Survey of Some Automotive Integrated-Starter-Generators and their Control

Transcription

1 Survey of Some Automotive Integrated-Starter-Generators and their Control DORIN DUMITRU LUCACHE Faculty of Electrical Engineering Gh. Asachi Technical University of Iaşi 53 D. Mangeron Blvd., Iaşi, ROMANIA Abstract: - The induction machine, the interior permanent magnet machine and the switched reluctance motor are the strongest candidates to be used as integrated-starter-generators in automotive applications. Their advantages and drawbacks are discussed. To get the requested ISG performances and flexibility, the vector control must be used. Two practical control schemes, using the induction machine and the interior permanent magnet machine consequently, are depicted. An indoor test bed based on a mean-torque-value engine simulator was used. Key-Words: - Integrated-starter-generator, Induction machine, Permanent magnet machine, Vector control, Engine simulator 1 Introduction Integrated starter generator (ISG) uses one machine to replace conventional starter and alternator onboard vehicles and provides greater electrical generation capacity and improves the fuel economy and emissions. The idea is not new, but needs a high complexity control system because of the differences between motoring and generating regimes, so that only modern motors and high-developed power electronics and digital signal processors made it practically possible. This paper reviews the opportunities and expected performances brought by the new resulted ISG-based automotive architecture. The electric motors types that are suited to play the role of an ISG are mostly the alternating current motors and two of their control algorithms are detailed and exemplified.. ISG structure and performances The main requirements of the ISG systems are to ensure: - the necessary cranking torque (usual ~150Nm) as starter in the most unfavorable conditions; - a constant output voltage irrespective of the input speed (typically between 600 and 8000 rpm) and load, as generator; - a high efficiency as generator in the speed range of rpm that corresponds both to I4 and V8 engines; - an acceptable cost. Other expected performances of the ISG systems are: the fault tolerant capability (operate with reduced performance if fault occurs), the withstand in harsh operating condition (high operating temperature of about 180 C, presence of oils, fluids, vapors, chemicals or vibrations), the low rotor inertia and noise, and the no maintenance. Fig.1 shows the principled architecture of a vehicular power system based on ISG. Beside its native functions, an ISG supplementary offers: improvement of the engine start-stop regimes, acceleration support, traction assist (additional torque for traction along with engine torque or lowspeed, short distance electric drive), torque support and vibration damping. BI- DIRECTIONAL CONVERTER 36V BATTERY 1V BATTERY 4V BUS ISG ICE HIGH POWER LOADS DC / DC CONVERTER BI- DIRECTIONAL CONVERTER 14V BUS LOW POWER LOADS Fig.1 The architecture of an ISG based automotive power system ISBN: ISSN

2 3. Motor selection The electric motor of an ISG system is desired to have a high starting torque ( Nm, overcoming a load torque of Nm) for initial acceleration (3 5 seconds), high power density and high efficiency to extend the battery range and a wide operating speed range as generator. The torque-speed characteristic of an electric machine has three regions: constant torque, constant power and natural characteristic regions. The constant torque region extends up to the rated speed or base speed of the machine, when the rated power condition of the machine is reached. The constant power region needs the field-weakening technique in order to allow operating above the rated speed. It has been shown [5] that the motor power and size is minimized in electric automotive application when the constant power of the motor is extended. Therefore, the design objective would be to minimize the constant torque region and extending the constant power region. So, a good efficiency in the field-weakening region is extremely important in automotive applications. Some motors types have the inherent property of operating with large extended speeds. The separately excited dc motors is an example. However, the principal problems of the DC machines are its commutators and brushes, which limit the maximum speed of the motor, create sparks and require regular maintenance. The Lundell claw-pole machines are widely used as alternators in the automotive industry. The airgap flux can be adjusted by controlling field current. The power range in most applications is up to.5 kw at 5,000 rpm. The power limitation is caused by high-rotor leakage flux between claw poles when the axial-lamination length is increased. The speed range of the Lundell machine is very wide because its flux can be adjusted through field current to keep power and voltage constant during motoring operation. However, the classical Lundell machines suffers from a low, approx 50%, electromechanical efficiency and are better suited for generation-only applications, for which a simple diode rectifier bridge is sufficient. A comparison by W. Cai [] between several electric machines types used for ISG applications shows that the most suited are the induction machine (IM), the permanent magnet (PM) machines and the switched reluctance motors. 3.1 Induction Machines (IM) The induction machine technology is an established technology with high efficiency, smooth torque and relative low inertia. The induction machine is an ISG machine candidate as usual a three-phase machine (preferable with cage rotor) and connected to the dc bus through a power electronics inverter/rectifier. Depending on its size, the induction motor has been shown to have a lower efficiency and lower power or torque density than PM and reluctance motors. But, except for the induction machine and trapezoidal PM machine, all other machines require a high-resolution position sensor although the sensorless technology is under development. Some promising cost-saving concepts have been investigated in recent years [4], but there appears to be a dearth of new ideas for making substantial improvements to efficiency. Only small incremental improvements may be expected in the performance characteristics (i.e. efficiency, power density etc.) of induction machines in the near future. Thus, high reliability and low technological risk will likely remain the primary advantages of this technology in the foreseeable future. 3. PM motors The PM synchronous machines are the most efficient (due to the absence of field-coil losses) but at higher cost than the others motors used in drive applications. However, they are superior to induction machines in life cycle cost. Based on the rotor structure, these can be categorized as the surface-mounted PM type or the interior PM (or buried magnet) type. Among all, the Interior Permanent Magnet (IPM) synchronous machines are better suited for ISG applications []. In contrast to surface-mounted PM machines (SPM), the IPM machines offer certain advantages, their suitability for high-speed operation being perhaps the most obvious. The fundamental difference is that buried magnets add a reluctance component to the torque produced by the motor and so, for similar overall dimensions, the total electromagnetic torque is higher, in comparison with the SPM. More, due to smaller airgaps it presents higher field weakening capability (which tends to improve the constant-power speed range), and power density. One drawback is the difficulty to be magnetized and manufactured. The other is the complexity of the control schemes, where several voltage and current limitations must be taken into account (see the sub-section 4.3). 3.3 Switched reluctance motors (SRM) The torque-speed characteristics of SRMs match well with the ISG characteristics. The SRM has high-speed operation capability with a wide constant power region. The motor has high starting torque and high torque-inertia ratio. Its efficiency is similar ISBN: ISSN

3 to a high-efficiency induction machine. The rotor construction is extremely simple without any windings or magnets, and is made usually by steel laminations to minimize the core losses. The SRM is a fault-tolerance machine, suited for harsh environments. The torque ripple, vibration, and acoustic noise are the main disadvantages of the SRM. The origin of the acoustic noise in SRMs are the radial forces that increase inversely with the airgap and manifests especially when the force frequency is near the stator resonant frequency []. The acoustic noise can be suppressed by proper design (higher pole number is better) as well as performing active noise control technology. The SRMs are excited by current pulses applied to each phase. The current pulses are applied based on precise rotor position and the motor creates torque in the direction of increasing inductance. Higher pole machines operate at higher electrical frequencies at the same mechanical speed, which will increase the frequency dependent losses. Unless new, sensorless technology is employed, the position sensors are a source of increased cost and reduced reliability. A wide choice of pole configurations and phase numbers are possible with SRMs. Lower number of phases reduces the converter cost, but increase the torque ripple. A higher pole machine has a reduced weight but has the disadvantage that requires a narrower air gap for the same performance (the torque decreases at given phase current with a large air-gap). The SRM is not suited for the crankshaftmounted applications because additional shaft vibration from the engine makes a bigger machine air-gap necessary. 4. Experimental study of two ISG systems 4.1 The engine emulator To perform experimental tests similarly to that on real vehicles, rather than an internal combustion engine (ICE), for the indoor-use test beds one needs an engine emulator. This is usual an electrical machine because is quieter than an ICE, does not emit exhaust and allow a greater flexibility to evaluate alternative engine characteristics. Because of their relatively simple nature, mean torque predictive engine models are often more appropriate for vehicle simulations than models that predict individual cylinder filling phenomena [6]. The model simulates a port-fuel-injected, sparkignition engine and captures the major dynamics (lag and delays) inherent in the spark ignition torque production process. It does not attempt to predict flow and torque pulsations due to individual cylinder filling events. It is a low-frequency model, generic enough to be used for a wide range of sparkignition engines. Our main purpose was to be used as a real-time engine model for the hardware-in-theloop testing of an ISG. In order to reduce the model complexity the engine was divided into functional blocks (Fig. ). The electronic controlled throttle body (ETB) block calculate the actual air mass flow rate m& th entering the intake manifold at given air mass flow rate request and intake manifold pressure: dm& th τ etb = m& th + m& th _ req and m & th m& th _lim it (1) where m& is the air mass flow rate request by th _ req the engine controller block. The Intake Manifold block s main purpose is to calculate the mean-value mass flow rates of air in the intake ports of the engine. The intake manifold pressure p m is modeled as a first order system and air mass flow rate entering the cylinders m& cyl is based on a regression model, which is function of engine speed and manifold pressure: dpm RTm RTm + m& cyl = m& th () Vm Vm m& cyl = M 1ωr + M pm + M 3ωr pm + M 4ωr pm (3) where R, T and V have the same meaning like in all thermodynamic equations (subscript m comes from manifold). The Combustion block calculate the engine net torque T ICE based on a regression model, which is a function of air mass flow rate with delay t d, spark advance SA and engine speed ω r : TICE = kt 0 + kt 1m& cyl ( t td ) / ωr + kt SA + kt 3SA + (4) + k ω + k SAω + k ω T 4 r T 5 r T 6 r π where t d =. ωr The following assumptions were considered: engine is always at warmed up condition and the angle pedal ω r Controller key on/off p m m th Electronic Controlled Throttle engine ON m th Intake Manifold Fig. ICE simulator 0 mcyl Combustion T ICE ISBN: ISSN

4 spark advance is a constant (0 for cruise and 30 for hard acceleration). The overall accuracy of the engine model depends to a large extend on the quality of the regression models, but more on the number of the previous table that are calibrated to represent the engine to interest. Tables are generally used for many of the engine specific parameters because they make a more generic model than using analytical expressions [6]. A special attention was paid to get real automotive engine dynamic characteristics from the engine simulator. The mechanical dynamics of an electrical machine (for example our PM synchronous motor) can be given by (the subscript sm refers to a synchronous motor): dωr( t ) J sm = Tsm( t ) Dsmωr ( t ) (5) where T sm is the electrical driving torque and ωr is the angular speed. The first problem is to cancel the moment of inertia J sm and viscous friction coefficient D sm that characterize the test bed and replace them with the ones proper to the modeled ICE [7]. If we wish to emulate the no-load condition for the tested motor, then the load machine must produce a compensatory torque T comp : dωr Tsm ( t ) = J sm Dsmωr = Tcomp( t ) (6) and the corresponding transfer function is ωr( s ) 1 = (7) Tcomp( s ) J sms + Dsm When the tested motor must overcome a virtual load characterized by the moment of inertia J em and viscous friction coefficient D em, the electromagnetic torque for the loading machine yields dωr( t ) Tsm( t ) = ( Jem J sm ) + ( Dem Dsm ) ωr( t ) = (8) = Tcomp( t ) + Tem( t ) and the new transfer function becomes: ωr( s ) 1 = (9) T ( s ) + T ( s ) ( J J )s + ( D D ) comp em em 4. The control of an IM-based ISG system The cage type induction motors remains industry s most popular choice in wide power range and the vector control seems to be universally used in the future [1]. The most suited for the ISG control system is the field-oriented control that transforms the control problem of the induction machine into the classical control problem of a separately excited dc machine and create independent flux and torque control loops. The stator current phasor is decomposed in two orthogonal components, one along the rotor sm em sm flux, and one in quadrature with it, when the rotor flux position is known [13]. The in-phase component is the reactive current and the quadrature component is the active current. The rotor flux position must be known on this purpose. Therefore, the accurate instantaneous position of the rotor flux is crucial for the success of the field-oriented structures. In the indirect control schemes the flux position is determined indirectly through the rotor speed and the slip estimation. The absence of the flux sensors and the ability to operate at low speeds has increased the popularity of the indirect control strategy. A vector control structure follows two objectives: the magnetic control of the machine and the electromagnetic torque control. An experimental testing structure is presented in Fig.3, where the ICE simulator is based on a PM synchronous machine driven by its own inverter. The whole test bed structure is controlled by a dspace 1104 board and ControlDesk software. In the motoring mode as starter, the electromagnetic torque is controlled. In the generating mode, the engine imposes the system angular speed and the torque (current) control is used to govern the output dc power. In this mode, the field oriented control technique is utilized for controlling energy flow rate so that, at different speed and load conditions, the rated output dc voltage is maintained. Depending on the engine operating-mode the control topology changes. During the start-up, the active current reference, i sq, is determined by the reference electromagnetic torque, T e, which has a greater value that the engine restoring torque. After a certain speed value the engine fires and becomes to develop an accelerating torque. For a short time the both motors generate a positive torque. Then, the ISG switch in the generating mode and the active current, i sq, is determined by the needed power pumped to the dc line load. Therefore, a dc bus PI voltage regulator can be used to determine them. The engine speed can vary at any time. This variation affects the ISG rotor speed, and the variation in rotor speeds affects the output voltage in generating mode, unless there is well-designed control system. In motoring application, all control scheme use constant flux for rotor speeds that are lower than the rated speed. The flux will be reduced inversely proportional to the speed when the induction motor is operated above its rated value which is the flux weakening mode. In induction generator the aim is to have a constant generated voltage. Of course, the ISBN: ISSN

5 angle pedal key on/off ICE simulator Dyn. char. emulator T ICE T comp + + VECTOR CONTROL PWM CONVERTER PWM INVERTER Voltage regulator VECTOR CONTROL i q Ψ r V T e engine ON PMSM IM Crankshaft model ω r Fig.3 The experimental test bed for the IM-based ISG system frequency of the generated voltage is dependent on the rotor speed but once it is rectified the dc voltage depends only on the magnitude of the ac voltage. Controlling reactive current, i sd, one can change the flux level of the induction machine. On the other hand, the generated no-load terminal voltage can be approximated by: e E = Cωrψr (10) where E is the electromotive force and C is a constant. If the product of the rotor speed and the flux linkage remains constant than the terminal voltage will not change. However, in practical applications there is a problem of core saturation. Since the maximum value of the flux linkage is determined by the saturation of the core, the flux linkage required at any speed is calculated based on this maximum flux linkage: ωr min ψ r = ψ r max (11) ωr When the rotor speed decreases to a value lower than ω r min, theoretically the flux linkage should increase to a value larger than ψ r max. In this case it is maintained constant. Some experimental results related to the behavior of the IM acting as an ISG are given for the following cases: the start-up and the ISG transition from the motoring to generating operating mode (Fig.4), and the operating regime at speed variations and load variations (Fig.5). Random acting on the driver pedal and dc load switch has produced both speed and load variations. The experimental results suggest that the controlled IM responds to the specific demands of an ISG system in different operating conditions, offers good dynamic performances and represents a cost effective solution. 4.3 The control of an IPM-based ISG system As shown before, the IPM motors due to their saliency develop supplementary reluctance torque and thus present a higher torque capability than the surface-mounted permanent magnet synchronous machine (SPM). In the following the control manner of these ones is developed. Usual, the control algorithm is based on the maximum torque-per-ampere strategy (minimum copper loss). In the d-q axis synchronous frame, the dynamic Fig.4 Start-up experimental results ISBN: ISSN

6 Fig.5 Experimental results for both random pedal commands and electric load variations equations of the IPM can be expressed as: disd usd = Rsisd + Ld ωelqi sq (1) disq usq = Rsisq + Lq + ωeld isd + ωeψm Symbols u and i denote voltage and current, ψ m the flux linkage of the permanent magnets in the d-axis rotor, L d is the d-axis self-inductance, L q is the q-axis self-inductance, ω e is the electrical speed, respectively, and index s denotes parameters and variables associated with stator. The developed electromagnetic torque t e in terms of stator currents is expressed as: te = p[ ψ misq ( Lq Ld ) isdisq ] (13) where p denotes the number of pole pairs. This has two components: the alignment torque produced by the flux linkage and the reluctance torque produced by the saliency. It is desirable that the reluctance torque should be properly utilized in order to increase the whole efficiency of the IPM drives. At the low speed, the back-electromotive force is small and so there is enough voltage to control the current to generate the torque. As the rotor speed increases, the marginal voltage to control the current is decreased and the torque becomes highly distorted so the flux weakening method should be applied [8]. The extension range of the speed is solely limited by the structure and the parameters of the motors under the given condition of voltage and current limitation. The imposed limits for the motor s voltage and current are: usd + usq U s max (14) isd + isq I s max Neglecting the stator resistance (when speed increases the term ω e becomes more important), from (1) the steady-state voltage limit equation yields: ψ + ( ) m L d U s max i sd + i sq (15) L d Lq ωelq From (14) and (15) it can be seen that the current limit equation determines a circle with a radius of I s max, while the voltage limit equation determines a series of nested ellipses (for the IPMSM, L q >L d ). Fig.6 shows the current-limit circle and the voltagelimit ellipses in the i sd -i sq plane. The voltage-limit ellipse decreases as the speed increases. At a so-called base speed ω base, the maximum torque point A is on the cross point of maximum torque-per-current trajectory and currentlimit circle. When the IPMSM is operated from the start up to the base speed in the constant torque region, the voltage-limit ellipse exceeds the maximum current boundary and no voltage limitation needs to be considered in this situation. But beyond the base speed, the IPMSM cannot be operated without flux-weakening control and so, to extend the speed range, a proper demagnetizing current has to be applied depending on the operating speed. Without a proper flux weakening at higher speed, the current regulators would be saturated and lose their controllability. Since the onset of current regulator saturation varies according to the load conditions and the machine parameters, the beginning point of the flux weakening should be varied. The late starting of the flux weakening may result in undesired torque drop, but the early starting Maximum Torque per Amp. Trajectory ω base A Voltage-limit ellipses ω>ω base B i sq I s max Current-limit circle Fig.6 Current-limit circle and voltage-limit ellipses in the i sd -i sq plane i sd ISBN: ISSN

7 deteriorates the acceleration performance [9]. As opposed to SPM, the i d =0 control method is not suitable for the IPM because the reluctance torque is not produced even if this kind of machine has a saliency. At low speeds, because the absence of voltage limitation, the current vector might be controlled to fully use reluctance torque in order to maximize the machine efficiency. To get the maximum torque control, the inclination angle γ of the current vector (Fig.7) depends on the load conditions, taking values between 0 and 45 [10]. The maximum torque-per-ampere strategy seeks to get a certain torque with the smallest possible stator current amplitude (so with minimum copper loss). The relation between i sd and i sq for the maxim torque per amper control is derived as i ψ m ψ sd = i sq + (16) m ( Lq Ld ) 4( L L ) This relation is shown as the maximum torqueper- ampere trajectory in Fig.6. Above the base speed, the normal operation is possible only applying the flux weakening and the maximum torque is obtained when the drive operates in the voltage and current limits. The flux weakening control of PM synchronous machines is conducted by injecting the d-axis current i sd negatively, which is different from induction machine where the flux is weakened by decreasing the d-axis current. In the present control scheme (Fig.8), two PI controllers, implemented in the synchronous reference frame, are used for the current control, followed by a decoupling circuit. In the constant power operation, a closed-loop voltage control is used to ensure an automatically onset of the accurate flux-weakening operation, depending on the load condition and machine parameters. Flux weakening mode is entered when the error ε = u smax ( usd + usq ) < 0 (17) becomes negative. This error leads to the d-axis current increases toward the negative direction to prevent saturation of the current regulators. The I controller output is limited (to avoid irreversible i sd i s γ Fig.7 Current phasor diagram in the d-q frame q i sq q d ψ m d demagnetization) so that the d-axis current to be between 0 and the minimal allowable i = I sd sd max. The magnitude of the injected stator current i s can be expressed as i s = isd + i (18) sq and taking into account (13), the d-q axis components of the current vector that ensures the maximum torque-per-ampere yields isq = sign( is ) is isd (19) where i sd ψ = m ψ 4 m + 8( Lq Ld ) ( L L ) q d is (0) The last equation is equivalent with (16) but expressing the d-axis current as a function of i s, taking into account (18). Thus, when the d-axis current is increased with respect to the voltage limit, the q-axis current is depressed in order to ensure the current limit condition. Moreover, a saturation block with adaptive limits take into account the flux weakening command i sd and forces, ones more, the operating point to not exceed the current-limit circle. In this way the current regulators regain the ability of regulating the d-axis and q-axis currents and the maximum torque-per-ampere is produced at the crossing point between the current limit circle and voltage limit ellipse, in the constant power region. Because this scheme utilizes for flux weakening the output voltage of the PI current regulators and the outer voltage-regulating loop instead of the IPMSM model, it becomes robust and insensitive to load conditions and load parameters. A commercial IPMSM (rated parameters in Table 1), operating at low voltage and high currents, was mechanical coupled to the internal combustion engine simulator depicted in sub-section 4.1 (Fig.8). The following figures present the experimental results for a time interval of two seconds. In the first moments, during 0.8 sec, the engine is cranked by the ISG from 0 up to 600rpm. Then the engine control system switched on the speed loop, Table1 IPM constructive characteristics Rated power [kw] 4 Peak current constraint [A] 160 Number of phases 3 Number of poles 1 R s [mω] 1 L d [mh] L q [mh] 0.1 ψ m [Wb] Rated speed [rpm] 000 Rated voltage [V] 11 ISBN: ISSN

8 Cranking torque 1 K s U DC t eisg is IPMSM U DC sign 1-1 isq = Eq. (19) isd = Eq.(0) I i sq i sd i sd L d u max Flux weakening PI PI ψ m LPF u sq + u sd ω e u sq u sd L q p θ e dq u a u b abc u c i sq dq i sa, i sb i sd θ e abc p 4 V bus IPMSM U DC Hall transd. ENGINE SIMULATOR key on/off angle pedal u a u b Vector u controller c i sa, i sb R/D ADS100 θ Ω ICE simulator t e SPMSM Restoring torque PI speed controller Ω ICE producing acceleration up to the reference cruise speed (in our case 100rpm). At this new speed level, the ISG control system changes from the motoring to generating regime. In the generating regime, the PWM inverter boosts up to about 38V on the dc link, charging the batteries pack with a low current (approx. 1.A) as in Fig.9. A special attention was paid to handle the nonsinusoidal currents produced by the IPMSM due to its specific geometry and construction. These currents are acquired and used to generate the reference voltages commands and consequently these present many oscillations, as we can see in Fig.10a and 10b. Fig.10c shows through estimation the ISG - generated electromagnetic torque. Once the start-up period was surpassed, the system will keep the engine-imposed speed and the same output voltage. While the battery pack is connected, this will keep the dc voltage approximately constant during non-high load variations. The tested system presents good dynamic performances and responds well also in more complex situations. Fig.8 The control system for an IPM acting as ISG 4 Conclusion The paper analyzes the characteristics of several electric machines types that are suited to be used in ISG applications, pointing out the advantages and drawbacks of each one. These all are ac electric machines and their use in ISG systems needs advanced vector control. The paper exemplifies by two practical applications, where an IM and an IPM play the ISG role, consequently. A test bed including a mean- Fig.9 External characteristics of the ISG system ISBN: ISSN

9 (a) (b) (c) Fig.10 The reference and measured i d, i q currents and the estimated electromagnetic torque t e torque-value engine simulator was used. The experimental results confirm the capability of acting like an ISG of both electric machine types. References: [1] Bimal Bose, Tutorial from the 7-th WSEAS Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Venice, Italy, Nov. 1-3, 007, conferences/007/venice/power. [] W. Cai, Comparison and Review of Electric Machines for Integrated Starter-Alternator Applications, Ind. Applicat. Conference, 004 IEEE Volume 1, 3-7 Oct. 004, pp [3] A. Walker et al., Automotive Integrated Starter Generator, Second IEE International Conference on Power Electronics, Machines and Drives, Edinburgh, UK, 31 March- April 004, p. v1-46. [4] R.H.Staunton et al., PM Motor Parametric Design Analyses for a Hybrid Electric Vehicle Traction Drive Application, Interim Report, Oak Ridge National laboratory, ORNL/TM- 004/10, 004. [5] I.Husain, M.S.Islam, Design, Modeling and Simulation of an Electric Vehicle Systems, International Congress and Exposition, Detroit, Michigan, March 1-4, [6] R.Weeks, J.Moskwa, Automotive Engine Modeling for Real-Time Control Using Matlab-Simulink, Society of Automotive Engineers, SAE 95417, [7] V.Horga et al., Control of an Interior PM Synchronous Machine for Operating as Integrated Starter-Generator, 3rd WSEAS International Conference on Dynamical Systems and Control, Arcachon, France, October 13-15, 007, pp [8] J. H. Song, J. M. Kim, and S. K. Sul, A new robust SPMSM control to parameter variations in flux weakening region, IEEE IECON, vol., pp , [9] Y. S. Kim, Y. K. Choi and J. H. Lee, Speedsensorless vector control for permanent-magnet synchronous motors based on instantaneous reactive power in the wide-speed region, IEE Proc-Electr. Power Appl., vol. 15, No. 5, pp , Sept [10] S. Morimoto, M. Sanada and K. Takeda, Widespeed operation of interior permanent magnet synchronous motors with high performance current regulator, IEEE Trans. Ind. Applicat., vol. 30, pp , July/Aug [11] J.Wai, T.Jahns, A new technique for achieving wide constant power speed operation with an interior PM alternator machine, Proc. of IEEE Ind. Appl. Society Annual Meeting, Chicago, IL, Oct [1] C-T. Pan, S-M. Sue, A linear maximum per ampere control for IPMSM drives over fullspeed range, IEEE Trans. on Energy Conv.,Vol.0, No., June 005, pp [13] P.Vas, Vector Control of AC Machines, Clarendon Press, Oxford, ISBN: ISSN