Chau, KT

Chen, M

Li, WL

Liu, C

Qiu, C

IEEE Transactions on Applied Superconductivity

HKU Scholars Hub

Title Design and analysis of a new magnetic gear with multiple gearratiosAuthor(s) Chen, M; Chau, KT; Li, WL; Liu, C; Qiu, CCitation IEEE Transactions on Applied Superconductivity, 2014, v. 24 n.3, article no. 501904Issued Date 2014URL http://hdl.handle.net/10722/202869Rights IEEE Transactions on Applied Superconductivity. Copyright ©IEEE.IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014 0501904Design and Analysis of a New Magnetic GearWith Multiple Gear RatiosMu Chen, K. T. Chau, Fellow, IEEE, Wenlong Li, Chunhua Liu, and Chun QiuAbstract—This paper proposes a new magnetic variable gearwhich offers different speed ratios. In order to provide the gear-ratio-changing ability, a high-remanence low-coercivity perma-nent magnet (PM), namely the aluminum–nickel–cobalt (Alnico),is utilized. By incorporating the concept of the magnetic gear andthe concept of memory machine, a new double-deck structure ofthe stationary ring is developed to locate the magnetizing windingwhich can dynamically magnetize or demagnetize the Alnico PM.By using finite element analysis, the electromagnetic performanceof the proposed magnetic gear at different gear ratios is analyzed.Hence, the corresponding validity can be verified.Index Terms—Alnico, finite element method, magnetic gear,memory machine, permanent magnet (PM), variable gear ratio.I. INTRODUCTIONM ECHANICAL gears and gearboxes, which play impor-tant roles in vehicular transmission systems, inherentlysuffer from the drawbacks of high transmission loss, bulkysize, annoying noise and wear-and-tear [1], [2]. Magnetic gearscan readily supersede mechanical gears because they offerthe advantages of physical isolation, maintenance free, silentoperation and inherent overload protection [3]–[5]. They canalso be integrated into electric machines to further improve thetorque density [6], [7]. However, the gear ratio of magneticgears cannot be adjusted, whereas the gear ratio of mechanicalgears can be flexibly varied via the gearboxes for differentdriving requirements and road conditions.Although the rare-earth permanent magnet (PM) materialssuch as neodymium-iron-born (NdFeB) and samarium-cobalt(SmCo) are widely adopted for magnetic gears, there is anincreasing concern on the price and supply of the rare-earthelements. Particularly, the use of non-rare-earth aluminum-nickel-cobalt (Alnico) PM material takes the merit of highercost-effectiveness than the NdFeB PM material for magneticgears [8]. Recently, by purposely employing the low coercivityof the Alnico PM material, the memory machines [9] canbe dynamically magnetized or demagnetized to flexibly varythe air-gap flux density, hence achieving wide constant-powerspeed range.The purpose of this paper is to present a new magnetic vari-able gear for vehicular transmission by combining the conceptManuscript received July 15, 2013; accepted November 12, 2013. Date ofpublication November 20, 2013; date of current version December 5, 2013. Thiswork was supported by a grant (Project No. HKU 710612E) from the ResearchGrants Council, Hong Kong Special Administrative Region, China.The authors are with Department of Electrical and Electronic Engineering,The University of Hong Kong, Pokfulam, Hong Kong (e-mail: ktchau@eee.hku.hk).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TASC.2013.2291972Fig. 1. Configuration of proposed magnetic variable gear.of magnetic gears and the concept of memory machines. Thekey is to propose a new double-deck structure of the stationaryring in such a way the magnetizing windings can be artfullyinserted to dynamically magnetize or demagnetize the AlnicoPM pieces in the outer and inner rotors. By using finite ele-ment analysis, the electromagnetic performance of the proposedmagnetic variable gear at different gear ratios is analyzed andsimulated. Hence, the corresponding validity can be verified.II. CONFIGURATION AND DESIGN PRINCIPLEThe basic configuration of the proposed magnetic variablegear consists of the outer rotor mounted with PM pieces, thedouble-deck stationary ring with magnetizing windings, andthe inner rotor mounted with PM pieces. As shown in Fig. 1,the outer rotor adopting a larger number of PM pole-pairs(7 pole-pairs) operates at a lower speed, whereas the inner rotoradopting a smaller number of PM pole pairs (4 pole-pairs)operates at a higher speed. The double-deck stationary ringfunctions to modulate the fluxes between the outer and innerrotors. The key design parameters of this magnetic variable gearare listed in Table I.First, in order to magnetize the inner-rotor and outer-rotorPM poles independently, the double-deck structure of the sta-tionary ferromagnetic ring is proposed. Since the magnetizingwinding only needs to carry a temporary current pulse tomagnetize or demagnetize PMs, it can be easily embedded intothe stationary ferromagnetic ring. The magnetizing windinglocated in the upper-deck is used to magnetize or demagnetizethe outer-rotor PM poles, while the magnetizing winding in thelower-deck is responsible for the inner-rotor PM poles. Smallair bridges are purposely incorporated between the upper-deckand lower-deck in such a way that the coils of the two decks1051-8223 © 2013 IEEE0501904 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014TABLE IKEY DATA OF PROPOSED MAGNETIC VARIABLE GEARTABLE IIPM MATERIAL COMPARISONhave their individual flux paths and can be excited indepen-dently.Secondly, the Alnico PM material, which has the charac-teristics of high remanence and low coercivity, is adopted inthe proposed magnetic variable gear to obtain the function ofdynamic magnetization. The Alnico PM is seldom used in elec-tric machines for industrial applications because its inherentlow coercivity may cause accidental demagnetization due tothe armature current field. Nevertheless, the proposed magneticvariable gear positively utilizes this drawback to effectivelyperform gear-changing. Most importantly, since there is noarmature current involved in the proposed magnetic variablegear during normal operation, the accidental demagnetizationof the Alnico PM is totally eliminated. While the Alnico PMoffers the definite advantage of using non-rare-earth elements,it possesses higher Curie temperature than other PM materials,which is essential when working at harsh vehicular environ-ment. Table II gives a comparison of natural properties amongdifferent PM materials.The key design criterion of the proposed magnetic variablegear is the selection of the number of inner-rotor PM pole-pairsNi, the number of inner-rotor PM pieces Nmi, the number ofouter-rotor PM pole-pairs No, the number of outer-rotor PMpieces Nmo, the number of stationary ring segments Ns, thenumber of the upper-deck magnetizing winding Nuw and thenumber of lower-deck magnetizing winding Nlw. Accordingto the magnetic gear operating principle, Ni, No, and Ns aregoverned by:Ns =Ni +No (1)Gr =NoNi(2)Fig. 2. Airgap flux density waveforms under Gr = 8/3. (a) Inner airgap.(b) Outer airgap.where Gr is defined as the gear ratio.Initially, Ns is kept constant. In order to vary Ni and No,various numbers of PM pieces are properly magnetized ordemagnetized to implement various numbers of pole-pairs, so-called the pole-changing ability. The necessary least number ofPM pieces for the magnetic variable gear can be defined as:M1 =(m11 m12 m1k . . . m1(Ns−1)) (3)where m1k is the least number of PM pieces of one possiblegear ratio which is calculated from the least common multipleof Ns − k and k as given by:m1k = [Ns − k, k] k = 1, 2, 3, . . . , (Ns − 1). (4)Then, the possible values of Nmi, Nmo, Nuw, and Nlw aredetermined by:Nmi = Nmo = Nuw = Nlw = 2n[m1j , . . . m1y︸ ︷︷ ︸selected gear ratios]n = 1, 2, 3, . . . j, y = 1, 2, . . . , (Ns − 1). (5)Considering the practicability of proposed magnetic vari-able gear, Nmi, Nmo, Nuw, and Nlw should be close to2n[m1j , . . .m1y].III. PERFORMANCE ANALYSISBy using finite element analysis, electromagnetic perfor-mances of the proposed magnetic variable gear are simulated.The outer rotor of the gear is coupled with the engine and theinner rotor of the gear serves as the output terminal. Accordingto above design principle, the parameters Nuw, Nlw, Nmo, andNmi are all set as 112, and Ns is set as 11.The airgap flux density waveforms and the resulting torquecharacteristics of the proposed magnetic variable gear undersix sets of Gr are simulated. Namely, the waveforms andcharacteristics under Gr = 8/3, Gr = 7/4, Gr = 6/5, Gr =5/6, Gr = 4/7, and Gr = 3/8 are depicted in Figs. 2–13,respectively. The first three sets of Gr are to scale up the enginespeed, whereas the last three sets are to scale down the enginespeed.From the airgap flux density waveforms, it can be observedthat the inner-airgap pole-pair number and the outer-airgappole-pair number can be changed according to the gear ratios.Hence, the engine torque or speed (namely the outer-rotortorque or speed) can be scaled down or up accordingly to fulfillCHEN et al.: DESIGN AND ANALYSIS OF NEW MAGNETIC GEAR WITH MULTIPLE GEAR RATIOS 0501904Fig. 3. Torque characteristics under Gr = 8/3. (a) Static torque. (b) Steady-state torque.Fig. 4. Airgap flux density waveforms under Gr = 7/4. (a) Inner airgap.(b) Outer airgap.Fig. 5. Torque characteristics under Gr = 7/4. (a) Static torque. (b) Steady-state torque.Fig. 6. Airgap flux density waveforms under Gr = 6/5. (a) Inner airgap.(b) Outer airgap.the torque or speed demand at the output shaft (namely theinner-rotor torque or speed).From the torque characteristics, it can be deduced that thesteady-state torque developed at the outer rotor and inner rotorare 21.6 Nm and 8.1 Nm, respectively, under Gr = 8/3. Thecorresponding torque ripples can be calculated as given by 1.1%and 16.1%, respectively. Consequently, the steady-state torquesand the torque ripples under other sets of Gr can similarly bededuced. All these data are listed in Tables III and IV. It can beobserved that the relationship between the torques developedFig. 7. Torque characteristics under Gr = 6/5. (a) Static torque. (b) Steady-state torque.Fig. 8. Airgap flux density waveforms under Gr = 5/6. (a) Inner airgap.(b) Outer airgap.Fig. 9. Torque characteristics under Gr = 5/6. (a) Static torque. (b) Steady-state torque.Fig. 10. Airgap flux density waveforms under Gr = 4/7. (a) Inner airgap.(b) Outer airgap.at the outer rotor and inner rotor agrees with the value of gearratios. For instance, the torque reduction from the outer-rotortorque of 21.6 Nm to the inner-rotor torque of 8.1 Nm is 2.67which agrees well with the gear ratio of 8/3.With the six workable gear ratios of the proposed magneticvariable gear, namely from 2.67 to 0.38, the vehicle can readilyadjust the gear ratio to fulfill different road conditions anddriving requirement. It should be noted that the dynamic mag-netization or demagnetization of the Alnico PM can be achieved0501904 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014Fig. 11. Torque characteristics under Gr = 4/7. (a) Static torque. (b) Steady-state torque.Fig. 12. Airgap flux density waveforms under Gr = 3/8. (a) Inner airgap.(b) Outer airgap.Fig. 13. Torque characteristics under Gr = 3/8. (a) Static torque. (b) Steady-state torque.by applying a positive or negative current pulse with a durationof only 1 ms. The associated power consumption for such gearchanging is insignificant.Finally, it can be observed that the torque ripples developedat the outer rotor are generally more significant than that atthe inner rotor, especially under Gr = 5/6 and Gr = 3/8. Thisdrawback can be alleviated by further increasing the numberof PM pieces or optimizing the arc shapes and dimensions.Nevertheless, since the outer rotor is coupled with the engine,the corresponding torque ripple is not so stringent as comparedwith that of the inner rotor which is used to drive the mechanicalload.IV. CONCLUSIONThis paper has presented a new magnetic variable gear whichcan provide controllable gear ratios for vehicular transmission.The key is to incorporate the concept of memory machinesinto the concept of magnetic gears in such a way that thePM pole-pair numbers of the inner and outer rotors can beindependently and flexibly controlled. While the Alnico PMmaterial is utilized to perform the desired dynamic magnetiza-tion or demagnetization, it takes the definite advantages of usingTABLE IIISTEADY-STATE TORQUE PERFORMANCETABLE IVTORQUE RIPPLE PERFORMANCEnon-rare-earth elements and possessing high Curie temperature.The key design criterion of the proposed magnetic variablegear has been discussed, namely the selection of the numberof inner-rotor PM pole-pairs, the number of inner-rotor PMpieces, the number of outer-rotor PM pole-pairs, the number ofouter-rotor PM pieces, the number of stationary ring segments,the number of the upper-deck magnetizing winding and thenumber of lower-deck magnetizing winding. The analysis andsimulation results confirm that the proposed magnetic gear canoffer six different gear ratios, namely from 2.67 to 0.38, whichcan readily enable the vehicle adjusting the gear ratio to fulfilldifferent road conditions and driving requirements.REFERENCES[1] K. Atallah and D. Howe, “A novel high-performance magnetic gear,” IEEETrans. Magn., vol. 37, no. 4, pp. 2844–2846, Jul. 2001.[2] L. Jian, K. T. Chau, Y. Gong, J. Z. Jiang, C. Yu, and W. Li, “Comparisonof coaxial magnetic gears with different topologies,” IEEE Trans. Magn.,vol. 45, no. 10, pp. 4526–4529, Oct. 2009.[3] E. Gouda, S. Mezani, L. Baghli, and A. Rezzoug, “Comparative studybetween mechanical and magnetic planetary gears,” IEEE Trans. Magn.,vol. 47, no. 2, pp. 439–450, Feb. 2011.[4] L. Jian, K. T. Chau, W. Li, and J. Li, “A novel coaxial magnetic gearusing bulk HTS for industrial applications,” IEEE Trans. Appl. Supercond.,vol. 20, no. 3, pp. 981–984, Jun. 2010.[5] W. Li, K. T. Chau, and J. Li, “Simulation of a tubular linear magneticgear using HTS bulks for field modulation,” IEEE Trans. Appl. Supercond.,vol. 21, no. 3, pp. 1167–1170, Jun. 2011.[6] K. T. Chau, D. Zhang, J. Z. Jiang, C. Liu, and Y. J. Zhang, “Design of amagnetic-geared outer-rotor permanent-magnet brushless motor for electricvehicles,” IEEE Trans. Magn., vol. 43, no. 6, pp. 2504–2506, Jun. 2007.[7] X. Li, K. T. Chau, M. Cheng, and W. Hua, “Comparison of magnetic-gearedpermanent-magnet machines,” Progr. Electromagn. Res., vol. 133, pp. 177–198, 2013.[8] M. Chen, K. T. Chau, W. Li, and C. Liu, “Development of non-rare-earthmagnetic gears for electric vehicles,” J. Asian Elect. Veh., vol. 10, no. 2,pp. 1607–1613, Dec. 2012.[9] F. Li, K. T. Chau, C. Liu, J. Z. Jiang, and W. Y. Wang, “Design and analysisof magnet proportioning for dual-memory machines,” IEEE Trans. Appl.Supercond., vol. 22, no. 3, p. 4905404, Jun. 2012.

Design and analysis of a new magnetic gear with multiple gear ratios

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Design and analysis of a new magnetic gear with multiple gear ratios

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