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Abstract In this era of electrified transportation, switched re-luctance motor (SRM) is emerging as a prospective replacementto traditional electric motors especially for large heavy duty ve-hicles such as the electric bus. This paper proposes the designand analysis of a novel outer rotor in-wheel SRM. The integra-tion of the motor housing inside the wheel rim saves significantspace and eliminates the need for additional mechanical partsused in the centralized drive. The developed concept of shortflux path configuration in this research manuscript has shownadditional important features compared to previous SRM de-signs and a substantial increase in efficiency is reported. Theprocedures of deriving the output power equation as a functionof the motor dimensions and parameters are explained in detail.Comparative finite element analysis (FEA) has been performedbetween the developed machine and a commercially availableconventional SRM to elicit the merits of the developed machine.The results obtained through FEA investigations show thatthere is a reduction of torque ripple and a considerable increasein motor efficiency.

Keywords Electric bus, finite element analysis, in-wheelouter rotor motor, switched reluctance machine.

I. NOMENCLATURE A s : Specific electric loading

A sp : Area of stator pole : Ratio of aligned to unaligned inductances B : Average flux density r , s : Rotor pole arc and stator pole arc : Step angle

g : Air-gap length I : Stator phase currentV : Single phase terminal voltageV s : Source voltagek e, k d : SRM efficiency and duty cycle factors

La, Lu : Aligned and unaligned inductances l m : Motor axial lengthm : Number of phases that are conducting simultaneously

N ph : Number of phases in SRM N : Number of turns per coil N r , N s : Number of rotor and stator pole P : Output power R : Phase winding resistance : Magnetic flux : Current conduction angle b : Base angular speed of the motor : Single phase flux linkage

II. I NTRODUCTIONOver the years, the increasing trend in CO 2 emissions from

the transportation sector has resulted in negative impacts tothe environment and human health. In urban areas, the diesel

particulate matter (PM) pollution is mainly attributed to urban buses [1]. A study performed by the British Columbia LungAssociation showed that a 1% improvement in ambient ultra-

fine PM and ozone concentrations is predicted to result inmillions of annual savings [2]. Introducing a fleet of all-electric buses is the best solution to this environmental hazard

because there is simply no exhaust pipe in these buses to re-lease any PM or CO 2 emissions in the air. As seen in [3], di-esel-fuelled buses make up 86.3% of the total transit busesand consumed nearly 560 millions of gallons of diesel. Giventhe conversion factor, this large volume of diesel fuel isequivalent to over six million tons of CO 2 emissions and willcontinue to increase in the future due to continuous demandfor public transit. Hypothetically, if half of the diesel transit

buses were replaced by non-polluting all-electric buses, thenthree million tons of CO 2 emissions could be instantly elimi-nated on an annual basis.

Permanent magnet synchronous machines have been wide-ly used in small/medium sized electric vehicles [4]. However,their application in heavy duty electric buses is limited due tothe increasing cost of permanent magnet material and theirsensitivity to heat and vibration [5]. Hence, the switched re-luctance machines (SRM) and the induction machines are

prospective for such applications as they are cost effectiveand rugged. Moreover, SRM can tolerate higher temperaturesince they do not have windings or magnets in the rotor.

However, the major bottleneck of SRM has been the torqueripple which limits the application of it in these commercialvehicles. Hence, this paper proposes an exclusive overall con-figuration of an in-wheel outer rotor SRM with reduction intorque ripple and an increase in the machine efficiency whencompared to the conventional SRM configurations.

Background literature obtained from [6]-[9] state that thein-wheel outer rotor motor has an edge over the conventionalmotor designs for the electric vehicle application as it savessubstantially large space previously occupied by the neces-sary mechanical components such as the transmission, speedreducer shafts and differential. The case study from [3] statethat the in-wheel motor was instrumental to improve the effi-

ciency of the electric bus as the bus could travel twice as faras a conventional bus on a litre of diesel. The in-wheel mo-tors conferred additional savings by eliminating the need fora transmission, differential, and related mechanical parts.That reduces both the overall weight of the bus and energylosses due to friction. The in-wheel motors also improvedtraction by allowing precise control over each wheel, andthey allowed greater flexibility in vehicle design since therewas no need to mechanically link the wheels to an engine.

Section 3 of this paper explains in detail the major featuresof the developed in-wheel outer rotor SRM (IOSRM). Sec-tions 4 and 5 explain in length the geometrical design andelectrical design of the developed machine. Finally, the de-

veloped IOSRM is then compared with a developed conven-tional SRM and FEA has been performed. Section 6 formu-lates the results of the investigations.

Outer Rotor Switched Reluctance Motor Design forIn-wheel Drive of Electric Bus Applications

1Anas Labak, Student Member , IEEE and 2 Narayan C. Kar, Senior Member , IEEECentre for Hybrid Automotive Research and Green Energy, University of Windsor, ON, Canada N9B 3P4

[email protected] and [email protected]

978-1-4673-0142-8/12/$26.00 2012 IEEE 418


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Fig. 1. The proposed IOSRM.

III. CONCEPT OF THE PROPOSED I N-WHSRM AND ITS FEATURES

Figure 1 shows the isometric perspectiv proposed SRM integrated in a wheel oshaft of the stator core is rigidly fixed tosuspension system. The outer rotor is monary components by a set of bearings thatning of the rotor. The rotor core is fir wheels rim by an arrangement of bars anshown in Fig. 2. This design has 18 rotor

poles. The rotor teeth are evenly distributspacing. The stator poles are formed of 8Fig. 3. The angular distance between thecent pair is 45 degrees. However, the atween the poles within the pair is 20 degre

The windings of each phase are splitcoils wound in series around 4 stator polediametrically. Upon excitation, the magn

phase is formed by a pair of stator polesrotor teeth as shown in Fig. 3. The torquedesign relies on the tendency of the exci

pull the nearby rotor teeth into alignment.ture of this design topology is that it off

path thus minimizing the iron loss withouthigh power capability of the motor. In adof four poles are energized at any given tthis machine at least double the torque cap

Fig. 2. The geometric parameters of the proposed IOS

Liqui

r

s

spp r or

r os

EL OUTER R OTOR

e illustration of theelectric bus. The

a beam of the rearnted on the statio-

facilitate the spin-ly fastened to thed two end rings asteeth and 16 stator

ed with 20 degrees pairs as shown inaxes of each adja-gular distance be-

es.on 4 concentrated

s, two on each sidetic circuit of each

facing two aligned production in thisted stator poles toAn important fea-

ers very short flux

compromising thedition, a minimumime which rendersability of the con-

RM.

conventional SRM. Additiondesign are summarized as follo The main advantage of usi

rotor electric drive is thatviously occupied by thenents such as the transmisdifferential.

The flux path is independThis particular feature giveincrease the torque by incwithout having to increaswhere F is the reluctanceand r is the radius of the air

In conventional SRM, oneall phases, and the windingsted together in one slot. Tcoupling between adjacentminimized in this design sident magnetic circuit, and tare further separated.

The direction of the flux insame. In other words, the

hence lowering the core loSRM [10]. Enough spacing between th

the stator makes it possibleas shown in Fig. 1. This paed by the stator coils, and trent rating which in turn inof the machine.

The insertion of cooling tubadvantage as it serves as atual coupling and the leakacircuits excited at the sameoverall efficiency of theformance.

IV. GEOMETRICAL DESIGThe goal of the design is to

the main well-known drawbacripple. This demerit is particul

Fig. 3. Cross-sectional view of the IOSand short magnetic path.

cooling tubes

r = s

Bar

Ph.

Ph.

al features of the proposedws:g an integrated in-wheel outerit saves substantial space pre-ecessary mechanical compo-ion, speed reducer shafts and

nt of the radius of the rotor.the designer the capability to

easing the radius of the rotore the flux path ( r F = ),force generated in the air-gapgap.magnetic circuit is shared byof two adjacent phases are fit-

hese contribute to the mutual phases. This disadvantage isce each phase has an indepen-he winding of different phases

the stator poles is always theflux reversal does not occur,

ses compared to conventional

e adjacent magnetic circuits onto include liquid cooling tubesifies most of the heat generat-herefore permits a higher cur-reases the output power rating

es has an additional importantflux barrier that limits the mu-ge flux between two magnetictime. It therefore increases theachine and improves its per-

OF THE PROPOSED IOSRM

provide a realistic solution toof SRMs, namely the torque

rly undesirable for the vehicle

RM showing the phases distribution

. BPh. C

Ph. D

419


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applications. Considerable research has been done to alle-viate this problem by proposing intelligent control schemes[11]. Yet, the geometrical structural solution is preferred overthe control scheme. Many literatures have suggested largenumber of phases or poles [12]. The proposed motor in this

paper is designed with outer rotor and large number of poles.Consequently, the number of strokes per revolution increas-es, and the torque ripple problem could be alleviated. The in-

creased number of poles requires larger diameter resulting ina greater flux-path length which in turn raises the losses andreduces efficiency. The solution for this was addressed in thisdesign by adopting a shorter flux-path as shown in Fig. 3.

A small step angle of 5 degrees is achieved by adoptingthe configuration explained in the previous section. The stepangle is calculated using the following equation:

r ph N N = 360

(1)

where, N ph and N r are the number of phases and the numberof rotor poles, respectively.

Generally, the initial design process goes through several

iteration steps. Several geometries have been calculated withvarying pole numbers and pole dimensions, keeping in mindthat a number of requirements need to be fulfilled such as;minimizing the step size ( ), the self starting capability, andthe optimum pole arcs. The stator arc ( s) should be greaterthan the step size in order to satisfy the self-starting require-ment. The optimum pole arcs, which are a trade-off betweenvarious conflicting requirements, should be made as large as

possible to maximize the aligned inductance and the flux lin-kage. However, if they are too wide there is not enoughclearance between the rotor and stator pole-sides in the un-aligned position. This restriction can be represented by:

sr r N

2 (2)

The optimum pole arcs are somewhere between theseextremes [12]. An adequate choice for this design was tohave both stator and rotor poles arcs equal. The maindimensions are presented in Table I, and illustrated in Fig. 2.The inductance profiles for all phases are obtained by

building the FEA model of the proposed design. Thesimulation results that are shown in Fig. 4 validate thecalculation of the step angle and the other geometric

parameters presented in this section.

V. OUTPUT POWER RATING ESTIMATION FOR THE PROPOSEDIOSRM

The typical and basic method of deriving the output powerhas been well covered in the literature [10], [13]. However,due to some dissimilarities in the design concept and topologyfrom the conventional SRMs, the final output power equationshould satisfy and include these changes.

The output power is a function of the specific electricloading, magnetic loading, motor speed, and the dimensions

TABLE I. DIMENSIONS OF THE PROPOSED IOSRM

Number of phases N ph 4 Air-gap length g 0.9 mm

Stator-rotor configuration 16/18 Motor axial length l m 180 mm

Rotor pole pitch r 20 Stator outer radius r os 134.1 mm

In-pair stator pole pitch s 20 Rotor inner radius r ir 135 mmStator pair-pair pitch spp 45 Rotor outer radius r or 180 mm

Stator pole arc s 8 Number of turns N 160

Rotor pole arc r 8 Step angle 5

Fig. 4. The inductance profiles for all the phases. of the machine [10], [13]. The voltage equation for one phaseis given by:

( ).

dt LI d

RI V += (3)

For the purpose of deriving the output power rating it can beassumed that the phase current is flat-topped during the phaseconduction period and the phase winding resistance is neglig-ible, (3) can be rewritten as:

dt dL

I V = (4)

[ ]

t

L L I V ua

= (5)

where, La is the inductance in the aligned position, Lu is theinductance in the unaligned position, and t is the time takenfor the rotor to move from the unaligned to aligned position.t can be related to the angular speed of the rotor and the sta-tor arc as follows.

.b

st

= (6)

The ratio of the aligned and unaligned inductances is:

.u

a

L L

= (7)

By inserting (6) and (7) into (5), (8) can be obtained.

( ) .11 s

ba L I V

= (8)

The flux-linkage at the aligned position is given by

==

N BA

I L

sp

a (9)

where, A sp is the stator pole area, N is the number of turns per phase, B is the average flux density at the stator pole face. Thevalue of this average flux density can be obtained from the B-

H characteristics of the material used.To derive the output power and voltage equations for this

design, the relationship that links all the relevant variables andgeometric parameters has to be found. The cross-sectionalarea for the stator pole A sp can be derived from Fig. 2 as:

m s sp rl A = (10)

Substituting (9) and (10) in (8) gives the voltage equation forthe proposed motor:

0

0.5

1 Phase A

0

0.5

1 Phase B

0

0.5

1 Phase C

0

0.5

1

0 5 10 15 20 25 30 35 40

Rotor position [mechanical degree]

Phase D

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( ).11 = N Brl V m (11)The phase current can be found for the specific electric load-ing condition as follows:

r mNI

A s = 4 (12)

where m is the number of phases that are conducting simulta-neously. The conventional output power equation for anSRM is defined as

VI k mk P d e= (13)where, k e is the efficiency factor. For SRMs, this factor isusually in the range of 0.8 and 0.94. Since it cannot be de-termined in advance, k e is arbitrarily given a value of 0.9. k d is the duty cycle and can be defined using (14).

=

2r phi

d

N N k (14)

where i is the current conduction angle which may be giventhe same value as the step-angle (5 o in this design). Moreover,due to the large number of poles, the phase-overlapping ratiois relatively high and thus, the value of k d may vary from 0.5to 1. Finally, the output power equation for the new SRM de-sign is found by substituting the voltage from (11) and thecurrent from (12) in (13).

( ).1123.0 2 = r Bl A P mb s (15)

VI. FINITE ELEMENT MODEL DEVELOPMENT R ESULTS ANDA NALYSIS

As known, SRMs are designed to operate in the saturatedregion to maximize the torque production and efficiency bymaximizing the energy transfer. This unavoidably gives riseto the non-linear issue of an SRM drive [13], [14]. Thus thefinite element analysis method is the optimum solution to de-rive its accurate characteristics. In this section, the FEA mod-el is built; the static analysis is used to derive the operatingcurrent, and to validate the correctness of the design.

A. The Optimum Magnetomotive ForceThe general expression for the torque produced by one

phase at any rotor position is

.const i

coW T =

= (16)

where W co is the co-energy, is the angular rotor position.At any position, the co-energy is the area below themagnetization curve so it can be defined as follows

diW ico = 10 (17)where, is the flux linkage at any rotor position as a

function of the current. The optimum MMF for this designcan be found by referring to equations (16) and (17) in whichit is proved that larger the increment of co-energy, higher theincrease in the torque. This is physically related to the levelof saturation in the core material. The FEA results for the co-energy variation between two rotor positions are obtained fora wide range of current. The co-energy increment iscalculated and plotted versus the magnetomotive force asshown in Fig. 5. The plot indicates that (1,400 AT) is theoptimum MMF. Finally, the optimum current can becalculated depending on the number of turns which isexplained in sub-section B.

B. Winding Design and Number of Turns DeterminationThe goal of this winding design is to determine the num-

ber of turns and the way of connecting the pole coils so thatthe required magnetomotive force MMF is produced, suffi-cient flux density is available inside the stator core parts as

well as in the air-gap, and minimal copper and iron losses areachieved. Several other requirements and restrictions have to

be satisfied, such as the voltage and current rating of the power converter, the motor speed, switching frequency, max-imum permissible current density, insulation, and the coolingmethods to be used.

A fundamental coil design flow chart, presented by theauthors in a previous work [16], is used here to determine all

the coil design details. The windings of each phase are spliton 4 concentrated coils wound, in series, around 4 stator poles, two on each side diametrically. The total number ofturns per phase is 160. FEA solution assists in verifying thecorrectness of thenwinding design. The sequential excitationof the motor phases causes the outer rotor rotation in thecounter clockwise direction. The color coding solution mapsin Fig. 6 shows satisfactory level of local saturation in theoverlapped poles regions.

C. Comparative Finite Element Analysis of the IOSRM andthe Developed Convetional SRMThe model of the proposed design IOSRM was built us-

ing MagNet Infolytica software to perform FEA. The magne-tization characteristics, as the most descriptive illustration ofthe motor performance and efficiency, are obtained and pre-sented in Fig. 7. It is clearly seen that the value of inductanceratio at the rated current is relatively high for a motor withshort axial length and large number of poles [16], [17]. By re-ferring to (15), high inductance ratio is directly reflected inthe output power, hence validating the design with respect tothe efficiency improvement.

An FEA model of 8/6 conventional SRM with similar sizeand power rating to the IOSRM is built for comparative anal-ysis purpose. The field solution of the developed conventionalSRM is as shown in Fig. 8. The output torque obtained during

single stroke of operation by both the machines is demon-strated in Fig. 9. Since their poles arcs are not equal, the rotor positions scales are taken as per unit quantities of each ma-chine.

The comparison in Fig. 9 clearly illustrates the improve-ment in the output torque and hence in the efficiency whichvalidates the proposed design.

Fig. 10 shows the output torque obtained by the individualconsecutive phases. The obvious large overlapping tells thatthere are no dead torque zones at the output. This is due to thesmall step angle in this design which is in turn governed bythe large number of poles and special configuration design ofthe poles. It can be concluded here that even with basic con-

trol techniques and without the need to boost the current at thelow torque regions this design can minimize the torque rippleto a very low level.

Fig. 11 illustrates the detailed view of all the in-wheelouter rotor components. Its power ratings are listed in table II.

Fig. 5. Increment of co-energy with respect to magnetomotive force atdifferent rotor positions.

00.20.40.60.8

11.21.41.6

0 500 1000 1500 2000 2500 3000 3500

C

o - e n e r g y

I n c r e m e n

t [ J ]

MMF [AT]

Coenergy increment at fullalignment (10 deg)

Coenergy icrement at 5 deg

421


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Phase A Phase B Phase C Phase D

Fig. 6. FEA solution for the sequential excitation of a ll phases and the corresponding motion of the outer rotor.

VII. CONCLUSION

This paper presents the design development and FEAof a novel in-wheel outer rotor SRM. The output powerequation of the proposed SRM design is derived in detail.FEA models of both the proposed IOSRM and aconventional SRM are built to perform a comparativeanalysis and elicit the merits of the proposed design. The

proposed machine is found to have reduced torque ripplesand higher efficiency than that of the conventional SRM.Hence, this design of high power SRM which has shortflux path configuration makes it applicable for heavy dutyelectric buses.

Fig. 7. The saturation characteristics of the proposed motor obtainedwhile varying the rotor angular position over one stroke by step of 2degree.

Fig. 8. The field solution of an 8/6 conventional SRM FEA model builtwith similar size and power rating to the IOSRM.

Fig. 9. Output torque obtained by both the machines over one singlestroke at rated current of the machines.

Fig. 10. Output torque showing large overlapping with no dead torquezones.

Fig. 11. The complete in wheel drive arrangement.

0

0.22

0.44

0.66

0.88

1.1

0 10 20 30 40 50

F l u x

L i n k a g e

[ W B ]

Current [A]

-20

0

20

40

60

80

100

120

140

0 18 36 54 72 90 108 126 144 162 180

T o r q u e

[ N . m

]

Rotor position [electrical degree]

Proposed SRM

Conventional SRM

0

20

40

60

80

100

120

140

0 5 10 15 20 25

T o r q u e

[ N . m ]

Rotor position [mechanical degree]

Phase A Phase B Phase C Phase D

Suspension

Stator andWinding

Rim and Bars

Ring

Rotor

Tire (Tyre)

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TABLE IIR ATING OF THE PROPOSED SRM.

Parameter Values Parameter

Output Power 24 kW Maximum powerRated torque 120 N.m Maximum torqueRated current 35 A Base speed

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IX. B IAnas Labak and ElectroniAleppo, Alep

M. A. Sc. deUniversity ofin 2009. He iin the ElectriUniversity ofHis researchanalysis and d

Narayan C.trical EngineeEngineering ain 1992 and ttrical engineeogy, Hokkaidtively. He is aand Compute

versity of WCanada Reseasystems. His research presently foctrol of permanent magnet synchr tance machines for hybrid electrictesting and performance analysis ozation techniques for hybrid energMember of the IEEE.

luctance Motors and Their Control s/Oxford Univ. Press, 1993, pp. 7-21.derations for the switched reluctance ppl. , vol. 31, no. 5, pp. 1079-- -1087,

ulation, and Control of Switched IEEE Transactions on Industrialc. 2005.

J.C. Balda, and R.M. Schupbach, SRM under multiphase excitation

s, presented at the 28th Annuallectronics Society, vol. 2, pp. 1072-

evelopment and Analysis of a Five-ed Reluctance Motor," in Proc. of the erence on Electrical Machines , Italy,

ic, and I. Salihbegovic, "Switched re-electric vehicles," in Proc. of the XIX

Electrical Machines (ICEM), pp.1-6,

OGRAPHIES eceived the B.Sc. degree in Electricalcs Engineering from University ofo, Syria, in 1996. And received the

gree in Electrical Engineering fromWindsor, Windsor, Ontario, Canadas currently pursuing the Ph.D. degreeal and Computer Engineering at theWindsor, Windsor, Ontario, Canada.interests are in machine modeling,esign for electric vehicle applications.

ar received the B.Sc. degree in Elec-ring from Bangladesh University ofnd Technology, Dhaka, Bangladesh,he M.Sc. and Ph.D. degrees in elec-ing from Kitami Institute of Technol-o, Japan, in 1997 and 2000, respec-n associate professor in the ElectricalEngineering Department at the Uni-

indsor, Canada where he holds therch Chair position in hybrid drivetrainuses on the analysis, design and con-nous, induction and switched reluc-vehicle and wind power applications, batteries and development of optimi- management system. He is a Senior

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