Overview of Stator-Permanent MagnetBrushless MachinesMing Cheng,Senior Member,IEEE,Wei Hua,Member,IEEE,Jianzhong Zhang,Member,IEEE,andWenxiang Zhao,Member,IEEEAbstract—Permanent magnet(PM)brushless machines having magnets and windings in stator(the so-called stator-PM machines) have attracted more and more attention in the past decade due to its definite advantages of robust structure,high power density, high efficiency,etc.In this paper,an overview of the stator-PM ma-chine is presented,with particular emphasis on concepts,opera-tion principles,machine topologies,electromagnetic performance, and control strategies.Both brushless ac and dc operation modes are described.The key features of the machines,including the mer-its and drawbacks of the machines,are summarized.Moreover,the latest development of the machines is also discussed.Index Terms—Doubly salient,flux reversal,flux switching, overview,stator-permanent magnet(PM).I.I NTRODUCTIONW ITH THE significant achievements of permanent mag-net(PM)materials and power electronic devices,brush-less machines excited by PMs are developing dramatically. Generally speaking,there are a total of four types of PM machines which have magnets located on the rotor,and they are the following:1)surface mounted;2)inset;3)radial interior; and4)circumferential interior.It should be emphasized that all of the PM machines mentioned have magnets located on the rotor,and they are referred to as“rotor-PM machines”in this paper,which predominate in the industry applications due to outstanding advantages[1],[2].However,magnets usually need to be protected from the centrifugal force by employing a retaining sleeve,which is made of either stainless steel or nonmetallicfiber.Rotor temperature rise may be a problem due to poor thermal dissipation,which may cause irreversible demagnetization of magnets and may ultimately limit the power density of the machine.Recently,in contrast,a new type of PMManuscript received March21,2010;revised June28,2010,October1, 2010,and December21,2010;accepted February16,2011.Date of publication March7,2011;date of current version September7,2011.This work was supported in part by NSFC under Projects59507001,50377004,50337030, 50729702,50807007,60974060,and50907031;by the Specialized Research Fund for the Doctoral Program of Higher Education of China under Projects 20050286020,200802861038,and20090092110034;by the High Tech Re-search Program of Jiangsu Province,China,under Project BG2005035;and by the Aeronautical Science Foundation of China under Projects20080769007and 20100769004.M.Cheng,W.Hua,and J.Zhang are with the School of Electrical Engineering,Southeast University,Nanjing210096,China(e-mail:mcheng@ ;huawei1978@;seuzjz@).W.Zhao is with the School of Electrical and Information Engineering, Jiangsu University,Zhenjiang212013,China(e-mail:zwx@). Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/TIE.2011.2123853Fig.1.Stator-PM alternator in1955[3].machine having magnets on the stator,nominated as stator-PMmachines,has re-emerged and has been developed,which canovercome the problems suffered by the rotor-PM counterparts.The purpose of this paper is to give an overview of thestator-PM machines.Thus,the state-of-the art technology ofthe stator-PM machines,including machine topologies,opera-tion principles,characteristics,control strategies,torque rippleminimization,and latest developments will be reviewed anddiscussed.II.C ONCEPTS OF S TATOR-PM M ACHINESThe idea of stator-PM machines can be traced back to1955when Rauch and Johnson proposed probably thefirst stator-PM topology,as shown in Fig.1[3],in which both the statortooth and rotor poles are salient so as to transfer thefluxescreated by the stator magnets from the stator to the rotoracross the air gap when the rotor poles face the stator teeth.Thus,the polarities of theflux linked in the coils in the statorreverse as the rotor aligns the alternative pair of stator teeth.However,due to the limitation of the magnet property at thattime,the stator-PM machines were overwhelmed by wound-field excitation machines,e.g.,synchronous and dc machines.Another origin of the stator-PM machine can be found fromthe well-known Law’s relay actuator shown in Fig.2[4].Inaddition,the doubly salient structure employed in electricalmachines can also originate from an induction-type generator,as shown in Fig.3[5],in which the stator assembly is a set ofpieces arranged in a cylinder laminated iron,with the armaturewindings wound.The special geometry allows the variation ofthe magnetflux that is linked into the“C”-shaped stator polepieces to induce electromotive force(EMF).0278-0046/$26.00©2011IEEEw’s relay actuator[4].Fig.3.Flux-switch“C”-segmented inductor alternator in1975[5]. Conceptually,stator-PM machines employ the polarized re-luctance principle,in which the torque and EMFs are the resultant from theflux-switching action of rotor saliencies on a unipolarflux produced by PMs in the stator,leading to an exceptionally simple and robust rotor structure.III.B ASIC T OPOLOGIES,P RINCIPLES,AND P ERFORMANCESBasically,there are three types of modern stator-PM ma-chines,namely,doubly salient PM(DSPM)machine[6],flux-reversal PM(FRPM)machine[7],andflux-switching PM (FSPM)machine[8],as shown in Fig.4.It should be noted that the operation principle and electromagnetic performance of the three machines are distinct,although they are all classified as stator-PM machines.A.DSPM MachineThe DSPM machine can be considered as a combination of switched-reluctance machine and stator magnets[6],in which the magnets are insetted in the stator back-iron and the con-centrated windings are employed,making the PMfluxes linked in the coils unipolar.Furthermore,by specifically designing the stator teeth and rotor pole arcs,the air-gap reluctance seen by magnets are invariant[9],and consequently,the PMflux-linkage linearly varies with the rotor position,as shown in Fig.5(a).Therefore,the ideal phase back-EMF waveform is trapezoidal,and the conventional brushless dc(BLDC)opera-tion mode can be adopted[10].On the other hand,a simple rotor-skewing method can be used[11],and a quasi-sinusoidal back EMF can be obtained. Consequently,the DSPM machine can also be run in the brushless ac(BLAC)mode,as shown in Fig.5(b).Fig.4.Re-emerged stator-PM machines.(a)DSPM machine.(b)FRPM machine[7].(c)FSPM machine[8].Fig.5.Operation principle of the DSPM machines.(a)Unskewed rotor.(b)Skewedrotor.Fig.6.Four/six-pole single-phase DSPM generator[12].Except for the three-phase six/four-pole(six-stator-teeth/ four-rotor-pole)topology,other feasible slot/pole combinations are presented,e.g.,one-phase four/six-pole[12](Fig.6),two-phase four/six-pole with dual stator[13](Fig.7),three-phaseCHENG et al.:OVERVIEW OF STATOR-PERMANENT MAGNET BRUSHLESS MACHINES5089Fig.7.Two-phase four/six-pole DSPM machine with dual stator[13].Fig.8.12/8-pole DSPM machine[14].Fig.9.Eight/six-pole DSPM machine [15].12/8-pole [14](Fig.8),and four-phase eight/six-pole configu-rations [15](Fig.9),revealing the different characteristics and the suitability for different applications.Additionally,power-dimension equations were derived analytically [16]–[18],which not only indicate that the output power is directly propor-tional to the ratio of the rotor poles to the stator teeth but also provide designers a practical way to make the initial calculation of the motor dimensions.In [19],a nonlinear varying-network magnetic circuit was built to analyze the electromagnetic per-formance of the DSPM machines more efficiently with enough accuracy.Moreover,a nonuniform air gap is employed to reduce the cogging torque [20].To regulate the uncontrollable PM flux and to extend the constant-power region,several schemes were proposed in [21](Fig.10),although some of them are some-what impractical in applications.Moreover,a new split-winding configuration was proposed,as shown in Fig.11[22].B.FRPM MachineThe FRPM machine has magnets located on the surface of the stator teeth and concentrated windings,as shown in Fig.12.Different from DSPM machines,the phase PM flux-linkage of an FRPM machine is bipolar,as shown in Fig.13.The BLDC operation mode is also suitable for the FRPM machines.Additionally,it is found that the FRPM machine exhibits fault-tolerance capability due to its natural isolation betweenphases,Fig.10.Schematic of the flux regulation in the DSPM machine [21].(a)By a movable magnetic shoring piece.(b)By moving magnets in the axial direction.(c)By a magnetic/nonmagneticcollar.Fig.11.Schematic of the split winding for a four-phase DSPM machine [22].and the variation of the inductances versus rotor position is small,which indicates that the reluctance torque is negligible [7].Similarly,the rotor-skewing method can also be used to improve the EMF waveform of a multipole FRPM machine to be more sinusoidal and to reduce the cogging torque for5090IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL.58,NO.11,NOVEMBER2011Fig.12.Stator magnets and winding configuration of the FRPMmachine. Fig.13.Operation principle of the FRPM machines.(a)Unskewed rotor.(b)Skewedrotor.Fig.14.Different magnet configurations of the FRPM machines[24].(a)Pole-PMs on stator.(b)Inset-PMs on stator.an automotive generator[23]or low-speed servo drives,as shown in Fig.14[24],respectively.However,the induced-eddy-current losses in the magnets and the low power factor may be the subject of further work,as addressed in[25].In addition,the magnet thickness in the magnetized direction increases the effective air-gap length between the stator teeth and rotor poles.Therefore,the magnet thickness,as well as rotor pole arc,air-gap length,etc.,has significant effects on the electromagnetic performance of the FRPM machine.Hence, the optimal designs on various dimensions were investigated, e.g.,employing the concave stator poles and additionalflux barriers,as shown in Fig.15[26],as well as a rotor-teeth-pairing technique to reduce the cogging torque,as shown in Fig.16[27].Additionally,it is found that the PWM control modes have an important influence on the performance of the FRPM machine,as revealed in[28].In[29],a single-phase two/three-pole FRPM was proposed,and to solve the starting torque problem,the cogging torque was used as a solution by proposing a rotor with a tapered air-gap structure,as shown in Fig.17.Moreover,an axialflux and an outer-rotor FRPM machine were proposed in[30]and[31],respectively.Fig.15.Concave stator pole andflux barrier for the FRPM machines[26]. Fig.16.Rotor-teeth-pairing technique for the FRPM machines[27].Fig.17.Rotor structure with a tapered air gap of a single-phase two/three-pole FRPM machine[29].Fig.18.Prototype of a12/10-pole FSPM machine.(a)Configuration.(b)Prototyped motor.C.FSPM MachineAmong the three stator-PM machines,the stator configura-tion of the FSPM machine is relatively complex,as shown in Fig.18.The stator contains12segments of“U”-shaped mag-netic cores,between which12pieces of magnets are inset and magnetized circumferentially in alternative opposite directions [32].As shown in Fig.19,a concentrated coil is wound around the adjacent stator teeth with a magnet sandwiched.Hence,CHENG et al.:OVERVIEW OF STATOR-PERMANENT MAGNET BRUSHLESS MACHINES5091Fig.19.Stator magnet and winding configuration of the FSPMmachines.Fig.20.Coils and phase EMF waveforms of a12/10-pole FSPM machine[33].(a)Predicted.(b)Measured.the polarity of the PMflux linkage in the coil reverses when the rotor pole aligns the alternative stator tooth that belongs to the same phase,i.e.,realizing the“flux-switching”action. It should be emphasized that,due to the magnetic reluctance difference between the two pairs of coils composing a phase, the resultant phase EMF waveforms are essentially sinusoidal without any additional measures[33],which is also one of the outstanding advantages of the FSPM machine over the other two counterparts.In addition,it is found that the optimal rotor pole arc is1.4times that of the stator teeth arc,and the resultant phase EMF waveform exhibits negligible harmonics,as shown in Fig.20[33].The design procedure and a general power dimension equa-tion were derived analytically for the design of the FSPM machine with arbitrary slot/pole combinations in[34].In addi-tion,to predict electromagnetic performance with satisfied ac-curacy,a nonlinear adaptive lumped parameter magnetic circuit model,a magnetic network model considering saturation effect, an alternative technique based on the harmonic or Fourier model,and an analytical hybrid modeling technique combin-ing the advantages of the magnetic equivalent circuit and the Fourier analysis were proposed in[35]–[38],respectively.It should be noted that,due to the stator-PM configuration, the end-effect caused by the axialflux leakage is relatively serious,and the predictions from a conventional2-D model must be modified.Hence,a simple and effective method in investigating the end-effect on the stator-PM machine was proposed and verified by the experimental results[39],[40]. Moreover,a3-D lumped model was built to predict the per-formance in[41].Furthermore,the eddy current loss in the frame and magnets,iron loss,and proximity loss of the FSPM machine were analyzed in[42]and[43],respectively.The topology of the FSPM machine is not limited in the three-phase12/10-pole structure.Three-phase12/14-pole and 6/5-pole FSPM machines were investigated in[44]and[45] [Fig.21(a)]and[46][Fig.21(b)],respectively.In addition, eight/six-pole two-phase[47]and four/two-pole one-phase FSPM machines are shown in Fig.22,while the asymmetry rotor pole is employed to improve the start torque[48].Fig.23 shows thefive-phase FSPM machines[49].Fig.21.FSPM machines.(a)12/14-pole three phase[44].(b)Six/five-pole three phase[46].Fig.22.FSPM machines.(a)Eight/six-pole two phase[47].(b)Four/two-pole one phase[48].Fig.23.Five-phase FSPM machines[49].(a)All poles wound.(b)Alternator poles wound.5092IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL.58,NO.11,NOVEMBER2011Fig.24.FSPM machines [50].(a)Conventional FSPM.(b)Alternator poles wound-1.(c)Alternator poles wound-2.(d)Alternator poleswound-3.Fig.25.Multitooth FSPM machines[52].Fig.26.Transverse flux FSPM machine [53].In addition,the combinations of stator and rotor pole num-bers,the winding connections,and the winding factors of the FSPM machines having all poles and alternate poles wound are analyzed in [50]and [51],as shown in Fig.24.The multitooth topologies of the FSPM machine are proposed and investigated in [52](Fig.25).Recently,a novel flux-switching transverse flux PM generator for low-speed wind power applications is proposed and analyzed by a 3-D finite element method [53],as shown in Fig.26.parison and SummaryThe comparisons between the three stator-PM machine them-selves and the stator-and rotor-PM machines have drawn a series of conclusions of significance [2],[54]–[58],which are summarized as follows.Similarities :1)Doubly salient structures are employed.2)The magnets are located on the stator,and conse-quently,the temperature rise of the magnets is much easier to control.3)The rotor is identical to that of an SR machine,namely,without windings or magnets,being mechan-ically robust,and able to operate at high speed.4)Concentrated windings are employed with shorter end parts,resulting in low copper consumption and winding resistance.Therefore,the stator-PM ma-chines are of high efficiency compared with the con-ventional overlapping winding machines.5)The electromagnetic torque is dominated by the PM excited torque,and the average reluctance torque is negligible.Differences :1)Due to different configurations of magnets,stator poles,and windings,the phase flux linkage in both FRPM and FSPM machines is bipolar,while in a DSPM machine,it is unipolar.2)The EMF per turn and the torque capability of an FSPM machine are significantly higher than those of the FRPM and DSPM machines due to the flux-concentration effect.3)The waveforms of the flux linkage and EMF of an FSPM machine are essentially sinusoidal with unskewed rotor,while in the DSPM and FRPM machines,they are more trapezoidal.Therefore,the DSPM and FRPM machines are more suitable for the BLDC drive,and an FSPM machine is more suitable for the BLAC drive.4)Due to flux concentration,flux switching,and smaller overlapping areas between the stator and rotor teeth in an FSPM machine,the magnetic saturation level in the teeth of the stator and rotor is very high,and it can reach 2.5T.Thus,the air-gap flux density in the FSPM machine can exceed 2.2T [57].5)In the FSPM machine,the coils are wound over two stator teeth between which one magnet is inset,while in the DSPM and FRPM machines,the coils are wound on each stator tooth.Hence,the end winding in an FSPM machine is slightly longer,while the amount of magnet materials is significantly higher.However,since the stator winding is concentrated,it is still significantly shorter than that in a conventional overlapping winding PM machine.In addition,flux concentrating can be utilized,and a low-cost magnet material,such as ferrite,can be used in an FSPM machine.CHENG et al.:OVERVIEW OF STATOR-PERMANENT MAGNET BRUSHLESS MACHINES 5093TABLE IQ UALITATIVE C OMPARISON OF THE T HREE S TATOR -PM MACHINES6)Among the three typical stator-PM topologies,the FSPM machine exhibits the highest torque capability and power density as revealed in [54],in which the electromagnetic torque of the FSPM machine is about 1.6times that of the DSPM machine under the same peak currents.However,the torque per magnets of the FRPM machine is higher than that of the FSPM machine [55].In addition,the cogging torque of the FSPM machine is the highest due to the flux-concentrated effect.7)In terms of demagnetization of the PM for the three stator-PM machines,briefly,the FRPM machine ex-hibits the weakest withstanding capability since the magnets are mounted on the stator teeth,and con-sequently,the MMF due to the armature currents is in series with that of the magnets,whereas for the FSPM and DSPM machines,since the armature reaction flux does not pass through the magnets,the irreversible demagnetization withstand capability is high,which makes it particularly suitable for flux-weakening operations.Table I summarizes the qualitative comparison among the key issues of the three stator-PM machines.IV .B ASIC C ONTROL S TRATEGIESThere are basically two types of operation modes for the stator-PM motor drives,namely,BLDC operation for the ma-chines with trapezoidal back EMF and BLAC operation for the machines with sinusoidal back EMF.In the BLDC operation,the control strategy of the stator-PM motor drive consists of two basic schemes,namely,current chopping control (CCC)and angle position control (APC),for constant-torque operation at speeds below the base speed and constant-power operation at speeds above the base speed,respectively,which are similar to those in an SR machine.Fig.27shows the typical current waveforms of an eight/six-pole DSPM motor [22].In the BLAC operation,field orientation control,i.e.,vector control,may be applicable to the stator-PM motor drive [59].Moreover,the current-hysteresis PWM,voltage space-vector PWM,and flux-weakening control were also adopted in the FSPM motor drives,respectively [60]–[62].Fig.28showstheFig.27.Typical current waveforms of a DSPM machine at the BLDC operation.(a)CCC mode.(b)APCmode.Fig.28.Measured current waveform of an FSPM machine at the BLACoperation.Fig.29.Stator interior PM topology [63].(a)Cross section.(b)Prototype.measured current waveform of an FSPM motor drive under vector control,showing a good sinusoidal phase current.V .T ORQUE R IPPLE M INIMIZATIONOwing to the nature of salient poles in both stator and rotor,the stator-PM machines suffer from higher torque ripple than traditional rotor-PM machines.Hence,different measures have been developed to minimize torque ripple in stator-PM machines.A.Improved Machine TopologyTo minimize the torque ripple,a modified topology (namely,the stator interior PM machine)was proposed,as shown in Fig.29,in which the conventional stator tooth shoe is employed and the magnets are inset [63].The predicted and experimen-tal results verify that the cogging torque can be significantly reduced.In addition,due to the simple rotor structure,the dummy slots can also be introduced to reduce the cogging torque,as shown in Fig.30for an FSPM machine [64].5094IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL.58,NO.11,NOVEMBER2011Fig.30.Rotor dummy slots in the FSPM machine.(a)Rotor topology with dummy slots.(b)Comparison of the cogging torque with and without dummyslots.Fig.31.Measured torque and current waveforms (25ms/div,2.2Nm/div,and 3.33A/div).(a)Unoptimized.(b)Optimizedcontrol.Fig.32.Measured torque (upper trace)and current (lower trace)waveforms (5ms/div,2Nm/div,and 2.4A/div).(a)V oltage space-vector PWM.(b)Proposed method.B.Conduction Angle ControlIn [65],the genetic algorithm was used to optimize the conduction angle proposed for the DSPM motor.Fig.31shows the torque and current waveforms.The torque ripple factor is re-duced from 81%to 21%by using the proposed control method.C.Harmonic Current Injection ControlIn [66]and [67],a new harmonic current injection method was proposed to minimize the torque ripple of the DSPM and FSPM motors,respectively.In [67],based on the harmonic spectrum analysis of the cogging torque,a series of specific harmonic currents is added into the q -axis reference current,resulting in additional torque components to counteract the fundamental and second-order harmonic components of the cogging torque.Both simulations and experiments confirmed the effectiveness of the proposed method.Fig.32shows the measured torque and current waveforms.It can be seen that the proposed harmonic current injection method can reduce the torque ripple by 50%as compared to the voltage space-vector PWMmethod.Fig.33.12/8-pole SDFDS machine.(a)Configuration.(b)Prototype[68].Fig.34.Six/four-pole HEDSPM machine.(a)Using ferrite magnets [69].(b)Using NdFeB magnets [70].VI.E MERGING T RENDSIn addition to the achievements earlier,new developments have been achieved on the stator-PM machines recently.A.Wound-Field and Hybrid-Excitation StructuresAlthough the stator-PM machines offer unique advantages,they cannot maintain a high efficiency or a constant output voltage over a wide speed range due to an uncontrollable PM flux,which are crucial for some applications such as electric vehicles (EVs).To enable flux control,the idea of stator dou-bly fed doubly salient (SDFDS)machine was proposed [68],in which the dc field windings replace the PMs to facilitate flux control and online efficiency optimization.Fig.33shows the configuration and prototype of the SDFDS machine.The efficiency of the machine can be improved by about 10%by online efficiency optimizing.Nevertheless,the SDFDS machine inevitably produces the extra loss in field windings,hence degrading machine effi-ciency.To incorporate both magnets and dc field windings,hybrid excited DSPM (HEDSPM)machines were proposed in [69]–[72](Figs.34–36),in which the dc field windings work forCHENG et al.:OVERVIEW OF STATOR-PERMANENT MAGNET BRUSHLESS MACHINES5095Fig.35.12/8-pole HEDSPM machine.(a)Cross section.(b)Efficiency–speed characteristic[71].Fig.36.Parallel configuration of an HEDSPM machine[72].Fig.37.Field-excitationflux-switching machine[73].flux control and efficiency optimization.The starting response can be improved by supplying a positive dcfield current to strengthen theflux,while the constant-power operating range can be extended by supplying a negative dcfield current to weaken theflux.In addition,a three-phase wound-fieldflux-switching ma-chine was proposed in[73](Fig.37).Recently,novel hybrid-excitationflux-switching(HEFS)machines were proposed in [74]–[76],in which thefield windings are accommodated in the room saved by the reduction of the magnets in the original FSPM machine.As stated in[75],the HEFS machine(Fig.38) exhibits excellentflux-regulation capability,in which the PM flux can be strengthened to twice that of the pure-PM topology and can be weakened to almost zero,indicating an infinite constant-power range.B.Redundant and Fault-Tolerant StructureIn some applications such as steering and fuel pumps,where a continuous operation must be ensured,reliability may be a critical requirement.Therefore,the need for a high degree of reliability in a motor drive system has inspired much researchin Fig.38.Configurations of the HEFS motor[75].Fig.39.Measured torque(trace1)and coil-B1and coil-C1current (traces2–3)waveforms before and after coil-A1fault(10ms/div,3Nm/div, and6A/div).the area.To achieve high reliability,redundant or conservative design techniques have been employed in many motor drives. Because of the magnetic independence between motor phases and the circuit independence of the converter phases,the stator-PM motor drives inherently possess a fault-tolerant character-istic.To improve the performance of the stator-PM motor drive with an open-circuited fault,fault compensation strategies were proposed[77],[78].Fig.39shows the measured torque and healthy-coil current waveforms during the fault occurrence of an FSPM machine under fault-tolerant control.It can be seen that the average torque can be maintained almost the same by controlling the current in the healthy coils when an open-circuit fault occurs in one coil.Moreover,redundant stator-PM topologies were proposed to improve the fault-tolerant capabil-ity,especially in the vehicle motor drives,as shown in Fig.40 [79].In[80],a novelfive-phase fault-tolerant doubly salient electromagnetic generator for directly driven wind turbine was proposed,as shown in Fig.41.5096IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL.58,NO.11,NOVEMBER2011Fig.40.Redundant stator-PM motor topologies.(a)Redundant DSPM motor.(b)Redundant FSPMmotor.Fig.41.Redundant stator-PM motor topologies [80].C.Memory PM Machine TopologyAlthough the dc field winding in both SDFDS motor and hybrid excited stator-PM motors enables the controllable air-gap flux,the use of dc field current inevitably causes additional power loss and degrades the efficiency.With the advent of the memory motors,the magnetization of PMs becomes online tunable.Hence,the concept of online-tunable flux-memory PMs was incorporated into the DSPM motor in such a way that the resulting flux-memory DSPM motor can offer an effective and efficient air-gap flux control [81].As shown in Fig.42(a),the memory DSPM motor adopts two-layer inner stator and outer rotor.In the stator,the armature windings are located in the outer layer,while both the PMs and magnetizing windings are placed in the inner layer,hence achieving a compact struc-ture.Since the outer rotor is simply composed of salient poles without PMs or windings,it is very robust.The PM material used in the motor is an aluminum nickel cobalt (AlNiCo)alloy.Fig.42(b)shows the measured back-EMF waveforms at different magnetization levels.It can be found that the back EMF can be effectively controlled for over four times.Due to the direct magnetization of PMs by a temporary current pulse in the magnetizing windings,the flux control is highly effective and highly efficient.D.Linear PM Machine TopologyThe linear PM motor is widely used because of its rapid dynamic response,high efficiency,and high power density.However,for long-stator applications such as underground traindrive systems,either the magnets or the armature windings,which are both expensive,have to be set on the stator.Recently,linear primary-PM machines,including the linear DSPM (LDSPM)and linear FSPM (LFSPM)types,have been pro-posed,which incorporate all of the merits of the rotary stator-PM machines.In [82]–[84],LFSPM machines were proposed,designed,and analyzed,such as feasible slot–pole number combinations and alternative stator winding configuration.In [85],a new structure of the LDSPM (Fig.43)was proposed and compared with the basic one.Three additional teeth and one piece of magnet are set at each end part of the two motors in order to reduce the end effect.The results show that the proposed method can avoid the asymmetry of the back-EMF waveforms and can reduce the cogging force and thrust force ripple,as shown in Fig.44.E.Advanced ControlDue to its strong nonlinear characteristics,the stator-PM mo-tors may not be exactly modeled.Hence,it is difficult to achieve high speed and torque performances by using the traditional control algorithm.Thus,some advanced control algorithms,such as sensorless control,vector control,and online efficiency optimization control,have been developed for the stator-PM machines.1)Position Sensorless Control:Similar to rotor-PM motor control,to turn on or off the phase windings at the spe-cific rotor positions,the rotor position information,which is traditionally measured by a physical sensor,is indispensable for the proper operation of the stator-PM motor.In order to avoid some difficulties resulting from the traditional sensor control method,the sensorless control of a DSPM motor drive was implemented [86].Detecting the induced back-EMF zero-crossing of unexcited winding was adopted.Fig.45shows the comparison of the position signals by physical sensor and by detecting circuits,illustrating that the real position signal lags behind the zero-crossing of the back EMF.Fig.46compares the speed rising process from standstill to the rated speed of 1500r/min between sensor and sensorless controls,showing that the DSPM motor drive with sensorless control offers almost the same performance as that with sensor control.To start the motor in sensorless control,the so-called “three-step startup。