Energy and Power En gineering, 2010, 2, 208-211
doi:10.4236/epe.2010.23030 Published Online August 2010 (http://www.SciRP.org/journal/epe)
The Design of New Sensorless BLDCM Control System for
Electric Vehicle
Zhibin Ren, Xiping Liu
School of Mechanical and Electrical Engineering, Jiangxi University of Science and Technology, Ganzhou, China
Received March 11, 2010; revised April 29, 2010; accepted June 3, 2010
Abstract
In order to meet the requirement of reliably running which is by Electric Vehicle for motor controller, the
paper is focused on a sensorless brushless DC motor controller design and a commutation point method. By
utilizing the saturation effect of stator iron core, six short voltage pulses are employed to estimate the initial
rotor position. After that a series of voltage pulses are used to accelerate the motor. When the motor reaches
a certain speed at which the back-electromotive force (EMF) method can be applied, the running state of the
motor is smoothly switched at the moment determined by the relationship between the terminal voltage
waveform and the commutation phases. “Lagging 90˚-α commutation” is bring forward to overcome the
shortages existing in the traditional method. The experimental results verify the feasibility and validity of the
proposed method.
Keywords: Brushless DC Motor, Electric Vehicle, Ensorless Control, Commutation Point
1. Introduction
Due to the advantages of high power density, robust stru-
cture, and ease of control, the brushless DC motor (BL-
DCM) has played an important role in many applications.
It is necessary for high-performance applications, such as
servos in machine tools and robotics, to use position sen-
sors for successful starting and operation. The issues on
reducing cost, low-performance applications, space-res-
tricted applications, and reliability of the position sensors
have motivated research on sensorless control [1-6]. In
addition, the fast and continuing improvements of pow-
erful and economical microprocessors and digital signal
processors (DSPs) have speeded up the development of
sensorless control technology. In fact, [7] enumerates
many applications of the BLDCM, for example, air-con-
ditioning compressor, engine cooling fan, fuel/water pump,
electric vehicle.
The sensorless control technology of the brushless DC
motor (BLDCM) based on the back-electromotive force
(EMF) detection method has been widely used in the
industrial and commercial fields. As we know, the mag-
nitude of the back-EMF is proportional to the motor
speed, so the back-EMF detection method cannot be ap-
plied properly when the motor is at standstill. In order to
solve this problem, many methods have been developed.
One of them, often referred to as a 3-step startup method,
is used to align the rotor first in a predetermined direc-
tion, and then accelerate the motor in an open-loop
scheme before the back-EMF method is applied. This
startup method is easy to implement but it tends to be
affected by the load and may temporarily cause reverse
rotation which is not allowed in some applications.
In this paper, the short pulse sensing method, which is
based on the saturation effect of the stator iron and will
not cause any reverse rotation or vibration during the
startup process. The key hardware implementation is the
current sensor detected by Ri and the resistance network
used as the voltage divider. The terminal voltage which
reflects the back-EMF information is sampled by the
A/D converter integrated in the micro-controller.
2. Initial Rotor Position Estimation
The phase inductance of the stator is determined by
L =Φ / I (1)
where I is the phase current and Φ is the flux due to
magnet rotor and stator coils and core. Figure 1 shows
the inductance of stator windings with nonlinear mag-
netization characteristics of the stator core, depending
upon the position of rotor. As the pole of magnet rotor is

Z. B. REN ET AL.
209
close to the stator winding, the ratio of the change of the
current in the stator winding flowing in the magnetizing
direction is larger than that in the opposite direction because
of the magnetic saturation of the stator core. So the value of
the current would be different according to the rotor posi-
tion if a constant voltage vector from inverter is applied to
the stator winding of the motor for a constant time period.
The estimation of the rotor position is based on strongly
magnetized stator field. Three situations of a magnetic pole
of permanent magnet of the rotor close to the stator core are
considered as shown in Figure 2. A smaller phase induc-
tance L(sat) is defined when the stat- or field is in phase
with rotor field, shown in Figure 2(a). Similarly, a larger
phase inductance L(linear) is defined when the stator field is
out of phase with rotor field, shown in Figure 2(b). Figure
2(c) depicts the case of middle value L(mid).
3. Initial Rotor Position Detection
Based on the operation principle mentioned above, six
voltage pulses are injected into the phase windings and
the peaks of the response current are compared with each
other to determine the rotor position.
As shown in Figure 3(a), the high-side power device
VT1 and the low-side power device VT6 are activated
first, which can be denoted as A+ B–. The resultant mag-
netic field is represented by arc line. Then the high-side
power device VT3 and the low-side powerdeviceVT2 are
activated, and are denoted as B+ C–, and arc line Figure
3(b) represents the resultant magnetic field. If the north
pole of the rotor is in the same direction as that of the
resultant magnetic field arc line and in the opposite dir-
ection from that of arc line, the peak of the response cur-
rent is greater when the north pole of the rotor is in the
i
F
L1
L2
L3
Figure 1. The inductance of stator windings with nonlinear
magnetization characteristics of the stator core.
s
V
i
s
N
N
V
i
s
N
s
V
i
s
N
N
(a) (b) (c)
Figure 2. Magnetic fields (a) saturated magnetic field, (b)
linear (non-saturated) magnetic field and (c) middle case.
A
B
C
A
B
C
A
B
C
(a) (b) (c)
Figure 3. The schematic diagram of initial rotor position
detetion. (a) Energized stator winding and rotor position
within180°; (b) Rotor position within 60°.
same direction as that of the resultant magnetic field arc
line and in the opposite direction from that of arc line,
the peak of the response current is greater when the north
pole of the rotor is in the same direction as that of the
resultant magnetic field arc line. Thus the north pole of
the rotor can be narrowed down to 60°, as shown in Fig-
ure 3(c).
When the initial rotor position is identified, the motor
is accelerated to a certain speed. Generally, a self-con-
trolled BLDCM with trapezoidal BEMF waveforms is
driven by a three-phase inverter with six-step commuta-
tion. Each conducting phase is called one step of two
phase conducting. The conducting interval for each
phase is 120° by electrical angle. Therefore, only two
phases conduct current at any time, leaving the third
phase floating. In order to produce maximum torque, the
inverter should be commutated every 60°, and the com-
mutations occur at 30° delay from the corresponding
zero-crossing points (ZCP) of the BEMF waveforms.
4. Lagging 90˚-α Commutation Method
According to intensive analysis of the zero-crossing detec-
tion of Back EMF, a detecting method with band-bass
filter is proposed. By analyzing the method in the detect-
ing Back EMF, zero point of Back EMF is lagged, see
Figure 4(a), so the corresponding correction method and
the new commutation approach are presented to improve
the performance of the BLDCM, see Figure 4(b). When
detecting the zero-crossing of Back EMF, the commuta-
tion approach is lagged for 90˚-α, see Table 1. The posi-
tion detection can be achieved over a wide speed range.
5. Experimental Results
The specifications of the test BLDCM are: 8 poles, 400
W. According to Figure 3 for the initial position detec-
tion and based on Table 1, Figure 5 display the response
of velocity waveform. To verify the proposed method,
we conducted some experiments. In experiment, the cur-
rents are sampled to verify the rotor position, the key
hard ware of the current sensor and powerful micropro-

Z. B. REN ET AL.
210
A phase
B phase
C phase
(a)
A phase
B phase
C
phase
(b)
Figure 4. Lagging 90˚-α commutation method.
Table 1. The commutation approach is lagged for 90˚-α.
zero-crossing
point
Delay
angle
Commute
point
Commute
switch
P1 90˚-α n1 V1 to V3
P2 90˚-α n2 V2 to V4
P3 90˚-α n3 V3 to V5
P4 90˚-α n4 V4 to V6
P5 90˚-α n5 V5 to V1
P6 90˚-α n6 V6 to V2
cessors of digital signal processors (DSPs) should be used
Traditional method of 3-step startup is shown in Figures
5(a, b) and new method is shown in Figure 5(c). In the
experiments, the results show that the use of the methods
makes the drive better, with better follow performance.
6. Conclusions
In this paper, new startup and smooth switching method
of a sensorless brushless DC motor is presented. By us-

Z. B. REN ET AL.
211
Tek View
(a)
Tek View
(b)
Tek View
(c)
Figure 5. Speed response curve.
ing this method, the rotor position at standstill can be es-
timated with a resolution of 60° and the motor is acceler-
ated to a certain speed at which the back-EMF detection
method can be applied. The method will not cause any
reverse rotation or vibration during the startup process.
The hardware implementation of the driving circuit is
simple. It is very suitable to use in the low-cost applica-
tions.
7. References
[1] R. Krishnan and R. Ghosh, “Staring Algorithm and Per-
formance of A PMDC Brushless Motor Drive System
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[3] T. Kim and M. Ehsani, “Sensorless Control of the BLDC
Motors from Near-Zero to High Speed,” IEEE Transac-
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1645.
[4] J. Holtz, “State of the Art of Controlled AC Drive with-
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[5] T. H. Kim, H. W. Lee, and M. Ehsani, “State of the Art
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[6] P. P. Acarnley and J. F. Watson, “Review of Position-
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