An Off-line Maximum Torque Control Strategy of Wound Rotor Synchronous Machine with Nonlinear Parameters

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1 J Electr Eng Technol.2016; 11(1): ISSN(Print) ISSN(Online) An Off-line Maximum Torque Control Strategy of Woun Rotor Synchronous Machine with Nonlinear Parameters Qi Wang*, Heon-Hyeong Lee *, Hong-Joo Park *, Sung-Il Kim** an Geun-Ho Lee Abstract Belt-riven Starter Generator (BSG) iffers from other mil hybri systems as the crankshaft of vehicle are not run off. Motor permits a low-cost metho of aing mil hybri capabilities such as start-stop, power assist, an mil levels of regenerative braking. Woun rotor synchronous motor (WRSM) coul be aopte in BSG system for HEV e-assiste application instea of the interior permanent magnet synchronous motor (IPMSM). In practice, aequate torque is inispensable for starter assist system, an energy conversion shoul be taken into account for the HEV or EV as well. Particularly, flux weakening control is possible to realize by ajusting both irect axis components of current an fiel current in WRSM. Accoringly, this paper present an off-line current acquisition algorithm that can reasonably combine the stator an fiel current to acquire the maximum torque, meanwhile the energy conversion is taken into consieration by losses. Besies, on account of inuctance influence by non-uniform air gap aroun rotor, nonlinear inuctances an armature flux linkage against current variation are propose to guarantee the results closer to reality. A computer-aie metho for propose algorithm are present an results are given in form of the Lookup table (LUT). The experiment shows the valiity of algorithm. Keywors: BSG, Fiel current control, Flux weakening control, Maximum torque control, WRSM 1. Introuction BSG is a kin of mil hybri system with simple structure an light-weight as shown in Fig. 1. It provies more power to traction system, an also be able to electrically start an assist the engine via belt. Recently, WRSM coul be a challenger to isplace the IPMSM that is frequently use in BSG system ue to plentiful merits such as high starting toque, wie constant power range, freeom fiel current control via external circuit substitute for permanent magnet, an controllable power factor. Inversely, there are some rawbacks as well. For instance, the exciting current shoul be injecte via slip ring-brush system, an external circuit makes the entire circuit more complex, shrinks the life cycle an increases the maintenance cost. Moreover, electromagnetic interference an electromagnetic compatibility are occurre ue to the outer circuit [1]. Despite these rawbacks, it is still worthy to o research on WRSM, because of its merits an the mentione problems coul be solve along with more research procee on. Literature [2] investigate the performances between the WRSM an IPMSM uner the same configuration, where iscloses that the former has the preferable efficiency in the high spee region, the poor Corresponing Author: Dept. of Automotive Engineering, Kookmin University, Korea. (motor@kookmin.ac.kr) * Dept. of Electrical an Electronic Engineering, Kookmin University, Korea. (arsenal@kookmin.ac.kr) ** Motor R&D Group, Digital Appliances, Samsung Electronics, Korea. (si1227.kim@samsung.com) Receive: July 14, 2015; Accepte: November 19, 2015 efficiency in the low spee region than the latter. WRSM has three control variables: -, q-axis components of current an fiel current. It allows the machine operating at an arbitrary power factor even unity power [3]. Currently, wiely use maximum torque control are solving partial ifferential an quaruplicate equation for acquiring the reference current where the inuctances are constant [4, 5]. However, the inuctances an armature flux linkage are impacte by non-uniform air gap aroun rotor in WRSM, which lea to the calculate current inaccuracy, an aversely affect its overall efficiency. In aition, the maximum torque control in the Flux-weakening (FW) region has to take into account of the controllable fiel current. As a consequence, this stuy presents an off-line control strategy to acquire the reference current for maximum torque control by nonlinear inuctances with consieration of losses. The algorithm is able to obtain the optimal combination of Fig. 1. BSG system structure Copyright c The Korean Institute of Electrical Engineers This is an Open-Access article istribute uner the terms of the Creative Commons Attribution Non-Commercial License ( licenses/by-nc/3.0/) which permits unrestricte non-commercial use, istribution, an reprouction in any meium, provie the original work is properly cite. 1921

2 An Off-line Maximum Torque Control Strategy of Woun Rotor Synchronous Machine with Nonlinear Parameters stator current an fiel current by means of computer-aie metho. The calculate currents refer to torque an flux comman are store in orer to force the flux linkage against the terminal voltage variation [5]. Therefore, the esire current coul respon rapily in case that the battery capacity is insufficient. The propose control strategy was euce in MATLAB with 6kW prototype WRSM. The strategy was emonstrate by simulation an experiment. 2.1 Nomenclature 2. Mathematical moel v s, v qs, v fr -, q-axis components of terminal voltage an rotor terminal voltage; v os, v oqs -,q-axis voltage components of iron loss; V 0 Terminal voltage at rate spee; i os, i oqs -, q-axis components of armature current with consieration of iron loss; i r, i qr, i fr -, q-axis component of amper wining current an fiel excitation current; i cs, i cqs -, q-axis current components of iron loss; i os, i oqs -, q-axis current components of torque generation; R s, R fr Armature are rotor resistance; R r, R qr -, q- axis components of amper wining resistance; R c Equivalent iron loss resistance; L s, L qs -, q- axis components of stator inuctance; L m, L mq Magnetizing inuctance; L r, L qr -, q-axis components of amper wining; L r, L qr -, q-axis components of amper wining; L r, L qr -, q-axis components of amper wining inuctance; L ls, L lfr Stator an rotor leakage inuctance; L lr, L lqr -, q-axis components of leakage inuctance of amper wining; ψ s, ψ qs -, q-axis components of flux linkage in stator sie; ψ r, ψ qr -, q-axis components of flux linkage in rotor sie; ψ f Excite flux linkage; ω e Electrical angular velocity; ω m Mechanical angular velocity; P n Motor pole pairs; s/ t Differential operator; P c Copper loss; P cs, P cr Stator an rotor copper loss; D Outer raius of rotor; L Stack length of motor; M Mass of rotor; K f Friction factor (1~3); n Motor spee. All the rotor variables are referre to the stator. 2.2 WRSM moel The performance of synchronous machine strongly epens on the vector control strategy. Equivalent circuits of WRSM base on -q moel are shown in Fig. 2. Base on them, the WRSM mathematical moel was escribe by Eq. (1) with consiering -, q-axis components of amper wining. vqs Rs + slqs ωels(1 + Rs Rc) slmq vs ωelqs(1 + Rs Rc ) Rs + sls ωelmq 0 slmq 0 Rqr + slqr 0 0 slm 0 v fr 0 slm 0 ωelm ω elm(1 + Rs Rc ) ioqs slm sl m i os 0 0 iqr (1) Rr + slr sl m i r slm Rfr + s( Llfr + Lm ) i fr It reveals that the electromechanical ynamics of WRSM is nonlinear Multiple Input Multiple Output system,which is very complicate for computation. Hence, in orer to intuitively analyze the system, the steay state moel is erive by equating all the time erivatives to zero an the power transferre to the rotor wining is zero, an all the power across the air gap is converte to mechanical power. Then the equations in steay state without consiering amper wining were (a) -axis (b) q-axis Fig. 2. Dynamic equivalent circuits of machine in the synchronously rotating coorinate 710 J Electr Eng Technol.2016; 11(1):

3 Qi Wang, Heon-Hyeong Lee, Hong-Joo Park, Sung-Il Kim an Geun-Ho Lee rewritten as follow. vs ios Rs vos Rs 1 v qs i + + oqs R v c oqs (2) vos 0 ωelq ios 0 v oqs ωel 0 + i oqs v fr (3) v ωψ ω L i (4) fr e f e m fr In Fig. 2, it is observe that armature current is consists of iron loss components an torque generate components. The torque generate current can be expresse as follows. Ri cs+ ωeli qoqs ios is ics Rc (5) Ri cqs ( ωeli os+ ωψ e f) ioqs iqs icqs Rc (6) R 2 2 cs i + ωelqqs i Rc ωelqψ f ios R2 2 c+ ωellq (7) Ri 2 c qs ωeli src ωψ e frc ioqs R2 2 c+ ωellq (8) I ( i + i ) 2 + ( i + i ) 2 (9) a os os oqs cqs Substitute (7), (8) into (5), (6), the equivalent iron loss currents are obtaine: i i cs cqs ω LLi ω Li R + ω Lψ 2 2 e q s e q qs c e q f R2 2 c+ ωellq ω LLi + ω Li R+ ωψ R 2 e q qs e s c e f c R2 2 c+ ωellq (10) (11) The torque generate current can be calculate as follows: T Pψ i + P ( L L ) i i (12) n a oqs n q os oqs Where ψ a is the phase flux linkage establishe by fiel current at no loa conition. Notice that magnetizing flux is involve in the excite flux linkage. Hence, the phase flux linkage at loa conition is use to replace the excite flux linkage ψ f. P P + P R ( i + i ) + R i (12) c cs cr s s qs f fr Iron losses are compose of the ey current loss an hysteresis loss in core. Literature [6] provies an iron losses calculation metho that only can be use for sinusoial variation. Due to the harmonic components in the magnetic flux ensity, the iron losses are one of the most ifficult phenomenon to moel accurately. Hereby, the aforementione iron loss resistance is utilize to calculate the iron losses refer to practical test. It is an operating frequency function as shown in Eq. (14), where the iron losses are proportional to the square of frequency, an flux linkage can be controlle by fiel current in the WRSM. Iron losses can be moele as a function of frequency an fiel current with finite element analysis metho by (15). Nevertheless, terminal voltage an total iron losses shoul calculate iron loss resistance at every operating conition, which results in the enormous an complicate computation since the conitions vary accoring to the current an its phase angle. Thus, the back electro-motive force (back-emf) in Fig. 3 an total iron loss at no-loa are use to calculate the iron loss resistance. V ( ωψ e 0) Rc (14) P P Fe Fe P f( ω, i ) (15) Fe Mechanical losses are not involve in the equivalent circuit, whereas these exist uring the machine running an shoul be taken into account in orer to estimate the accurate efficiency. In WRSM, it mainly consists of raft loss an frictional loss on the rotor resistance an internal friction among mechanical components, respectively. It is harly possible to estimate mechanical losses precisely. Normally the mechanical losses are less than 1~2% of total losses. Hence, a simplifie moel was aopte on the basis of experiment in this stuy. The mechanical losses are relate to motor spee for the synchronous motor [7]. Herewith mechanical losses are moele as a function of motor spee accoring to the test an expresse as: fr 2.3 Loss moel In orer to consier the efficiency, loss moel is necessary. The ominant losses in WRSM mainly consist of copper losses, iron losses, an mechanical losses. As seen from equivalent circuit, copper losses are cause by stator an rotor wining resistance in the form of heat issipation. Hereby, copper losses are formulate as: Fig. 3. Back-EMF versus fiel current 711

4 An Off-line Maximum Torque Control Strategy of Woun Rotor Synchronous Machine with Nonlinear Parameters P (2 D L+ K M) n *10 (16) mech f The motor efficiency is expresse by: η eff ωmte ω T + P + P + P m e c Fe mech 3. Algorithm realization (17) Generally, BSG motor operates in constant torque region an constant power region accoring to operating spee. In this section, an off-line algorithm is analyze accoring to the operating region. One feature of WRSM which iffers from IPMSM is - axis inuctance larger than q-axis, therefore it is able to control in arbitrary quarant. The propose current vector operating in the first an secon quarant is shown in Fig. 4. In the low spee region, motor operates at the point A in the first quarant where both -, q-axis component of currents are positive, an maximum fiel current is maintaine to ensure the torque maximization. As spee increase, the maximum terminal voltage is reache ue to output capability of inverter. Thereby flux weakening control is execute to exten the spee on account of terminal voltage limitation, which is implemente by ajusting both -axis components of current an fiel current. The motor operating point goes towars the secon quarant along the negative irect axis from point B to C to maximize the torque when the terminal voltage restriction is consiere. Then the total flux is ecrease along the C to the centre M as the spee increasing. Note the point M is controllable by means of fiel current control [3]. Despite of there are corresponing equation for each control algorithm [8], but the characteristic analysis is much ifficult to be ealt by them for WRSM ue to the nonlinear inuctances, that impacte by several controllable variables such as excitation current, armature current an phase angle. This paper suggests a computer aie metho which uses the iteration loop to calculate the motor characteristics an the reference currents for motor controlling. Before the base spee, the loop conition is the maximum torque an current ue to maximum torque per ampere (MTPA) control. After base spee, the maximum power is the loop conition. Once loop conition is Fig. 4. Current vector of propose control strateg Fig. 5. The flowchart of the propose algorithm 712 J Electr Eng Technol.2016; 11(1):

5 Qi Wang, Heon-Hyeong Lee, Hong-Joo Park, Sung-Il Kim an Geun-Ho Lee satisfie, the calculate ata is store. The proceure of the algorithm implementation is shown in Fig. 5. The currents are calculate by an offline program for eriving the look-up tables. Four pre-prepare ata such as -, q-axis components of inuctance, iron loss an back- EMF are employe as program calle files. Maximum fiel current is utilize to maximize the flux for proviing the maximum torque before the motor rate spee. In the flux weakening region, the algorithm extracts the maximum torque by means of comparing the torque generate by ientical -axis component current with the reuce fiel current graually, an the torque generate by ientical fiel current with the varying -axis component current. On the other han, the ifferent combination of the stator current in each angle an fiel current are compute to seek which combination coul prouces the maximum torque with nonlinear parameters. Terminal voltage suppression is the execution conition an Eqs. (13) ~ (17) use to the losses calculation. Assume the space vector moulation is aopte, where the maximum output voltage is V c-link / 3, the total magnetic flux linkage is calculate as follow. Where v represents the function value, then interpolate along the y-axis an obtaine: w i (1 y ) + i y w j y j y (1 ) + 2 Finally, interpolate along the z-axis an obtaine: (21) vi w (1 x ) + w x (22) 1 2 where vi is the esire value of unerlying 3-D function v at the points in arrays. The result of the tri-linear interpolation is not relate to the interpolating calculation s orer which obeys the commutative law of tensor prouct. In this paper, x, y, z are I a, β an i fr, respectively. vi is the esire point such as L, L q, an ψ a. Fig. 6 show the calculate -, q-axis components of inuctance an phase flux linkage profiles. It is evient that L q has not much change in total current istribution, while the L behaves significant nonlinearity with saturation. Fig. 7 show the calculate -, q-axis components an Vc link ψ 0 (18) 3ω Eventually, the reference current relate to the torque comman an total flux comman are store as LUT automatically. e 4. Simulation In particular, saliency of WRSM varies owing to the nonlinear characteristics that are inuce by magnetic saturation. Consequently, the nonlinear parameters are employe by Finite Element Analysis Metho, which aims to obtain more realistic characteristic. Tri-linear interpolation metho is utilize to buil these parameters for reference current computation as follows. (a) -axis components of inuctance x ( x x0) / ( x1 x0) y ( y y0)/ ( y1 y0) z ( z z ) / ( z z ) (19) (b) q-axis components of inuctance (x, y, z ) are esire point, x 0 an x 1 represent the point below an above x in the cubic space lattice an similarly for y 0, y 1, z 0 an z 1. For fining the 3-D point, firstly interpolate along the x-axis an obtaine: [,, ] (1 ) [,, ] [,, ] (1 ) [,, ] [,, ] (1 ) [,, ] [,, ] (1 ) [,, ] i v x y z x + v x y z x i v x y z x + v x y z x j v x y z x + v x y z x j v x y z x + v x y z x (20) (c) Phase flux linkage Fig. 6. Nonlinear inuctances an phase flux linkage 713

6 An Off-line Maximum Torque Control Strategy of Woun Rotor Synchronous Machine with Nonlinear Parameters fiel current profiles. Fig. 7 (a) shows the file current is much utilize for flux weakening control with consiering losses at the high spee an lower torque requirement. Otherwise, the negative -axis component current is more utilize an the fiel current keep constant control in orer to sustain the higher torque at the high spee. The variations can be seen from Fig. 7 (a) an (c). Fig. 7 (b) which shows the variation of q-axis component current accoring to the variation of the - axis component current an fiel current. 5. Test Bench an Experiment 6kW prototype machine was utilize to verify the aforesai algorithm in the rive system as shown in Fig. 8. Fiel current is regulate by H-brige circuit via a slip ring, which has the rapi response than the traitional chopper circuit [2]. Table 1 provies the specification of the machine an river system. Several performance evaluations with propose rive system were teste. On account of the real situation, maximum torque was Table 1. Specifications (a) -axis components of current (b) q-axis components of current Items Rate/Max.power Rate spee Teste Max spee Rate /Max.torque DC link voltage Max.phase current Max.line to line voltage Max.fiel current D-axis average inuctance PhiA average inuctance Q-axis average inuctance Stator resistance Rotor resistance Pole pairs number Cooling system Value 4.25/6 [kw] 2150 [r/min] 7500 [r/min] 17.75/27 [Nm] 115 [V] [Arms] [Vrms] 3 [A] 0.42 [mh] [Wb] 0.33 [mh] [Ohm] [Ohm] 3 Water-cooling Table 2. Torque comparison Items (c) Fiel current Fig. 7. Current mapping accoring to Flux & Torque Max. Torque Max. Power Sim Test Motor Temperature [ C] Spee [RPM] 2150 Torque [Nm] Efficiency [%] Fiel current [A] 3 3 (Sim is the abbreviation of simulation.) Fig. 8. WRSM rive system with propose algorithm 714 J Electr Eng Technol.2016; 11(1): Max. power Max. spee Sim Test

7 Qi Wang, Heon-Hyeong Lee, Hong-Joo Park, Sung-Il Kim an Geun-Ho Lee evaluate an test results are shown in Fig. 9 an Fig. 10. The comparison between teste results an esigne value in ifferent conition is provie in Table 2. The first test is uner the conition of maximum power an torque, an the secon is uner the conition of maximum power an spee. Water-cooling system is use to maintain the motor temperature. The teste torque is approximate to the simulate ata at 20 C. At the higher temperature of 65 C, the maximum torque have less than 5% ecline that can be acceptable. Fig. 11 is the vector plane of teste current in the flux weakening control region. Accoring to the spee increase, the more negative -axis current is use till to the -70A, an q-axis current is ecrease to about 20A at high spee region. Fig. 12 is the maximum torque test in ifferent fiel current conition. As seen, the fiel current have a significant contribution for maximum torque generation at the low spee region, an make a contribution to flux weakening with -axis components of current at the high spee region. Current control is implemente accoring to the torque an flux comman in Fig. 13. The start moe is activate when the loa starts, an after the motor starte with the require torque an reache to 2500 rpm, torque e-assist was shut up an current commans become zero. Fig. 14 is the torque assist moe with targete engine where 2.5:1 pulley ratio is selecte. The engine spee is 800 rpm where motor spee is 2000rpm. With the 24 Nm Fig. 12. Experimental maximum torque characteristic in fiel current control Fig. 9. Maximum torque comparison Fig RPM in Start-up moe Fig. 10. Maximum torque characteristic in ifferent temperature Fig. 11. Experimental i - i q plane in the FW region Fig. 14. Torque e-assist moe for BSG system 715

8 An Off-line Maximum Torque Control Strategy of Woun Rotor Synchronous Machine with Nonlinear Parameters Fig. 15. Test bench with loa motor Fig. 16. Teste efficiency an generation power map torque starte, the maximum fiel current an phase current were given to follow the engine commans. While the engine spee rise up to the 800 rpm, the torque e-assist was release promptly, meanwhile currents were release to follow the torque comman as well. Fig. 15 is 8kW loa motor that is assemble to establish the efficiency an regenerative power map. The coolant temperature was fixe at 25 C. In aition, in orer to test the regeneration power, constant fiel current 2A was employe in rate operating an the coolant temperature was fixe at 65 C. Since the power generating is proceee uring the riving. Fig. 16 shows the teste results, where maximum efficiency an maximum regenerative power were 96% an 5.2 kw in the motoring an generating moe, respectively. 6. Conclusion This stuy presente an offline strategy to acquire the reference current with taking losses an torque maximization into account for BSG application. It provies a computeraie metho to implement the -axis components of current an fiel current istribution reasonably for flux weakening control. With the algorithm, the torque maximization an preferable efficiency coul be achieve which have been verifie by experiment. Generally, the constant or online etection parameters are frequently use in the previous stuies. However, the motor parameters are influence by a number of factors such as current an temperature in practice. With this consieration, the nonlinear parameters that coul be aopte for algorithm evelopment an etermine by stator current, fiel current an phase angle. It contributes the algorithm to agree better with the reality. The experiment results emonstrate the obtaine maximum torque was in accorance with the simulate ata at normal temperature status. However, the temperature effect cannot isregar which cause about 5% torque ecline because a number of seasons such as thermal loss. Due to the BSG characteristic, the motor usually have the contribution in low spee an high torque region. It implies this motor is often use to be starting torque assistant. Test results reveal the WRSM have approximate efficiency with the IPMSM, but the manufacturing cost lower in the absence of permanent magnet, an the performances in each operating region is more suitable for BSG e-assist application such as the higher torque in the start moe. The payoff of propose algorithm is practical, not merely theoretical. Unquestionable, some of the unesirable current harmonics are involve in the alternative current line that lea to the converter efficiency reuction. Meanwhile, inuce voltage istortion in the alternating current supply that causes the electromagnetic compatibility problems. Therefore, further work is still nee to procee. Sorting these problems have assistance to improve efficiency an compatibility. Acknowlegements This research was financially supporte by the Clean iesel automobile key components inustry promotion program through the Ministry of Trae, Inustry & Energy (MOTIE) an Korea Institute for Avancement of Technology (KIAT). Meanwhile, it was supporte partially by the research fun from Kookmin University. References [1] Ganesan R, Das S. K, Sinha B. K, Analysis an control of EMI from single phase commutator motor use in house-hol mixer, IEEE. Maras EMI/C International Conference, pp , Dec [2] Geun-Ho Lee, Heon-Heyong Lee, Qi Wang Development of Woun Rotor Synchronous Motor for 716 J Electr Eng Technol.2016; 11(1):

9 Qi Wang, Heon-Hyeong Lee, Hong-Joo Park, Sung-Il Kim an Geun-Ho Lee Belt-Driven e-assist System, Journal of Magnetics. vol. 18, pp , Nov [3] Rossi. C, Casaei. D, Pilati. A, an Marano. M, Woun Rotor Salient Pole Synchronous Machine Driver for Electric Traction, IEEE. Trans. In. Appl, vol. 3, pp , October [4] Thomas. M.J, GERALD. B.K, an Thomas. W.N, Interior Permanent-Magnet Sychronous Motors for Ajustable-Spee Drives, IEEE. Trans. In. Appl, vol. IA-22, pp , July/August [5] Bon-Ho Bae, Seung-Ki Sul, New Fiel Weakening Technique for High Saliency Interior Permanent Magnet Motor, In Conf. Rec. IEEE-IAS, vol. 2, pp , October [6] Lonel, D.M, Popescum, M, McGilp, M.I, Miller, T.J.E, Dellinger, S.J, Heieman, R.J, Computation of core losses in electrical machines using improve moels for laminate steel, IEEE. Trans. In. Appl, vol. 43, pp , Nov/Dec [7] Jacek F. Giera, Mitchell Wing, Permanent Magnet Motor Technology, Design an Applications, 2n e. Marcel Dekker, Inc, 2000, pp [8] Shigeo Morimotor, Yoji Takea, Takao Hirasa, Current phase control methos for permanent magnet synchronous motors, IEEE. Trans. Power Electron., vol. 5, no. 2, pp , April Sung-Il Kim He receive the B.S. an M.S. egrees in electrical engineering from Changwon National University, Changwon, Korea, in 2003 an 2005, respectively, an the Ph.D. egree in automotive engineering from Hanyang University, Seoul, Korea, in From 2011 to 2013, he was a Research Staff Member with the Samsung Avance Institute of Technology, Yongin, Korea. Since 2013, he has been a Senior Engineer with the Compressor an Motor Business Team, Samsung Electronics, Suwon, Korea. His research interests inclue the esign an analysis of various electromagnetic evice for vehicles an home appliances. Geun-Ho Lee He receive his B.S. an M.S. in Electrical Engineering an his Ph.D. in Automotive Engineering in 1992, 1994 an 2010, respectively, from the Hanyang University, Seoul, Korea. From 1994 to 2002, he joine LG Inustrial Research Institute where he evelope inverter system for elevators. Since 2011, he became professor for Automotive Engineering at the Kookmin University. His current research interests inclue the avance control of electric machines, an electric vehicles. Qi Wang He is currently pursuing integrate M.S. an Ph.D. egree of automotive engineering in Kookmin University, Korea. His interesting research fiel is synchronous machine control system. Heon-Hyeong Lee He is currently pursuing M.S. an Ph.D. egree in Kookmin University, Korea. His interesting research fiels are alternator an EPS control system. Hong-Joo Park He is currently pursuing integrate M.S. an Ph.D. egree in Kookmin University, Korea. He is intereste in high performance AC motor rive control system