Automotive App

IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 12, DECEMBER 2008 4591

A Coreless Axial-Flux Permanent-Magnet Generatorfor Automotive ApplicationsSaeid Javadi�� and Mojtaba Mirsalim��

Electrical Engineering Department, Amirkabir University of Technology, Tehran 15916, IranCenter of Excellence in Power Engineering, Amirkabir University of Technology, Tehran 15916, Iran

Electrical Engineering Department, St. Mary’s University, San Antonio, TX 78228 USA

We present a modified structure of an axial flux permanent magnet generator for an automotive application. The generator has acoreless double-stator structure; one is stationary, and the other can be moved to achieve the desired performance. This provides asimple mechanical way to weaken flux in order to extend the speed range of the generator for a constant output power. The excitationis from permanent magnets mounted on the rotors, and the windings are concentrated with simple circular form to achieve a sinusoidalwaveform with very low harmonic contents. We have designed and analyzed the generator by both the three-dimensional finite-elementmethod and the analytic approach. The calculated values coincide very well with the experimental results.

Index Terms—Automotive, axial flux, coreless, flux weakening, permanent magnet generator.

I. INTRODUCTION

THERE has been a continuous increase in electrical powerdemand in automobiles as a result of the electrification of

existing loads and the introduction of new vehicle functions.One consequence of this increase in demand is that improve-ments in automotive generators are becoming necessary. Also,the progress in the new high magnetic field rare-earth permanentmagnets (PMs) such as Neodymium-Iron-Boron (Nd-Fe-B), themost powerful permanent magnets available today, has showngreat opportunities for novel topologies of electric machines[1], [2]. As an example, a prototype Nd-Fe-B manufactured byHitachi Metals, Ltd., Tokyo, Japan has a residual flux densityof 1.35–1.43 (T), coercivity of 1018–1123 (kA/m), and a max-imum BH-product of 342–390 [1]. Using rare earthPM materials, it is possible to get high power density in a smallvolume. Rare earth Nd-Fe-B magnet helps designers to producehigh performance machines with minimum loss and materials.Coreless machines with conventional structures can as well berealized using rare earth PM materials.

As of today, Lundell-type generators with diode rectifiersare almost universally used in automotive applications [3],[4]. Since classical Lundell-type alternators are reaching theirpower limits, new devices are actively investigated in orderto replace the existing ones, and satisfy the increasing energydemand. However, achieving compact, low-cost, light-weight,and high-efficiency devices is one of the most challengingrequests for designers. Axial-flux machines are among the mostsuitable candidates for several automotive applications due totheir high compactness and lightness, together with their highefficiency [5].

Coreless configurations eliminate any ferromagnetic ma-terial, and thus eliminating the associated eddy current and

Digital Object Identifier 10.1109/TMAG.2008.2004333

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

hysteresis losses. Because of the absence of core losses, acoreless stator axial-flux permanent-magnet (AFPM) machinecan operate at higher efficiency than conventional machines [6],[8]–[11]. Adoption of ironless windings results in a significantreduction in the stator weight, so that a small size actuator canbe employed for shifting of one of the windings. Moreover, byimplementing an iron-less stator core, no cogging torque wouldbe produced.

The development of a coreless three-phase generator consid-ered in this paper is focused on the design, and analysis of amodified structure of an AFPM generator. The machine type se-lection is based on several key advantages, including high powerdensity, being brushless, and low manufacturing cost relative tothe conventional Lundell generator and other proposed PM ma-chines [7]. The dc load voltage of the designed generator underloadings at different speeds is assumed to be 12-V, and the wiresize used is comparable to its Lundell countertype. The designand performance analysis is done by a 3-D finite element, andspace harmonic analytical methods. It will be shown that due tothe simple structure, controllable output voltage, low amount ofharmonics, and higher efficiency, this generator can be a suit-able choice for automobiles and an alternative to the conven-tional Lundell generator.

II. MODELING AND DESIGN

The schematic structure of the proposed AFPM generatorshown in Fig. 1 consists of two stator and three rotor parts.The two ironless stator windings are placed in the air gap be-tween the rotor discs with 1 mm mechanical clearance from therotor on each side. The rotor discs consist of circular flat-shapedhigh energy Nd-Fe-B magnets glued on surfaces, mild steelback plates (yokes), and rotor supporting parts. Fig. 2 shows theschematic rotor poles with an opposite arrangement (N-S type)and the associated flux paths for the proposed generator.

Concentrated armature coils of each stator are glued ontononmagnetic, nonconducting material such as bony fiber that isresistive against temperature and pressure. As previously men-tioned, one stator is stationary while the other can be turned.

0018-9464/$25.00 © 2008 IEEE

4592 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 12, DECEMBER 2008

Fig. 1. Proposed machine structure.

Fig. 2. Flux paths.

By connecting the two stator windings in series, the resultantvoltage can be obtained as in the following [6]:

(1)

where is the mechanical phase shift between the two wind-ings and is the number of pole pairs. If the phase shift is equalto zero, then maximum voltage is induced which is suitable forlow-speed ranges. At higher speeds, the phase shift between thetwo stator windings is increased to obtain the desired voltage atthe output terminals of the generator. In other words, an approx-imately fixed output voltage can be generated for a wide speedrange. Using this configuration a mechanical field weakening isimplemented that is cost effective without power electronic de-vices. To do so, it was necessary to form “gear teeth” on one of

the bony fiber plates (the last part of Fig. 8) to make it turn bya pivot.

The three-phase output voltage of the generator is then recti-fied by diodes to obtain the required 12-V dc output voltage. Theopen circuit dc voltage of the generator is obtained as follows:

(2)

where , , and are output frequency, the number ofcoil turns in series per phase, and the magnetic flux per pole,respectively. Of course, the voltage drop of the diodes and totalresistances should also be taken into account.

The output volt-ampere ratings VA, of the machine can beobtained by [10]:

(3)

where , , , and are the output power coeffi-cient, the mean diameter of stator, the axial length, and the syn-chronous speed of the generator, respectively. The output coeffi-cient is related to magnetic and electric loading of the machine.The electric loading is similar to the Lundell generator [3], [4].

By using circular shape magnets and coils, maximum voltageand minimum resistance and inductance can be achieved. Thenumber of concentrated coils required is calculated by [13]

(4)

From (2)–(4), and the volume limitation the designer is facedwith, the authors chose eight poles and six concentrated coilsfor the proposed three-phase generator.

The schematic of the linearized analytic model and the mod-ified assembly of rectangular PMs to circular shape are shownin Figs. 3 and 4, respectively. The analytic formula for normalcomponent of magnetic flux density in the middle of air gap isas follows [13]:

(5)

where , , , , and are pole pitch at differentradii, the width of PM in circumferential direction, the PMthickness, the residual flux density, and the relative permeabilityof PM, respectively.

To calculate the induced voltage, the variation of flux in thecoils is assumed sinusoidal

(6)

JAVADI AND MIRSALIM: CORELESS AXIAL-FLUX PERMANENT-MAGNET GENERATOR 4593

Fig. 3. Schematic for the analytic model.

Fig. 4. Modified assembly of rectangular PMs to circular shape.

Fig. 5. Coil geometry.

Fig. 6. DC load and the equivalent circuits.

(7)

where and are the inner and outer radii of the armaturecoils, respectively.

Fig. 5 shows the coil geometry of the stator. To calculate theresistance we assumed that the turns per coil is located inmean radius of a coil. And, for leakage inductance calculation, itis assumed that the resultant magnetic flux density produced bya coil in the air gap is in axial direction [13]. Due to the large air

Fig. 7. Prototype generator (in EMTR Laboratory).

gap (distance between the two rotor back irons) flux is mostlyin axial direction, but here, we make this assumption to simplifythe calculation of inductance.

The resistance and inductance of each coil is obtained as fol-lows [13]:

(8)

where and are wire cross-sectional area and mean radiusof the coil, respectively.

Because of a weak mutual effect between the coils, the valuesof stator inductance and resistance are obtained as follows:

(9)

where , , and are the resistance, inductancevalues per phase, and the total number of turns of the coils inseries per phase, respectively.

Fig. 6 shows the circuit and its equivalent model for auto-motive applications. The equivalent circuit parameters and thegenerator efficiency are obtained as in the following [13]:

(10)


Fig. 8. Detailed structure of the machine.

where , , , , and are equivalent dc resis-tance, equivalent resistance due to diode commutating induc-tance, open circuit dc voltage, output dc voltage, and efficiency,respectively [13].

III. PROTOTYPE GENERATOR AND SIMULATION

A prototype generator was constructed for verification. It wastested for various loads at different speeds to evaluate its perfor-mance. The generator and detailed structure of the machine isshown in Figs. 7 and 8. Two parallel coils per phase on eachside are connected in series with each other according to (1). Itshould be noted that one of the stators can be turned by a smallactuator to get the desired voltage and performance. The designdata are summarized in Table I.

TABLE IDESIGN DATA OF THE PROPOSED MACHINE

Fig. 9. Meshed model of half of the generator.

The meshed model of half of the generator is shown in Fig. 9.It is sufficient to analyze half of the machine structure due tosymmetry. For 3-D FEM calculation with motion, the model isdivided in two moving and stationary parts. In this model, thewinding has been placed in the meshed volume and turned whilethe rotor is stationary. This is done arbitrarily to simplify theprocess of simulation.

Fig. 10 shows the relative position of the two windings withrespect to each other. One stator is stationary and another ismovable to get the desired dc output voltage. The required phaseshift in electrical degrees for the proposed generator to get 12dc at no load from zero to 12 000 rpm is depicted in Fig. 11. Itis clear that to get 12 V in loading condition, the curve is shifted


Fig. 10. Models of windings in zero and nonzero mechanical phase shift.

Fig. 11. Phase shift angle versus speed for the proposed generator.

to the right side because of the voltage drop on generator andrectifier circuit elements.

Figs. 12 and 13 show magnetic flux density values in backiron and the PMs. The variations are not smoothly distributedin some parts of the figures due to the coarse mesh density andnumerical errors. It is observed that the flux density in back ironis below 1.8 T which is lower than the field saturation value ofthe magnetic iron.

The comparison between the experimental and theoreticalno-load output voltage at 1000 r/min for a zero phase-shift be-tween the windings is shown in Fig. 14. The results are approx-

Fig. 12. Magnetic flux density distribution in back iron.

Fig. 13. Magnetic flux density distribution in PMs.

imately the same. The harmonic analysis for the experimentalno-load output voltage of Fig. 14 is shown in Fig. 15. The totalharmonic voltage (THD%) is under 7.8%, which is suitable.

The magnetic flux density versus circumferential distance bythe 3-D FEM, and analytical methods are shown in Figs. 16, and17, respectively. It is deduced from the figures that the resultscompare very well. The maximum value of flux density is 0.63 Tfor a 10-mm air gap between permanent magnets, suitable forair core electric machines.

The efficiency and the dc output voltage at 1000 r/min are, re-spectively, shown in Figs. 18 and 19. The maximum discrepancyis less than 8%. The main source of losses in the machine is fromstator copper losses. As it was expected, the graphs are linear,because armature reaction is insignificant in ironless stators.

Assuming an average speed of 2000 r/min for automotivegenerators, a dc voltage of 12 V at rated load is desired. Usu-ally the starting speed of the automotive generator is 2000 (rpm)and the desired dc voltage under loading is 12 V. The windingis of air winding type, which means we can have higher currentvalues for the same condition as in a conventional Lundell gen-erator. Fig. 20 shows the experimental no-load voltage at 2000r/min which is approximately sinusoidal. The theoretical outputvoltage and efficiency at this speed for different dc load currentsare, respectively, depicted in Figs. 21 and 22. One can deduce


Fig. 14. Comparison of the experimental, 3-D FEM, and analytical no-loadvoltages at 1000 r/min. (a) Experimental; (b) 3-D FEM; and (c) analyticalmethod.

Fig. 15. Harmonic spectrum of the voltage at 1000 r/min.

from the plots that up to a load of 25-A, the output voltage ishigher than 12-V and the efficiency for a load current of 10-A(normal current) is higher than 80%. The phase-shifting be-tween the two stators is required to maintain the output voltageat a 12-V value.

Fig. 16. Magnetic flux density variations versus circumferential distance using3-D FEM.

IV. CONCLUSION

The main objective of this paper was to introduce, analyze,and verify a modified structure of axial flux generators for auto-motive applications. The main advantages achieved were simpleand robust structure of the proposed machine, and lower mate-rial and manufacturing cost with better performance. The gen-erator is a brushless PM machine which is an advantage espe-cially at high-speeds. Because of its simple structure, control-lable output voltage, and very low harmonic components, theproposed generator can be a suitable choice and an alternativeto the conventional Lundell generator. Due to coreless stator, thearmature reaction is very small, which is advantageous in shortcircuit conditions especially at high speeds. It also has lowerlosses and temperature rise, and higher efficiency.


Fig. 17. Magnetic flux density variations versus circumferential distance usinganalytical method.

REFERENCES

[1] J. F. Gieras, R. J. Wang, and M. J. Kamper, Axial Flux PermanentMagnet Brushless Machines. Norwell, MA: Kluwer, 2004.

[2] W. Mo, L. Zhang, A. Shan, L. Cao, J. Wu, and M. Komuro, “Improve-ment of magnetic properties and corrosion resistance of NdFeB mag-nets by inter-granular addition of MgO,” Trans. Alloys Comp., vol. 461,no. 1, pp. 351–354, Aug. 2007.

[3] L. M. Lorrilla, T. A. Keim, J. H. Lang, and D. J. Perrault, “Topologiesfor future automotive generators—Part I: Modeling and analytics,” inProc. IEEE Conf. Vehicle Power and Propulsion, 2005, pp. 819–830.

Fig. 18. Comparison between theoretical and experimental efficiencies versusdc load current at 1000 r/min.

Fig. 19. Comparison between theoretical and experimental dc output voltageversus dc load current at 1000 r/min.

Fig. 20. Experimental no-load output voltage at 2000 r/min.

[4] L. M. Lorrilla, T. A. Keim, J. H. Lang, and D. J. Perrault, “Topologiesfor future automotive generators part II: Optimization,” in Proc. IEEEConf. Vehicle Power and Propulsion, 2005, pp. 831–837.

[5] M. Mirzaei, M. Mirsalim, and S. E. Abdollahi, “Analytical modelingof axial air gap solid rotor induction machines using a quasi-three-di-mensional method,” IEEE Trans. Magn., vol. 43, pp. 3237–3242, Jul.2007.

[6] S. M. Hosseini, M. Mirsalim, and M. Mirzaei, “Design, prototyping,and analysis of a low cost axial-flux coreless permanent-magnet gen-erator,” IEEE Trans. Magn., vol. 44, no. 1, pp. 75–80, Jan. 2008.

[7] M. Comenscu, A. Keyhani, and M. Dai, “Design and analysis of42-V permanent-magnet generator for automotive applications,” IEEETrans. Energy Convers., vol. 18, pp. 107–112, Mar. 2003.

[8] L. D. Ferraro, F. G. Capponi, R. Terrigi, F. Caricchi, and O. Honorati,“Ironless axial flux PM machine with active mechanical flux weak-ening for automotive applications,” in Proc. IEEE Industry Applica-tions Conf., 2006, pp. 1–6.


Fig. 21. Theoretical output voltage versus load current at 2000 r/min.

Fig. 22. Theoretical efficiency versus load current at 2000 r/min.

[9] F. Caricchi, F. Crescimbini, O. Honorati, G. L. Bianco, and E. Santini,“Performance of coreless-winding axial-flux permanent-magnet gener-ator with power output at 400 Hz, 3000 r/min,” IEEE Trans. Ind. Appl.,vol. 34, pp. 1263–1269, Dec. 1998.

[10] F. Crescimbini, A. D. Napoli, L. Solero, and F. Caricchi, “Compact per-manent-magnet generator for hybrid vehicle applications,” IEEE Trans.Ind. Appl., vol. 41, pp. 1168–1177, Sep. 2005.

[11] L. Del Ferraro, F. Caricchi, F. Giulii Capponi, and G. De Donato,“Axial-flux PM starter/alternator machine with a novel mechanical de-vice for extended flux weakening capabilities,” in Proc. IEEE 39th IASAnnu. Conf., Oct. 3–7, 2004, vol. 3, pp. 1413–1419.

[12] E. S. Hamdi, Design of Small Electrical Machines. New York: Wiley,1994.

[13] J. R. Bumby and R. Martin, “Axial-flux permanent-magnet air-coredgenerator for small-scale wind turbines,” Proc. Inst. Elect. Eng., vol.152, pp. 1065–1075, Sep. 2005.

Manuscript received April 14, 2008; revised August 05, 2008. Currentversion published January 08, 2009. Corresponding author: S. Javadi (e-mail:[email protected]).

Saeid Javadi was born in Aran and Bidgol, Iran, on March 21, 1969. Hereceived the B.S. degree in communication engineering and the M.S. degreein electrical power engineering from Amirkabir University of Technology,Tehran, Iran, in 1992 and 1999, respectively. He is pursuing the Ph.D. degreein the Department of Electrical Engineering, Amirkabir University of Tech-nology, Tehran. His research interests are numerical and analytical analysis ofelectromagnetic fields, and electrical machines.

Mojtaba Mirsalim was born in Tehran, Iran, on February 14, 1956. He receivedthe B.S. degree in EECS/NE and the M.S. degree in nuclear engineering fromthe University of California, Berkley, in 1978 and 1980, respectively, and thePh.D. degree in electrical engineering from Oregon State University, Corvallis,in 1986.

Since 1987, he has been at Amirkabir University of Technology, where hehas served five years as the Vice Chairman and more than seven years as theGeneral Director in Charge of Academic Assessments, and currently is a FullProfessor in the Department of Electrical Engineering where he teaches coursesand conducts research in energy conversion, electrical machine design, and hy-brid vehicles, among others. His special fields of interest include the design,analysis, and optimization of electric machines, FEM, renewable energy, andhybrid vehicles. He is the author of more than 100 international journal and con-ference papers and three books on electric machinery and FEM. He is Founderand Director of the Electrical Machines and Transformers Research Laboratory(http://www.ele.aut.ac.ir/~emtrl)

Automotive App

Documents