shown in Fig.9.The torque/ripple ratio defined as the difference of peak-to-peak torque value to average torque value for the DSAFPMM is 5.5%,whereas it is 10%for the DSRFPMM resulting from the smaller air gap.The volumetric torque/power density of both proposed motors have the same value,whereas the DSAFPMM has higher torque/power-weight ratio than the DSRFPMM due to less usage of magnets and the iron core.The results of the performance estimation by FEM are summarized in Table II.C.Demagnetization AnalysisThe irreversible demagnetization of the ferrite magnets in both of the proposed motors was evaluated at the current density of 18Arms/mm 2under the rated speed condition.The demagnetizing ratio is defined asδ=E 1−E 2E 1(3)where E 1is the rms value of the terminal voltage in phase before feeding with current excitation,and E 2is the rms value of the terminal voltage in phase after feeding with current excitation as shown in Fig.10.The demagnetization ratio for the DSRFPMM is 3%,whereas it is 3.2%for the DSAFPMM,which demonstrates that both of the proposed8104404IEEE TRANSACTIONS ON MAGNETICS,VOL.50,NO.11,NOVEMBER2014Fig.10.Terminal voltage with no load and fullload.Fig.11.Constant torque and constant power region by FWcontrol.Fig.12.Copper loss and iron loss during the constant power region.motors have good endurance against the irreversible demag-netization,despite the fact that only a slight demagnetization occurs at the edge of the ferrite magnets.D.Operation Range,Loss,and EfficiencyFig.11shows the simulated characteristics of the torque and power over speed range by the flux weakening (FW)control strategy.The results demonstrate that both proposed motors have the capability of producing 60kW of mechanical power over a 5:1speed range.The loss and efficiency are investigated under the constant power load as shown in Figs.12and 13,respectively.The copper loss is determined byP copper =3I 2rmsR (4)where I rms is the rms value of the armature current,and R isthe stator winding resistance.The iron loss is obtained by FEM modeling employing the commercial software JMAG,which is based onP iron =N element i[P hi (B ,f e )+P ei (B ,f e )](5)Fig.13.Efficiency during the constant power region.where N element is the element number,P hi (B ,f e )is the hysteresis loss of each element,P ei (B ,f e )is the joule loss of each element,B is the magnetic flux density,and f e is the frequency.The efficiency is herein defined asη=P outP out +P copper +P iron .(6)IV.C ONCLUSIONIn this paper,two traction motors with low cost ferrite magnets adopting a similar novel structure have been proposed for HEVs.The two proposed motors,named as DSRFPMM and DSAFPMM,have been verified to have competitive per-formance regarding high torque/power density and efficiency with respect to a referenced rare earth magnet motor employed in the HEV (Toyota Prius 2010)by FEM.It is also found that both of the proposed motors have good endurance against the irreversible demagnetization in the ferrite magnets.Conse-quently,it is expected that the proposed ferrite magnet motors could be good alternatives for traction application replacing the need for rare earth magnets.A CKNOWLEDGMENTThis work was supported by the 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