Download Optimal Commutation of a BLDC Motor by Utilizing the Symmetric

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
1
Professor : Ying-Shieh, Kung
Student ID : Yi-Chun,Chen
SN : M9920206
Date : 2011.03.23
Outline
 Abstract
 I. INTRODUCTION
 II. DETECTION OF COMMUTATION POSITION OF A
BLDC MOTOR
 III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION
 IV. CONCLUSION
2
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(1/2)
 A BRUSHLESS DC (BLDC) motor is used in various applications of
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.
4
I. INTRODUCTION(2/2)
 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.
5
II. DETECTION OF COMMUTATION POSITION OF A
BLDC MOTOR(1/6)
 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 zerocrossing 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 to PWM switching.
6
II. DETECTION OF COMMUTATION POSITION OF A
BLDC MOTOR(2/6)
 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.
 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(3/6)
 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.
 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(4/6)
 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(5/6)
 The slope of the filtered terminal voltage of the nonenergized phase is linear
near the zero-crossing position so that and VBF,VEF 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(6/6)
 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(1/5)
 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.
 A PWM signal is digitally generated by using timers. A low-pass filter is
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.
15
16
III. SYSTEM IMPLEMENTATIONS AND
EXPERIMENTAL VERIFICATION(2/5)
 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(3/5)
 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(4/5)
 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(5/5)
 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
Thanks for your attention!
25