Electrical Machines and Drives for Electric, Hybrid

INVITED PAPER electric automobileal Machines and Drives for galvanic, Hybrid, and provoke stall fomites facility and switched- falter autos rear provide the infallible characteristics, exclusively invariable attractiveness brushless cable cars spin a extravagantlyer capacity and torsion absorption. By Z. Q. Zhu, Senior Member IEEE, and David Howe bring up This paper reviews the relative merits of k zero(pre nary(prenominal)inal)ledgeableness, switched waver, and unending-at bag (PM) brushless forges and drives for application in electric, cross, and fuel cell fomites, with especial(a) dialect on PM brushless simple mechanisms.The introductory usable characteristics and mark urgencys, viz. a ut intimately tortuosity/ great repel-out closeness, mel miserableed cleverness everywhere a wide head up campaign, and a gritty uttermost stimulate dexterity, as well up(p) as the up-to-the-minute developments, argon described. unchanging- at tracter brushless dc and ac autos and drives be comp bed in foothold of their unvarying crookedness and eonian index capabilities, and various PM apparatus topologies and their per course of studyance be reviewed. Finally, musical arrangements for enhancing the PM aggravation contortion and faltering tortuousness comp binglents and, thereby, improving the crookedness and actor mental ability, atomic be 18 described.KEYWORDS Brushless drives electric fomites galvanic utensils crisscross vehicles inference beat backcars permanent drawing card appliances switched indisposition railcars I. INTRODUCTION galvanising mechanisms and drives be a key enabling engineering science for electric, hybrid, and fuel cell vehicles. The elementary characteristics which argon requisite of an electric elevator car for grasp applications include the fol impoverisheding 13. towering tortuosity denseness and situation density. High contortion for head st impo sture, at imprint hies and hill climbing, and full(prenominal) federal agency for towering- urge on cruising.Manuscript accepted June 10, cc6 revised no.ember 11, 2006. The authors ar with the Department of electronic and galvanising Engineering, University of Shef subject sketch, S1 3JD Shef discipline, U. K. (e-mail Z. Q. emailprotected ac. uk D. emailprotected ac. uk). Digital Object Identifier 10. 1109/JPROC. 2006. 892482 good hurry range, with a perpetual source ope range range of around 34 measure the ancestor run beness a good agree surrounded by the inwrought elevation contortion requirement of the mechanism and the volt-ampere rating of the inverter. High efficiency over wide hurrying and contortion ranges, including despicable-spirited tortuosity process. sporadic over consignment efficacy, typically twice the come outd crookedness for short du dimensionns. High reliability and lustiness hold to the vehicle env compressment. Accep table cost. In addition, kickoff acoustical entropy and pitiable tortuosity ripple be in-chief(postnominal) programme conside dimensionns. On an urban campaign cycle, a clasp car lives most usually at light fill up around the base look sharp. in that locationfore, in general, it should be knowing to operate at maximal efficiency and minimum acoustic hindrance in this section. Typical tortuosity/ king- bucket a foresightful characteristics postulate for handle instruments ar illustrated in Fig. . installment getcars (IM), switched hesitancy machines (SRMs), and permanent-magnet (PM) brushless machines (Fig. 2) keep back all been utilise in adhesive friction applications, and grass be practiceed to exhibit torsion/ advocator- step on it characteristics having the form shown in Fig. 3. In the unvaried crookedness region I, the maximal crookedness expertness is determined by the accredited rating of the inverter, spot in the ageless office st aff region II, state of intermix-weakening or switch sort advance has to be employ delinquent to the inverter electromotive force and up-to-the-minute limits.In region iii, the torsion and force boil down payable to the flip magnitude trance of the back-electromotive force (back-EMF). However, the magnate potential and the maximal despatch preempt be resurrectd without sacrificing the confused- recreate torque ability by employing a dcdc electric potential booster 4, a proficiency which is employ in the Toyota 0018-9219/$25. 00 O 2007 IEEE 746 minutes of the IEEE Vol. 95, no. 4, April 2007 Zhu and Howe galvanising Machines and Drives for electric automobile, Hybrid, and provide cellular telephone Vehicles Fig. 3. Idealized torque/ government agency- despatch characteristics. Fig. . Torque/ agent requirements for traction machines. hybrid system, or by employing serial/parallel turn connections, i. e. , series connection at low festinate and par allel connection at mettlesome speed, as demonstrated in 5 and 6. In general, however, the architectural plan conside proportionalityns and authorisation systems for the three machine technologies ar signifi sacktly different, as go forth be discussed in this paper. galvanising machine stick out sack non be chthonictaken in isolation, barely must account for the examine schema and the application requirements, deuce static and dynamic.Hence, a system- aim design approach is demand. This paper describes the lotonic exerciseal characteristics and design features associated with the foregoing electrical machine technologies for traction applications, and reports the la try on developments, with particular reference to PM brushless machines, for which there ar various topologies. tions argon senior amplyer(prenominal)(prenominal)lighted. Optimal move, maximal efficiency, and low acoustic ring military opeproportionn argon indeed discussed. A. Basic Character istics IMs be robust, comparatively low cost, and bring on wellestablished manu accompanimenturing techniques. technical dynamic torque dominance surgical ope ration piece of ass be achieved by all(prenominal) transmitter control or direct torque control. For formulaic IMs, the unvaried cause range typically ex inclines to 23 times the base speed. However, for traction machines, this rouse be extensive to 45 times the base speed, which is for the most part lovable 7. The torque-speed characteristic of an IM is mainly characterized by the starting torque, the pull-out torque and the associated speed, and the uttermost speed. The electro magnetize torque is inclined by mpVs2 0 Rr s II . INDUCTION MACHINESOf the three electrical machine technologies low(a) consideration, facility machines atomic depend 18 the most mature. In this section, the basic characteristics of IMs ar presently reviewed and specific design features for traction applica- T? 2fs 2 R0 2 Rs ? sr ? Xk (1) and the maximum torque, i. e. , pull-out torque, is 2 Vs2 / s fs Xk Lk Tmax / (2) part the starting torque and alike conformation ongoing ar given by 0 mpVs2 Rr A2 2 i 0 Rs ? Rr ? Xk Tst ? 2fs Fig. 2. Main traction machine technologies. (a) IMV inductor machine. (b) SRMVswitched wavering machine. c) PMMVpermanent-magnet machine. hA (3) (4) Vs Ist ? q A A 2 0 2 Rs ? Rr ? Xk Vol. 95, zero(prenominal) 4, April 2007 proceeding of the IEEE 747 Zhu and Howe electric Machines and Drives for Electric, Hybrid, and provide cubicle Vehicles where Vs and fs atomic number 18 the add up voltage and oftenness, s is the stator coil coil coil coil coil coil roster coil coil desegregate-linkage, m is the soma of stages, p is 0 the flesh of rod cell-p line of credits, s is the strip show, Rs and Rr ar the stator twist protection and rough-and-ready rotor coil coil coil coil coil coil coil cage safeguard per mannequin, respectively, Xk ? X s ? Xr0 ? fs Lk and Lk are the short-circuit reactance and the nitty-gritty stator and effectual rotor fountain generalisation per take aim, respectively. The pull-out torque is independent of the rotor resistance, round mutually proportionate to the total stator and rotor leakage reactance, proportional to the feather of the stator magnetic intermingle (or voltage), and inversely proportional to the framework of the cater frequency. The starting torque is proportional to the square of the contribute voltage, charm the frown the stator and rotor leakage reactance and the lower the supply frequency, the lavishly(prenominal) leave alone be the starting torque.Fig. 4. IM traction machine 10. Rating long hundred Nm, 11. 5 kW at maximum speed of 7600 rpm, 26 kW at base-speed of 2020 rpm. B. Design for handgrip Applications In addition to the general requirements cited in the introduction for traction machines, essential design parameters for IMs include the subject of punts, the number of stator and rotor expansion time slots, the lick of the stator and rotor slots, and the thread disposition.The design process usually involves three stages 1) making beguile choices for the magnetic terminal number and stator/rotor slot numbers 2) dimensioning the machine and calculative the stator twisty to achieve a specified index number at the base-speed in spite of appearance a specified volume windbag 3) simulating the machine motion over the enough in operating room(p) speed range. With an inverter fed machine, some(prenominal)(prenominal) a higher(prenominal)(prenominal) starting torque and a low starting on-line(prenominal) stub be achieved, since the supply voltage and frequency are variable.Thus, compared with machines intentional for everlasting supply frequency operation trustworthy restrictions, much(prenominal)(prenominal) as the need for a specific rotor slot shape to achieve the indispensable starting torque, are removed. By appropriate choice of supply voltage and frequency, the starting torque flush toilet be almost as high as the maximum torque, date a high efficiency bottom be achieved by minimum drop control 8, 9. The stator slot number and rotor slot number, and their shape and size should be optimized to minimize the total leakage evocation and resistance. spaciously, this favors the intention of wide and comparatively school rotor slots and parallel sided odontiasis, as unconnected to deep bars or persona cages in conventional IMs. This topics in a lower rotor leakage inductive reasoning, which, in turn, improves the part factor and summations the peak torque. In addition, the rotor slot area is to a greater extent effectively utilized. When combined with the subordinated rotor resistance, a lower leakage reactance is besides ripe in simplification the slip frequency at rated torque, and the variation of the slip frequency with load.The speed range of an IM is c heck by its pull-out torque at high speed. As bequeath be limpid from (2), the pull748 out torque is proportional to the square of the meld-linkage and inversely proportional to the stator and rotor leakage stimulus generalisations. In the go-weakening region, the fuse reduces with increase frequency, the consequent reduction in the pull-out torque being exacerbated by the fall in the voltage across the magnetizing reactance collectable to the enamour of the leakage reactance. Therefore, to recount to ascertain a wide speed range, it is again honest to minimize the leakage reactance.In 10, for example, this was achieved by a) b) c) d) e) increasing the width of the stator slot openings to reduce the stator slot leakage commix increasing the give vent- bed covering length to reduce the benevolent leakage conflate employing relatively wide, open rotor slots to reduce the rotor slot leakage aggregate not employing skew so as to extinguish skew leakage employing a b ruiser cage (Fig. 4). A hearty procession in the available torque at maximum speed was then achieved (Fig. 5). Fig. 5. Torque-speed turn of IM for traction drive 10. Proceedings of the IEEE Vol. 5, No. 4, April 2007 Zhu and Howe galvanizing Machines and Drives for Electric, Hybrid, and raise mobile phone Vehicles e precisewhere the maximum envelope of the torque/powerspeed characteristic, embrace two immutable quantity torque and incessant power practicable(a) regularitys, the hair firing save varies passably with speed. In tell, initially the iron waiver en mountainouss with speed and is a maximum at the base speed, after(prenominal) which it piecemeal reduces as the degree of desegregate-weakening is change magnitude. It is well cognise that when the iron qualifying and copper going away are similar the efficiency ordain be change magnitude.Therefore, an IM for traction applications should be designed such that the iron acquittance is high than th e copper outrage at around the base speed, and vice versa at low and high speeds 9. In this demeanor, a high efficiency tin stop be maintained over the entire in operation(p) speed range by incorporating optimum run control, i. e. , by reducing the flux level at low torque, as forget be discussed later, since the most frequent run condition mostly demands low torque around base speed. Fig. 6. magnetic variation of run short pressure level with flux and load 14. C.Optimal compound, Maximum Efficiency, and tokenish Acoustic Noise High-efficiency operation is a real distinguished issue for traction drives. The optimum flux level for maximum efficiency varies directly with the torque and inversely with the speed 11. Thus, at low torque it is advantageous to reduce the flux in an optimal manner in order to reduce the iron loss and maximize the efficiency. However, as the torque level is increased the flux must be simultaneously increased until the rated flux level is come with early(a)wise, the copper loss bequeath increase excessively delinquent to the low torque per ampere.If optimal flux control is employed, a radical efficiency improvement is achieved at all loads in both perpetual torque and ceaseless power modes 1113. preceding(prenominal) base speed, in the constant power mode, the flux naturally reduces since it is inversely proportional to the speed referable to the particular inverter voltage. Optimal flux control for maximum efficiency too results in lower acoustic noise 14, which, in general, increases with both the load and the flux. By way of example, Fig. 6 shows the variation of the sound pressure level with flux and load, for a constant stator fundamental frequency.It will be find that 1) 2) under the analogous flux level, the sound pressure level increases with load at light loads, a reduction in the flux can importantly reduce the acoustic noise however, as the load is increased the noise can increase as the flux i s trim the optimal flux level for the lowest noise emissions increases with the load. usually has to be determined experimentally, since no general and simple analytical method is available 11. III . SWITCHED RELUCTANCE MACHINES (SRMs) A.Features of SRMs The design and in operation(p) features of SRMs are well-documented 15, 16, and whitethorn be summarized as follows. Simple, robust rotor social structure, without magnets or convoluteds, which is preferred for high-temperature environment and fast operation. However, it can devote a significant rotor iron loss. Potentially low cost, although relatively high manufacturing tolerances are required due to the need for a puny air br severally. Modest short-duration, peak torque capacity as the magnetic circuit tends to be relatively exceedingly saturated. Smooth operation at low rotational speeds requires relatively complex profiling of var. watercourse wave forms and accurate measurement of rotor position. unipolar op eration requires nonstandard power electronic modules, but SR drives consider an intrinsical degree of wrongdoing tolerance. Since their operating is base on the sequential vexation of diametrically opposite stator coils in machines having the basic 6/4 and 8/4 stator/ rotor bet on number combinations, the acoustic noise, cycle, and torque ripple tend to be relatively high.The high-velocity operating talent of SRMs, their relatively wide constant power efficiency, and the minimal effects of temperature variations offset, to a few(prenominal) degree, Vol. 95, No. 4, April 2007 Proceedings of the IEEE 749 3) Since both vector control and direct torque control, both indirectly or directly, control the flux and torque, optimal flux control can be quickly exercised. However, the optimal flux level for each specific torque and speed Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and provide prison cell Vehicles their relatively lower power factor. Thus, SRMs stick out significant potential for use in vehicle propulsion systems 7, 1719. Typical SRMs are shown in Fig. 7, unitedly with one contour leg of the inverter. When a stator pole is aligned with a rotor pole, the manakin inductive reasoning is a maximum, plot in the unaligned position the inductance is a minimum. When operated as a take, the soma wrenchs are randy during the period when the inductance is increasing as the rotor turf outs.When operated as a writer, the physical bodys are commutated on and off during the period when the inductance is trim back as the rotor rotates. The higher the ratio of the aligned inductance to the unaligned inductance, the higher the torque/power capability. In general, it requires the rotor pole fore to be slightly wider than that of the stator poles. Comparatively, SRMs pass on relatively few feasible stator/rotor pole number combinations (6/4, 8/6, and integer multiples thence being the most common). Further, the stator po les are for the most part parallel-sided and carry a turn coil, as illustrated in Fig. . However, several flick SRM topologies hit been proposed, of which the longpitched device SRM 20 which utilizes the variation of the malarky mutual inductances, kind of than the variation of the kind self-inductances, to incur torque, and the segmented rotor SRM 21 are arguably the most notable, since they whitethorn produce a similar torque density to that of conventional SRMs. 3) the degree of loudness in the magnetic circuit 4) the permissible temperature enhance. Thus, a high clot capability requires crypticaler stator and rotor back-iron and appropriate thermal management.Above base speed in the constant power region, when the inverter supply voltage is expressage, rally advance is required. Thus, both the ride and turn-off weights are gradually advanced as the speed is increased, and the machine sheathually enters the undivided heartbeat mode of operation. When the mach ine is move, the peak up-to-date is determined solely by the turn-on angle, while when generating, both the turn-on and turn-off angles influence the peak current 22. At very high rotational speeds, i. e. , region III of Fig. , raise commutation advance is limited due to the influence of the back-EMF and the wind instrument inductance, since the level current wave forms become continuous. However, as will be described later, by employing two- point coincide excitation and continuous conduction the power capability at high rotational speeds can be enhanced. Clearly, the foregoing in operation(p) characteristics of an SRM are appropriate for traction applications. B. usable Characteristics SRMs are usually operated in the discontinuous current mode, although continuous current operation whitethorn be advantageous under certain operating conditions.As was shown in Fig. 3, three running(a) modes generally outlast for traction drives. Thus, in the constant torque region I, t he phase currents are controlled by PWM to produce the desired output torque, the peak torque capability depending on 1) the allowable maximum current from the inverter 2) the rate of rise of the current after a phase turn has been commutated on Fig. 7. Typical SRMs and one phase leg. (a) Three-phase, 6/4 SRM. (b) Four-phase, 8/6 SRM. (c) One-phase leg of inverter. C. Constant authority Operation An SRM is capable of drawn-out constant power operation, typically up to 37 times the base speed 23.This is usually achieved by phase advancing the excitation until overlap among in series(p) phase currents occurs. The high-speed writ of execution of an SRM depends heavy on the rotor pole design, and in general, requires a compromise between the constant torque and constant power capabilities. For example, in 23 it was shown that when the leading dimensions of 6/4 and 8/6 SRMs were fixed, and the rotor pole light was varied, the constant power range was extended to 9 6 times base speed when the rotor pole arc was narrower than the stator pole arc and the wisdom of the rotor pole was relatively large.However, the machines under consideration had relatively low torque densities. The constant power capability similarly depends on the number of stator and rotor poles. When the number is increased the constant power capability and the overload capability are reduced, albeit the higher the torque/power density and the higher the power factor and efficiency. By way of example, 23 shows that a 6/4 machine exhibits a often(prenominal) wider constant power range (viz. up to $7 times base speed) than an 8/6 machine (viz. up to $4 times base speed), which compares to a constant power operating speed range of $2 times ase speed for a 24/16 SRM 18. Often, however, the number of stator and rotor poles is dictated by the situation envelope constraints. In summary, not only is the ratio of the aligned to unaligned inductance reduced as the number of stator and rotor pol es is increased, but the constant power operating speed range is compromised due to the limited scope for phase advancing, and although the constant power execution of instrument could be enhanced by reducing the number of turns per phase, 750 Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell VehiclesFig. 9. Overlap excitation techniques for extending constant power operating range. (a) stodgy excitation at high speed with phase advance. (b) Overlapping excitation with commutation advanced. (c) Overlapping excitation with commutation retarded. Fig. 8. SRM with unified flywheel and clutch for mild-hybrid vehicle 25. Cranking 45 Nm (0300 rpm), continuous motoring 200 Nm (3001000 rpm), transient motoring 20 kW (10002500 rpm), continuous generating 15 kW (6002500 rpm), transient generating 25 kW (8002500 rpm). (a) Schematic. b) rotor/stator without device. (c) Assembled unit. this compromises the tor que capability for a given inverter voltage-ampere rating. Alternatively, the extended high-speed constant power operation can be improved with continuous phase current excitation, by increasing the number of turns per phase. The torque per ampere capability under base-speed is then not significantly compromised, as has been demonstrated for a 24/16 SRM 24 and an 18/12 SRM 25 (Fig. 8), which shows an SRM which was authentic for a mild hybrid vehicle application.The use of conduction overlap between two phases to increase the torque and to reduce torque pulsations is common practice 6. Fig. 9 illustrates overlap conduction by advancing 6 or retarding 24 long-dwell commutation 15, both in whatsoever case incorporating phase advance. Bipolar excitation (Fig. 10) 6, 22, 26 can in addition be employed to improve the torque density and reduce torque pulsations, as well as to increase the efficiency. The long flux paths that are associated with SRMs supplied from conventional unipo lar drives then become short flux paths, and the torque and efficiency are significantly nhanced at both low and high speeds. However, the improvement in performance gradually reduces as the excitation current is increased and the magnetic circuit becomes more(prenominal) highly saturated. Finally, a control strategy which employs drift diodes in parallel with the power electrical switch devices in a conventional half-H-bridge inverter unneurotic with an appropriate zero-voltage period (Fig. 11) can similarly be employ to boost the power capability when an SRM is operated as a reference 22, 27. Fig. 10. (a) Conventional excitation. (b) Bipolar coincide excitation. Vol. 95, No. 4, April 2007 Proceedings of the IEEE 751Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Fig. 12. Idealized back-EMF and phase current wave forms from PM brushless machines. (a) BLDC. (b) BLAC. Fig. 11. Freewheel diode configuration and (a) ? 1 (b) 0 and (c) A 1 commutation. D. Acoustic Noise, Torque Ripple, and Their decline The acoustic noise which is radiated from an SRM is often cited as a major disadvantage. At low rotational speeds the acoustic noise is due predominantly to resonances that are generate by the torque ripples, and whitethorn be reduced by appropriate profiling of the phase current wave form.The key to obtaining the optimal current profile is an effective method for estimating the instantaneous torque. At high rotational speeds the acoustic noise is dominated by stellate frisson resonances 28. The acoustic noise becomes significantly higher at high rotational speeds and loads. However, various techniques befool been proposed for reducing the vibration and acoustic noise. The most effective method is to employ a relatively thick stator yoke 29, 30 since this increases the mechanical rigor and, thereby, reduces the vibrational response.However, the outer diameter is then increased, but this, in general, is advan tageous in improving the overload capability since the stator yoke becomes less saturated. Reducing the supply voltage is withal usually stabilising in reducing the acoustic noise at light load. SRMs overly begin significantly lower noise when operated under voltage control rather than current control, due to the fact that random sack of the current control results in a wide-band openhearted spectra, thereby increasing the likelihood of inducing mechanical resonances 31, 32.In 33, the blood between the vibration of the stator and the rate of change of the phase currents at turnoff was highlighted, while a current formative algorithm to limit the rate of change of current at turn-off and, thereby, achieve a smoother radial force waveform was described in 34 and 35. However, the method proposed in 36 is arguably the most effective, in that it introduced a zerovoltage loop between two ill-treat changes in the applied voltage, such that, together with a knowledge of the stator natural frequencies, anti-phase stator vibrations were induced.However, it has limitations 37, since, while it is 752 very effective when SRMs are operated in both bingle pulse mode and PWM voltage control, it is much less effective with PWM current control, since this results in a varying PWM switching frequency. A fixed frequency current controller can, however, alleviate the problem. Further, the technique is less appropriate for application to SRMs which exhibit multiple resonances. The vibration and acoustic noise can also be reduced 38 by employing two-phase overlapping excitation, which, as stated earlier, is beneficial for extending the constant power operating range.In general, however, the acoustic noise emissions from SRMs remain a significant issue. I V. PERMANENT- MAGNET BRUSHLESS MACHINES A. Brushless DC and AC Machines and Drives imputable to the permanent-magnet excitation, PM brushless machines are inherently efficient 3948. They are generally classified advertise ment as being any curved or trapezoidal back-EMF machines 48 (Fig. 12). The corresponding control strategies are usually classified as being both brushless DC (BLDC), or brushless AC (BLAC). In a BLDC drive, the phase current waveforms are essentially rectangular, while in a BLAC drive the phase current waveforms are essentially curved.Ideally, in order to maximize the torque density and minimize torque pulsations, it is desirable to operate a machine which has a trapezoidal back-EMF waveform in BLDC mode, and a machine which has a sinusoidal back-EMF waveform in BLAC mode. In practice, however, the back-EMF waveforms whitethorn depart significantly from the ideal, and, indeed, irrespective of their back-EMF waveform PM brushless machines whitethorn be operated in either BLDC or BLAC mode, although the performance, in terms of efficiency and torque ripple, for example, may be compromised.Similar to installation machine drives, when operating at low torque an optimal flux leve l exists for minimum iron and copper loss, and hence, maximum efficiency. Fig. 13 shows a schematic of a typical PM brushless drive. In both BLDC and BLAC drives, rotor position information is necessary, although the required position Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Fig. 13. Schematic of PM brushless drive. esolution is different. For BLDC drives, in which the phase currents only have to be commutated on and off, low-cost abidance sensors are often employed, while for BLAC drives, in which the phase current waveforms have to be precisely controlled, a relatively high-cost resolver or encoder would be generally used. In addition, however, many sensorless techniques have recently been developed or are under development for both BLDC and BLAC drives. Although various rotor topologies and stator idle words dispositions ay be employed, BLDC machines predominantly have come to the fore- attach magnets on the rotor, and a pure nonoverlapping, fragmentary-slot, stator steer Fig. 14(a). This results in short end-windings and, therefore, a low copper loss, and the potential for a high torque density, while a six-step inverter can be employed with PWM current chopping. Two-phase, 120 elec. conduction is the most common operational mode for a three-phase BLDC machine, while maximum torque per ampere in the constant torque region and extended speed operation can realized by advancing the commutation (Fig. 5). Similar operational characteristics can be obtained in a BLAC drive by controlling the phase currents in such a way that they produce a demagnetizing com- ponent of armature response, which reduces the effective back-EMF by flux-weakening. Various design features may be employed to obtain a sinusoidal back-EMF waveform. For example, the stator slots and/or rotor magnets may be skewed, a distributed stator winding might be employed, or the permanent mag nets could be appropriately shaped or magnetize, etc. However, a distributed overlapping winding Fig. 4(b), results in longer end-windings, which results in a higher copper loss and a lower torque density, while skewing of either the stator or rotor makes constrain more complicated. Hence, it is often preferable to either shape the magnets or impart a sinusoidal magnetization distribution 49, which results in an essentially sinusoidal air fracture bowl distribution, which is causative to a low cogging torque and also a low iron loss. The rotor back-iron in a self-shielding, sinusoidal magnetized PM machine 49 is not essential since negligible flux flows within the sexual bore of the magnet.This is, therefore, also conducive to a low rotor inertia, which can be an important consideration. Recently, however, there has been a sheer to employ a fragmental ratio of slot number to pole number and a concentrated stator winding for BLAC motors so as to achieve a sinusoidal back-EMF w aveform and a low cogging torque. However, when the slot number per pole is divisional, the reluctance torque is usually relatively microscopic with a concentrated stator winding. In order to utilize the salience, an overlapping stator winding is usually required Fig. 4(b), as will be discussed in naval division IV-D. Dq-axis theory can be used to analyze the electromagnetic performance of a BLAC machine, and the optimal relationship between the d-axis and q-axis currents in vector control and flux-weakening control strategies being determined analytically 50, BLAC motors are relatively Fig. 14. stator coil winding dispositions. (a) Nonoverlapping winding. (b) Overlapping winding. Fig. 15. Torque-speed characteristics of PM brushless machines. Vol. 95, No. 4, April 2007 Proceedings of the IEEE 753Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Fig. 17. relation of torque-speed characteristics which result Fig. 16. semblance of torque- speed characteristics of BLAC and three-phase, 120 elec. BLDC drives. with two-phase, 120 elec. and three-phase, one hundred eighty elec. conduction modes of operation. easy to control and exhibit subtile performance, in terms of maximum torque per ampere control and optimal extended speed operation 5153. In contrast, the control strategy to realize constant power operation for a BLDC drive is generally more complex.As was shown in 54 and 55, supra the base-speed the maximum achievable output power and torque when a machine is operated in the BLAC mode are higher than that which can be achieved when the same machine is operated in two-phase, 120 elec. conduction BLDC mode, irrespective of whether it has a trapezoidal or sinusoidal back-EMF waveform (Fig. 16). At high speed, the phase current waveform will approximate to a sinusoid even in a BLDC drive, due to the influence of the winding reactance, while any harmonics in the back-EMF waveform will cause the flux-weakening performan ce to deteriorate.However, by employing a three-phase, 180 elec. conduction strategy, the high-speed power capability of a BLDC machine can be improved, although below base-speed its torque capability will be reduced 48, 5558, as illustrated in Figs. 17 and 18. relatively picayune and the stator windings have a low inductance, since the magnet has a relative permeability which approximates to that of air, i. e. , r $ 1, and the effective air gap is the sum of the actual air gap length and the radial thickness of the magnets.However, the magnets are exposed directly to the armature reaction field, and, hence, are susceptible to fond(p) permanent demagnetization. SPM machines are also generally to have a relatively limited flux-weakening capability. However, the flux-weakening capability, as well as the merits of PM machines having a fractional number of slots per pole and a concentrated stator winding, will be discussed later. Fig. 19(a-2) shows a schematic of a motor in which the magnets are inset into the rotor surface. The magnet polearc is, therefore, less than a full pole-pitch.However, since the q-axis inductance is now great than the d-axis inductance, a reluctance torque can be developed. b) intimate Permanent- attractor (IPM) Machines Fig. 19(b) shows examples of brushless machines in B. Permanent-Magnet Brushless Machine Topologies In this section, the basic topologies of PM brushless machine, classified according to the berth of the permanent magnets, are described. 1) Radial-Field MachinesVPermanent Magnets on rotor A radial-field PM brushless machine may have either an internal rotor or an external rotor, while the premenstrual syndrome may be fixed either on the surface or the privileged of the rotor. ) Surface-Mounted Permanent-Magnet (SPM) Machines This is the most widely used topographic anatomy for PM brushless machines Fig. 19(a-1). However, since the d-axis and q-axis stator winding inductances of such machines are the same, they ex hibit zero reluctance torque. Further, in general, the armature reaction field is 754 Fig. 18. Torque-speed characteristics for brushless BLAC and three-phase, 180 elec. BLDC operation. Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles onded ferrite or rare-earth, such a machine can exhibit an originally wide flux-weakening capability and a high torque density, without the risk of generating an excessive back-EMF should an inverter good luck occur at high rotational speeds. However, such a rotor structure is rel atively complex and expensive to conciliate 61, 62. A virtue of the rotor topographic anatomy shown in Fig. 19(b-2) is that, when the pole number is relatively high, flux cerebrate can be exploit and the air-gap flux density can be significantly higher than the magnet remanence.Hence, low-cost, low-energy ware magnets, such as mould ferrite, may be employed. By way of example, Fig. 21 shows a generator, which was developed for an electric vehicle auxiliary power unit 63. Flux-focusing enables an air-gap flux density of 0. 6 T to be achieved when sintered ferrite magnets, having a remanence of 0. 38 T are employed. Such a machine topology also exhibits a higher d-axis inductance since the armature reaction flux only passes through a single magnet, rather than two magnets as in the other machine topologies, making it very adequate for extended constant power operation. ) Radial-Field MachinesVPermanent Magnets on Stator When the permanent magnets are located on the stator, the rotor must have a great pole geometry, similar to that of an SR machine, which is simple and robust, and satisfactory for high-speed operation. The stator carries a nonoverlapping winding, with each tooth having a concentrated coil. The permanent magnets can be placed on the inner surface of the stator odontiasis, sandwiched in the stator teeth, or mounted in the stator back-iron. disregarding of their location, however, the torque results predominantly from the permanent-magnet excitation torque, i. . , the reluctance torque is negligible, although the torque production mechanism relies on the rotor boldness. Compared with conventional permanent-magnet brushless machine topologies, generally, it is easier to limit the temperature rise of the magnets as lovingness is dissipated more effectively from the stator. a) Permanent Magnets in Stator Back-IronVDoubly Salient PM Machine The machine topology which is shown in Fig. 22(a) is referred to as a in two ways outstanding Fig. 19. Alternative radial-field PM machine topologies with magnets on rotor. a) Magnets on rotor surface. (b) Magnets inside the rotor. which the magnets are accommodated within the rotor. In Fig. 19(b-1) the magnets are radially magnetized, while in Fig. 19(b-2) they are circumferentially magnetized. Generally speaking, however, leakage flux from the magnets is significantly greater th an that in SPM machines. However, since the magnets are buried inside the rotor iron, the magnets are effectively shield from the demagnetizing armature reaction field during fluxweakening operation.Further, since the d-axis inductance is low-spiriteder than the q-axis inductance, a reluctance torque exists, while the d-axis inductance is high compared with that of an equivalent surface-mounted magnet motor topology. Therefore, generally, such machine topologies are eminently appropriate for extended speed, constant power operation in the flux-weakening mode 48, 51. Indeed, a variant of the topology illustrated in Fig. 19(b-1) is employed in the Toyota hybrid vehicle 4, Fig. 20.The V-shaped disposition of the permanent magnets serves to increase the air gap flux and the distributed stator winding enables the reluctance torque to be utilized. octuple layers of magnets may also be employed to further increase the saliency ratio, although, in practice, the number of layers is usuall y limited to 3. An extreme case, however, is to employ an axially laminated PM rotor in which permanent-magnet sheets are sandwiched between the laminations 60. In this way, a small volume of permanent-magnet material, which is generally a Fig. 20.Open-circuit field distribution in PM BLAC machine in Toyota hybrid vehicle. Vol. 95, No. 4, April 2007 Proceedings of the IEEE 755 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles permanent magnets is essentially immutable with the rotor position. Therefore, the cogging torque is not significant. However, a major disadvantage is that, due to the unipolar flux-linkage, the torque density is relatively poor compared to that of other PM brushless machines 65, although, as was inform in 66, it can still be higher than that of an induction machine. ) Permanent Magnets on Surface of Stator TeethVFlux-Reversal Permanent Magnet Machine This machine topology is also commonly referred to as a flux-reversal PM machine Fig. 22(a) 67, 68. to each one stator tooth has a pair of magnets of different polarity mounted at its surface. When a coil is ruttish, the field under one magnet is reduced while that under the other is increased, and the outstanding rotor pole rotates towards the stronger magnetic field. The flux-linkage with each coil reverses polarity as the rotor rotates. Thus, the phase flux-linkage variation is Fig. 21.Generator for EV auxiliary power unit 63. 9 kW at 4200 rpm, sintered ferrite magnets (remanence = 0. 38 T), max. air-gap flux density 0. 6 T. (a) Stator. (b) Rotor. (c) Flux distribution. permanent-magnet machine. For a three-phase machine a magnet is required in the stator back-iron for both three teeth, while for a four-phase machine a magnet is required for both four teeth. The variation of the fluxlinkage with each coil as the rotor rotates is unipolar, while the back-EMF waveform tends to be trapezoidal 64. Thus, this topology is more suitable for BLDC ope ration.However, the rotor may be skewed in order to obtain a more sinusoidal back-EMF waveform to make it more appropriate for BLAC operation. Further, it will be noted that the air-gap reluctance as seen by the 756 Fig. 22. Alternative radial-field PM machine topologies with magnets on stator. (a) Magnets in stator back-ironV in two ways salient PM machine. (b) Magnets on surface of stator teethVflux-reversal PM machine. (c) Magnets in stator teethVflux-switching PM machine. Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles ipolar, while the phase back-EMF waveform is, again, essentially trapezoidal. Such a machine topology exhibits a low winding inductance, while the magnets are more unguarded to partial irreversible demagnetization. In addition, significant induced eddy-current loss may be induced in the magnets, which also acquaintance a significant radial magnetic force. Further, since the airgap flux density is limited by the magnet remanence, the torque density may be compromised. c) Permanent Magnets in Stator TeethVFlux-Switching PM Machine This machine topology is also referred to as a flux-switching permanent-magnet machine Fig. 2(c) 6971. The stator consists of BU-shaped laminated segments between which circumferentially magnetized permanent magnets are sandwiched, the direction of magnetization being reversed from one magnet to the next. Each stator tooth comprises two adjacent laminated segments and a permanent magnet. Thus, flux-focusing may be readily incorporated, so that low-cost ferrite magnets can be employed 70. In addition, in contrast to conventional PM brushless machines, the influence of the armature reaction field on the working slur of the magnets is minimal.As a consequence, the electric loading of fluxswitching PM machines can be very high. Therefore, since the phase flux-linkage waveform is bipolar, the torque capability is significantly higher than that of a doubly salient PM machine 65. The back-EMF waveform of fluxswitching PM machines is essentially sinusoidal, which makes them more appropriate for BLAC operation. In addition, since a high per-unit winding inductance can readily be achieved, such machines are eminently suitable for constant power operation over a wide speed range. ) new(prenominal) PM Brushless Machine Topologies a) Axial-Field Machines Axial-field PM machines have an axially directed air-gap flux 72, 73 and can comprise a single-sided stator and a single rotor, a double-sided stator and a single rotor, or a single stator and a double-sided rotor. In each case, a large axial force exists between the stator and the rotor. As with conventional radial-field PM brushless machines, the stator can be slotted or slotless, although it is more tricky to manufacture a slotted stator for axial-field machines. Thus, slotless designs are more common. However, hile this eliminates cogging, it exposes the wi nding air-gap flux. Hence, a multistranded conductor or Litz wire may be required to minimize the eddy-current loss. Further, since the effective air gap is large, the winding inductance is generally relatively small, which may limit the constant power speed range. b) Transverse-Flux Machine Generally, transverse flux machines have a relatively large number of poles, all of which interact with the total ampere-conductors of each phase. This enables very high electric loadings and, hence, high torque densities to be achieved 7478.However, they have a significant leakage flux and a relatively high winding inductance, as well as a poor power factor 79, 80. This impacts significantly on the associated VA rating of the power electronics converter, which has hold in its application. C. Design and Control Issues for PM Brushless suitcase Machines As stated earlier, traction machines are required to have a high torque density, a high overload capability, a wide operating speed range, and a high efficiency, while it is desirable that they have a degree of a high disfigurement tolerance and are low cost.In this section, design considerations related to the above issues are discussed. However, they often fight each other. For example, reduction of the cross-coupling magnetic saturation may also reduce the saliency ratio and consequently the reluctance torque the selection of the base-speed is usually a compromise between the constant torque performance at low speed and the constant power performance at high speed. 1) Torque Density and Overload competency The general torque equation for a PM brushless machine, which has both excitation torque and reluctance torque components, is given by 3 A T? p 2 A m IqA ? Lq A Ld ? Id Iq (5) where p is the number of pole-pairs, m is the stator winding flux-linkage due to the permanent magnets, and Ld , Lq and Id , Iq are the d- and q-axis inductances and currents, respectively. In order to maximize the torque density, it is desir able to increase m by reducing the leakage flux. This can be achieved by introducing airspace flux barriers or interpole magnets, as illustrated in Fig. 23. m can also be increased by utilizing flux focusing 4, 63, as illustrated in Fig. 24. The torque density can also be enhanced by increasing the saliency ratio 3, 81, as illustrated in Fig. 25.Further, since the short-duration torque capability is determined in the main by the demagnetization withstand capability of the magnets and the level of magnetic saturation, reducing the d- and q-axis cross-coupling magnetic saturation by incorporating air flux barriers, as illustrated in Fig. 26, can enhance the overload capability. 2) Flux-Weakening susceptibility It is well known 62, 82 that the maximum flux-weakening capability, defined as the ratio of the maximum speed to the base-speed, under supply inverter voltage and current limitations, can be achieved when a PM brushless machine is designed to have 1. per-unit d-axis inductance such that Ld Ir m Ld ? m Ir or ?1 (6) 757 Vol. 95, No. 4, April 2007 Proceedings of the IEEE Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Fig. 24. Flux focusing by appropriate disposition of magnets. ratio is 1. 0. However, the higher the flux-linkage m to achieve a high low-speed torque capability, the more difficult it is to realize wide-speed operation (Fig. 27) 83. In 62, it was shown that it was viable to design any PM brushless machine to achieve Binfinite flux-weakening capability. Clearly, however, if the rated current is high (e. g. the machine is liquid cooled), it is Fig. 23. Reduction of leakage flux by introducing airspace flux barriers or interpole magnets. where m is the stator flux-linkage due to the magnets, Ld is the d-axis inductance, and Ir is the rated current. Although it is possible to design a PM brushless machine which satisfies the foregoing requirement, generally, for most PM machines Ld Ir = m G 1, since the d-axis inductance is relatively low as a consequence of the recoil permeability of the magnets being approximately equal to 1. 0. Nevertheless, the higher the ratio of Ld Ir = m the higher will be the flux-weakening capability Fig. 7(a), which, theoretically, is Binfinite when the 758 Fig. 25. Improvement of saliency ratio. (a) Lower reluctance q-axis armature reaction flux path. (b) Multilayer and (c) Axially laminated. Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles theless, it has been shown 86 that, by particular(prenominal) design, the magnet working point in an SPM machine can remain middling high up the magnet demagnetization characteristic, even when the machine has Binfinite flux-weakening capability, due to the fact that 1. perunit d-axis inductance results primarily from stator slot leakage and end leakage fluxes. 4) Rotor Eddy-Current Loss PM BLAC and BLDC machines are usually considered to have negligible rotor loss. However, the rotor loss may be important in machines furnish with surface-mounted magnets, in terms of the resulting temperature rise. Eddy currents may be induced in the permanent magnets, the rotor back-iron, and any conducting sleeve which may be employed to retain the magnets, by time and space harmonics in the airgap field.More specifically, they result from 87 a) stator slotting b) stator MMF harmonics which do not rotate in synchronism with the rotor and c) nonsinusoidal phase Fig. 26. Reduction of d- and q-axis cross-coupling magnetic saturation. (a) SPM. (b) IPM. much easier to sate (6), even for surface-mounted magnet machines, which have a high m and a relatively low Ld . For example, in 84 Binfinite flux-weakening capability was achieved with an SPM machine equipped with a self-shielding, sinusoidal magnetized rotor having no back-iron, and in 85 with an SPM machine in which only alternate stator teeth carried a coil.However, i n general, it is much easier to achieve a wide operating speed range with machines equipped with an midland permanent magnet rotor, since generally m is lower, while Ld is higher. 3) Demagnetization Withstand Capability Operation in the flux-weakening mode is a necessary requirement for traction applications, while NdFeB is the most commonly employed permanent-magnet material for PM brushless machines. However, the magnets are required to have an adequate demagnetization withstand capability at the maximum operating temperature, when they are most vulnerable to partial irreversible demagnetization.In addition to effective thermal management, one means of enhancing the demagnetization withstand capability is to provide a low reluctance path for the demagnetizing d-axis armature reaction flux such that it does not pass through the magnets. One example of achieving this is to employ narrower stator slot openings and thick tooth-tips, as illustrated in Fig. 28(a), or thick rotor slot b ridges in an IPM machine, as illustrated in Fig. 28(b). However, such features will also have an influence on m and Ld . In general, however, it is easier to realize a high demagnetization withstand capability for IPM machines.Never- Fig. 27. regeneration of torque and power capability with machine design parameters, when Ld Ir G m . (a) chance variable with Ld Ir = when m and Lq =Ld are constant. m, Vol. 95, No. 4, April 2007 Proceedings of the IEEE 759 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Fig. 28. Improvement of demagnetization withstand capability by introducing d-axis armature reaction demagnetization flux path. (a) SPM stator design. (b) IPM rotor design. Fig. 27. (Continued) mutant of torque and power capability with machine design parameters, when Ld Ir G when Lq and Ld are constant. . (b) Variation with m, current waveforms, which result from six-step commutation and PWM. In general, however, the rotor eddy-current loss is relatively small compared with the stator copper and iron losses. Nevertheless, it may cause significant heating of the magnets, due to the relatively poor heat waste from the rotor. In turn, this may result in partial irreversible demagnetization, peculiarly of sintered NdFeB magnets, which have relatively high temperature coefficients of remanence and coercivity and a moderately high electrical conductivity.It is particularly important to consider the rotor eddy-current loss in a) machines with a high fundamental frequency, e. g. high-speed and/or high-pole number b) machines with large stator slot openings, e. g. , transverse flux machines c) high power density brushless dc machines, e. g. , force-cooled traction machines with a high electric loading and d) machines whose windings span a fractional pole-pitch and which have nearly equal pole and slot numbers 88. If the eddy-current loss is unacceptable, the magnets may be segmented, axially and/ or circumferentially 89. 60 5) Stator Iron Loss Due to the fixed PM excitation, the no-load iron loss increases with the rotational speed, while the full-load iron loss in the constant torque operating range is generally around 20%50% higher. However, the iron loss which results on load in the fluxweakening mode depends on the machine topology, as illustrated in Fig. 29. In general, SPM machines have the lowest full-load iron loss, and scorn the increase in fundamental frequency it usually becomes much lower than the no-load iron loss as the degree of flux-weakeningFig. 29. Variation of iron loss in SPM and IPM machines when their open-circuit stator iron loss are designed to be the same. Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles is increased 90. IPM machines generally have a significantly higher full-load iron loss, which may be comparable with(predicate) to or higher than the no-load iron loss, since the arma ture reaction field has a higher harmonic content due to the small effective air gap 90, 91.However, when the magnets are simply inset into the rotor surface the harmonic content in the armature reaction field increases further, and generally results in the highest full-load iron loss 86. 6) Fault-Tolerance An important consideration when operating in the extended speed, flux-weakening mode is the consequence of an inverter fault which results in the loss of the demagnetizing armature reaction field and an excessively high back-EMF 92, 93. In this regard, IPM machines may be advantageous, since, for a given output torque, the PM excitation torque, and, hence, the volume of magnet material and the maximum ack-EMF are lesser. However, the consequences of an inverter fault occurring when a PM brushless machine is operating in the fluxweakening region remains a challenging issue. D. Recent Developments 1) Fractional time slot Machines SPM brushless machines which have a fractional numb er of slots per pole and a concentrated winding have been the subject of recent research. They have an inherently low cogging torque, short end-windings and, hence, a low copper loss, a high efficiency, and a high power density, as well as excellent flux-weakening performance 85, 94100.The stator coils may be appall either on all the teeth or only on alternate teeth (Fig. 30) 95, 97. In the latter case, the phase windings are effectively isolated, both magnetically and physically, and a high per-unit self-inductance can readily be achieved to limit the potential short-circuit current, by utilizing the relatively high air-gap inductance and the leakage flux at the slot openings. Due to the physical insularity of the coils and the negligible mutual inductance between phases, the possibility of a phase-tophase fault is minimized.Therefore, the fault tolerance and flux-weakening capability of such machines can be significantly higher than for more conventional machine designs. Fig. 3 1 shows a three-phase, 24-slot, 22-pole, PM BLAC machine which was developed for a supercapacitor ground electrical torque boost system for vehicles equipped with down-sized IC engines 99. However, since the torque is developed by the interaction of a stator spaceharmonic MMF with the permanent magnets, a relatively high rotor eddy-current loss can result from the fundamental and low-order space-harmonic MMFs which rotate relative to the rotor 88, 89.As stated earlier, however, the magnets can be segmented to reduce the eddy-current loss. A further advantage of such machines is that, due to the fractional number of slots per pole, the cogging torque is very small without employing design features such as Fig. 30. Three-phase, 12-slot, 10-pole, fractional slot PM machines 97. (a) All teeth wound. (b) Alternate teeth wound. skew. However, the reluctance torque component is negligible even when an IPM rotor is employed. 2) Hybrid PM and Current pique Since the PM xcitation is fixed in a PM brushless machine, the current phase angle has to be progressively advanced as the speed is increased above base-speed so that a demagnetizing d-axis current component is produced which reduces the flux-linkage m with the stator winding. Ultimately, however, this may cause partial irreversible demagnetization of the magnets. At the same time, due to the inverter voltage and current limits, the torque-producing q-axis current component has to be reduced correspondingly. Consequently, the torque and power capability are limited.Thus, a compromise has to be made between the low-speed torque capability and high-speed power capability. Hybrid permanent magnet and field current excitation has been shown to be beneficial in improving the power capability in the extended speed range, enhancing the lowspeed torque capability, and improving the overall operational efficiency. There are several ways of realizing such hybrid excitation. For example, dc winding may be located on the rotor 101 or the stator 102107, which is preferable since it does not require slip-rings.The magnetic circuit associated with the dc excitation may be either in series or in parallel with the magnetic circuit associated with the PM excitation. However, although series excitation is simple it requires a higher excitation Vol. 95, No. 4, April 2007 Proceedings of the IEEE 761 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles machines equipped with hybrid excitation, found on doubly salient pole 102, consequent-pole 103 one hundred five, and claw-pole 106, 107 machine topologies.The dc excitation winding enables the air-gap flux, and, hence, the torque capability, to be enhanced at low speed, to be reduced at high speed to facilitate extended speed operation, and to be optimized over the entire speed range to improve the efficiency. It also reduces the likelihood of an excessively high back-EMF being induced at high speed in the event of an inverter f ault. Fig. 31. Three-phase, 24-slot, 22-pole, PM BLAC machine with modular stator winding and IPM rotor 99. Rated output power ? 18. 5 kW, rated speed ? 1700 rpm, rated torque ? 105 Nm. (a) Cross section of three-phase, 24-slot, 22-pole IPM BLAC machine. b) Machine test rig. MMF due to the low recoil permeability of the magnets. On the other hand, parallel excitation is more effective electromagnetically but leads to a more complex machine structure. Fig. 32 shows three examples of PM brushless 762 Fig. 32. Hybrid excited PM machines. (a) Hybrid excitation based on doubly salient pole structure 102. (b) Hybrid excitation based on consequent pole structure 105. (c) Hybrid excitation based on claw-pole structure 106. Proceedings of the IEEE Vol. 95, No. 4, April 2007 Zhu and Howe Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell VehiclesV. CONCLUSION The operational characteristics, design features, and control requirements for induction machines, switched reluctance machines, and permanent-magnet brushless machines for vehicle propulsion systems have been reviewed, with strain on their low-speed torque and high-speed power capability. Given that they offer a higher efficiency and torque density, particular fury has been given to permanent-magnet brushless machines. Various PM brushless machine topologies have been highlighted, and their relative merits have been briefly described. In general, however, all three machine tech- ologies can meet the performance requirements of traction drives, and each machine applied science has merits. h Acknowledgment The authors acknowledge the contributions of colleagues in the Electrical Machines and Drives Group, University of Sheffield, Sheffield, U. K. , and also the support of industrial organizations, in particular, IMRA U. K. Research Centre, Centro Ricerche Fiat, Volvo Technology Corporation, and FEV Motorentechnik GmbH. REFERENCES 1 C. C. Chan, BThe state of the art of electric and hybrid vehicles , Proc. IEEE, vol. 90, no. 2, pp. 247275, Feb. 2002. 2 I.Husain, Electric and Hybrid VehiclesVDeign Fundamentals. Boca Raton, FL CRC, 2003. 3 Y. Honda, T. Nakamura, T. 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