Extended Summary 本文は pp.639–645

Permanent Driven by PWM Inverter with Voltage with Regenerating Capability Augmented by Double-Layer Capacitor

Kichiro Yamamoto Member (Kagoshima University, [email protected]) Katsuji Shinohara Member (Kagoshima University, [email protected]) Shinya Furukawa Student Member (Kagoshima University, [email protected]) Keywords: permanent magnet motor, PWM inverter, voltage booster, double-layer capacitor and regeneration

1. System Configuration double-layer capacitor unit is charged while the terminal voltage of An interior permanent magnet (IPM) system which the double-layer capacitor unit eDL is less than the limit voltage of has regenerating capability augmented by double-layer capacitors is EBT + 8.0 V. And regeneration to the battery starts when eDL reaches proposed. The configuration of proposed system is demonstrated to the limit voltage. The charging current to the battery is controlled ∗ = − in Fig. 1. The motor is driven by a PWM inverter with voltage to the value of constant reference iBT 5 A by the charging cur- booster. The voltage booster is used to control the dc link voltage rent controller and the charging current controller operates correctly in high speed region to improve the system efficiency. Furthermore, in accordance with eDL and direction of iL. the double-layer capacitor as a storage element is combined with the PWM inverter with voltage booster to gain the efficiency for the regenerating operation. In this system, normally, the regenera- tive power does not return to a battery directly but is stored in the double-layer capacitors for the next motoring action to suppress the excessive regenerative current to battery (see Fig. 2), and the regen- erative power returns to the battery when the regenerative energy is larger than a certain value (see Fig. 3). The charging current to the battery is controlled to a constant value to extend the life-time of the battery. 2. Simulated and Experimental Results The transient and steady state characteristics of the system for 1.5 kW IPM motor are investigated by both simulation and exper- iment and the results are illustrated in Figs. 2 and 3, respectively. From these figures, one finds the following: - It is clear that there is good agreement between the simulated and experimental results. - From behavior of the current to the double-layer capacitor unit iDL, it is also clear that the double-layer capacitor is charged during the decelerating period in Fig. 2. The current from battery iBT is reduced during the accelerating period due to using of the stored energy. -FrombehaviorofiDL, iBT and eDL in Fig. 3, one finds that the

(a) Simulated results (b) Experimental results Fig. 2. Acceleration and deceleration characteristics

(a) Simulated results (b) Experimental results Fig. 3. Characteristics for steady state regenerating Fig. 1. System configuration of proposed system operation (average regenerative power is 200 W)

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Permanent Magnet Synchronous Motor Driven by PWM Inverter with Voltage Booster with Regenerating Capability Augmented by Double-Layer Capacitor

∗ Kichiro Yamamoto Member ∗ Katsuji Shinohara Member ∗ Shinya Furukawa Student Member

An interior permanent magnet (IPM) motor drive system which has regenerating capability augmented by double- layer capacitors is proposed. The motor is driven by a PWM inverter with voltage booster. The voltage booster is used to control the dc link voltage in high speed region to improve the system efficiency. Furthermore, the double-layer capacitor as a storage element is combined with the PWM inverter with voltage booster to gain the efficiency for the regenerating operation. In this system, normally, the regenerative power does not return to a battery directly but is stored in the double-layer capacitors for the next motoring action to suppress the excessive regenerative current to bat- tery, and the regenerative power returns to the battery when the regenerative energy is larger than a certain value. The charging current to the battery is controlled to a constant value to extend the life-time of the battery. The transient and steady state characteristics of the system for 1.5 kW IPM motor are investigated by both simulation and experiment. Finally, the effectiveness of the system is demonstrated by the simulated and experimental results.

Keywords: permanent magnet motor, PWM inverter, voltage booster, double-layer capacitor and regeneration

constant torque region. It makes the motor design much 1. Introduction easier in terms of the maximum current of the motor. The permanent magnet (PM) motors form an increasingly The purpose of our research is to improve the efficiency of important class of high performance drives and are the PM motor drive system. We use the PWM inverter with used for not only industrial applicati ons but also electric ve- dc link voltage control circuit to extend the speed range of hicle and another applications. the interior permanent magnet (IPM) motor without the flux There are two main operations of the PM motors over wide weakening. We have investigated the stability of the PWM speed range. These are performed by 1) the flux weakening inverter with voltage booster (5) and the characteristics of the operation at high-speed region (1) (2) or 2) a PWM inverter with drive system (6) and have compared the flux weakening oper- a dc link voltage control circuit (3)–(5). ation with the operation by the PWM inverter with voltage The flux weakening operation is more conventional booster analytically (7). The analytical results for both oper- method. However, this operation needs additional current to ations demonstrated that the operation by the PWM inverter reduce the magnet flux of motor and the current increases with voltage booster gave higher efficiency than that given by copper loss. the flux weakening operation in the constant power region. A PWM inverter with a dc link voltage control circuit is To increase the efficiency of the drive system, regener- another solution. This drive system has several advantages ative energy is also important. In the system of electric over the drives with flux weakening operation as follows: vehicles, however, the regenerative energy is consumed by - This drive system does not need the current to reduce the breaking resistor to extend the life-time of high perfor- the magnet flux of the motor. Thus, no additional copper mance expensive batteries, generally. We paid attention to loss by the current is generated. the double-layer capacitor as a new energy storage element - If the gating signal is suddenly removed from the in- to absorb the regenerative energy. This capacitor has a lot verter switches during high-speed operation, the ampli- of advantages such as no maintenance, long life-time and tude of the line-to-line back-emf generated by the spin- quick charge/discharge characteristics with large current, etc. ning PM is kept under the dc link volt- Some papers for applications of the double-layer capacitor age. Therefore, this system has no probability of serious have been published. In reference (8), the double-layer ca- faults. pacitor was used for power smoothing and absorbing regen- - In spite of variation of the battery voltage, this system erative energy in elevator applications. In reference (9), the can keep the value of a constant dc link voltage in the capacitors were attached to the dc link of adjustable-speed drive system through the flyback converter to compensate ∗ Department of Electrical and Electronics Engineering, Kagoshima University the dc link voltage drop during short-term power interrup- 1-21-40, Korimoto, Kagoshima 890-0065 tions. We also have proposed a voltage sag compensator

電学論 D，126 巻 5 号，2006 年 639 using double-layer capacitor (10). layer capacitor unit to store regenerative energy temporarily. In this paper, an IPM motor drive system which has re- The IPM motor is 1.5 kW and has six poles. This system op- generating capability augmented by double-layer capacitors erates as a conventional PWM inverter when the dc link volt- ∗ is proposed. We combine the double-layer capacitor as a age reference edc is less than the battery voltage EBT and op- storage element with the PWM inverter with voltage booster erates as a PWM inverter with variable dc link voltage when ffi ∗ to gain the e ciency for the regenerating operation. In the edc is greater than EBT . system in reference (8), double-layer capacitors are used to The double-layer capacitor unit and the charging current reduce the high power peaks appearing on the power grid. controller are located between the voltage booster and the However, the regenerative energy in the system is not re- battery. For the motoring operation, the energy needed by the turned to the power grid. On the other hand, in our sys- motor is supplied by the battery through the diode D7.Forthe tem, the stored regenerative energy in the double-layer capac- temporary regenerating operation, the regenerative energy is itors is used for not only the next motoring operation but also stored in the double-layer capacitor unit and is used for the charging battery with constant current in accordance with the next motoring operation. For the long time regenerating oper- voltage level of the double-layer capacitors. At first, the sys- ation, the regenerative energy charges the battery with a con- tem configuration is explained. Next, the transient and steady stant current when the terminal voltage of the double-layer state characteristics of the system for 1.5 kW IPM motor are capacitor unit exceeds a limit voltage. The charging current investigated by simulation, and the validity of the simulated to the battery is controlled by a hysteresis controller. results is confirmed by the experimental results. Finally, the The nominal values of parameters of a double-layer capaci- effectiveness of the system is demonstrated. tor are 2.3 V and 6000 F. We made two capacitor banks. Each bank consists of 50-series-connected capacitors. The two 2. System Configuration banks were connected in parallel and a double-layer capac- Fig. 1 shows the configuration of the proposed system. itor unit was made. We assumed the equivalent circuit for the This system consists of a conventional PWM inverter, a volt- capacitor unit as illustrated in Fig. 2 and decided the equiv- age booster which can control the dc link voltage and double- alent capacitance CDL and the equivalent internal resistance

Fig. 1. System configuration of PWM inverter with voltage booster with double-layer capacitor for IPM motor drive

640 IEEJ Trans. IA, Vol.126, No.5, 2006 PWM Inverter with Voltage Booster with Double-Layer Capacitor

the nearest to the origin of the id-iq plane. The trajectory for the maximum torque per ampere operating point is plotted in Fig. 3. The trajectory in the third quadrant is for the regener- ating operation. We used an approximation for the trajectory with the following equation. ∗ = − ∗2 ······································ id Kiq (3) The value of K was decided as 0.01456 by the least-square ∗ method. Using this idc can make the motor current minimum Fig. 2. Equivalent circuit for the double-layer capacitor unit even if the motor is in the regenerating operation. ∗ v The dc link voltage reference edc is calculated from sd and vsq to make the current control possible. The parameter Kedc is used to avoid partial saturation for the PWM voltage. ∗ When edc becomes higher than battery voltage EBT , the volt- age booster begins to operate to increase the dc link voltage edc.Andedc is controlled to the value which is enough to control the motor currents. The feedforward of the reactor current iL is used to stabilize the voltage booster. The charging current to battery is controlled in accordance with the terminal voltage of double-layer capacitor unit eDL and the direction of the reactor current iL. The details will be described in the next chapter. The values of the parameters are listed in Table 1. 3. Regenerating Operation In the proposed system, normally, the regenerative power does not return to a battery directly but is stored in the double- layer capacitor unit for the next motoring action to suppress the excessive regenerative current to battery (Mode I). And the regenerative power returns to the battery when the regen- erative energy is larger than a certain value (Mode II). The charging current to battery is controlled to a constant value to extend the life-time of the battery. We decided the condition to switch between Mode I and Mode II as the following: 1. If (EBT + 8V)< eDL, then the controller begins charging battery. (Mode I → Mode II) 2. If (EBT + 2V) > eDL, then the controller stops charging Fig. 3. Trajectory for the maximum torque per ampere operation battery. (Mode II → Mode I) EBT is the battery voltage and the value was 100 V, and RDL by measurement. The values of CDL and RDL were 207 F e is the terminal voltage of electric double-layer capacitor Ω DL and 0.06 , respectively. unit. If the terminal voltage increases from 102 V to 106 V, An inverter controlled 15 kW, 6 poles IPM motor was used the stored energy in the electric double-layer capacitor unit as a load. ∗ increases as follows, In the controller, the d-axis current reference id makes the maximum torque per ampere operation possible. The opera- 1/2 × 207 × (1062 − 1022) = 86112 J. tion is explained briefly. The electromagnetic torque of a PM machine is expressed as This energy corresponds to the regenerative power of   1.5 kW for one minute. Practically, there are a lot of factors P ff T = (φ + Ldid)iq − Lqiqid , ···················(1) which a ects to EBT and eDL such as voltage drop in switch- 2 ing devices, change of voltage across the internal resistance where P is the pole number, φ is the flux of field, Ld and for current direction, etc. Taking these factors into account, Lq are the d-andq-axes inductance. Solving (1) for iq,the the hysteresis band for switching between Mode I and Mode following equation is obtained. II was chosen as 6 V (from 102 V to 108 V). The voltage of 2 T 2 V is voltage margin. iq = ··························(2) P φ + (Ld − Lq)id 4. Simulated and Experimental Results This equation expresses a hyperbola for a certain torque in 4.1 Temporary Regenerating Operation Simula- the id-iq plane. For the maximum torque per ampere opera- tion of the proposed system for regenerating operation was tion, we choose the operating point on the hyperbola which is performed. The values of the parameters used for the

電学論 D，126 巻 5 号，2006 年 641 Table 1. Values of parameters

(a) Simulated results (b) Experimental results Fig. 4. Acceleration and deceleration characteristics

between the simulated and experimental results except simulation are listed in Table 1. At first, we investigated the some noises in the experimental results. temporary regenerating operation. The acceleration and de- - From behavior of the current to the double-layer celeration for high inertia load were simulated and the re- capacitor unit iDL, it is also clear that the double-layer sults are shown in Fig. 4(a). The experimental results corre- capacitor is charged during the decelerating period. The sponding to the simulated results are also shown in Fig. 4(b). current from battery iBT is reduced during the accelerat- The inertia of moment was 0.058 kg-m2 (tested IPM motor: ing period due to using of the stored energy. 2 2 0.004 kg-m , load IPM motor: 0.044 kg-m and couplings: -Thed-axis current id changes so as to perform the max- 0.01 kg-m2) and the acceleration and deceleration were per- imum torque per ampere operation in accordance with formed between 100 r/min to 1800 r/min. The averaging re- the q-axis current iq. generative power is nearly 500 W during the decelerating pe- - In the experimental results, the terminal voltage of the riod. For the terminal voltage of the double-layer capacitor double-layer capacitor unit eDL changes smaller than unit of 100 V, the voltage booster operates for the speed be- that in the simulated results. tween 500 r/min to 1800 r/min. In addition, D/Aconverter 4.2 Long Time Regenerating Operation We inves- output signal was used for the experimental eDL because eDL tigated the long time regenerating operation. Steady state re- had a lot of small ac components on a large dc component. generating operations were simulated and the simulated re- For the change of experimental eDL, the ac coupling of the sults are represented in Figs. 5(a) and 7(a). The enlarged digital oscilloscope was also tried but it was not available be- waveforms corresponding to A in Fig. 5(a) and C in Fig. 7(a) cause eDL had some very low-frequency components. are illustrated in Figs. 6(a) and 8(a), respectively. The regen- From Fig. 4, one finds the following: erative power was produced by a 15 kW IPM motor with in- - It is clear from this figure that there is good agreement verter control in the experimental system. The battery current

642 IEEJ Trans. IA, Vol.126, No.5, 2006 PWM Inverter with Voltage Booster with Double-Layer Capacitor

(a) Enlarged waveforms (b) Enlarged waveforms (a) Simulated results (b) Experimental results corresponding to A corresponding to B Fig. 5. Characteristics for steady state regenerating operation (Simulated results) (Experimental results) (average regenerative power is 200 W) Fig. 6. Enlarged waveforms corresponding to A and B in Fig. 5

∗ = − reference was set to iBT 5 A (charging operation) and EBT was 100 V. The simulated results in Figs. 5(a) and 7(a) are for - Regeneration to the battery starts when eDL reaches to two values of the regenerative power of 200 W and 700 W, the limit voltage. The direction of iDL is switched respectively. The regenerative power of 200 W was chosen from positive (charge) to negative (discharge) for the so that the absolute value of reactor current iL might be less regenerative power of 200 W in Fig. 5(a) and (b). On than the absolute value of battery current iBT and 700 W was the other hand, the direction of iDL does not change for chosen so that the absolute value of iL might be greater than the regenerative power of 700 W in Fig. 7(a) and (b). the absolute value of iBT . - The charging current to the battery is controlled to the ∗ = − Experimental results corresponding to Figs. 5(a), 6(a), 7(a) value of constant reference iBT 5 A by the charging and 8(a) are shown in Figs. 5(b), 6(b), 7(b) and 8(b), respec- current controller and the charging current controller op- tively. The same value of hysteresis band was set for both erates correctly in accordance with eDL and direction of simulation and experiment to control the charging current to iL. the battery iBT . For the experiment, however, effective hys- - From the enlarged waveforms of eDL in Fig. 6(b), the teresis band became wider than that for the simulation be- variation of eDL is coursed because of the variation of cause of the delay of comparator device. As a result, the ex- iL in the experimental system and the variations of eDL perimental iBT and iDL included less high frequency compo- and iL become seriously for large iL in Fig. 8(b). nents than the simulated results. In addition, the experimental - In comparison Figs. 5 and 6 with Figs. 7 and 8, it is clear eDL also included less high frequency components than the that the d-axis current id changes properly in accordance simulated results because D/A converter output signal from with the change of the q-axis current iq so that the maxi- DSP system was used for the experimental eDL. mum torque per ampere operation may be accomplished. From Figs. 5–8, one finds the following: 5. Conclusion - There is good agreement between the simulated and ex- perimental results. An IPM motor drive system which has regenerating capa- -FrombehaviorofiDL, iBT and eDL, it is clear that the bility augmented by double-layer capacitors was proposed. double-layer capacitor unit is charged while the terminal The double-layer capacitor as a storage element was com- voltage of the double-layer capacitor unit eDL is less than bined with the PWM inverter with voltage booster to gain the limit voltage of EBT + 8.0 V in Fig. 5 and Fig.7. the efficiency for the regenerating operation. Normally, the

電学論 D，126 巻 5 号，2006 年 643 (a) Simulated results (b) Experimental results (a) Enlarged waveforms (b) Enlarged waveforms corresponding to C corresponding to D Fig. 7. Characteristics for steady state regenerating operation (Simulated results) (Experimental results) (average regenerative power is 700 W) Fig. 8. Enlarged waveforms corresponding to C and D in Fig. 7 regenerative power does not return to a battery directly but is References stored in the double-layer capacitors for the next motoring ac- tion to suppress the excessive regenerative current to battery, ( 1 ) T.M. Jahns: “Uncontrolled Generator Operation of Interior PM Synchronous and the regenerative power returns to the battery when the Machines following High-Speed Inverter Shutdown”, Conf. Rec. IEEE-IAS Annu. Meeting, pp.395–404 (1998) regenerative energy is larger than a certain value. The charg- ( 2 ) S. Morimoto, Y. Takeda, T. Hirasa, and K. Taniguchi: “Expansion of operat- ing current to the battery is controlled to a constant value to ing Limits for Permanent Magnet Motor by Current Consid- extend the life-time of the battery. The transient and steady ering Inverter Capacity”, IEEE Trans. Ind. Appl., Vol.26, pp.866–871 (1990) state characteristics of the system for 1.5 kW IPM motor were ( 3 ) J. Jelonkiewics and S. Linnman: “High Efficient Wheelchair Drive with PM Synchronous Motor”, Conf. Rec. EPE’93, pp.145–149 (1993) investigated by both simulation and experiment, and the ef- ( 4 ) H. Jonokuchi, M. Hirata, S. Hashimoto, H. Ishihara, M. Umeda, H. Itoh, S. fectiveness of the system was demonstrated by the simulated Hashizume, and S. Kakuchi: “EV Drive System with Voltage Booster”, Conf. and experimental results. Rec. EVS-13, pp.287–294 (1996) ( 5 ) K. Yamamoto, T.A. Lipo, K. Shinohara, and Y. Sueyoshi: “Power Loss We conclude from the results the following. Reduction and Optimum Modulation Index of PWM Inverter with Voltage - The validity of the simulated results was confirmed by booster for Permanent Magnet Synchronous Motor Drive”, Conf. Rec. IPEC- comparing the simulated results with that obtained from TOKYO 2000, pp.147–152 (2000) the experimental. ( 6 ) K. Yamamoto, K. Shinohara, and T. Nagahama: “Characteristics of Per- manent Magnet Synchronous Motor Driven by PWM Inverter with Voltage - The fact that for two modes of the regenerating oper- Booster”, IEEE Trans. Ind. Appl., Vol.40, pp.1145–1152 (2004) ation the system works properly, is confirmed by both ( 7 ) K. Yamamoto, K. Shinohara, and H. Makishima: “Comparison between Flux simulation and experiment. Weakening and PWM Inverter with Voltage Booster for Permanent Mag- The proposed system can recovery the regenerative energy net Synchronous Motor Drive”, Conf. Rec. PCC-OSAKA 2002, pp.161–166 ff (2002) e ectively and would be able to contribute for the extension ( 8 ) A. Rufer and P. Barrade: “A Supercapacitor-Based Energy-Storage Sys- of the life-time of the battery. tem for Elevators with Soft Commutated Interface”, IEEE Trans. Ind. Appl., Acknowledgment Vol.38, pp.1151–1159 (2002) ( 9 ) J.L. Duran-Gomez, P.N. Enjeti, and A. von Jouanne: “An Approach to This research was partially supported by the Ministry of Achieve Ride-Through of an Adjustable-Speed Drive with Flyback Converter Education, Culture, Sports, Science and Technology, Grant- Modules Powered by Super Capacitors”, IEEE Trans. Ind. Appl., Vol.38, in-Aid for Scientific Research (C)(2), 15560249, 2004. pp.514–522 (2002) (Manuscript received May 11, 2005, (10) K. Yamamoto, K. Shinohara, and F. Wada: “Maximum Load Capacity and Main Circuit Design of Voltage Sag Compensator Using Double-Layer Ca- revised Dec. 27, 2005) pacitor”, Conf. Rec. PEMD2004, pp.588–593 (2004)

644 IEEJ Trans. IA, Vol.126, No.5, 2006 PWM Inverter with Voltage Booster with Double-Layer Capacitor

Kichiro Yamamoto (Member) was born in 1965. He received the Shinya Furukawa (Student Member) was born in 1981. He re- B.Eng. and M.Eng. degrees from Kagoshima Univer- ceived the B.Eng. degree from Kagoshima University, sity, Kagoshima, Japan, in 1987 and 1989, respec- Kagoshima, Japan, in 2004, and he is currently work- tively, and the Dr.Eng. degree from Kyushu Univer- ing toward the M.Eng. degree at Kagoshima Univer- sity, Fukuoka, Japan, in 1996. He was a Research sity, Kagoshima, Japan. His current research is con- Associate at Kagoshima National College of Tech- cerned with inverter system for AC motor drives. nology, Japan, from 1989 to 1993. Since 1993, he has been with the Department of Electrical and Elec- tronics Engineering, Kagoshima University, where he is a Research Associate. His research interests are ac motor drives and power converter circuits.

Katsuji Shinohara (Member) was born in Japan in 1943. He re- ceived the B.Eng. and M.Eng. degrees from Kyushu Institute of Technology, Fukuoka, Japan, in 1966 and 1968, respectively, and the Dr.Eng. degree from Kyushu University, Fukuoka, Japan, in 1983. He was with Yaskawa Electric Manufacturing Company, Ltd. from 1961 to 1977 and with Kyushu Institute Tech- nology from 1977 to 1985. Since 1985, he has been with the Department of Electrical and Electronics En- gineering, Kagoshima University, Kagoshima, Japan, where he has been a Professor since 1992. His current research interests are ac machine drives, power converter circuits, and electric machines.

電学論 D，126 巻 5 号，2006 年 645