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On the Feasibility of Four-Switch Three-Phase
BLDC Motor Drives for Low Cost Commercial
Applications: Topology and Control
IEEE 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
Student : Chien-Hung,Chen
Professor : Ming-Shyan,Wang
Date : 24th-DEC-2010
Outline
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Abstract
Introduction
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
SIMULATION AND EXPERIMENTAL RESULTS
CONCLUSION
REFERENCES
Abstract
• The main purpose of this paper is to describe a low cost four-switch brushless dc
(BLDC) motor drive for commercial applications. For effective utilization of the
developed system, a novel direct current controlled pwm scheme is designed and
implemented 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
operational principle of the four-switch BLDC motor drive and the developed control
scheme are theoretically analyzed and the performance is demonstrated by both
simulation and experimental results.
Introduction 〔1/4〕
VARIABLE-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].
Introduction 〔2/4〕
• 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].
• 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 a three-phase BLDC motorIt results in the possibility of the fourswitch configuration instead of the six switches, as shown in Fig. 2.
Fig. 1. Conventional six-switch threephase BLDC motor drive systems
Fig. 2. Proposed four-switch converter
topology for three-phase BLDC motor.
Introduction 〔3/4〕
• 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 developed. The
solutions can be obtained from a modification of the conventional voltage controlled
pwm strategies, such as the space vector pwm. 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.
Introduction 〔4/4〕
• 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.
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔1/11〕
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 degrees conduction and 60
degrees 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 degrees conducting current profiles is inherently difficult. Thiscan be
explained as follows:
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔2/11〕
(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
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔3/11〕
• 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 thatthe lower switch is
turned on and “1” the upper switch is turned on. The two switches never turn on and off
simultaneously. In the case of the six-switch converter, switching status (0, 0) and (1, 1)
are regarded as zero-vectors, which cannot supply the dc-link voltage to the load, so that
current cannot flow through the load. However, in the four-switch converter, one phase of
the motor is always connected to the midpoint of the dc-link capacitors, so that current is
flowing even at the zero-vectors, as shown in Fig. 3(a) and (b). Moreover, in the 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 other two phases flow through this
phase. If the load is ideally symmetric, there is no current in the (0, 1) and (1, 0) vectors.
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔4/11〕
As a result of the operation using four switching vectors, one can depict the phase
voltage or current waveforms as shown in Fig. 4. From Fig. 4, it is noted that obtaining
the 120 conduction and a 60 nonconducting period current profile is inherently difficult
based on the “asymmetric voltage pwm.” It means that conventional pwm schemes for
the four-switch induction motor drives cannot be used directly for BLDC motor drives.
Therefore, in order to use the four-switch converter topology for the BLDC motor drive, a
new control scheme should be developed.
Fig. 4. Voltage and current waveforms of four-switch converter based on
four switching vectors
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔5/11〕
B. Operational Principle of Direct Current Controlled PWM
• From the motor point of view, even though the BLDC motor is supplied by the fourswitch converter, ideal back-EMF of three-phase BLDC motor and the desired current
profiles can be described as shown in Fig. 5. From the detailed investigation of the fourswitch configuration and back-EMF and current profiles, we could come up with a pwm
control strategy for the four-switch three-phase BLDC motor drives as follows:
Fig. 5. Back EMF and current profile in the four-switch converter forthree-phase BLDC motor drives.
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔6/11〕
• Under a balanced condition, the three-phase currents always satisfy the following
condition:
Then, (1) can be modified as
• In the case of the ac induction motor drive, at any instant there are always
three phase currents flowing through the load, such as
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔7/11〕
However, in the case of the BLDC motor drive, (3) is not valid anymore. Note that in Fig. 5
phase A and B currents are only controllable and phase C is uncontrollable. According
to the operating modes, one can derive the following current equations: Table I implies
that due to the characteristics of the BLDC motor, such as two-phase, only two phases
(four switches) needed to be controlled, not three phases. Therefore, based on Table I,
one can develop a switching sequence using four switches as follows:
TABLE I
DETAILED CURRENT EQUATIONS ACCORDING TO THE OPERATING MODES
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔8/11〕
As shown in Table II, the two-phase currents need to be directly controlled using the
hysteresis current control method by four switches. Hence, it is called the direct
current controlled pwm scheme. Based on the direct current controlled pwm,
implementation of the switching sequence and current flow are depicted in Fig. 6.
TABLE II
SWITCHING SEQUENCES OF THE FOUR-SWITCH CONVERTER
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔9/11〕
(a)
(c)
(b)
(e)
(f)
(d)
Fig. 6. Implementation of the direct current controlled pwm strategy. (a) Mode I (S ). (b) Mode II (S and S ). (c) Mode III (S ). (d) Mode IV
(S ). (e) Mode V (S and S ). (f) Mode VI (S ).
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔10/11〕
C. Current Regulation
• Based on the switching
sequences in Table II, the
current regulation is actually
performed by using hysteresis
current control. The purpose of
regulation is to shape
quasisquare waveform with
acceptable switching (ripple)
band. The detailed waveforms
and switching sequences are
described in Fig. 7.
Fig. 7. Current regulation and detailed switching sequences.
FOUR-SWITCH THREE-PHASE BLDC
MOTOR DRIVE 〔11/11〕
TABLE III
VOLTAGE AND CURRENT EQUATIONS
SIMULATION AND EXPERIMENTAL
RESULTS 〔1/6〕
• In order to verify the developed four-switch converter system, a computer
simulation has been performed along with the conventional six-switch
converter. As shown in Fig. 10(b), the 120 degrees conducting quasisquare
shaped current profiles are successfully obtained using the developed direct
current controlled pwm scheme, which are comparative with the ones
of conventional six-switch converter.
(a)
Fig. 10. Phase currents waveforms. (a) Six-switch converter. (b) Four-switch
converter.
(b)
SIMULATION AND EXPERIMENTAL
RESULTS 〔2/6〕
• A prototype drive was designed and developed in the laboratory at Texas A&M
University, as shown in the block diagram of Fig. 11. A 1HP Power Tec BLDC motor,
rated 160 V and 3000 RPM, is used and a permanent magnet dc machine, rated 1 HP,
90 V, 1725 RPM, is used as a constant torque load. The load is changed by varying the
value of the resistor
. The entire system is controlled by the Texas Instrument (TI)
TMS320F243 digital signal processor.
Fig. 11. Block diagram of experimental test bed.
SIMULATION AND EXPERIMENTAL
RESULTS 〔3/6〕
• First of all, the conventional six-switch converter was built and tested. Fig. 12 shows the
experimental phase current waveforms, using hysteresis current control with 160 V dclink voltage. The current command is set at 2 A and every 20 kHz (50 s) pwm period the
actual current is compared with the command value and determines which switches
should be turned on or off. As explained earlier, in the six-switch converter, the threephase BLDC motor is operated by quasisquare shaped current profile.
Fig. 12. Experimental phase current waveforms of conventional six-switch converter (from top to bottom:
Ia , Ib , Ic ; 5 A/div., 20 ms/div.).
SIMULATION AND EXPERIMENTAL
RESULTS 〔4/6〕
• The performance of the developed four-switch converter for the three-phase BLDC
motor with the direct current controlled pwm strategy is examined as follows: Fig. 13
shows the back-EMF problem of phase C, which was explained earlier in Section II-D.
From Fig. 13
Fig. 13. Back-EMF problem of silent phases (from top to bottom: Ia , Ib , Ic ; 2 A/div., 50 ms/div.).
SIMULATION AND EXPERIMENTAL
RESULTS 〔5/6〕
• In these cases, only positive current is sensed to generate pwm signal, so that the current
distortion appears on the negative uncontrolled current. With the developed direct current
pwm control strategy, considering back-EMF compensated solution, Fig. 14 shows the
characteristics of the four-switch converter. As phases A and B are controlled
independently, one can obtain the successful current profile as shown in Fig. 14(a). Even
though it contains more current ripplethan with the six-switch converter, it can be acceptable
and also can be reduced by controlling the hysteresis band size. As shown in Fig. 14(a), as
phases A and B are activated (modes II and IV), these phases are supplied by the full of dclink voltage of and , so that in the one pwm period (50 s), the current is increased more than
the other operating modes. Moreover, independent control of phases A and B results in
current ripple in phase C during the silent periods, which is the difference between the phase
A and phase B currents. The detailed switching signal waveforms can be observed from Fig.
14(b) and (c). As shown in Fig. 14(c), it is noted that the switching signals of and are not
identical. It means that the phase A and B currents are controlled independently to prevent
the effect of back-EMF of phase C during modes II and V.
SIMULATION AND EXPERIMENTAL
RESULTS 〔6/6〕
(a)
(b)
(c)
Fig. 14. Experimental voltage and current waveforms of the developed four-switch three-phase BLDC motor drives. (a) Phase current
profiles (50 ms/div., 2 A/div.). (b) Phase A and B currents with the gating signal of S1 and S2 (from top to bottom: Ia , Ib , S1 , S4 ; 10
V/div., 2 A/div., 20 ms/div.). (c) Expanded waveforms of (b) (10 V/div., 2 A/div., 5 ms/div.).
PERFORMANCE COMPARISON
〔1/3〕
• The overall operating modes of the four-switch BLDC drive are divided into six modes,
as show in Fig. 6. According to the voltage utilization, these modes are classified into two
groups: one is full dc-link voltage utilization (modes II and V) and other is half dc-link
voltage (modes I, III, IV, and VI). This irregular voltage utilization distinguishes the fourswitch converter from the six-switch one in terms of current dynamics, slow
, and
speed limitation: During the half dc-link voltage period, the motor phases are energized
by half value ( ) of the full dc-link voltage (2
), so that it produces the slower .
Therefore, in a pwm period, the rate of current incensement is less than the full dc-link
voltage period. This irregular current shape can cause torque ripple, but it can be
controllable by adjusting hysteresis band size and fundamentally do not affect any
changes in the operation of the BLDC motor drive, such as low speed and four quadrant
operations.
PERFORMANCE COMPARISON
〔2/3〕
• The other affect of the irregular voltage utilization is speed limitation. In case of the
conventional six-switch converter, all motor phases are excited by the full dc voltage.
However, in case of the four-switch converter, mainly only half dc voltage is utilized
through all operations. This voltage utilization makes the four-switch BLDC motor drive
have speed limitation. The relation between dc-link voltage, back-EMF, and speed can be
observed as
• As shown in (4), the operating speed range is determined by the back EMF and dc-link
voltage. In case of the four-switch converter, the dc-link voltage is half of the six-switch
one, so that a BLDC motor can be operated by the half speed.
PERFORMANCE COMPARISON
〔3/3〕
• The above-mentioned two problems, such as slow and speed limitation, are the
inherent characteristics and main drawbacks of the four-switch configuration. However,
those problems can be overcome in conjugation with voltage-doublers as
shown in Fig. 15. Using the half-bridge configuration of diode rectifier, one can obtain
double value of the dc voltage from the same ac source. Also if the front-end is replaced
with active power semiconductor switches, the dc-link voltage can be
controlled to the desired value. The half-bridge diode rectifier can be a low cost and
effective solution; otherwise the active voltage-doubler has additional advantage, such
as unity power factor correction.
CONCLUSION
• In this paper, the four-switch converter topology is studied to provide a possibility for the
realization of low cost and high performance three-phase BLDC motor drive system.
From the observation, one should note that the development of the proper pwm control
strategy should be accompanied with the reduced parts converter. As a solution, we
propose the direct current controlled pwm and examine the performance. With the
developed control scheme, it is expected that the proposed system can be widely used in
commercial applications with a reduced system cost.
REFERENCES
[1] H.W. Van Der Broeck and J. D. VanWyk, “A comparative investigation of a threephase induction machine drive with a component minimized voltage-fed inverter under
different control options,” IEEE Trans. Ind. Applicat., vol. 20, pp. 309–320, Mar./Apr. 1984.
[2] F. Blaabjerg, D. O. Neacsu, and J. K. Pedersen, “Adaptive SVM to compensate dc-link
voltage ripple for four-switch three-phase voltagesource inverter,” IEEE Trans. Power
Electron., vol. 14, pp. 743–752, July 1999.
[3] G. T. Kim and T. A. Lipo, “VSI-PWM rectifier/inverter system with a reduced switch
count,” IEEE Trans. Ind. Applicat., vol. 32, pp. 1331–1337, Nov./Dec. 1996.
[4] J. I. Itoh and K. Fujita, “Novel unity power factor circuits using zerovector control for
single-phase input systems,” IEEE Trans. Power Electron., vol. 15, pp. 36–43, Jan. 2000.
[5] P. Pillay and P. Freere, “Literature survey of permanent magnet ac motors and drives,”
in Proc. IEEE IAS Rec., 1989, pp. 74–84.
[6] J. R. Hendershot and T. J. E. Miller, Design of Brushless Permanent-Magnet Motors.
Oxford, UK: Oxford Science, 1994.
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