On Feasibility Of Four-switch Three-phase BLDC Motor Drives For Low Cost Commercial

Application: Topology And Control

IEE TRANSACTIONS ON POWER ELECTRONICS, VOL.18,NO. 1, JANUARY 2003

Byoung-Kuk Lee, member, IEEE, Tae-Hyung Kim, student Member, IEEE, and Mehrdad Ehsani, Fellow, IEEE

Teacher : Prof. Ming-Syhan Wang

Student : Nana SutarnaID Student: DA230208

Department of Electrical EngineeringSouthern Taiwan University of Science and Technology

Outline:AbstractI. Introduction

II. Four-Switch Three-phase Converter for BLDC motor Drive a. Investigation of the Four-Switch Converter for BLDC Motor Drives b. Operational Principle of Direct Current Controlled PWM c. Current Regulation d. Back EMF Compensated PWM Control Strategy

III. Simulation And Experimental Results

IV. Performance Comparison

V. Conclusion

Abstract:

To describe a low cost four-switch brushless dc (BLDC) motor drive for commercial applications.

Direct current controlled pwm scheme is designed and implemented to produce the desired dynamic and static speed-torque characteristics.

The feasibility of the four-switch converter is extended to two-phase BLDC motor drives and six-switch converter for power factor correction and speed control.

The operational principle of the four-switch BLDC motor drive and developed control scheme are theoretically analyzed and the performance is demonstrated by both simulation and experimental result.

Variable speed drives, employing a PWM voltage which used in consumerproducts and industrial applications has some advantages, but some researchers are becoming aware and exploring the possibility of their cost reduction. There are two approaches to take these issues: 1). The topology approach, how many minimum number of switches are required for the converter circuit.2). Control approach, how to design and implement the algorithm in junction with a reduced component converter to produce the desired speed- torque characteristics.Many different converter topologies have been developed andvarious pwm control strategies have been proposed to enhance theperformance of the system [1]-[4]. Thus, based on investigation, it is possibleto reduce part converter for BLDC motor drives with advanced control technique.The conventional six-switch converter is redundant to drive a three-phase BLDC motor, as shown in Fig.1. It results in the possibility of the four-switch configuration,as shown in Fig.2.

I. Introduction

Fig.1 Conventional 6 switch 3 phase BLDC motor drive system Fig.2 the four- switch converter topology for 3 phase BLDC motor drive system

The problem :1. The conventional pwm for the four-switch induction motor drive

cannot be directly used for BLDC motor drive because its limited voltage vectors which have generation 120o conducting current profile. This problem is well known as “asymmetric voltage pwm”.

2. How to build pwm control technique based on the currentcontrolled pwm method.

II. Four-Switch Three Phase BLDC Motor Drive

A. Investigation of four switch converter for BLDC motor drive

1. A BLDC motor needs squasisquare current waveforms.

2. In the four-switch configuration only two phases are conducting and the other phase is inactive. As shown in Fig.3, such as (0,0), (0,1), (1,0), and (1,1).

3. There are two switches never turn on and turn off simultaneously for switching status (0,0) and (1,1). It is called as zero vector.

4. One phase of the motor is always connected to the midpoint of the dc-link capacitor, so that current is flowing event at zero vector, as shown in Fig. 3(a) and (b).

Fig.3 voltage vector of 4-switch converter

5. In case of (0,1) and (1,0), the phase which is connected to the midpoint of dc-link capacitors is uncontrolled and only the resultant current of the two phases flow through this phase.

B. Operational principle of direct Current Controlled PWM

Ideal back-EMF of three-phase BLDC motorand the desired current profiles can bedescribed as shown Fig. 5. Under balancedcondition, three phase current sare:

Fig. 4 voltage and current waveform of four-switch converter based on four switching vectors.

6. Period current conduction for using four-switch converter are 120o conduction and a 60o non conducting, shown in Fig. 4. This period current profile is difficult based on the asymmetric voltage pwm.

or

Only phase A and B current are controllable and phase C is uncontrollable.

(1)

(2)

According to period mode (table I), That only two-phases (four-switches)neededto be controlled, not three-phase.

In table II, shown two-phase currents are directly controlled using hysteresis current control method by four switches..

The implementation of the switching sequence and current flow are depicted in Fig. 6

Direct current controlled PWM

Overall operating modes are 6, but these modes are classified into 2 groups: full dc-link voltage (mode II and V) and half dc-link voltage (modes I, III, IV and VI) .

Fig. 6 Implementation of direct current controlled pwm strategy.

Current regulation can be explained asfollow:

Mode II: Ia > 0; Ib < 0; and Ic = 0, as shown in

Fig. 6(b), switch S1 and S4 are used.

S1 and S4 turn on for supplying dc

link energy to increase current untilIa (Ib )reaches the upper limit. S1 and

S4 turn off to decrease the current

through the anti parallel diode D2

and D3 . At that time, the reverse

bias (negative DC-link voltage) isApplied to the phases, resulting indecreasing the current.

Mode III:Only current Ia can be controllable.

It means only switch S1 can be used

(Fig. 6(c)). When Ia increase, S1 is

turn on and the other case S1 is turn

off.

C. Current Regulation

The purpose of current regulation is to shapequasisquare waveform with ripple band. Detail of This current is described in Fig.7.

Fig. 7 Current regulation and detailed switching sequence

D. Back EMF compensated PWM Control StrategyBecause only phase A and phase B are conducting current (in mode II and mode V), sothat is expected that there is no current in phase C, but the back EMF of phase C cancause an additional and unexpected current which can make current distortion inphase A and phase B.

Fig. 8 simplified equivalent circuit of modes II and V. (a) ideal case. (b) actual case when the back EMF causes current in phase C

To make phase A and phase B as independent current sources and no more influence coming from back-EMF of phase C, so the direct current controlled pwm to phase A and phase B must be controlled independently as shown in Fig. 9

Fig. 9 PWM strategy for compensating the back EMF problem

In Fig. 10 is the block diagram of the experimental test bed, A 1 HP power Tec BLDC motor rated 160 V and 3000 RPM (as motor), and permanent magnet dc machine rated 1 HP, 90 V, 1725 RPM (as constant torque load)

III. Simulation and experimental Results

A 1 HP power Tec BLDC motor rated 160 V and 3000 RPM (as motor), and permanent magnet dcmachine rated 1 HP, 90 V, 1725 RPM (as constant torque load)

Fig. 10 Block diagram experimental

Fig. 11 phase current waveforms of conventional six-switch converter (Ia, Ib, Ic: 5A/div, 20 ms/div)

Fig. 12 performance of 4-switch converter for 3-phase BLDC motor. There is Back EMF problem of silent phase ( Ia, Ib, and Ic 2A/div, 50 ms/div)

After developing direct current pwm controlstrategy, Phase A and phase B are controlled independently.

Fig. 13 Experimental voltage and current waveforms of the developed four-switch three phase BLDC motor drives. (a) Phase current profiles (50ms/div., 2 A/div). (b) Phase A and B current with the gating signal of S1 and S4 (from top to bottom: Ia, Ib, S1 , S4 ; 10 V/div., 2A/div, 20 ms/div). (c) Expanded waveform s of (b) (10 V/div., 2 A/div., 5 ms/div.).

IV. Performance ComparisonThe distinguish four-switch converter than six-switch converter are :

-. current dynamics, -. Slow di/dt-. Speed limitation

During the half dc-link voltage period (1Vd), so it produces the slower di/dt also the rate of current is less then the full dc-link voltage period (2 Vd). This irregular current shape can cause torque ripple, but it can be controlled by adjusting hysteresis band size.

The four-switch BLDC motor drive have speed limitation because of half dc voltage

The relation between dc-link voltage, back EMF, and speed can be observed as:

(3)

The draw-backs of the four-switch converter configuration can be overcome with voltage-doublers as shown in Fig. 14.

Fig. 14 Front-end voltage doublers. (a) half bridge diode voltage-doublers, (b) active voltage doublers

Conclusion:1. The four-switch converter topology is a possibility for realization of low cost and high performance three phase BLDC motor drive system. 2. The development of proper pwm control strategy should be accompanied with the reduced part converter. 3. As a solution to use the direct current controlled pwm and examine the performance.4. In the future, this system can be widely used in commercial applications with a reduce system cost.

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