Send Orders for Reprints to [email protected] 1652 The Open Automation and Control Systems Journal, 2014, 6, 1652-1660 Open Access Design and Implementation of the Control System for a Hybrid Excitation Synchronous Machine

Guo Xinjun*, Huang Mingming, Huang Quanzhen, Liu Yuping and Lina

School of Electrical Information Engineering, Henan Institute of Engineering, Zhengzhou, 451191, China

Abstract: This paper proposed a novel kind of hybrid excitation synchronous machine (HESM) driving system, which possesses the properties of low speed high torque and has wide speed range. Firstly, the construction, the basic working principle and the concept of design were overviewed. Following this , the principle of two-speed-region control algorithm was presented based on the space voltage vector control. The proposed control algorithm includes two aspects. 1) In the low speed region, the current control strategy was adopted with or without excitation current for adjusting speed accord- ing to the load of HESM. 2) In the high speed region, the current control strategy adopts a combination strategy of holding q-axis components of Back-EMF unchanged and the copper loss minimization algorithm. The proposed methods were verified by the contrastive simulations and the experimental results through different current control methods.

Keywords: Flux weakening control, hybrid excitation synchronous machine, vector control, wide speed range.

1. INTRODUCTION current for flux enhancement) or by using non-excitation method for adjusting speed under the rated speed. After the Hybrid excitation synchronous machine (HESM) is a motor speed exceeds the flux weakening base speed (typical- wide speed range machine consistings of the permanent ly rated speed), its back-EMF approaches to the DC bus magnet synchronous motor (PMSM). Its inner side contains voltage and if the speed needs to be further improved the two magnetic potential sources: permanent magnet and exci- flux weakening control method is adopted. Currently, the tation windings. The permanent magnet in the motor gener- research papers on HESM control system mainly include the ates a main air gap magnetic flux and excitation winding following. A hybrid excitation brushless DC motor fuzzy generates an auxiliary air gap magnetic flux. Because of control scheme was presented by academician Qingquan excitation windings, it can increase or reduce the air-gap Chen, used to regulate the armature current and excitation magnetic field by changing the size and direction of the cur- current by using the fuzzy controller [8]. A common coordi- rent, which make the motor have the characteristics of a low nate system based dynamic vector control model was pro- speed high torque and a wide speed operation range. There- posed by Japanese scholars Shinji Shinnaka [9], and a copper fore , its application is very extensive and valuable in indus- loss minimization vector control method based on id=0 was trial and agricultural production, and is particularly suitable proposed for the non-salient pole HESM [10]. A HESM con- for machines which need high speed operation capability, trol method was simplified by Qianfan Zhang after which, like the electric vehicles, machine tools, servo systems, and the quantity and direction of coil’s current are controlled by so on [1-3]. the stator winding current and the back EMF which are the Because of structural and performance similarities in same phase of square wave [11]. In the paper [12], professor HESM and PMSM, and updated control technology and Deng proposed a novel maximum torque/copper loss control products of PMSM, the HESM can fully reference the basic method. A relatively simple method for partitioning control principles and control methods of the PMSM [4-7]. But, the strategy and its implementation was presented by Dr. Yang HESM generates an additional controllable excitation cur- Chengfeng, and the no excitation current control mode was rent to adjust the air gap magnetic field of the motor, there- also compared [13]. All of the above documents did not con- fore a reasonable allocation of the excitation current and the sider the role of d-axis current for regulated speed, therefore, armature currents must be considered to achieve optimal it is very difficult to obtain adequate the motor efficiency control of efficiency, static and dynamic characteristics. and speed range. HESM’s basic control principle is to make the motor acquire In order to obtain the desired properties of low speed operation capabilities of a higher starting torque and low high toque and wide speed range operation, this paper pro- speed high torque by increasing air gas magnetic field inten- posed a novel control strategy based on two-speed region sity (Combination of excitation current and the d-axis control, with or without excitation current for regulating

speed in the low speed region, and adopted flux weakening control based on copper loss minimization method in the *Address correspondence to this author at the No. 1 Xianghe Road, high speed region. Designing the flux weakening control Longhu Town, Xinzheng City, Zhenzhou City, China, Postcard: 451191; Tel: 13526587276; E-mail: [email protected]

1874-4443/14 2014 Bentham Open Design and Implementation of the Control System The Open Automation and Control Systems Journal, 2014, Volume 6 1653

(a) The structure of HESM (b) The rotor 1. permanent magnet; 2. the iron pole; 3. the iron core of stator; 4. the outer covering; 5. the claw pole; 6. the cover; 7. the axis; 8.the exciting winding; 9. the armature winding. Fig. (1). The model of HESM. algorithm in high speed region is an important feature of the The magnetic flux generated by each current component HESM control system. According to the flux weakening flows with least reluctance, and their magnetic flux path is operating characteristics of HESM in the high speed region almost same with the permanent magnet. However, their and based on vector control of two current allocation algo- difference is mainly due to the fact that the most magnetic rithms, the combination of the excitation current and the d- flux generated by the currents is bypassed through the high axis current for air-gas magnetic flux regulation, and the magnetic conductivity material (iron core) paralleled with excitation current for air-gas magnetic flux regulation, were permanent magnet., thus it does not pass the permanent proposed. Comparing the above two algorithms with no flux magnet and will not cause permanent damage to permanent weakening control method in the simulation, the results magnets. Due to the low reluctance of the whole magnetic show that the suggested methods, based on copper loss min- circuit , a good magnetic-flux-adjusting effect is obtained by imization, combined the d-axis currents and excitation cur- regulating excitation current and the d-axis current. rents for flux weakening control, had much higher speed In summary, the paths of magnetic flux generated by the range than another two methods. Finally, the experiment permanent magnet, the excitation current and the d-axis cur- proved the effectiveness of the proposed control methods. rent are substantially identical. Therefore, the air-gas mag- netic field of HESM can be approximated linear superim- 2. THE FRAMEWORK AND MATHEMATIC MODEL posed, and can also be adjusted effectively by changing the OF HESM size and direction of excitation current and the d-axis current. The motor of the proposed HESM drive system is a claw- To simplify the analysis, the effects of the armature volt- pole asymmetric interlaced HESM, which is shown in Fig. age, harmonic component, temperature change, iron loss, (1). The motor has dual stator structure and outer stator is stray loss, and magnetic saturation were ignored when math- similar to the general PMSM stator. Three-phase armature ematic model and driver system model were built according windings are installed inside and a chute structure is used to to the above illustrated HESM structure. Several basic equa- reduce cogging torque. Inner stator is consists of a coil with tions of mathematic model of HESM can be obtained as iron core, namely excitation coil, which is fixed to the end follows: cap of the motor. The rotor has a claw-pole shape. The Circuit equation: magnetically conductive irons (namely iron core pole) and the permanent magnets (i.e., permanent magnet pole) are ! $ !u $ R + sL '( L sM !i $ placed on the adjacent claw-pole staggered. (AUTHOR: # d & # s d e q sf & # d & # & Should this word “staggered” be removed?) In the inner #uq & = ( eLd Rs + sLq ( e Msf #iq & # & # & # & surface of HESM, magnetic flux generated by permanent #u & # sM 0 R + sL & #i & magnet pole is almost unchanged during motor operation. " f % "# sf f f %& " f % (1) Except for some of the magnetic flux leakage. The magnetic ! 0 $ flux flows in the path as follows: # & + # ( e) pm & permanent magnet N-pole → air gap → stator teeth → stator # & 0 iron core → stator teeth →air gap → adjacent permanent "# %& magnet S-pole → S-pole claw pole → magnet yoke disc of Flux linkage equation: claw pole → rotor magnet yoke → additional air gap in rotor → inner stator core → claw pole yoke ring → N-pole claw pole → permanent magnet N-pole. 1654 The Open Automation and Control Systems Journal, 2014, Volume 6 Xinjun et al.

" % the control algorithms, and to improve the reliability of " % id HESM’s operation, a vector control algorithm based on id=0 " % Ld 0 Msf $ ' " % ! d $ ' ! pm $ ' = $ i ' + $ ' (2) was adopted due to the inductor values Ld, Lq, and Msf $ '$ q ' $! q ' 0 L 0 $ 0 ' changed with the currents id, iq, and if,. The torque equation # & $ q '$ ' # & # & i f can be obtained as: #$ &' 3 Torque equation: Teref = p[! pm + Msf i fref ]iqref (4) 2 3 Tpii=()ψψ− Where, T is the reference value of electromagnetic torque, eqddq2 eref (3) iqref is the reference value of q-axis current component, ifref is 3 =+pi[()]ψ i L− L+ M i the reference value of excitation current. When Teref is less 2 qddqsffpm than or equal to the base value In the formula (1) - (3), the symbol meanings are as fol- 3 TpI= ψ (5) lowing: NqN2 pm ud-- d-axis voltage component, Where, IqN is the base value of q-axis current. In this condi- uq-- q-axis voltage component, tion, no excitation current is needed for speed regulation, only the current iqref is required to be adjusted according to id-- d-axis current component, the value of Teref changed during HESM’s operation, that is iq-- q-axis current component, 2Teref ψd-- d-axis flux linkage, iqref = (6) 3p! pm ψq-- q-axis flux linkage,

ψpm-- The flux linkage generated by permanent magnet , Such, when Teref is larger, and

uf -- excitation voltage, 3 Teref > p! pm IqN (7) if -- excitation current, 2 R -- armature resistance, s After this, iqref is adjusted to maintain stable operation of HESM which will lead to motor overheat. By adopting flux Rf -- excitation winding resistance, strengthening control, let iqref=IqN, and Teref can be obtained Ld -- d-axis inductance, by

Lq -- q-axis inductance,

Msf -- mutual inductance between armature windings and (8) excitation windings, then ωe -- electrical angular velocity,

p -- number of pole pairs of the motor, 2Teref ! 3p" pm IqN i = (9) T -- electromagnetic torque. fref 3pM I e sf qN In summary, when T ! T , the reference currents can 3. THE PRINCIPLE OF TWO-SPEED-REGION CON- eref N TROL be obtained as: According to the HESM’s speed operation range, the proposed control strategy divides the whole speed range into two regions. Region I ( nr ≤ nb , nr is the rotor speed, nb is the base speed) is the low speed region, uses the method that adopts current with or without excitation for adjusting speed and region II( nr > nb ) is the high speed region, adopting (10) flux weakening control by excitation current and d-axial cur- rent for adjusting speed based on copper loss minimization. when Teref > TN, and the reference currents are

# 2T ! 3p" I 3.1. The Control Algorithms in the Low Speed Region % eref pm qN i fref = % 3pMsf IqN When the HESM’s speed is less than or equal to the base % i 0 (11) speed, it adopts current with or without excitation, for adjust- $ dref = ing speed according to its load torque. In order to simplify % i I % qref = qN % &% Design and Implementation of the Control System The Open Automation and Control Systems Journal, 2014, Volume 6 1655

3.2. Flux Weakening Control Algorithms For HESM Where, idref, and ifref are the reference values of d-axis cur- rents and excitation currents, respectively. Thus, it can be The motor enters into the high-speed operation mode obtained when the HESM’s speed nr exceeds the base speed nb, and the back-EMF approaches to the DC-bus voltage (Udc). To ! (n " n ) further elevate the speed, flux weakening methods must be (L i + M i ) = pm b r (18) d dref sf fref n used. The regulation of the armature current and excitation r current is restricted to the voltage limit ring, which is similar The copper loss equation in high speed regions can be to PMSM flux weakening control method in terms of con- obtained by using copper loss minimization principle to car- stant power region. When the operation is stable, the voltage ry out flux weakening control vector magnitudes have to meet the following condition:

2 2 2 2 3 2 2 u = u + u ! U (12) Pcu-ref = idref Rs + ifref Rf (19) s d q lim 2

Where, Ulim is the voltage vector limit whose value depends Where, in order to simplify the computation, the value of iqref on Udc of the drive circuit. In the formula (12), us depends on is directly determined by the reference electromagnetic uq, while uq is determined by Eq (q-axis component of the torque Teref given as output by speed controller. Therefore, it back EMF). According to the above analysis, a flux weaken- does not include the copper loss produced by q-axis compo- ing control method based on maintaining the back EMF in- nent current iqref in the formula (19), and variable is presented, i.e.

iqref = kiTeref (20) E = E (13) q base Where, ki is the q-axis current to torque ratio coefficient. Where, Ebase is the corresponding back-EMF in the no-load Combining equation (18) and (19), the Lagrange multiplier condition when nr reaches nb. was adopted to determine the reference current values based on copper loss minimization control method. E = pn ! " / 30 (14) base Bdec pm L(idref ,ifref ,!) = Pcu_ref + ![(Ld idref + Msf ifref ) The expression of Eq is the following as (21) # pm (nb " nr ) p n " ] ! r n Eq = (" pm + id Ld + i f Msf ) (15) r 30 To solve partial derivatives of idref, ifref and λ for equation Where, it is known that nr is the linear relationship with Eq (21), respectively: when id = if = 0. Therefore, when using conventional vector control, the maximum speed nmax in no-load and no- % !L excitation current are also in a linear relationship with Udc ' = 3i R + "L !i dref s d Where, nb is restricted by nmax, while nmax can be obtained ' dref ' by experiments with id=if=0 vector control in the no-load. ' !L Following this, the relationships of n , n and U can be = 2i R + " M (22) b max dc & !i fref f sf expressed using the following formula ' fref ' !L $ pm (nb # nr ) ! n = k U + N ' = (Ldidref + Msf ifref ) # # max v dc 0 !" n " (16) (' r n k n # b = b max $ !L !L Where, for the experimental prototype, the maximum speed In the formula, assuming that = 0 , = 0 , !idref !ifref to voltage ratio coefficient takes kv = 5.69, offset value N0 = - 13. To ensure the utilization rate of DC bus voltage and the !L motor efficiency in flux weakening operating state, the flux = 0 , then the values of idref and ifref are as following : !" weakening base speed coefficient kb is set a range of 0.7~0.9, where, kb = 0.75. Therefore, when the DC bus voltage value # Ld IS Udc = 300V, then nb = 1270rpm. % idref = ! " % 3Rs Therefore, in order to maintain the back EMF with a $ (23) higher value and unchanged in-flux weakening running M % i = ! sf " states, and to combine equation (13~16), it is assumed that: % fref 2Rf & ! # id = idref Where, " (17) i i # f = fref $ 6Rs Rf" pm (nr # nb ) ! = (24) n (2R L2 + 3R M 2 ) r f d s sf 1656 The Open Automation and Control Systems Journal, 2014, Volume 6 Xinjun et al.

PWM Id Ud u g TL PID + SV- Udc C Ipark u PWM B C HESM - Iq A PID Uq Armature B PWMf Optical iq theta Driver A encoder id g + If Uf+ idref iqref ifref A i Current f PWM - B Uf- Distributor i If Driver f i U i f d nr dc Teref iB i theta theta Speed d Signal i PID α Processor Park Clarke iA nr iq i Δn β n ref nr nref

Fig. (2). The control system model of HESM.

Fig. (3). Current allocation module.

4. MODELING AND SIMULATION In summary, two current allocation algorithms were used In order to verify the effectiveness of flux weakening in the flux weakening control for the HESM. control method, simulation models of HESM were estab- Algorithm 1, adopted the flux-weakening control based lished by applied MATLAB/SIMULINK software, shown in on copper loss minimization algorithm, and the reference Fig. (2). The main function modules include HESM, Clarke, value for each current component are presented as follows. Park, Ipark, Speed PID, Current Distributor, Id PID, Iq PID, SVPWM, If PWM, Armature Driver, and If Driver module. Compared with the traditional PMSM vector control system, # i = k T % qref i eref HESM control system adds 3 functional modules, If PWM, If Driver and Current Distributor. If PWM gives output con- % Ld % idref = ! " trol signal to the control If Driver which is a single-phase 3R $ s (25) bridge inverter circuit. Current Distributor divides the entire % M operation region of HESM into a low speed zone and a high % sf speed zone by using different partition control strategies. Its ifref = ! " % 2Rf model is shown in Fig. (3). The control strategies of Current & Distributor are used to coordinate allocation of the armature Algorithm 2, used only the excitation current for flux current and excitation current to realize reasonable distribu- weakening control, namely based on idref=0 vector control tion of the current reference values for the motor operations algorithm to adjust speed. From the equation (18) and (20), between two speed zones in enhanced or reduced flux state, the current component reference values were obtained. to ensure stable, reliable, and efficient operational state of the motor. # i = k T According to the control system model shown in Fig. (2), % qref i eref % i 0 the following simulation analysis was completed in detail. % dref = The simulation results of the three control methods were $ (26) compared, which specifically included non-excitation current % ! pm n i ( b 1) operation method, with only excitation current operation % fref = " Msf nr method and minimum copper loss method that used the d- &% Design and Implementation of the Control System The Open Automation and Control Systems Journal, 2014, Volume 6 1657

Table 1. Characteristic parameters of the prototype.

Parameters Value Parameters Value

PN (W) 700 Udc (V) 300

nN (rpm) 500 Rs (Ω) 2.7

TN (Nm) 13 Rs (Ω) 33.0

IN (A) 5 Lq(mH) 27

Ψpm (Wb) 0.243 Msf (mH) 76

p 4 Ld (mH) 38

IfN (A) 1.0 Lf (H) 0.57

Notes: IfN+ rated field enhancing excitation current; IfN- rated field weakening excitation current.

5000 4000 iiand co-action 3000 df / 2000 r n1000 rpm i = 0 ii==0 0 d df 0 2 4 6 8 10 (a) t/s 200 150 100 iidf==0 / iidfand co-action EV i 0 50 d = 0 0 2 4 6 8 10 (b) t/s 15

iidf==0

/ 10 e TNm id = 0 5 iidfand co-action 0 0 2 4 6 8 10 (c) t/s 8 6 iidf==0 / 4 q iA i = 0 iiand co-action 2 d df 0 0 2 4 6 8 10 (d) t/s

Fig. (4). Simulation waveforms of the HESM. axis current and excitation current together for flux weaken- Fig. (4) shows comparison of simulation waveform of ing operation. The simulation parameters of the HESM were HESM that used three kinds of control strategies: idref=ifref=0, set according to the actual parameters of the prototype, idref=0, and combined id and if for field weakening control to shown in Table 1. adjust speed. The HESM load was set to 1Nm, with the ref- erence speed of 6000rpm. 1658 The Open Automation and Control Systems Journal, 2014, Volume 6 Xinjun et al.

Fig. (5). The experiment system of HESM.

Fig. (4) (a) shows the speed curve of HESM obtained by encoder disk, TAMAGAWA OIH48-2500, which is built in three different control methods. By adopting no excitation the motor. The architecture of control circuit board is based current control, the maximum speed observed was 1650rpm. on TMS320F2812+ AT89C55WD. In addition, it includes if alone adjusted speed of flux weakening with the maxi- two driving board of armature and excitation, and a control mum speed of 2350rpm, and by combining id and if for flux and display circuit board. weakening control, the maximum speed of HESM was ob- Fig. (6) shows the starting current waveforms of HESM served to be more than 4600rpm. with load 1Nm while the given speed was 2800rpm, greater Fig. (4) (b) shows the back EMF waveform of HESM than the flux weakening base speed (1270rpm) of the motor. under three different control methods. The back EMF with In order to improve the starting torque of the motor, and for no field weakening current control reached fastest to the increased excitation winding inductance, a positively rated extreme, and flux weakening for adjusting speed and com- excitation current command was implemented to enhance the bined id and if increased the most slowly. Finally they be- air-gap magnetic field before the armature current started at came stable between 160V~170V. 0.5s. With the increase in the excitation current, motor speed decreased gradually. When the speed reached n , the rate of Fig. (4) (c) shows the i vs time curves obtained with b q flux weakening control for adjusting speed increased, along three different control methods. with if and id which continually increased negatively with the Fig. (4) (d) shows the curve of the electromagnetic rising speed . torque Te changed with increase in the speed for three dif- ferent control methods. Fig. (7) shows steady current waveforms in HESM weak magnetic operation. The given speed was 2800rpm, with the load torque of 1Nm and excitation current if of -0.8A, which 5. EXPERIMENTAL basically remains constant. Moreover, phase current was sine wave, amplitude was 4A, and the waveform was According to the proposed control algorithms and the smooth with low harmonics. simulation results of HESM, the HESM controller based on TMS320F2812+AT89C55WD architecture was set up, and Fig. (8) shows the curves of maximum output power in the corresponding motor drive experiment was carried out. three different control strategies with changing speed. By adopting no field current mode (id=if=0), the speed was ap- Fig. (5) shows the experimental picture of driving sys- proximately up to 700~1300 r/min in the constant power re- tem for the HESM. Rated DC bus voltage for experimental gion. In addition, only the excitation current i was used with- prototype was 300V. The motor torque characteristics test- f out d-axis current(id=0) for regulating flux. The constant ing adopted TS-7700 Torque Station Pro with MT-6425 power region was extended to 450~1600 r/min while in the torque detectors. This set of equipment can be used to accu- combined the d-axis current and the excitation current for rately measure the motor speed, torque, input and output regulating flux, the constant power region extended to power and efficiency etc. The prototype of HESM is an 450~2000 r/min. However, when the speed exceeded to 2000 asymmetric interlaced construct, and an incremental optical

Design and Implementation of the Control System The Open Automation and Control Systems Journal, 2014, Volume 6 1659

i i A f 2A/div ’vertical’ 500mA/div ’vertical’ 500mS/div ’horizontal 500mS/div ’horizontal

Fig. (6). Test waveforms of starting current.

Fig. (7). Steady-state current waveforms in flux weakening operations.

Fig. (8). Maximum output power at different speeds. 1660 The Open Automation and Control Systems Journal, 2014, Volume 6 Xinjun et al.

r/min, although the output power could not be kept constant, [2] C. F. Yang, H. Y. Lin, J. Guo and Z. Q. Zhu, “Design and analysis as it decreased slowly when the speed increased to 4000 of a novel hybrid excitation synchronous machine with asymmetri- r/min and the output power dropped to 200W. The experi- cally stagger permanent magnet”, IEEE Transactions on Magnet- ics, vol. 44, pp. 4353-4356, 2008. mental results show that applying the flux weakening control [3] C. C. Chan, K. T. Chau, and J. Z. Jiang, “Novel permanent magnet technology which combined id and if based on copper loss motor drives for electric vehicles,” IEEE Transactions on Industri- minimization control can effectively extend the speed range al Electronics, vol. 43, pp. 331-339, 1996. of HESM. [4] J. J. Chen and K. P. Chin, “Minimum copper loss flux weakening control of surface mounted permanent magnet synchronous mo- tors”, IEEE Transactions on Power Electronics, vol. 18, no.4, pp. CONCLUSION 929-936, 2003. [5] J. S. Lawler, J. Bailey, and J. McKeever, “Minimum current mag- In this paper, according to the magnetic field adjusting nitude control of surface PM synchronous machines during con- property of HESM, based on the space voltage vector control, stant power operation”, IEEE Power Electronics Letters, vol. 3, no. 2, pp. 53-56, 2005. a new kind of two-speed-region control algorithm for [6] E. F. Fuchs, and M. H. Myat, “Speed and torque range increases of HESM was presented. The proposed control method divid- electric drives through compensation of flux weakening”, In: Inter- ed the whole speed range into two region. In the low speed national Symposium on Power Electronics, Electrical Drives, Au- region, the current control strategy was adopted with or tomation and Motion, Pisa, Italy, 2010, pp. 1569-1574. without excitation current for adjusting speed according to [7] J. J. Chen, and K. P. Chin, “Automatic flux-weakening control of the load torque while in the high speed region, the current permanent magnet synchronous motors using a reduced-order con- troller”, IEEE Transactions on Power Electronics, vol. 15, no.5, control strategy with a combination strategy of holding q- pp. 881-890, 2000. axis components of Back-EMF unchanged and the copper [8] C. C. Chan, R. Zhang, and K. T. Chau, “Optimal efficiency control loss minimization algorithm was adopted. Both simulation of PM hybrid motor drives for electrical vehicles”, In: 28th Annual and experimental results were given to verify the validity of IEEE Power Electronics Specialists Conference, pp. 363-368, the proposed control strategy. 1997. [9] S. Shinnaka, “New dynamic mathematical model and new dynamic vector simulators of hybrid-field synchronous motors”, IEEE Inter- CONFLICT OF INTEREST national Conference on Electric Machines and Drives, pp. 882- 889, 2005. The authors confirm that this article content has no con- [10] S. Shinnaka, “New optimal current control methods for energy- flicts of interest. efficient and wide speed-range operation of hybrid-field synchro- nous motor”, IEEE Transactions on Industrial Electronics, pp. 54, No. 5, pp. 2443-2450, 2007. [11] Q. F. Zhang, H. X. Wu, and S. K. Cheng, “Flux weakening control ACKNOWLEDGEMENTS of axial and radial air-gap hybrid magnetic-circuit multi-coupling This work was supported by Scientific and technological motor”, Journal of Harbin Institute of Technology, vol. 38, no. 10, pp. 1654-1656, 2006 (in Chinese). project of Henan Science and Technology Agency [12] T. T. Zhu, Z. Q. Deng, and Y. Wang, “Research on hybrid-excited (142102210403)，Project Supported by the National Natural flux-switching machine and the current vector control strategy”, In: Science Foundation of China (61403123). Proceedings of the CSEE, vol. 32, no. 15, pp. 140-148, 2012. (in Chinese). [13] C. F. Yang, H. Y. Lin, and X. P. Liu, “Control strategy and simula- REFERENCES tion for hybrid excitation synchronous machine”, Electric Ma- chines and Control, vol. 12, no. 1, pp. 27-33, 2008. (in Chinese). [1] E. Spooner, S. W. Khatab, and N. G. Nicolaou, “Hybrid excitation of AC and DC machine”, Journal of University of Manchester In- stitute of Science and Technology, vol. 3, pp. 48-52, 1989.

Received: September 16, 2014 Revised: December 23, 2014 Accepted: December 31, 2014

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