A Novel Hybrid Brushless dc motor/Generatorfor Hybrid Vehicles Applications

E. Afjei, H. Toliyat, Senior Member, IEEE, and H. Moradi

Abstract--The Brushless dc motor is a simple and robustmachine, which has found application over a wide power andspeed ranges in different shapes and geometries. This paper

presents a new configuration for an integrated starter-generatorsystem based on brushless dc motor without permanent magnettechnology. The proposed novel motor consists of twomagnetically dependent stator and rotor sets (layers), where eachstator set includes nine salient poles with windings wrappedaround them while, the rotor comprises of six salient poles withno windings. The magnetic field passes through a guide to therotor then the stator and finally completes its path via the motorhousing. To evaluate the motor performance, two types ofanalysis, namely numerical technique and experimental studyhave been utilized. In the numerical analysis the finite elementanalysis is employed, where as in the experimental study, a

proto-type motor has been built and tested. The calculatedresults compare favorably with the test results. Due to theruggedness of the proposed motor in comparison with theconventional and brushless dc motors it looks very promising forhybrid vehicle.

Index Terms-- Brushless dc motor, dc motor, hybrid vehicle.

I. INTRODUCTION

UE to the increasing demand for higher power and lesslJ fuel consumption in cars, the concept of starter-generator

integrated into the flywheel has been considered over thepast several years. It is intended to provide the starter for thethermal engine and the generator for charging the car batteryand supplying the on board equipment. Significantenhancement of vehicle driving performance andimprovements in fuel economy and exhaust emissions hasbeen demonstrated by the introduction of more-electric driveconcepts for road transportation. Although the application ofelectrical machines and drives systems in all-electric andhybrid-electric vehicles has been widely reported in recentyears [1-4], there has been a relatively slow progress in thesetechnologies due to the cost of major vehicle technologicalchange. However, mild hybrid solutions have been recognized

E. Afjei is with the Department of Electrical and Computer Engineering,Shahid Beheshti University, Tehran, Iran (e-mail: e-afjeinsbu.ac.ir).

H A. Toliyat is with the Department of Electrical Engineering, TexasA&M University, TX., USA. (e-mail: toliyat0ee.taru.edu).

H. Moradi is with the Shahid Beheshti University.(e-mail:hmchl580(,yahoo.com).

0-7803-9772-X/06/$20.00 (©2006 IEEE

as a next solution, since they are viable within the existingautomotive infrastructure. Switched Reluctance Generator(SRG) is an attractive solution for worldwide increasingdemand of electrical energy. It is low cost, fault tolerant witha rugged structure and operates with high efficiency over awide speed range. Merits of using SRG have been proved forsome applications like starter/generator for gas turbine ofaircrafts [5, 6], windmill generator [7, 8] and as an alternatorfor automotive applications [9]. In [10], principle of operationof SRG has been presented and the necessity for closed loopcontrol is proved. In [11], the control of excitation of SRG formaximum efficiency at single pulse mode of operation hasbeen presented. Turn on and turns off angles are defined ascontrol variables, turn on angle is set based on the outputpower and the turn off angle is selected to achieve optimalefficiency at each power level and speed [12]. Fig. 1 showsthe general block diagram of a hybrid vehicle.

Thermal HybridEnlgine ; t0en

COFFUI 1-e

Fig. 1. The general block diagram of a hybrid vehicle.

Block diagrams contains the combustion engine, theelectric motor [integrated starter-generator (ISG)], the batterypack and the power electronics system. The traction systemalso contains the gearbox. The main advantage of an ISGsystem over a conventional starter motor can be found in thefast engine stop/start behavior and the potential of reducednoise, vibration, and harshness during actively controlledengine startup and shutdown procedures [13].

This paper presents construction, numerical analysis,experimental results of a new type of hybrid dcmotor/generator.

II. MOToR/GENERATOR DESCRIPTION

The proposed novel motor/generator consists of twomagnetically dependent stator and rotor sets (layers), whereeach stator set includes nine salient poles with windingswrapped around them while, the rotor comprises of six salientpoles without any windings. Every stator and rotor pole arcs isabout 30. The two layers are exactly symmetrical with respectto a plane perpendicular to the middle of the motor shaft. This

is a three phase motor/generator, therefore, three coilwindings from one layer is connected in series with the otherthree coil windings in the other layer. Fig. 2 shows the shapeof the stator and the rotor laminations.

Fig. 2. (a) Stator lamination. (b) Rotor lamination.

There is a stationary reel, which has the field coils wrappedaround it and is placed between the two-stator sets. This reelhas a rotating cylindrical core, which guides the magneticfield. The magnetic flux produced by the coils travels throughthe guide and shaft to the rotor and then to the stator poles,and finally closes itself through the motor housing. Therefore,one set of rotor poles is magnetically north and the other set ismagnetically south. In this motor, the magnetic field has beeninduced to the rotor without using any brushes. A cut view ofthe motor/generator is shown in Fig. 3.

Mo±or FIeldhousing col1 stotoL

WaveguldeRotorpoLes

Fig. 3. The cut view of the motor/generator.

In order to get a better view of the motor/generatorconfiguration, the complete motor /generator assembly isshown in Fig. 4.

Fig. 4. The complete motor/generator assembly.

There are two stators and rotors sections placed on bothsides of the field coil assembly which has the rotor shaft as its

main core and two front / end caps plus the motor housing. Aset of photo interrupters are also place in the back of themotor for the detection of rotor position. One of the mostwidely used methods for analysis of any types motor orgenerator is the finite element technique [7].

III. NUMERICAL ANALYSIS

One side or layer of the motor/generator cross section isshown in Fig. 5.

Fig. 5. One layer of the motor/generator cross section.

As seen from Fig. 5, this motor has nine stator poles aswell as six rotor poles, which will be engaged in the torqueproduction mechanism. The design of the motor becomescomplicated due to complex geometry and material saturation.The reluctance variation of the motor has an important role onthe performance; hence an accurate knowledge of the fluxdistribution inside the motor for different excitation currentsand rotor positions is essential for the prediction of motorperformance. The motor can be highly saturated under normaloperating conditions. To evaluate properly the motor designand performance a reliable model is required. The finite-element technique can be conveniently used to obtain themagnetic vector potential values throughout the motor in thepresence of complex magnetic circuit geometry and nonlinearproperties of the magnetic materials. These vector potentialvalues can be processed to obtain the field distribution,torque, and flux leakage.

In order to be able to analyze this motor in 2-D case, onlyone side of the motor which is symmetric to the field coil isconsidered, normal boundary conditions are applied to theouter and inner borders of the motor, and a magnet producingthe same magnetic field density is considered to act as thefield coil. It is worth mentioning that the other options for thefield coils could have been using small coil windings on therotor poles.

The field analysis has been performed using a MagnetCAD package [8], which is based on the variational energyminimization technique to solve for the magnetic vectorpotential. The partial differential equation for the magneticvector potential is given by [9-10].

a (OA> a OAI- = (1)ax oxax) ay K ay)Where, A is the magnetic vector potential.In the variational method (Ritz) the solution to (1) obtained

by minimizing the following functional

F(A) Iff_[e,)2 (aA2]dQ ffJA dQ (2)2Q o ay Q

Where Q is the problem region of integration.In order to be able to analyze the motor in two dimensions,

the field normal boundary conditions are used over the innerand outer boarders of the motor. In the finite element analysissecond order triangular elements with dense meshes at placeswhere the variation of fields are greater have been used.

Figs. 6 and 7show the magnetic flux and the magnetic fielddensity for aligned and non-aligned cases when the machineacting as a motor.

Shaded PlotIBI smoothedE 0.71t291

Fig. 6. Plots of magnetic field density and magnetic flux for aligned case.

|Shaded Plot|IBI smoothed

0 3.9C99|jj0;312781lp02SZ69111 0.15e402

h| nSAS7212

constant current in two phases of the motor in appropriateswitching cycle.

There is a good discussion on the static torque vs. positionfor BLDC motor in [11] which models the effects of skewingin BLCD motor on its performance. It is worth mentioninghere that, the stator and rotor cores are made of a non-orientedsilicon steel lamination. The magnetization curve is takenfrom manufacturer's data sheet for M-27 steel. In the Fig. 8zero degree is considered as unaligned case.

In the generator mode, the plots of magnetic field densityand magnetic flux for only field coil considered to be turnedon are shown in Fig. 9.

Shaded PltIBI smoothed

020ff889

Fig. 9. Plots of magnetic field density and magnetic flux.

IV. EXPERIMENTAL RESULTS

The motor has been fabricated and tested for performanceand functionality in the laboratory. Fig. 10 illustrates the novelbrushless dc motor fabricated in the laboratory.

Fig. 7. Plots of magnetic field density and magnetic flux for non-aligned case.

The plot of static torque versus rotor positions developedby the hybrid brushless dc motor is shown in Fig. 8. Fig. 10. The actual brt

0.7

0.6E

0.52,s 0.40*0 0.30w 0.2co

0.1

0 5 10 15 20

Rotor position (Deg.)

Fig. 8. Static torque versus rotor angle.

The static torque of the motor was obtained by blockingthe motor at different angle. The average static torque for arated current of 3A was measured to be about 46 N.cm overthe stator pole arc (O to 300). It suddenly went to zero at thestart of stator to rotor complete overlap. It was observed thatthe static torque shows lower value than computed which isexpected, since, the silicon sheet steel material used to buildthe motor is not quite what is used for the numerical analysis.

25 30 35 Using a motor generator assembly, the dynamic torque forthe motor versus speed has been measured by loading themotor. The torque speed characteristics of the motor for twodifferent field currents is shown in Fig. 11.

The Torque versus angle characteristics of the motorobtained from using the finite element method by giving

E0z

0

12

108

64

2

0

A Field current.25A

* Field current.5 A

0 1000 2000 3000 4000

speed (RPM)

Fig. 11. Dynamic torque of the motor versus speed.

The power curve fitting has been used for the data points.The torque speed characteristics of the motor behave like aseries dc motor and switched reluctance motor.

Fig. 12 shows the plot of the motor torque versus currentunder different loads for the motor.

E0z

0I-a0

121086420

iField current.5AField current.25 A

Fig. 13. Opto- interrupters with Slotted Disk.

The output signals come from the photo-interruptersmounted on the back of the motor. There are three 300 pulsesproduced by the motor shaft position sensors and each pulseappears 6 times in one rotation. Fig. 14a shows the resultingpulses produced by the sensing unit for 300 duration while14b shows two consecutive photo-interrupters.

0 2 4

Current (A)

Fig. 12. Plot of the motor torque versus current.

As seen from the Fig. 12 the torque is proportional to thesquare of motor current, which resembles the switchedreluctance motor. The static torque versus rotor position isalso obtained using a torque meter which in general agreeswith the one found numerically. There is a good discussion onthe static torque vs. position for BLDC motor in [11] whichexplains the torque curvature produced under differentskewing in this type of motor.

In BLCD motors, each individual phase excitation must besynchronized with the rotor position, which necessitates theneed for a position sensing scheme. In general there are twotypes of rotor position sensing method namely, direct andindirect. In the direct position sensing method usually, amechanical shaft position transducer, such as opto-couplerswith a slotted disk, Hall-effect sensors and embeddingpermanent magnets within the teeth of the slotted disk, or ahigh precision encoder is mounted on the motor housing toproduce the necessary and accurate rotor position informationfor the proper motor operation. Fig. 13 shows opto-couplerswith slotted disk together used in detecting the rotor positionfor the new brushless dc motor.

Fig. 14a. The output signals from the photo-interrupters.Fig. 14b. Resulting pulses from two consecutive photo-interrupters.

In Fig. 14a there is no overlapping between the phases andeach photo-interrupter works one third of the period. In Fig.14b the overlapping of two photo-interrupters output signalswhich corresponds to two of the motor phases to be on at onetime is clearly shown. It is possible to adjust the unit for anyoverlapping needed.

The shaft of the motor/generator machine is connected to amotor to act as prim mover. The speed of the motor keptconstant first at 1000 and then at 2000 rpm under differentloads for various field current. The results of these tests areshown in Figs. 15 and 16.

8-

6-

5-

4

3-

2-

Field Current- 1

ield Current = 0.76Field Current = 0.5

IField Current = 0.25

0 0.5 1 1.5 2

Power (P)

Fig. 15. Output voltage vs. power (1000 rpm).

u

* 1

* 0.75

0.5

0.25

20

18

16

14

2 12

109 o

> 8

64

4-

* 0.25

----X_--------------\II

=Field

Field Current = 0.5

Field Current = 0.25

0 2 4 6 8 10 12 14 16

Power (P)

Fig. 16. Output voltage vs. power (2000 rpm).

In these Figs. curve fitting (power) has been used for betterpresentation of the data points. The actual output voltages fortwo consecutive phases are also shown in Fig. 17.

Fig. 17. output voltages for two consecutive phases.

Figs. 1 8a and 1 8b show the output voltages from one phase

of the generator for a field current of O.25A and O.5A,

respectively

Fig. 18. Output voltage from one phase of the generatorA) Field current of .25 A B) Field current of .5 A.

The voltages have harmonics which are due to the shape ofthe stator and rotor poles. These voltages are then rectified bya three phase full bridge configuration.

V. CONCLUSION

In this paper a novel motor/generator was fabricated in thelaboratory. Some of the motor parameters numericallycomputed and experimentally measured and tested. The mainobjective of this paper namely, introduction of a new

motor/generator configuration with out using any permanentmagnet to increase its durability has been achieved. Theexperimental analysis shows the functionality of themotor/generator in its new configuration, meaning, it has theability and the potential of becoming a motor potentially beused in hybrid vehicle.

VI. REFERENCES[1] K. M. Rahman, B. Fahimi, G. Suresh, A. V. Rajarathnam, and M.

Ehsani, "Advantages of Switched Reluctance Motor Applications to EVand HEV: design and control issues", IEEE Trans. Industry.Applications, vol. 36, pp. 111-121, Jan./Feb. 2000.

[2] Emadi, " Low-Voltage Switched Reluctance Machine Based TractionSystems for lightly Hybridized vehicles", Society of AutomotiveEngineers, pp. 1-7, 2001.

[3] Koch, J. Lehmann, G. Probst, and H. Schafer, "The Integrated StarterGenerator as Part of the Power Train Management", AachenerKollequium Fahrzeug- und Motorentechnik, pp. 11-11, 2000.

[4] S. A. Long, N. Schofield, D. Howe, M. Piron and M. McClelland,"Design of a switched reluctance machine for extended speedoperation," Proceedings of IEEE International Electrical Machines andDrives Conf. (IEMDC), Wisconsin, pp. 235-240, 1-4 June 2003.

[5] S. R. MacMinn and J. W. Sember, "Control of a Switched-ReluctanceAircraft Starter-Generator Over a very wide Speed Range," in Proc.Intersociety Energy Conversion Engineering Conf., 1989, pp. 631-638.

[6] C.A Ferreira., S.R Jones, W.S. Heglund, W.D. Jones, "Detailed Designof a 30-kW Switched Reluctance Starter/Generator System for a GasTturbine Engine Application", IEEE Transactions on IndustryApplications, Volume: 31, Issue: 3, May-June 1995.

[7] M.A Mueller, "Design ofLow Speed Switched Reluctance Machines forWind Energy Converters", Electrical Machines and Drives, 1999. NinthInternational Conference on (Conf. Publ. No. 468), 1-3 Sept. 1999, pp.60 - 64.

[8] R. Cardenas, W. F. Ray, and G. M. Asher, "Switched ReluctanceGenerators for Wind Energy Applications," in Proc. IEEE PESC'95,1995, pp. 559-564.

[9] B. Fahimi, A. R.B. Emadi, Jr. Sepe, "A Switched Reluctance MachineBased Starter/Alternator for More Electric Cars" Energy Conversion,IEEE Transactions on Energy Conversion, Volume: 19, Issue: 1 , March2004, pp.1 16 - 124.

[10] A. Radun, "Generating With the Switched-Reluctance Motor," in Proc.IEEE APEC'94, 1994, pp. 41-47.

[11] Y. Sozer, D.A. Torrey, "Closed Loop Control of Excitation Parametersfor High Speed Switched-Reluctance Generators", IEEE Transactions onPower Electronics, Volume: 19, Issue: 2 , March 2004, pp. 355 - 362.

[12] B. Fahimi, S. Dixon, "Enhancement of Output Electric Power inSwitched Reluctance Generators", IEEE International on ElectricMachines and Drives Conference, 2003, IEMDC'03, Volume: 2 , 1-4June 2003,pp.849 -856.

[13] Ion Boldea,, Lucian Tutelea, and Cristian I. Pitic, "PM-AssistedReluctance Synchronous Motor/Generator (PM-RSM) for Mild HybridVehicles: Electromagnetic Design", IEEE Transactions on IndustryApplications, Vol. 40, No. 2, March, 2004, pp. 492-498.

VII. BIOGRAPHIES

Ebrahim S. Afjei received the B.S. and M. S. degrees in electricalengineering from the University of Texas in 1984 and 1986, and the Ph.D.degree from New Mexico State University, Las Cruces, in 1991. He iscurrently a professor in the Department of Electrical & ComputerEngineering, Shahid Beheshti University, Tehran, Iran. His research interest isin switched reluctance motor drives.Hamid A. Toliyat (S'87-M'91-SM'96) received the B.S, degree from SharifUniversity of Technology, Tehran, Iran, in 1982, the M.S. degree from WestVirginia University, Morgantown, in 1986, and the Ph.D. degree from theUniversity of Wisconsin, Madison, in 1991, all in electrical engineering.Following receipt of the Ph.D. degree, he joined the Faculty of FerdowsiUniversity of Mashhad, Mashhad, Iran, as an Assistant Professor of electricalengineering. In March 1994, he joined the Department of ElectricalEngineering, Texas A&M University, where he is currently a Professor. Hismain research interests and experience include multi-phase variable speeddrives for traction and propulsion applications, fault diagnosis of electricmachinery, analysis and design of electrical machines, and sensorless variablespeed drives. He has published over 190 technical papers in these fields.Dr. Toliyat received the Texas A&M Select Young Investigator Award in1999, the Eugene Webb Faculty Fellow Award in 2000, the Space Act Awardfrom NASA in 1999, the Schlumberger Foundation Technical Awards, in2000 and 2001, and the 1996 IEEE Power Engineering Society Prize PaperAward. He is a member of Sigma Xi and an Editor of the IEEETRANSACTIONS ON ENERGY CONVERSION. He is a member of theEditorial Board of the Electric Machines and Power Systems Journal. He isalso Vice-Chairman ofIEEE-IAS Electric Machines Committee.

o

H. Moradi received the B.S. and M. S. degrees in electrical engineering fromthe University of Esfahanin 2002 and 2006, respectively. His research interestis in motor drives and power electronics.