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symmetrical PWM ac chopper designed to operate with single- ... of this converter is evaluated. ... Index Terms— AC chopper, duty cycle, harmonic distortion,.
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 46, NO. 5, OCTOBER 1999

A New Configuration of Single-Phase Symmetrical PWM AC Chopper Voltage Controller Nabil A. Ahmed, Kenji Amei, and Masaaki Sakui, Member, IEEE

Abstract—With the increased availability of power MOSFET’s and insulated gate bipolar transistors, a new generation of simple choppers for ac inductive loads is foreseen. These new power semiconductors ease the use of forced commutations of thyristor switches to improve the supply power factor, even with highly inductive loads. The ac controllers with thyristor technology can be replaced by pulsewidth modulation (PWM) ac chopper controllers which have important advantages. In this paper, a symmetrical PWM ac chopper designed to operate with singlephase inductive loads with a reduced number of controlled switches is described. The operation as a variable voltage source of this converter is evaluated. This includes the conversion characteristics, harmonic generation, harmonic distortion factor, and input power factor. By digital simulation, these characteristics are investigated theoretically, and to correlate the measurements with theory, an experimental setup is presented to confirm the analytical analysis. Index Terms— AC chopper, duty cycle, harmonic distortion, switching function, symmetrical pulsewidth modulation, voltage controller.

NOMENCLATURE Chopper duty cycle. Switching function of the chopper switch. Switching frequency. Instantaneous inductor current. Instantaneous load current. Instantaneous supply current. Load inductance. Load resistance. Total harmonic distortion factor. Peak value of the sinusoidal supply voltage. Instantaneous supply voltage. Instantaneous output voltage. Instantaneous intermediate modulated chopper voltage. Supply angular frequency (rad/s). Switching angular frequency (rad/s). Load power factor angle at the supply frequency. Displacement angle. Filter inductor. Manuscript received March 23, 1998; revised September 29, 1998. Abstract published on the Internet June 18, 1999. N. A. Ahmed is with the Electrical and Electronic Department, Faculty of Engineering, Toyama University, Toyama 930-8555, Japan, and is also with the Electrical and Electronic Department, Faculty of Engineering, Assiut University, Assiut 71516, Egypt. K. Amei and M. Sakui are with the Electrical and Electronic Department, Faculty of Engineering, Toyama University, Toyama 930-8555, Japan (e-mail: [email protected]). Publisher Item Identifier S 0278-0046(99)07252-4.

Filter capacitor. Input capacitor, for the purpose of power factor improvement. I. INTRODUCTION

T

HE ac voltage regulator is used as one of the power electronic systems to control an output ac voltage for power ranges from a few watts (as in light dimmers) up to fractions of megawatts (as in starting systems for large induction motors). Phase-angle control line-commutated voltage controllers and integral-cycle control of thyristors have been traditionally used in these type of regulators. Some techniques offer such advantages as simplicity and the ability of controlling a large amount of power economically. However, they suffer from inherent disadvantages, such as retardation of the firing angle, causing a lagging power factor at the input side, in particular, at large firing angles, and high low-order harmonic contents in both load and supply voltages/currents [1]. Moreover, a discontinuity of power flow appears at both the input and output sides. The recent developments in the field of power electronics make it possible to improve the electrical power system utility interface. Line-commutated ac controllers can be replaced by pulsewidth modulation (PWM) ac chopper controllers, which have better overall performance, and the above problems can be improved if these controllers are designed to operate in the chopping mode [2]–[6]. In this case, the input supply voltage is chopped into segments, and the output voltage level is decided by controlling the duty cycle of the chopper switching function. The advantages to be gained include nearly sinusoidal input–output current/voltage waveforms, better input power factor, better transient response, elimination of the loworder harmonics and, consequently, smaller input–output filter parameters [7]–[10]. However, little attention has been given to the input power factor of ac chopper controllers. Most researchers who deal with ac choppers have not considered the variation of the input power factor of such controllers [9], [11]–[13]. Others [14] insisted that the input power factor can be made to coincide with the load power factor and that it is independent of the duty cycle. In fact, this claim is not true from the practical and theoretical points of view due to the higher order harmonic contents in the line current resulting from the nature of the switching processes, in particular, at low values of duty cycle. On the other hand, control by switching is often accompanied by extra losses due to the switching losses. The reduction in the number of switches is essential for control simplicity,

0278–0046/99$10.00  1999 IEEE

AHMED et al.: SINGLE-PHASE SYMMETRICAL PWM AC CHOPPER VOLTAGE CONTROLLER

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(a)

(b)

(c) Fig. 1. Proposed PWM ac chopper voltage controller. (a) Circuit configuration. (b) Switching patterns of the gating signals. (c) Block diagram of the control circuit.

cost, reliability, and high switching frequency with good efficiency [11], [12]. This paper describes a new configuration of a symmetrical PWM ac chopper voltage controller for single-phase systems

[15], [16]. The proposed circuit employs only three switches. The modulated chopper switch is placed across a diode rectifier bridge connected in series with the load, and two parallel switches are connected for the freewheeling purpose. The

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 46, NO. 5, OCTOBER 1999

Fig. 2. Equivalent circuits for the fundamental and harmonic voltages.

TABLE I SWITCHING SEQUENCE OF THE DRIVING SIGNAL

Fig. 3 Design factors of the output filter versus the duty cycle.

proposed controller is more economical, owing to a smaller number of controlled switches and fewer switching losses. II. CIRCUIT DESCRIPTION AND PRINCIPLE OF OPERATION Fig. 1(a) shows the circuit configuration of the proposed single-phase symmetrical PWM ac chopper. This circuit has the following characteristics. The circuit can operate directly from a single-phase line, the voltage across each switch is limited to the line voltage, and the number of switches has been reduced to three. In the present scheme, the power circuit is composed of a dc chopper switch across a diode bridge rectifier connected in series with the load and two and connected switches with two freewheeling diodes in parallel across the load. The series-connected switch is used periodically to connect and disconnect the load to the supply, i.e., it regulates the power delivered to the load. The and provide a freewheeling path for parallel switches the load current to discharge the stored energy of the load inductance when the series switch is turned off. The basic reason to use a diode in series with each parallel switch is to enable it to be used in a circuit where a reverse voltage is encountered and to complete the freewheeling current paths. The scheme of the present paper uses insulated gate bipolar transistor (IGBT) devices as controlled switches, and gating of these switches is based on equal PWM technique or constant pulsewidth method. The switching patterns of the controlled switches are decided by the polarity of the source voltage and the load current in such a way as to provide a path for the load

Fig. 4. Normalized intermediate chopper voltage.

Fig. 5. Normalized output voltage.

AHMED et al.: SINGLE-PHASE SYMMETRICAL PWM AC CHOPPER VOLTAGE CONTROLLER

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Fig. 6. Harmonic spectra of intermediate chopper and output voltages.

Fig. 7. Percentage THD of the output voltage, output, and inductor currents.

current whatever its direction. Table I gives the sequence of closure and opening of switches, in which the chopper switch is always modulated with a constant duty cycle. When the supply voltage and the load current are of equal polarity, normal switching takes place, in which one of the parallel or is completely turned on and the other is switches completely turned off according to the polarity of the supply voltage. In other words, when the supply voltage and the is turned on and is turned load current are positive, off, and vice-versa. This is not the case when voltage and current are of different polarity, where the on switch from the parallel switches is gated by the complementary signal of the modulated switch instead of continuous conduction. Normal switching is resumed at the instant when the load current reverses its direction, as shown in Fig. 1(b). By such switching patterns, a continuous current path always exists, regardless of the load current direction. Since only a single switch is modulated and due to the fact that a single freewheeling switch is turned on during the majority of the half period of the voltage source, the switching losses are significantly reduced and, consequently, high efficiency can be approached. The operation modes are divided into two modes: active and freewheeling modes. The active mode is defined when the modulated switch is turned on; during the active mode, the inductor current is forced to flow through the voltage source during its on-state periods. The via the modulated switch

freewheeling mode is defined when the modulated switch is turned off and the inductor current paths can be formed by the direction of the load current, i.e., in the freewheeling mode, the load current freewheels and naturally decays through the with the help of the body diode of or through switch with the help of the body diode of switch the switch according to the direction of the load current. The logic circuit for actuating the controlled switches is shown in Fig. 1(c). For the design of the control circuit, the following requirements must be satisfied: 1) generating gating signals synchronized with the supply and the load current, as shown in Fig. 1(b) and Table I; 2) giving the ability of changing the duty cycle of the gating pulses. III. ANALYSIS The input supply voltage is defined as (1) are the angular frequency and the peak value where and of the input voltage, respectively. with a switching frequency When a switching function and constant duty cycle , defined by the Fourier series of (2) [14], is applied to the chopper switch , the chopper appears in a PWM form at the load modulated voltage

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 46, NO. 5, OCTOBER 1999

terminals (2) Then, the chopper intermediate (modulated) voltage can be given by

(3) From (3), the peak value of the fundamental component of the intermediate chopper voltage and its total harmonic , can be expressed as contents,

(4)

According to (3) and (4), the fundamental component of the intermediate chopper voltage is proportional to the duty cycle , which is defined by the ratio of the on time to the total . The dominant harmonics modulated period are suppressed in proportion to the sum/difference between the frequencies of the switching signal and the source voltage . The value of the fundamental output voltage can be adjusted according to the required duty cycle , which can be obtained by comparing a triangular waveform with a dc reference signal. and setting , (3) shows By defining that the lowest order harmonics of the chopper modulated and, at least voltage occurs at the frequencies of in theory, the size of the input/output filter components is inversely proportional to the value of . This implies that the switching frequency in this type of controller should be kept high enough to raise the order of the dominant harmonics to a high level. The output filter reduces the harmonic contents in the output voltage from that of the intermediate voltage, given by (3). The equivalent circuits of the fundamental and harmonic voltages are shown in Fig. 2. The fundamental component of the output voltage is given by (5)

Fig. 8. Experimental gating signals for the control switches

S; S

1; and S 2.

impedances are approximated as follows:

where (7) For the fundamental component and , therefore, the fundamental output voltage can be simplified by

The harmonic components of the output voltage are given by

(6) If the switching frequency is much higher than the supply , the harmonic frequency and the harmonic frequency,

(8) where

.

AHMED et al.: SINGLE-PHASE SYMMETRICAL PWM AC CHOPPER VOLTAGE CONTROLLER

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(a)

(b) Fig. 9. Comparison of experimental and computed results of a resistive load. (a) and (b) Waveforms of the load voltage (upper trace), the load current 1:8 kHz without the output filter. (lower trace), and the input current for fs

=

For a high switching frequency, , (8) can be simplified to

For the harmonic components of the inductor current

and

(9)

(13)

The total harmonic contents of the output voltage are given by (10) In the same way, the equation for the fundamental compocan be derived as follows: nent of the inductor current (11) and , then the peak value If of the fundamental component is simplified as

and , then the peak values of If the harmonic components are simplified by

(14)

The total harmonic contents of the inductor current are given by

(15)

(12) where

.

Using the above equations, the percentage THD’s of the and the inductor current can be output voltage

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 46, NO. 5, OCTOBER 1999

(c)

(d) Fig. 9. (Continued.) Comparison of experimental and computed results of a resistive load. (c) and (d) Waveforms of the load voltage (upper trace), the 10 kHz with the output filter. load current (lower trace), and the input current for fs

=

represented by

(16)

where is the input capacitor current, placed across the supply terminals for the purpose of power factor improvement, and is the load power factor at the . supply frequency From (18), the fundamental component of the input current is (19) and the effective value is

(17) The corresponding input current has a PWM form distributed over a whole cycle, and the difference between the sinusoidal load current and the supply current is the current of the freewheeling path. Then, the analytical expression for the input current can be expressed as the product of the inductor current times the switching function which can be put in the form

(18)

(20) and are expressed here in peak values. Both is defined as the angle between the fundamental If is called the component of input current and voltage, displacement angle, then, the input power factor of the chopper can be expressed as (21)

AHMED et al.: SINGLE-PHASE SYMMETRICAL PWM AC CHOPPER VOLTAGE CONTROLLER

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(a)

(b) Fig. 10.

Harmonic spectra of the load voltage/current and input current of a resistive load for fs

= 10 kHz. (a) Unfiltered output. (b) Filtered output.

where

where (22)

(26)

The output filter parameters and can be designed within and allowable in the system. the THD’s From (17), the filter inductor is given by

and versus the duty Fig. 3 shows the design factors for a switching frequency of 10 kHz. cycle In the same way, the input filter reduces the harmonic contents in the input current given by (18). It can also be designed within the THD required in the input current by assuming that all the injected harmonics from the chopper flow through the input filter capacitor.

IV. FILTER DESIGN CRITERIA

(23)

V. PERFORMANCE OF TESTED CONTROLLER where (24)

Substituting in (16), the filter capacitor

is obtained as (25)

An experiment on a 1.1-kVA (110 V, 10 A, 60 Hz) laboratory model was performed in order to verify the feasibility of the circuit and to investigate the validity of the simulated results for the proposed ac chopper. The load parameters used mH and . were Using Fig. 3 and (23)–(26), the output filter parameters for kHz were selected as mH and F to % and % at a duty keep the THD’s and to maintain % over all the cycle of . control range

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(a)

(b)

(c)

(d) Fig. 11. Results of an inductive load ( = 45 ) for fs = 10 kHz. (a) Comparison of experimental and computed waveforms of output voltage (upper trace) and load current (lower trace). (b), (c), and (d) Harmonic spectra of the load voltage, load, and input currents.

Although a proper firing sequence between chopper switch and freewheeling switches was provided, a small capacitor of 1 F, as a voltage suppressor, was placed across the freewheeling path in order to avoid problems of high-voltage transients that can occur if both switches are left open in the presence of a reactive load. The test results for different load conditions came close to the predicted values. The obtained

calculated and experimental results will be discussed in the following. Fig. 4 shows the variation of the normalized value of the intermediate voltage over a complete range of control, to , including its fundamental component , , and the harmonic contents in the total harmonic contents according to (4) and (10). filtered load voltage

AHMED et al.: SINGLE-PHASE SYMMETRICAL PWM AC CHOPPER VOLTAGE CONTROLLER

Fig. 5 demonstrates the normalized value of the output versus the duty cycle for an inductive load with voltage . It is clear that the relation between the fundamental component of the output voltage and the duty cycle is almost linear over most of the control range. Fig. 6 shows the calculated harmonic contents of the intermediate chopper voltage, given by (4), and the filtered output of 0.7 and a switching frequency voltage for a duty cycle of 10 kHz. The relation between the THD’s of the output voltage , load current and filter inductor current versus the duty cycle , calculated according to (16) and , are shown in Fig. 7. (17) for an inductive load A switching frequency of 10 kHz is considered for the calculations. Fig. 7 shows a low total harmonic distortion in the output voltage and current, less than 2% in both; this means that the harmonic contents of the output current are almost negligible, which enhances the assumption that the load current is approximately a sinusoidal current. The experimental gating signals from the control circuit to and for pure resistive and actuate the switches inductive loads are shown in Fig. 8. When the load is purely resistive, as in the control of a heater and light dimming, and the freewheeling path and the parallel switches can be redundant. Therefore, the control circuit in this case becomes very simple and the logic gates shown in Fig. 1(c) are dispensed with. Fig. 9(a) shows a comparison of the dynamic simulation and experimental waveforms of the load voltage (upper trace) and the load current (lower trace) for a resistive load at a switching frequency of 1.8 kHz and a duty cycle without connecting the output filter to the load terminals; a low switching frequency of 1.8 kHz is used for explanation. Fig. 9(b) shows the computed and experimental supply current waveforms for the same conditions of Fig. 9(a). Fig. 9(c) and (d) shows the computed and experimental waveforms of filtered output voltage, load, and supply currents at a switching . frequency of 10 kHz and a duty cycle Fig. 10(a) and (b) demonstrates the respective rms harmonic spectra of the load voltage/current and supply current waveforms of filtered and unfiltered output at a switching frequency of 10 kHz. These figures indicate that the waveforms of the output voltage and current are close to pure sine waves under the filtering conditions. Unlike the load current, the supply current is significantly distorted by the higher order harmonics, as shown in Fig. 10(c). The behavior of the chopper circuit with an inductive load at a switching frequency of 10 kHz and a duty cycle is shown in Fig. 11. The computed and experimental waveforms of the load voltage and load current with its rms harmonic spectra are shown in Fig. 11(a)–(c), respectively. It is clear that the harmonic contents of the load voltage and currents are almost negligible, and they practically coincide with a sinusoidal waveform, as predicted. Fig. 11(d) shows the rms harmonic spectrum of the input current for the same conditions. In addition to the fundamental component, the line current contains harmonics near the switching frequency and its multiplies.

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Fig. 12. Variation of the input power factor, resistive, and inductive loads.

Figs. 10(b) and 11(d) corroborate the important predicted result, given by (18), that the order of the lower harmonic where . spectrum in the input current is The same conclusion can also be drawn from Fig. 6, which depicts the same quantities, but for the intermediate chopper voltage. It is worth noting that the chopper shifts the line current harmonic contents toward high-frequency values, when high chopping frequency is chosen. These harmonics may adversely affect the supply performance if not filtered out, in particular, at low values of the duty cycle. The variations of the input power factor versus the duty cycle over the complete range of control and for different phase and inductive load angles, pure resistive at the supply frequency, are shown in Fig. 12. Input power factor measurement is done by recording the input power and rms values of the voltage and current at the supply side. It should be noted that the symmetrical PWM ac chopper shows a poor power factor due to the presence of supply current distortion, but it remains still better than that obtained by thyristor switches. More analysis and tests will be performed in the hope of identifying the best performance.

VI. CONCLUSION This paper has presented an ac chopper circuit for singlephase systems with a reduced number of controlled switches (only three switches) and a simple control circuit. The circuit under consideration provides a full range of ac power control Besides the wide and continuous range of control, the relation between the fundamental component of the output voltage and the duty cycle is almost linear over most of the control range. Due to the nature of the switching process, high switching frequency and lower order harmonics both in the load and supply sides are reduced, while harmonics near the switching frequency and its multipliers are canceled by a filter tuned with the switching frequency and practically sinusoidal voltage and current waveforms can be obtained. The performance of the proposed circuit has been illustrated as applied to a single-phase voltage regulator by an example of pure resistive and inductive loads to verify the feasibility of

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the proposed technique. A good agreement is obtained between the experimental and predicted results. It should be recalled that typical applications of the proposed ac chopper consist of light dimming, heat control, and speed control of single-phase induction motors. It is predicted that the chopper method allows also improvements in the motor efficiency, as it is not submitted to undesired harmonics at low frequency, which is the subject of future work. REFERENCES [1] M. H. Rashid, Power Electronics: Circuits, Devices and Applications, 2nd ed. Upper Saddle River, NJ: Prentice-Hall, 1993. [2] B. W. Williams, “Asymmetrically modulated AC choppers,” IEEE Trans. Ind. Electron., vol. IE-29, pp. 181–185, Aug. 1982. [3] S. A. Bhat and J. Vithayathil, “A simple multiple pulse width modulated AC chopper,” IEEE Trans. Ind. Electron., vol. IE-29, pp. 185–189, Aug. 1982. [4] S. Iida and S. Miyairi, “Effects of PWM applied in single-phase ac chopper control,” Trans. Inst. Elect. Eng. Jpn., vol. 103-B, no. 1, pp. 7–14, Jan. 1983. [5] G. Roy, P. Poitevin, and G. Olivier, “A Comparative study of singlephase modulated AC choppers,” IEEE Trans. Ind. Applicat., vol. IA-20, pp. 1498–1506, Nov./Dec. 1984. [6] G. H. Choe, A. K. Wallace, and M. H. Park, “An improved PWM technique for AC choppers,” IEEE Trans. Power Electron., vol. 4, pp. 496–505, Oct. 1989. [7] S. A. Hamed, “Steady-state modeling analysis, and performance of transistor controlled AC power conditioning,” IEEE Trans. Power Electron., vol. 5, pp. 305–313, July 1990. [8] D. A. Deib and H. W. Hill, “Optimal harmonic reduction in ac/ac chopper converters,” in Proc. IEEE PESC’93, 1993, pp. 1055–1060. [9] M. Mazzuccheli, L. Puglisi, G. Sciutto, and P. Tenti, “Improving the performance of AC/AC static converters with high frequency AC chopper control,” in Proc. POWERCON 9, 1982, vol. I-3, pp. 1–9. [10] D. H. Jang, J. S. Won, and G. H. Choe, “Asymmetrical PWM method of ac chopper with improved input power factor,” in Proc. IEEE PESC’91, 1991, pp. 838–845. [11] L. Salazar, C. Vasquez, and E. Weichmann, “On the characteristics of a PWM ac controller using four switches,” in Proc. IEEE PESC’93, 1993, pp. 307–313. [12] P. D. Ziogas., D. Vincenti, and D. Joos, “A practical PWM ac chopper topology,” in Proc. IEEE IECON’91, 1991, pp. 880–887. [13] K. E. Addoweesh and A. L. Mohamadein, “Microprocessor based harmonic elimination in chopper type AC voltage regulators,” IEEE Trans. Power Electron., vol. 5, pp. 191–200, Apr. 1990. [14] B. H. Kwon, B. D. Min, and J. H. Kim, “Novel topologies of ac choppers,” Proc. Inst. Elect. Eng.—Elect. Power Applicat., vol. 143, no. 4, pp. 323–330, July 1996. [15] N. A. Ahmed, K. Amei, and M. Sakui, “Improved circuit of AC choppers for single-phase systems,” in Proc. Energy Conversion Conf. (PCC’97), Nagaoka, Japan, Aug. 3–6, 1997, pp. 907–9012.

[16] N. A. Ahmed, K. Amei, and M. Sakui, “A symmetrical PWM AC chopper controller fed single-phase induction motor,” in Proc. AL-Azhar 5th Int. Conf. (AEIC’97), Cairo, Egypt, Dec. 19–22, 1997, pp. 123–134. [17] G. Joos and P. D. Ziogas, “A PWM AC controller-high current power supply,” in Proc. IEEE IECON’92, 1992, pp. 554–559. [18] B. Cotta, M. Mazzuccheli, and G. Sciuto, “AC chopper regulation using power transistors,” in Proc. POWERCON 8, 1981, vol. G1-4, pp. 1–8.

Nabil A. Ahmed was born in Sohag, Egypt, in 1966. He received the B.S. (with distinction) and M.S. degrees in electrical engineering from Assiut University, Assiut, Egypt, in 1989 and 1994, respectively. He is currently working toward the Ph.D. degree in electrical engineering in the Electrical and Electronics Department, Toyama University, Toyama, Japan. In 1989, he joined the Department of Electrical Engineering, Assiut University, as a Teaching Assistant, where he is currently an Assistant Lecturer. His research interests include power electronics and electric drives.

Kenji Amei was born in Toyama, Japan, in 1966. He received the B.S. and M.S. degrees in electrical engineering from Nagaoka University of Technology, Nagaoka, Japan, in 1989 and 1991, respectively. He was an Engineer with Nissin Electric Company Ltd., Kyoto, Japan, from 1991 to 1994. Since 1994, he has been with Toyama University, Toyama, Japan, where he is currently an Assistant Professor. He is engaged in research on power electronics. Mr. Amei is a member of the Institute of Electrical Engineers of Japan.

Masaaki Sakui (M’88) was born in Toyama, Japan, in 1949. He received the B.S. and M.S. degrees in electrical engineering from Toyama University, Toyama, Japan, in 1972 and 1974, respectively, and the Ph.D. degree from Tokyo Metropolitan University, Tokyo, Japan, in 1988. Since 1974, he has been with Toyama University, where he is currently a Professor. He is engaged in research and education on power electronics. Dr. Sakui is a member of the Institute of Electrical Engineers of Japan and Institute of Electrical Installation Engineers of Japan.