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164 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 1, JANUARY 2003 On the Feasibility of Four-Switch Three-Phase BLDC Motor Drives for Low Cost...

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164 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 1, JANUARY 2003 On the Feasibility of Four-Switch Three-Phase BLDC Motor Drives for Low Cost Commercial Applications: Topology and Control Byoung-Kuk Lee , Member, IEEE , Tae-Hyung Kim , Student Member, IEEE , and Mehrdad Ehsani , Fellow, IEEE Abstract— The main purpose of this paper is to describe a low cost four-switch brushless dc (BLDC) motor drive for commer- cial applications. For effective utilization of the developed system, a novel direct current controlled pwm scheme is designed and im- plemented to produce the desired dynamic and static speed–torque characteristics. Also, the feasibility of the four-switch converter is extended to two-phase BLDC motor drives and the six-switch converter for power factor correction and speed control. The op- erational principle of the four-switch BLDC motor drive and the developed control scheme are theoretically analyzed and the per- formance is demonstrated by both simulation and experimental results. Index Terms— Brushless dc motor, direct current control, four- switch converter, power factor correction, two-phase BLDC motor. I. INTRODUCTION V ARIABLE-SPEED drives, employing a pulsewidth modulation (pwm) voltage-fed inverter, are being used for various purposes in consumer products and industrial applications. Although their technical advantages are generally acknowledged, researchers are becoming aware of their cost and are exploring the possibility of cost reduction. The cost reduction of variable-speed drives is accomplished by two approaches. One is the topological approach and the other is the control approach. From a topology point of view, minimum number of switches is required for the converter circuit. In the control approach, algorithms are designed and implemented in conjunction with a reduced component converter to produce the desired speed–torque characteristics. As a result, many different converter topologies have been developed and various pwm control strategies have been proposed to enhance the performance of the system [1]–[4]. Until now, the reduced part converters have been applied mainly to ac induction motor drives However, these days, the BLDC motor is attracting much interest, due to its high efficiency, high power factor, high torque, simple control, and lower maintenance [5], [6]. Thus, we have been investigating the possibility of the reduced part converter for BLDC motor drives with advanced control techniques. Consequently, we found that one switch leg (two switches) in the conventional six-switch converter, as shown in Fig. 1, is redundant to drive Manuscript received February 21, 2002; revised October 21, 2002. Recom- mended by Associate Editor P. C. Luk. The authors are with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128 USA (e-mail: [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/TPEL.2002.807125 Fig. 1. Conventional six-switch three-phase BLDC motor drive systems. Fig. 2. Proposed four-switch converter topology for three-phase BLDC motor. a three-phase BLDC motor. It results in the possibility of the four-switch configuration instead of the six switches, as shown in Fig. 2. Compared with the four-switch converter for the induction motor [1], it is identical for the topology point of view. However, in the four-switch converter, the generation of 120 conducting current profiles is inherently difficult due to its limited voltage vectors. This problem is well known as “asymmetric voltage pwm.” It means that conventional pwm schemes for the four-switch induction motor drive cannot be directly used for the BLDC motor drive. Therefore, in order to use the four-switch converter topology for the three-phase BLDC motor drive, a new control scheme should be devel- oped. The solutions can be obtained from a modification of the conventional voltage controlled pwm strategies, such as the space vector pwm. However, it naturally requires lots of 0885-8993/03$17.00 © 2003 IEEE
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LEE et al. : FOUR-SWITCH THREE-PHASE BLDC MOTOR DRIVES 165 (a) (b) (c) (d) Fig. 3. Voltage vectors of four-switch converter. (a) (0, 0) vector, (b) (1, 1) vector, (c) (1, 0) vector, and (d) (0, 1) vector. equations for the transformation of voltage and current vectors, such as and frames. As a result, the current control block becomes much more complicated. Moreover, in order to handle the complicated calculations in one sampling period, a high-speed digital processor is also necessary, which increases the manufacturing cost. Therefore, for the low cost BLDC motor applications, voltage vector pwm schemes cannot be regarded as a good solution for cost effective purpose. In this paper, we propose a novel pwm control technique based on the current controlled pwm method, instead of the voltage controlled pwm, which will be called “direct current controlled pwm.” The developed direct current controlled pwm method is not grounded on a bunch of equations, but on a keen and detailed observation of the overall operation, so that it dramatically reduces equations from the conventional control scheme and is simple to implement from the hardware and software points of view. Therefore, based on the direct current controlled pwm, the four-switch three-phase BLDC motor drive could be a good alternative to the conventional six-switch counterpart with respect to low cost and high performance. The theoretical operating principle of the four-switch converter for the three-phase BLDC motor drive and the proposed pwm control scheme are explained. Also, the feasibility of the four-switch converter is extended to the two-phase BLDC motor drive and the six-switch converter for power factor correction. The validity of the proposed system is verified by simulation results, compared with the conventional ones and a prototype 1HP BLDC motor test-bed with the high-speed DSP, TI TMS320C F243, is built to provide the experimental results. II. FOUR-SWITCH THREE-PHASE BLDC MOTOR DRIVE A. Investigation of the Four-Switch Converter for BLDC Motor Drives A BLDC motor needs quasisquare current waveforms, which are synchronized with the back-EMF to generate constant output torque and have 120 conduction and 60 nonconducting regions. Also, at every instant only two phases are conducting and the other phase is inactive. However, as mentioned earlier, in the four-switch converter, the generation of 120 conducting current profiles is inherently difficult. This can be explained as follows: In the four-switch configuration, there are four switching status as shown in Fig. 3, such as (0, 0), (0, 1), (1, 0), and (1, 1), in which the motor load is replaced by a resistive load and the switches are replaced by simple ideal switches. “0” means that
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