外文翻译---电动汽车外转子定子pm无刷电机驱动器的比较.docx

Comparison of Outer-Rotor Stator-Permanent-Magnet Brushless Motor Drives for Electric Vehicles K.T. Chau1, Senior member IEEE, Chunhua Liu1, and J.Z. Jiang2 1 Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China 2 Department of Automation, Shanghai University, Shanghai, 200072, ChinaAbstractIn this paper, two emerging outer-rotor stator-permanent-magnet (PM) brushless motor drives, namely the doubly-salient PM motor drive and the PM hybrid brushless motor drive, are firstly quantitatively compared, which are particularly attractive for serving as in-wheel motor drives for electric vehicles. In order to enable a fair comparison, these two motor drives are designed with the same peripheral dimensions and based on the same outer-rotor 36/24-pole topology. By utilizing the circuit-field-torque time-stepping finite element method for analysis, their steady-state and transient performances are critically compared. Moreover, the cost analysis of these two machines is conducted to evaluate their cost effectiveness.Index Terms Electric vehicle, Finite element method, Machinedesign, Permanent-magnet motor drive. I. INTRODUCTIONIn recent years, permanent-magnet (PM) brushless motordrives have been widely used in electric vehicles (EVs) 1-2.The doubly-salient PM (DSPM) motor drive and PM hybridbrushless (PMHB) motor drive are two emerging stator-PMbrushless motor drives which offer high mechanical integrityand high power density, hence suitable for EV applications 3.Their outer-rotor motor structures are particularly attractive fordirect driving of EVs, especially for serving as in-wheel motordrives for EVs 4. However, a quantitative comparison of thesewo motor drives is absent in literature. The purpose of this paper is to newly compare two emergingouter-rotor stator-PM brushless motor drives, namely the DSPMand PMHB types. Based on the same peripheral dimensions,both motor drives are designed with the identical outer-rotor36/24-pole topology. By using the circuit-field-torquetime-stepping finite element method (CFT-TS-FEM) 5, thesteady-state and transient performances of both motor drives arecompared and analyzed. Moreover, the corresponding costeffectiveness will be revealed and discussed.Section II will introduce the motor drive systems and their configurations. In Section III, the design and cost effectiveness of two motor drives will be compared. Section IV will discuss the analysis approach of these two motor drives. The comparison of their performances will be given in Section V. Finally, a conclusion will be drawn in Section VI.II. SYSTEM CONFIGURATION AND OPERATION MODES Fig. 1 shows the schemes of these two outer-rotor stator-PM motor drives when they serve as the in-wheel motor drives for EVs, especially for motorcycles. It can be seen that these in-wheel motor drives effectively utilize the outer-rotor nature and directly couple with the tire rims. So, these topologies can fully utilize the space and materials of the motor drives, hence greatly reducing the size and weight for EV applications. Fig. 1. Topologies of proposed in-wheel motor drives. (a) DSPM. (b) PMHBThe two motor drives configurations are shown in Figs. 2 and 3. It can be found that they have the similar three-phase full bridge driver for the armature windings; while the difference is the H-bridge driver for the DC field windings of the PMHB motor drive. Hence, their operation principles are very similar, except that the controllable field current of the PMHB motor drive. For both motor drives, when the air-gap flux linkage increases with the rotor angle, a positive current is applied to the armature windings, resulting in a positive torque. When the flux linkage decreases, a negative current is applied, also resulting in a positive torque. For the PMHB motor drive, it can accomplish online flux regulation by tuning the bidirectional DC field current.When these two motor drives act as in-wheel motor drives and are installed in the EVs, they operate at three modes within the speed range of 01000rpm, namely the starting, the cruising, and the charging.When the EV operates at the starting mode, it needs a high torque for launching or accelerating within a short time. For the DSPM motor drive, since its PM volume is much more than that of PMHB motor one, it can provide a sufficiently high torque for the EV starting. For the PMHB motor drive, the positive DC field current will be added to produce the magnetic field together with the PM excited field, hence it also able to offer the high torque for the EV to overcome the starting resistance and the friction force on the road.When the EV runs downhill or works in braking condition, it works in the charging mode. In this mode, these two machines can play the role of electromechanical energy conversion, which recover or regenerate the braking energy to recharge the battery module. Furthermore, for the PMHB machine drive, it can fully utilize its flux controllable ability to maintain the constant output voltage for directly charging the battery, which is more flexible than the DSPM machine drive. When the EV runs in the cruising mode or in the steady speed, these stator-PM motor drives will enter the constant-power region. This speed range usually covers 400rpm1000rpm for the DSPM in-wheel motor drive. But for the PMHB motor drive, it not only can effectively extend its operating speed range up to 4000rpm which is enough to cover the conventional speed range requirement, but also can regulate its magnetic field situation which can make the power module working at the optimal operation point. Fig. 2. Configuration of DSPM motor drives.Fig. 3. Configuration of PMHB motor drive.III. COMPARISON OF MOTOR DRIVES STRUCTURES AND FEATURESThe two stator-PM motor drives structures are shown in Figs. 2 and 3. It can be seen that they have the same peripheral dimensions and the identical outer rotor, as well as the same 36/24 pole and armature windings. The major difference is their stators and field excitations. The DSPM motor drive is simply excited by PMs, which is located in the stator. But for the PMHB motor drive, it has double-layer stator and double excitations. Its outer-layer stator accommodates the armature windings, whereas its inner-layer stator contains PMs and DC field windings together to produce the magnetic field 6. Their similar structures achieve many advantages when they serve as the in-wheel motor drives for EVs. The outer-rotor nature can make the machine directly connect with the tire rim, which totally eliminates the mechanical gear transmission and processes high mechanical integrity. Hence, it reduces the power loss, the system complication, and the total cost. These motor drives fully utilize the whole space, which makes them compact and effective. They arrange the stator to locate the windings and excitations, hence resulting in the robust outer rotor. The concentrated armature windings with 36/24 fractional-slot structure can shorten the magnetic flux path and the span of end-windings, which lead to reduce both iron and copper materials. Moreover, this arrangement of windings can significantly reduce the cogging torque which usually occurs at conventional PM motor drives. Their different constructions also make them have distinct features. For the DSPM motor drive, it has simpler structure than the PMHB one. Also its control strategy is simpler. But this simple structure limits its flexibility due to its uncontrollable airgap flux. For the PMHB motor drive, since it fully takes advantage of double excitations (both PMs and DC field windings), it can offer flexible airgap flux control, including flux strengthening or weakening. In addition, the air-bridge is present to shunt with each PM, hence amplifying the flux weakening ability. The corresponding field excitation inevitably causes additional power loss. Nevertheless, this reduction of efficiency can be partially compensated by the efficiency improvement due to airgap flux control. By properly tuning the airgap flux density, the efficiency can be online optimized at different speeds and loads. Fig. 4. Control strategies. (a) DSPM. (b) PMHB.Fig. 4 shows the control strategies of these two stator-PM motor drives, indicating that the PMHB motor drive has an additional flux controller to regulate the airgap flux. The pole selection of the DSPM motor drive is governed by the following equations: N s = 2mk and N r = N s - 2k （1）where m is the number of phases, k the integer, N s the number of stator poles, and N rthe number of rotor poles. The pole selection of the PMHB motor drive is given by: 4mp and 2Ns/m (2) where p is the number of pole pairs of the DC field windings.Therefore, when the suitable parameters are selected, namely m= 3, p= 3, and k= 6 , the poles of these stator-PM motor drives lead to be 36， and 24 . It can be found that for three-phase armature windings of the PMHB motor drive, all the other parameters can be obtained according to the value of p. Hence, the aforementioned equation (2) can be used to simply determine the other possible slot-tooth combination for the PMHB motor drive.IV. ANALYSIS APPROACH The CFT-TS-FEM can be used to analyze the steady-state and ransient performances of both machine drives. For each machine drive, the mathematic model consists of three sets of equations: the electromagnetic field equation of the machine, the circuit equation of the armature windings, and the motion equation of the motor drive. The electromagnetic field equation of both machine drives is given by 7:where is the field solution region, v the reluctivity, the electrical conductivity, J the current density, A the magnetic vector potential component along the z axis, and and the PM remanent flux density components along the x axis and y axis, respectively. It should be noted that for the PMHB machine drive, the DC field current excitation is regarded as a component added together with the PM component as the magnetization. The circuit equation of the armature windings at motoring is governed by:where u is the impressed voltage, R the resistance per phase winding, i the phase current, Le the inductance of the end winding, l the axis length of iron core, S the conductor area of each turn of phase winding, and the total cross-sectional area of conductors of each phase winding. The motion equation of both motor drives is given by:where is the moment of inertia, the electromagnetic torque, the load torque, the damping constant, and the mechanical speed.After discretization, the above three sets of equations can be solved at each step. Hence, the steady-state and transient performance of both machine drives can be deduced. Fig. 5 shows the no-load magnetic field distributions of both machine drives. It can be seen that the DSPM machine has a constant field pattern, whereas the PMHB machine exhibits different field patterns at different field excitations (350 A-turns, 0 A-turns, and +1000 A-turns). It verifies that PMHB motor drive has the flux controllable ability.Fig. 5. Magnetic field distributions. (a) DSPM. (b) PMHB with 350 A-turn. (c) PMHB with 0 A-turn. (d) PMHB with +1000 A-turns.V. COMPARISON OF MOTOR DRIVE PERFORMANCES Based on the same peripheral dimensions and the identical outer-rotor configuration, the two stator-PM motor drives are designed. Their corresponding design data are listed in Table I.Since the DSPM motor can accommodate more PMs than the PMHB one, its power density is 167% of the PMHB one. However, this merit in power density is offset by the high cost of PMs. From Table I, it can be seen that the DSPM motor utilizes the PM volume up to 502% of the PMHB one. Based on the present international rates, the PM material cost of the DSPM motor is US$116.3 as shown in Table II, which is much higher than the US$22.3 of the PMHB one. Hence, it leads to the total material cost of the DSPM motor is over 174% of the PMHB one. The corresponding cost per unit power and per unit torque of the PMHB motor is significantly less than that of the DSPM one. Thus, the PMHB motor is much more cost-effective than that of the DSPM one. TABLE I PARAMETERS OF DSPM AND PMHB MOTOR DRIVES Armature current density5 A/mm2Armature phases3Rotor outside diameter270.0 mmRotor inside diameter 221.2 mmAir-gap length0.6 mmStack length 80.0 mmPM excitationNd-Fe-BPM remanent flux density1.1TField excitation-DC field windingsRated power 3.2 kW2 kWRated torque34 Nm20 NmRated voltage380 V220 VShaft diameter 70 mm40 mmSpeed900 rpm0-4000 rpmPower density122 W/kg73 W/kgPM volume 284.8 cm354.7 cm3DC winding volume -230.2 cm3TABLE IICOSTING OF DSPM AND PMHB MACHINESItemsDSPM machinePMHB machinePM cost 116.3 USD22.3 USDDC winding cost -20.0 USDArmature winding cost27.8 USD27.8 USDIron cost25.8 USD27.1 USDTotal material cost169.9 USD97.2 USDCost per unit torque5.0 USD/Nm4.9 USD/NmCost per unit power53.1 USD/kW48.6 USD/kWBy using the CFT-TS-FEM, the electromagnetic characteristics of the two motor drives are calculated and compared. Fig. 6 shows the airgap flux density distributions of both motor drives, indicating that the PMHB motor drive can offer a very wide range of flux control (up to 9 times). Then, the flux linkage of the DSPM machine at full magnetization level is shown in Fig. 7(a), whereas those of the PMHB machine are computed at different magnetization levels with various field currents and shown in Fig. 7(b). It can be seen that the two motor drives have the similar forms of flux linkages, but have different amplitudes. Due to the use of more PMs, the DSPM motor drive can definitely produce higher torque than the PMHB motor one. Nevertheless, as shown in Fig. 8, the PMHB motor drive can utilize flux strengthening to achieve the torque up to 85.7% of the DSPM motor one, even though its PM volume is only 19.2% of the DSPM one. Also, since the PMHB motor drive inherently provides low airgap flux density than the DSPM motor one while they have a similar tooth-slot structure, the PMHB motor drive can offer significantly lower cogging torque than that the DSPM motor one as depicted in Fig. 9. It also illustrates that the cogging torque of both motor drives is small due to the use of concentrated armature windings with 36/24 fractional-slot structure. When the two motor drives run in the starting mode, their transient torque responses (normalized by the rated values) are compared as shown in Fig. 10. When they start a load torque of 40 Nm, their armature currents can still be limited to 2 times the rated value. It can be also found that the PMHB motor drive can produce much higher starting torque in the presence of flux strengthening at 750 A-turn. When both of the stator-PM machines work in the generation mode, their no-load EMF waveforms at different speeds are shown in Fig. 11. Because of uncontrollable flux, the DSPM machine generates speed-dependent EMF waveforms. On the contrary, the PMHB machine can uniquely achieve constant-amplitude EMFs by the use of flux strengthening at 250 rpm and flux weakening at 1000rpm, which covers all the constant-power speed range of the in-wheel EV drive. Hence, the PMHB machine can keep the constant output voltage for directly charging the battery.Fig. 6. Airgap flux density distributions. (a) DSPM. (b) PMHB.VI. CONCLUSIONTwo emerging stator-PM motor drives (the DSPM and the PMHB types) have been quantitatively compared. Based on the same peripheral dimensions and outer-rotor 36/24-pole topology, the two motor drives have undergone detailed performance analysis. Compared with the DSPM motor drive, the PMHB motor drive takes the definite merit of flux controllability, hence achieving better constant-power profile, lower cogging torque, higher starting torque and constant voltage generation over a wide speed range. Also, with the view of material cost, the PMHB machine is more cost-effective than the DSPM one to produce the desired power. ACKNOWLEDGMENTThis work was supported and funded by a grant (HKU 114/06E) from the Research Grants Council, Hong Kong pecial Administrative Region, China.REFERENCES1 K.T. Chau and C.C. Chan, “Emerging energy-efficient technologies for hybrid electric vehicles,” IEEE Proceedings, Vol. 95, No. 4, April 2007, pp. 821-835. 2 Z.Q. Zhu and D. Howe, “Electrical machines and drives for electric, hybrid and fuel cell vehicles,” IEEE Proceedings, Vol. 95, No. 4, April 2007, pp. 746-765. 3 K.T. Chau, C.C. Chan, and C. Liu, “Overview of permanent-magnet brushless drives for electric and hybrid electric vehicles,” IEEE Transactions on Industrial Electronics, Vol. 55, No. 6, June 2008, pp. 2246-2257. 4 C.C. Chan and K.T. Chau, Modern Electric Vehicle Technology. Oxford: Oxford University Press, 2001. 5 Y. Wang, K.T. Chau, C.C. Chan, and J.Z. Jiang, “Transient analysis of a new outer-rotor permanent-magnet brushless dc drive using circuit-field-torque time-stepping finite element method,” IEEE Transactions on Magnetics, Vol. 38, No. 2, March 2002, pp. 1297-1300. 6 C. Liu, K.T. Chau, J.Z. Jiang, and L. Jian, “Design of a new outer-rotor permanent magnet hybrid machine for wind power generation,” IEEE Transactions on Magnetics, Vol. 44, No. 6, June 2008, pp. 1494-1497. 7 S.J. Salon, Finite Element Analysis of electri