IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 8104404

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

Ferrite Magnets for Traction ApplicationWenliang Zhao1, Thomas A. Lipo2, Life Fellow, IEEE, and Byung-Il Kwon1

1Department of Electronic Systems Engineering, Hanyang University, Ansan 426-791, Korea2Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706-1691 USA

This paper proposes two novel traction motors with ferrite magnets for hybrid electric vehicles (HEVs), which have competitivetorque density and efficiency as well as operating range with respect to a referenced rare earth magnet motor employed in thethird-generation Toyota Prius, a commercialized HEV. The two proposed traction motors, named as dual stator radial flux permanentmagnet motor (DSRFPMM) and dual stator axial flux 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-groupconcentrated-coil windings for the purpose of increasing torque density and reducing torque ripple. A finite element method isutilized 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 isperformed under the same operating condition. As a result, it is demonstrated that both of the proposed ferrite permanent magnetmotors could be good alternatives for traction application, replacing the rare earth magnet motors.

Index Terms— Axial flux permanent magnet motor (AFPMM), dual stator, ferrite permanent magnet motors, finite elementmethod (FEM), hybrid electric vehicles (HEVs), radial flux permanent magnet motor (RFPMM), torque density, tractionmotors.

I. INTRODUCTION

RECENTLY, the demand for electrical propulsion systemswith high torque density, high efficiency, and low cost

for hybrid electric vehicles (HEVs) is becoming more andmore urgent due to environmental and energy considerations.In order to satisfy the demand, permanent magnet synchronousmotors (PMSMs) with rare earth magnets have been primarilyemployed as the main traction motors, benefitting from theexcellent properties of rare earth magnets. However, the rareearth materials, such as neodymium and dysprosium, in rareearth magnets exhibit the problem of high cost and limitedsupply, which introduces a serious challenge for the massproduction of HEVs. Therefore, continuation of research fordevelopment of high-performance synchronous motors withless or no rare earth magnets is of importance.

Accordingly, several substitute motors with a reducedamount 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 bebetter candidates, considering their efficiency and vibrationproperties, and especially their feature of making possibleuse of both reaction torque and reluctance torque. However,conventional ferrite magnet motors generally have muchpoorer torque/power density than rare earth magnet motorsdue to the lesser properties of ferrite PMs. Although someimproved ferrite PM motors, such as a motor with a segmentedrotor 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 replacingrare earth magnet motors, their performance in terms oftorque/power density, efficiency, and field-weakening capa-bility, as well as irreversible demagnetization durability, arerestricted when compared with the target performance required

Manuscript received March 7, 2014; revised April 26, 2014; acceptedMay 30, 2014. Date of current version November 18, 2014. Correspondingauthor: B.-I. Kwon (e-mail: bikwon@hanyang.ac.kr).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2014.2329506

Fig. 1. Design concept.

to replace the rare earth magnet motors under the sameoperation condition. In [6], a novel structure for an axial fluxPM motor (AFPMM) incorporating the unaligned arrangementof two stators together with the use of a spoke-type permanentmagnet array and phase-group concentrated-coil windings wasexplored for the purpose of increasing torque density andreducing torque ripple. In this paper, the idea is extended toradial flux PM motors (RFPMMs) and a comparative study isperformed between a dual stator RFPMM (DSRFPMM) anddual stator AFPMM (DSAFPMM).

As traction motors using ferrite permanent magnets, theproposed two motors are designed to be competitive with thetarget performance values from the rare earth magnet motormounted in a commercialized HEV of the third-generationToyota Prius [1]. The investigation of motor performance isbased on the same outer diameter and axial length, as well asthe same ratings such as rated speed and power, as those of thereferenced rare earth magnet motor. The main characteristics,such as back electromotive force (EMF), cogging torque, elec-tromagnetic torque, iron loss, and efficiency, of both proposedmotors are predicted by the finite element method (FEM).

II. MOTOR TOPOLOGIES AND OPERATING PRINCIPLE

A. Basic Design Rules

Based on the design concept as shown in Fig. 1, a specificcombination of stator slots and magnet poles depending on thestator winding construction was deduced as expressed in [6].

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8104404 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

Fig. 2. Winding configuration and resultant EMF vector.

Fig. 3. Topologies of DSRFPMM and DSAFPMM.

The number of slots in one stator for three phases is

Q = 3 n1n2. (1)

The number of magnet poles is

2 p = 3 n1n2 + n2 (2)

where n1 is defined as the number of coils for one phase group,and n2 is the number of groups for one phase. It is notedhere that the slot and teeth width within one phase group aredesigned to be π /2 (elec.), whereas the slot width betweentwo different phases is 5π /6 (elec.), for producing a three-phase balanced back EMF. Assuming that the combination ofn1 = 4 and n2 = 2 is selected, the resultant EMF vector isshown in Fig. 2. Due to the opposite winding direction of theadjacent coils and the alternating magnetization directions inpermanent magnets, the induced EMF of every coil will followthe 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 valuesin [1] are illustrated in Table I. Both motors feature the sameouter diameter and axial length as the target motor as wellas considering the dimension of winding end turns. Ferritemagnets are circumferentially magnetized with alternatelyreversing magnetization direction in polarity and arrangedas a spoke-type array for air gap flux magnification. Phase-group concentrated-coil stator windings are wound aroundeach stator tooth and the two stators adopt axially unalignedconfigurations displaced by one tooth width. The DSRFPMMadopts a smaller inner air gap than the outer air gap, aimingat obtaining similar magnetic flux density in each air gap forsimilar 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 isproduced, the condition within one phase group is displayed.When the rotor pole rotates to become aligned with the teeth of

TABLE I

SPECIFICATIONS OF DESIGNED DSRFPMM AND DSAFPMM

Fig. 4. Operating principle.

upper stator and slot of lower stator, almost the total PM fluxwill flow into the upper air gap, reaching the maximum of fluxas illustrated in case 1. As the rotor rotates, the PM flux flowsinto two air gaps simultaneously as shown in case 2. After90o (elec.) rotation from case 1, the same effects as case 1will occur in the lower air gap as shown in case 3. Assumingthat sinusoidal currents are injected into the coils, a higherresultant torque can be obtained than the conventional motorsin which two air gaps work independently as is verified in [6].

III. FEM RESULTS AND DISCUSSION

A. Magnetic Flux Density, Back EMF, and Cogging TorqueThe magnetic flux density distribution for the no load case

with the DSRFPMM and DSAFPMM was analyzed at therated speed as shown in Fig. 5(a) and (b), respectively. Threepositions are displayed corresponding to the cases in Fig. 4.In case 1, almost the total PM flux of the proposed motorsflows into the outer/upper air gap corresponding to phaseA 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 1 will occur inthe inner/lower air gap. Combining with the same principlein other two phases, a three-phase balanced back EMF isproduced, as shown in Fig. 6.

Fig. 7 demonstrates the advantage of unaligned arrangementof two stators for cogging torque minimization. The peak-to-peak values of cogging torque in the proposed DSRFPMMand DSAFPMM are 1.826 and 1.135 Nm, respectively, whichare 0.9% and 0.5% with respect to their rated torque. TheDSRFPMM contains higher cogging torque than theDSAFPMM 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 DSAFPMM 8104404

Fig. 5. (a) Magnetic flux density distribution in DSRFPMM. (b) Magneticflux density distribution in DSAFPMM. Magnetic flux density distribution atno load case.

Fig. 6. Three-phase back EMF.

Fig. 7. Cogging torque.

Fig. 8. Average torque with respect to current phase angle.

Fig. 8 shows the relationship between the average torque andcurrent phase angle. The maximum average torque for theDSRFPMM and DSAFPMM is the same (208.8 Nm) at thecurrent phase angle of 20°and 15°, respectively. Thus, theoutput power for both motors is the same (60.5 kW), which

Fig. 9. Electromagnetic torque with respect to rotational angle.

TABLE II

PERFORMANCE ESTIMATION FOR DSRFPMM AND DSAFPMM BY FEM

slightly exceeds the target value. The instantaneous torquewith respect to the rotational angle for both of the proposedmotors is shown in Fig. 9. The torque/ripple ratio defined asthe difference of peak-to-peak torque value to average torquevalue for the DSAFPMM is 5.5%, whereas it is 10% for theDSRFPMM resulting from the smaller air gap.

The volumetric torque/power density of both proposedmotors have the same value, whereas the DSAFPMM hashigher torque/power-weight ratio than the DSRFPMM due toless usage of magnets and the iron core. The results of theperformance estimation by FEM are summarized in Table II.

C. Demagnetization Analysis

The irreversible demagnetization of the ferrite magnets inboth of the proposed motors was evaluated at the currentdensity of 18 Arms/mm2 under the rated speed condition. Thedemagnetizing ratio is defined as

δ = E1 − E2

E1(3)

where E1 is the rms value of the terminal voltage in phasebefore feeding with current excitation, and E2 is the rmsvalue of the terminal voltage in phase after feeding withcurrent excitation as shown in Fig. 10. The demagnetizationratio for the DSRFPMM is 3%, whereas it is 3.2% for theDSAFPMM, which demonstrates that both of the proposed

8104404 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

Fig. 10. Terminal voltage with no load and full load.

Fig. 11. Constant torque and constant power region by FW control.

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 demagnetizationoccurs at the edge of the ferrite magnets.

D. Operation Range, Loss, and EfficiencyFig. 11 shows the simulated characteristics of the torque and

power over speed range by the flux weakening (FW) controlstrategy. The results demonstrate that both proposed motorshave the capability of producing 60 kW of mechanical powerover a 5:1 speed range. The loss and efficiency are investigatedunder the constant power load as shown in Figs. 12 and 13,respectively. The copper loss is determined by

Pcopper = 3I 2rms R (4)

where Irms is the rms value of the armature current, and R isthe stator winding resistance.

The iron loss is obtained by FEM modeling employing thecommercial software JMAG, which is based on

Piron =Nelement∑

i

[Phi(B, fe) + Pei(B, fe)] (5)

Fig. 13. Efficiency during the constant power region.

where Nelement is the element number, Phi(B, fe) is thehysteresis loss of each element, Pei(B, fe) is the joule lossof each element, B is the magnetic flux density, and fe is thefrequency.

The efficiency is herein defined as

η = Pout

Pout + Pcopper + Piron. (6)

IV. CONCLUSION

In this paper, two traction motors with low cost ferritemagnets adopting a similar novel structure have been proposedfor HEVs. The two proposed motors, named as DSRFPMMand DSAFPMM, have been verified to have competitive per-formance regarding high torque/power density and efficiencywith respect to a referenced rare earth magnet motor employedin the HEV (Toyota Prius 2010) by FEM. It is also foundthat both of the proposed motors have good endurance againstthe irreversible demagnetization in the ferrite magnets. Conse-quently, it is expected that the proposed ferrite magnet motorscould be good alternatives for traction application replacingthe need for rare earth magnets.

ACKNOWLEDGMENT

This work was supported by the BK21PLUS Programthrough the Ministry of Education, National ResearchFoundation of Korea.

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