IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
Optimal Commutation of a BLDC Motor
by Utilizing the Symmetric Terminal
Voltage
G. H. Jang and M. G. Kim
Precision Rotating Electromechanical Machine Laboratory (PREM), Department of Precision
Mechanical Engineering,Hanyang University, Seoul 133-791, Korea
Professor : Ming-Shyan,Wang
1
Student ID : Yi – Chun,Chen
SN : M9920206
Date : 24th Dec.2010
Outline
 Abstract
 I. INTRODUCTION
 II. DETECTION OF COMMUTATION POSITION OF A




2
BLDC MOTOR
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
IV. CONCLUSION
ACKNOWLEDGMENT
REFERENCES
Abstract
 This paper presents a method to determine the improved commutation
position of a brushless DC (BLDC) motor in such a way as to generate the
symmetric terminal voltages of the nonenergized phase at the beginning and
end of the commutation period.
 If the BLDC motor is energized at the exact commutation position, the
terminal voltage of the nonenergized phase at the beginning of commutation
is equal to the terminal voltage at the end of commutation, and the
waveform of the terminal voltage should be symmetric.
 This paper also develops a DSP-based sensorless BLDC motor controller to
implement the proposed method and to verify its effectiveness
experimentally.
 This research shows that the proposed method improves the performance of
3
a BLDC motor by increasing torque and efficiency.
I. INTRODUCTION
 ABRUSHLESS DC (BLDC) motor is used in various applicationsof
electromechanical systems because of its high efficiency and good
controllability over a wide range of speed.
 The position information of a rotor is identified by using the sensors or the
sensorless algorithms in order to energize the correct phase of windings at
the exact commutation position.
 One of the popular sensorless algorithms utilizes the zero-crossing of the
back-emf voltage of nonenergized phase [1], [2].
 However, this method may involve the detection error of a rotor position,
and results in the reduction of maximum torque and efficiency of the BLDC
motor.
4
I. INTRODUCTION
 Some researchers have proposed several methods to detect the exact rotor
position for optimal commutation. Chen and Liaw have proposed an
intelligent commutation tuning method by minimizing winding current [3].
 Song and Choy presented a new rotor position estimation method based on
neutral voltage compensation [4]. Their methods have disadvantages to
monitor both current and terminal voltage, and the latter did not present the
experimental verification.
 This paper presents a method to detect the improved commutation position
of a BLDC motor by utilizing the symmetric terminal voltages of the
nonenergized phase at the beginning and end of the commutation period. It
also develops a digital signal processor (DSP)-based sensorless BLDC
motor controller to implement the proposed method and to verify its
effectiveness experimentally.
5
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 When a BLDC motor is running, back-emf is induced in each phase
and the position information of a rotor can be identified by monitoring
the zero-crossing of the back-emf voltage where the voltage of neutral
point is equal to the terminal voltage of nonenergized phase.
 However, it may not be easy to identify the zero-crossing position
because high frequency components due to pulse width modulation
(PWM) switching are involved in the terminal voltage.
 This method generally uses the low-pass filter to remove high-
frequency components of the terminal voltage due toPWMswitching.
6
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 However, it results in the detection error of rotor position, i.e., the phase
delay between the exact and the detected position information of a rotor,
which is mainly affected by rotor speed, PWM frequency, and
characteristics of the low-pass filter.
 Fig. 1 shows the back-emf waveform and the terminal voltage due to the
change of commutation position. EB , EE, VB, and VE are the back-emf and
the terminal voltage of nonenergized phase at the beginning and end of the
commutation period, respectively.
 Subscripts of B and E denote the beginning and end of the commutation
period. Theoretically, the exact commutation should start at the shift angle
of 30 electrical degrees after the rotor passes the exact zero-crossing
position.
7
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 If the BLDC motor is energized at the exact commutation position, EB
and VB should be equal to EE and VE due to the trapezoidal shape of the
back-emf voltage. The waveform of the terminal voltage should be
symmetric as well.
 On the other hand, if there is a phase delay due to the low-pass filter,
the actual commutation will start at the shift angle of 30 electrical
degrees after the rotor passes the detected zero-crossing position. In this
case, as shown in Fig. 1, EB, EE, VB, and VE change to EB’, EE’, VB’, and
VE’, respectively.
 They result in the asymmetric waveform of the terminal voltage of the
nonenergized phase, which is dominantly affected by the commutation
position.
8
9
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 This paper proposes a method to detect the improved commutation position
of a BLDC motor in such a way as to generate the symmetric terminal
voltages of the onenergized phase at the beginning and end of the
commutation period.
 Fig. 2 shows the asymmetric waveform of the filtered terminal voltage when
a BLDC motor is energized at the incorrect commutation position. VBF and
VEF are the filtered terminal voltages of the nonenergized phase at the
beginning and end of the commutation period, respectively.
 Measured values of VBF and VEF are not symmetric not only because the
low-pass filter distorts the actual shape of the terminal voltage when it
changes abruptly, but also because there exists the freewheeling current in
the switching of the commutation circuit.
10
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 The slope of the filtered terminal voltage of the nonenergized phase is
linear near the zero-crossing position so that and , without taking the
effect of the low-pass filter and the freewheeling current into account,
can be estimated with the following equations:
 where T, Z, and A are the shift angle, magnitude, and slope of the
filtered terminal voltage at the zero-crossing position, respectively.
11
12
II. DETECTION OF COMMUTATION
POSITION OF A BLDC MOTOR
 Fig. 3 shows the algorithm of the proposed method. First, T, Z, and A
are measured and calculated before and after the commutation period,
respectively, and they are averaged during N electrical rotations in
order to minimize the measurement error. Then, VBF and VEF are
calculated by (1).
 If VBF is smaller than VEF, commutation has begun earlier than the
exact commutation position. Then the shift angle increases in such a
way as to increase VBF and decrease VEF so that the commutation
position moves back.
 In the opposite case, the shift angle decreases so that the commutation
position moves forward. This procedure repeats until VBF is equal to
VEF and the terminal voltage is symmetric in the improved commutation
of a BLDC motor.
13
14
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 Fig. 4 shows a DSP-based sensorless BLDC motor controller to implement
the proposed method and to verify its effectiveness [5]. The position
information of a rotor is detected by monitoring the back-emf voltage, and a
DSP controls the switching of the inverter circuit and the speed of the motor
with the proportional - integral (PI) control.
 The DSP is TMS320LF2407A by Texas Instruments. Its computing speed is
25 ns/instruction, and it has a 32-bit accumulator, three timers, and two 10bit 375-ns analog-to-digital (A/D) converters.
 A PWM signal is digitally generated by using timers. A low-pass filter is
15
used in order to remove high-frequency components of the terminal voltage
due to PWM switching. Speed, back-emf, and terminal voltage are directly
monitored in the computer through communication circuits and the user
interface program.
16
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 The proposed method is verified for a BLDC motor used in a computer
hard disk drive. It has 8 poles and 12 slots, threephases, Y-winding, and
the rated operating speed of 7200 r/min with a 3.5-in disk by a bipolar
drive.
 It is driven by the supplied voltage of 12 V. Fig. 5 shows the measured
filtered terminal and neutral voltages at the speed of 7200 r/min. The
DSP determines the zero-crossing position by comparing the voltage of
the neutral point with the terminal voltage of the nonenergized phase.
 The voltage at the zero-crossing position, ZB, is also measured by the
DSP. The slope of the terminal voltage, , is calculated by measuring the
terminal voltages four times every PWM period (12.8 s) from the zero
crossing position.
17
18
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 The shift angle, TB, is determined by using the timer of the DSP. Then
the DSP estimates the filtered terminal voltage of the nonenergized
phase at the beginning of the commutation period, VBF, by using (1).
 The procedure is repeated to estimate the terminal voltage of the
nonenergized phase at the end of the commutation period, VEF, and the
improved commutation is determined by using the algorithm in Fig. 3.
 For the improved commutation of the given motor, the shift angles from
the detected zero-crossing position (which is adjusted in the developed
controller automatically) are 22 electrical degrees at 7200 r/min and 19
electrical degrees at 9200 r/min.
19
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 Fig. 6 shows the variation of terminal voltage and phase current by using
the conventional method and the proposed method at 7200 r/min,
respectively.
 The terminal voltages of the nonenergized phase at the beginning and end of
the commutation period are determined to be 11.67 V and 7.94 V in the
conventional method, and 10.68 V and 10.32 V in the proposed method,
respectively.
 This shows that the terminal voltage of the proposed method is much more
symmetric than that of the conventional method. The current ripples (peakto-peak value of phase current) of the proposed method and the
conventional method are 0.23 A and 0.33 A, respectively, and the former is
smaller than the latter by 30.3%.
20
21
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 The reduction of current ripple may contribute to the decrease of the ripple
of torque and speed, and consequently vibration and noise.
 Fig. 7 shows the variation of input power due to the shift angle. Because the
speed of the rotor is controlled constantly under the constant load, the
output power is assumed to be constant regardless of shift angle.
 By applying the proposed method, the input power is reduced from 2.68 W
to 2.59 W (3.4%) at 7200 r/min and from 4.38 W to 4.17 W (4.8%) at 9200
r/min.
 This shows that the proposed method increases the output torque by
22
selecting the improved commutation position so that it results in the
reduction of input power and the increase of efficiency consequently.
23
IV. CONCLUSION
 This paper has presented a method to detect the improved commutation
position of a BLDC motor by utilizing the symmetric terminal voltages
of the nonenergized phase at the beginning and end of the commutation
period.
 It also develops a DSP-based sensorless BLDC motor controller to
implement the proposed method and to verify its effectiveness
experimentally.
 The proposed method can be effectively applied to improve the
performance of a BLDC motor.
24
ACKNOWLEDGMENT
 This work was supported by a research fund of Hanyang University
(HY-2005-I).
25
REFERENCES
26

[1] K. Iizuka, H. Uzuhashi, M. Kano, T. Endo, and K. Mohri, “Microcomputer control for
sensorless brushless motor,” IEEE Trans. Ind. Appl., vol. IA-21, no. 4, pp. 595–601, May/Jun.
1985.

[2] S. Ogasawara and H. Akagi, “An approach to position sensorless drive for brushless DC
motors,” IEEE Trans. Ind. Appl., vol. 27, no. 5, pp. 928–933, Sep. 1991.

[3] H. C. Chen and C. M. Liaw, “Current-mode control for sensorless BDCM drive with
intelligent commutation tuning,” IEEE Trans. Power Electron., vol. 17, no. 5, pp. 747–756,
Sep. 2002.

[4] J. H. Song and I. Choy, “A rotor position sensorless control based on neutral voltage
compensation,” Proc. 35th IEEE Power Electronics Specialist Conf., vol. 2, pp. 1431–1437,
Jun. 2004.

[5] G. H. Jang, J. H. Park, and J. H. Chang, “Position detection and start-up algorithm of a
rotor in a sensorless BLDC motor utilizing inductance variation,” Proc. Inst. Elect. Eng.,
Electric Power Applicat., vol. 149, no. 2, pp. 137–142, Mar. 2002.
Thanks for your attention!
27