Flux-switching permanent-magnet (FSPM) motors are a newly developed brushless AC (BLAC) machine having magnets in the stator. This paper proposes and implements a stator-flux-oriented (SFO) control strategy for fault-tolerant operation of FSPM motors. The key is to set the q-axis component of armature current invariant before and after the fault. In the rotor reference frame, by building a SFO-dq equation of the FSPM motor, the fault-tolerant control strategy is deduced. The finite-element method and the field-circuit cosimulation method are employed to analyze the performance of the FSPM motor drive. Finally, a dSPACE-based FSPM motor drive platform is built for experimental verification. Both the steady-state and dynamic performances at normal and fault-tolerant operations are tested, confirming that the proposed fault-tolerant operation can keep the output torque invariant while offering good dynamic performance during fault. © 2011 IEEE.published_or_final_versionThe IEEE International Magnetic Conference (INTERMAG2011), Taipei, Taiwan, 25-29 April 2011. In IEEE Transactions on Magnetics, 2011, v. 47 n. 10, p. 4191-419

Chau, KT

Cheng, M

Hua, W

Ji, J

Jia, H

Li, W

Zhao, W

IEEE Transactions on Magnetics

HKU Scholars Hub

Title Stator-flux-oriented fault-tolerant control of flux-switchingpermanent-magnet motorsAuthor(s) Zhao, W; Cheng, M; Chau, KT; Hua, W; Jia, H; Ji, J; Li, WCitationThe IEEE International Magnetic Conference (INTERMAG2011),Taipei, Taiwan, 25-29 April 2011. In IEEE Transactions onMagnetics, 2011, v. 47 n. 10, p. 4191-4194Issued Date 2011URL http://hdl.handle.net/10722/155669Rights©2011 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 4191Stator-Flux-Oriented Fault-Tolerant Control of Flux-SwitchingPermanent-Magnet MotorsWenxiang Zhao , Ming Cheng , K. T. Chau , Wei Hua , Hongyun Jia , Jinghua Ji , and Wenlong LiSchool of Electrical Engineering, Southeast University, Nanjing 210096, ChinaSchool of Electrical and Information Engineering, Jiangsu University, Zhenjiang 212013, ChinaDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Hong KongFlux-switching permanent-magnet (FSPM)motors are a newly developed brushless AC (BLAC)machine havingmagnets in the stator.This paper proposes and implements a stator-flux-oriented (SFO) control strategy for fault-tolerant operation of FSPMmotors. The keyis to set the -axis component of armature current invariant before and after the fault. In the rotor reference frame, by building a SFO-equation of the FSPMmotor, the fault-tolerant control strategy is deduced. The finite-element method and the field-circuit cosimulationmethod are employed to analyze the performance of the FSPM motor drive. Finally, a dSPACE-based FSPM motor drive platform isbuilt for experimental verification. Both the steady-state and dynamic performances at normal and fault-tolerant operations are tested,confirming that the proposed fault-tolerant operation can keep the output torque invariant while offering good dynamic performanceduring fault.Index Terms—Fault-tolerant control, flux switching, permanent-magnet motor, stator-flux oriented (SFO).I. INTRODUCTIONF AULT tolerance of motor drives is important for criticalapplications desiring continued operation following afault such as electric vehicle applications [1]. In recent years,some fault-tolerant machines, namely the switched reluctance(SR) machine [2] and the conventional permanent-magnet(PM) brushless machine having magnets in the rotor (therotor-PM machine) [3], have been developed. However, the SRmachine still suffers from relatively low power density, whilethe rotor-PM machine is still hindered by weak mechanicalstructure [4]. Recently, a new class of brushless machines withPMs located in the stator, namely the stator-PM machine, hasbeen proposed, which offers high power density and goodmechanical integrity [5]–[7]. It has been identified that thedoubly-salient PM (DSPM) motor can inherently offer faulttolerance [8], [9]. On the other hand, recent research has ver-ified that the power density of a flux-switching PM (FSPM)motor is significantly higher than that of a DSPM motor [10].Hence, how to improve the fault-tolerance capability of FSPMmotors has attracted more and more attention.In recent years, some fault-tolerant FSPM motors have beendeveloped. By incorporating the modular structure, alternatepoles, and redundant windings into the existing motor design[11]–[13], the fault-tolerance capability can be achieved. How-ever, differing from using design approaches, the use of con-trol strategies takes the advantages of flexible and directly ap-plicable to the existing FSPM motors.This paper presents a stator-flux-oriented (SFO) fault-tolerantcontrol strategy for FSPM motors, which can keep the outputtorque invariant while offering good dynamic performanceduring fault. The remedial operation mode will be investigatedand then verified by both simulation and experimentation. ItManuscript received February 21, 2011; accepted May 11, 2011. Date ofcurrent version September 23, 2011. Corresponding author: W. Zhao (e-mail:zwx@ujs.edu.cn).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2011.2157106Fig. 1. Three-phase 12/10-pole FSPM motor. (a) Schematic. (b) Prototype.should be noted that the SFO control for fault-tolerant operationof FSPM motors is absent in literature.II. ANALYSIS AND CONTROLA. FSPM MotorA three-phase 12/10-pole FSPM motor is shown in Fig. 1, inwhich the rotor has only solid-iron salient poles while the statorincorporates both PMs and armature windings. Since there areno PMs, brushes, or windings in the rotor, the FSPM motor of-fers the advantages of simple rotor configuration and mechan-ical robustness. By adopting high-energy PM as excitation, theFSPM motor retains the merits of the rotor-PM motor, such ashigh efficiency and power density. Also, since all PMs are lo-cated in the stator, the problems of cooling difficulty and thermalinstability in the rotor-PM motor can be eliminated.Moreover, the finite-element method (FEM) is employed toanalyze the characteristics of the FSPM motor. Fig. 2 comparesthe flux vectors due to the PMs and the armature winding cur-rent. It can be seen that their flux paths are in parallel, whichdiffers from the series nature of those flux paths in other PMmotors. Thus, the possibility of demagnetization of PMs by ar-mature reaction flux under the short-circuit fault is eliminated,providing the capability of fault tolerance.0018-9464/$26.00 © 2011 IEEE4192 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011Fig. 2. Comparison of flux vectors. (a) PMs only. (b) Armature current only.Fig. 3. Measured three-phase no-load EMF waveforms (1 ms/div, 50 V/div).Fig. 4. No-load field distribution and axis definition.B. Stator-Flux OrientationFig. 3 shows the measured no-load EMF waveforms of theFSPM motor, where their shapes are approximately sinusoidal.Thus, the FSPM motor is particularly suitable for brushlessAC (BLAC) operation and the vector control can be employed.However, since the FSPM motor does not generate rotationalmagnetomotive force (MMF), the traditional rotor-flux-ori-ented control strategy cannot be applied to the FSPM motordirectly.In order to enable vector control in this FSPM motor, aSFO- equation of the motor in the rotor reference frameis developed. Fig. 4 shows the FEM-based no-load magneticfield distribution, in which the relative positions between the- and -axes, and the stator teeth and rotor poles are depicted.It can be found that the rotor position of the maximum PMflux linkage is defined as the -axis, while the -axis advancesthe -axis by 90 in electrical degrees (equivalent to 9 inmechanical degrees). The development of the -axes is crucialfor the development of this SFO fault-tolerant control strategy.C. Fault-Tolerant Control StrategyIt has been known that the - and -axis components of ar-mature currents correspond to the production of torque and flux,respectively. When the -axis component of armature current ismaintained, the stator-PM flux-linkage will be constant. Thus,by setting the -axis component of armature current invariantbefore and after the fault, the electromagnetic torque of theFSPM motor can be maintained.Based on the proposed SFO, the armature currents of theFSPM motor during normal operation can be expressed as(1)where is the rotor angle, , and are the phase currents,and and are the transformed currents in the synchronousreference frame. The - -0 to - - transformation can bewritten as(2)where and are the transformed currents in the fixed statorreference frame, and is the zero-sequence current.When the phase-A winding is open circuited, the phase-Acurrent vanishes so that the motor is fed by two healthy phases.The zero-sequence component of armature current can be ob-tained as(3)Then, by substituting (3) into (2), it deduces(4)The - to - transformation can be written as(5)Then, by substituting (5) into (4), the fault-tolerant controlstrategy can be deduced as(6)Fig. 5 shows the control block diagram of the FSPM motordrive, in which the SFO fault-tolerant control is incorporated.By employing a proportional–integral (PI) regulator, the differ-ence between the reference speed and the actual speed deter-mines the reference electromagnetic torque, which is directlyproportional to the -axis component of armature current. Thehysteresis PWM controller is employed to drive the inverter.During the fault-tolerant operation, the FSPMmotor operates inthe two-phase mode. The -axis component of armature currentZHAO et al.: STATOR-FLUX-ORIENTED FAULT-TOLERANT CONTROL OF FLUX-SWITCHING PERMANENT-MAGNET MOTORS 4193Fig. 5. Control block diagram of fault-tolerant FSPM motor drive.Fig. 6. Cosimulation model of FSPM motor drive.can be set invariant before and after the fault, thus maintainingthe average torque during the fault-tolerant operation.III. RESULTS AND VERIFICATIONA. SimulationTo evaluate the proposed remedial control strategy, a transientcoupling field-circuit cosimulation model of the FSPM motordrive is developed as shown in Fig. 6. The cosimulation methodsimultaneously employs both the magnetic field solver and theelectric circuit solver, thus providing an accurate system levelsimulation [8].For the purpose of quantitative evaluation of torque perfor-mance of the FSPMmotor drive under normal and fault-tolerantoperations, a torque ripple factor is defined as100% (11)where , and are the maximum, minimum, andaverage values of the output torque, respectively.During the normal and fault-tolerant operations of the FSPMmotor drive, the cosimulated torque and current waveforms areobtained as shown in Fig. 7. It can be seen that under the normaloperation, the corresponding and of the motor driveare 6.7 Nm and 58.9%, respectively. It should be noted thatthe torque ripple is mainly caused by the cogging torque ofthe FSPM motor [7]. Quantitatively, the and of theFSPM motor drive under fault-tolerant operation are 6.6 Nmand 62.7%, respectively. It can be concluded from these resultsthat the torque performances under the normal and fault-tol-erant operation are practically unchanged. A slight discrepancyis mainly due to the nonideal sinusoidal no-load EMFs of theFSPM motor.B. ExperimentationA prototype of the FSPM motor drive is built for experi-mental verification of the proposed fault-tolerant control. Thetest bench consists of the FSPM motor, a DC dynamometer toserve as variable load, a dynamic torque transducer to measureFig. 7. Simulated torque and current waveforms. (a). Normal operation. (b)Fault-tolerant operation.Fig. 8. Measured steady-state torque (trace 1) and current (traces 2–4) wave-forms (25 ms/div, 4 Nm/div, 4.8 A/div). (a) Normal operation. (b) Fault-tolerantoperation.the torque waveform, and a dSPACE-based digital controller toimplement the control scheme. In addition, an optical positionencoder and three Hall-effect current sensors are used for mea-surement.First, the proposed remedial operation of the FSPM motoris assessed during steady-state operation. Fig. 8 compares thesteady-state torque and current waveforms under normal andfaulty conditions. It can be seen that the measured waveformsagree with the simulated ones as shown in Fig. 7, verifying thatthe proposed remedial operation can maintain the same torqueduring the loss of one phase.Then, the proposed fault-tolerant control strategy is evaluatedduring dynamic operation. Fig. 9(a) shows the healthy phasecurrent responses before and after fault-tolerant operation. It canbe observed that the motor drive can online switch to the fault-tolerant mode.Furthermore, the self-starting capability under the open-cir-cuit fault is particularly assessed, which is essential for those ap-4194 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011Fig. 9. Measured dynamic current responses of healthy phases. (a) Before andafter fault (50 ms/div, 4.8 A/div). (b) Startup (25 ms/div, 12 A/div).Fig. 10. Measured speed (trace 1) and current (trace 2) responses under suddenload changes (5 s/div, 200 rpm/div, 2.4 A/div).plications such as electric vehicles desiring frequent start–stop.Fig. 9(b) shows the startup current responses under fault-tol-erant operation, confirming that the motor drive can success-fully perform self-starting during the loss of one phase.Finally, the dynamic load test of the developed motor driveat fault-tolerant operation is performed. Fig. 10 depicts the dy-namic responses of speed and current under sudden changes ofload torque (from 7.2 to 2.5 Nm and then back to 7.2 Nm) whenthe motor operates in the fault-tolerant mode. It can be foundthat the transient changes in speed are quite small, while thespeed regulation is very good.IV. CONCLUSIONIn this paper, the SFO control strategy has been newly ap-plied for fault-tolerant operation of FSPM motors. By settingthe -axis component of armature current invariant before andafter the fault, the output torque can be maintained constantwhile offering good dynamic performance during fault. Boththe field-circuit cosimulation and experimental results confirmthe validity of the proposed fault-tolerant control strategy forFSPM motors. This control strategy can readily be extended toother types of stator-PM motors that can operate at the BLACmode.ACKNOWLEDGMENTThis work was supported in part by the National NaturalScience Foundation of China (Projects 60974060, 50907031,and 51007031), by the Aeronautical Science Foundation ofChina (Project 20100769004), and by the Professional Re-search Foundation for Advanced Talents of Jiangsu University,China (Project 10JDG089).REFERENCES[1] K. T. Chau and C. C. 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Magn., vol. 45, no. 10, pp. 4704–4707, Oct. 2009.[7] Z. Q. Zhu and J. T. Chen, “Advanced flux-switching permanentmagnet brushless machines,” IEEE Trans. Magn., vol. 46, no. 6, pp.1447–1453, Jun. 2010.[8] W. Zhao, K. T. Chau, M. Cheng, J. Ji, and X. Zhu, “Remedial brushlessAC operation of fault-tolerant doubly-salient permanent-magnet motordrives,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2134–2141, Jun.2010.[9] C. Liu, K. T. Chau, and W. Li, “Comparison of fault-tolerant oper-ations for permanent-magnet hybrid brushless motor drive,” IEEETrans. Magn., vol. 46, no. 6, pp. 1378–1381, Jun. 2010.[10] J. Zhang, M. Cheng, Z. Chen, and W. Hua, “Comparison of stator-mounted permanent-magnet machines based on a general power equa-tion,” IEEE Trans. Energy Conv., vol. 24, no. 4, pp. 826–834, Dec.2009.[11] M. Jin, C. Wang, J. Shen, and B. Xia, “A modular permanent-magnetflux-switching linear machine with fault-tolerant capability,” IEEETrans. 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Stator-flux-oriented fault-tolerant control of flux-switching permanent-magnet motors

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Stator-flux-oriented fault-tolerant control of flux-switching permanent-magnet motors

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