2013 Third International Conference on Instrumentation, Measurement, Computer, Communication and Control
A Kind of Simplified Structure Direct Torque Control
Method for Brushless DC Motor
Pan Haipeng
Institute of Automation
Zhejiang Sci-Tech University
Hangzhou, China
e-mail: pan@zstu.edu.cn
Gu Minming
Institute of Automation
Zhejiang Sci-Tech University
Hangzhou, China
e-mail: guminming@163.com
Gu Junjie
Institute of Automation
Zhejiang Sci-Tech University
Hangzhou, China
e-mail: gujunjie.248@163.com
Abstract—A simplified direct torque control method is
introduced in order to solve difficult situations in flux linkage
observation and control of the direct torque control of brushless
DC motor. This simplified direct torque control adopts a
simplified structure without flux linkage observation and control.
To calculate torque of motor in back electromotive force shape
function method is simple and effective, which will help
realization of the project. The system is simulated with
MATLAB/SIMULINK. Results of the experiment show that this
simplified method provides better limit to the torque ripple of
brushless DC motor comparing with traditional direct torque
control method and dynamic response becomes faster.
Motor’s after-flow time and turn-off time, puts forward HSVDSC theory for Brushless DC Motor. Since the above
mentioned direct torque control methods for Brushless DC
Motor contain flux linkage and the amplitude of stator flux
linkage changes with its spatial positions change all the time,
it’s extremely hard to detect and control. In order to solve this
issue, the paper puts forward a structure-simplified direct
torque control method with non-flux linkage and simplified
control procedure for Brushless DC Motor to directly control
torque instantly, to effectively restrain torque pulsation and to
provide rapid dynamic response by torque controller only.
Keywords-brushless DC motor; direct torque control; torque
ripple;
II.
Brushless DC Motor normally adopts each two phases
connecting method, in which, during the after-flow time, in
addition to all three phases are connected, each two phase will
be connected as well. During normal operation, the upper
bridge pipe of a phase is connected to the lower bridge pipe of
the other phase, while the upper bridge pipe and lower bridge
pipe of the third phase are disconnected. Therefore are 2
switchers to control connection. There are six connected stats
for six different non-zero voltage spatial vector: V1 (100001),
I.
INTRODUCTION
The current control strategies for brushless DC motor are
mainly to indirectly control torque by controlling the current,
and it is known as the open-loop control with the slow torque
response and remarkable torque pulsation. The Direct Torque
Control (DTC) is a kind of closed loop control for torque. By
take the instant torque of motor as control object and pulsation
for measurable disturbance, it realizes instant direct torque
control through torque controller on the basis of torque
tolerance. Therefore, it’s highly dynamic in torque control.
V2 (001001), V3 (011000), V4 (010010), V5 (000110) and
V6 (100100), as well as a disconnected state corresponding to a
1 zero voltage vector, namely the V0 ˄ 000000 ˅ . The 1
The widely adoption of DTC in Asynchronous Motor[1]
and Permanent Magnet Synchronous Motor[2] control has
proved its excellent torque control with the advantages of rapid
torque control and effective torque pulsation restriction. In
recent years, some scholars have introduced the DTC into
control system for Brushless DC Motor[3-4] for the sake of
gaining excellent torque feature. Literature[5-6] optimize
switch table by classifying vector space intensively. Literature
[7] designs a torque adjuster based on fuzzy logic and it can
fuzzily choose voltage vector in accordance with flux linkage
tolerance, torque tolerance and flux linkage’s spatial angle.
Literature[8], by disregarding the ratio between Brushless DC
978-0-7695-5122-7/13 $26.00 © 2013 IEEE
DOI 10.1109/IMCCC.2013.330
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standards for connect while 0 standard for disconnect. For
example, voltage’s spatial vector 100001 stands for that VT1
and VT6 are connected while other switchers are off, indicating
that Phase A and Phase C is connected while Phase B is not
connected and current will be input from Phase a and output
from Phase C.
The calculating formula for spatial voltage vector is as
below:
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u
2
ua ube j 2 /3 uc e j 4 /3 (1)
3
Ea Assuming that Phase A and Phase C is connected, after
idealizing phase voltage, we can get:
0
1
x / 2 S
1
S
1
1
ua u d , uc ud 2
2
(2)
ub 0
(6)
Te K Ea ia Eb ib Ec ic (8)
(4)
In the equation K 60k . This torque function only
2
contains current and back electromotive force shape function
rather than rotation speed. In case that Phase A and Phase B is
connected while Phase C is disconnected, Ea is 1 and Eb is -1
while Ec is 0. The torque can be further simplified to be:
7+(&$/&8/$7,212)(/(&7520$*1(7,&72548(
Te K ia ib 2 K ia (9)
Literature[9-10] put forward the back electromotive force
shape function to simplify the calculation of electromagnetic
torque. The back electromotive force shape function adopts
piecewise function to established the trapezoid wave shape
functions Ea Eb DQG Ec with 120˚ mutual difference,
120˚ top width and constant 1 amplitude. Therefore, in the real
time calculation of back electromotive force, except that
rotating speed and position of rotator are variables, other
parameters of motor are constant and can be detected offline.
This method has eliminated the calculus term in the function
and simplified the relationship between torque and current to
be linear relationship. It’s simple, effective and practical.
IV.
THE SIMPLIFIED DIRECT TORQUE CONTROL METHOD
The ordinary direct torque control gets appropriate spatial
voltage vector by checking switcher table on the basis of
electromagnetic torque tolerance, stator flux linkage tolerance
and section of stator to reduce electromagnetic torque pulsation.
In order to solve the linkage detection and control detection of
direct torque control of Brushless DC Motor, this paper adopts
non-flux linkage and control procedure to simplify structure for
the sake of realizing excellent direct torque control for
Brushless DC Motor.
The gradient of trapezoid is
S x / 2 3 x / 2
3 x / 2 3 x / 2
3 x / 2 2
On the basis of above equation, we can kown:
Similarly, we can get vectors for other five non-zero
voltages as indicated in Formula 4.
III.
ea k Ea n
eb k Eb n (7)
ec k Ec n
1
ud e j / 6 (3)
3
1
1
V1 (100001) ud e j /6 , V2 (001001) ud e j / 2
3
3
1
1
V3 (011000) ud e j 5 /6 , V4 (010010) ud e j 7 /6
3
3
1
1
V5 (000110) ud e j 3 / 2 , V6 (100100) ud e j11 /6
3
3
x / 2
Since the position of rotator is determined by the shape, the
amplitude of EMF is positively related to the products of speed
and shape equation.
According to Formula 1, we can get voltage spatial vector
as below:
V1 (100001) x / 2
S
2
x
The shape function Ea of Phase A is as below formula.
Similarly, we can get Phase b back electromotive force wave
function Eb and Phase C back electromotive force wave
function Ec :
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The Brushless DC Motor speed / torque double closed-loop
control established in this paper is indicated in Figure 1. The
operational process of direct torque control for Brushless DC
Motor: the speed controller give amplitude limitation to
determine torque value; the given torque value reduces actual
torque to get torque tolerance; input torque tolerance to torque
controller; the output control signal, by taking rotator’s position
information into consideration, selects a voltage vector from
switcher table to directly control inverter’s state.
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the dynamic response to torque in 0.6 when load disturbance
appears is only 0.01s
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Figure 1. The Simplified Direct Torque Control for Brushless
DC Motor
In this control system, the difference between given torque
value Tr and actual torque value T f is compared in hysteresis
loop. In case that the given value is larger than actual value, the
torque controller will output =1 for selecting non-zero
voltage spatial vector to increase torque. In case that given
torque is smaller than actual value, the torque controller will
output =0 for selecting zero voltage spatial vector to reduce
torque. Finally, the system will select voltage spatial vector
through torque output value from torque controller and
rotator’s position Section S. Therefore, the switcher table is as
indicated in Table ĉ.
Figure 2. The Speed and Torque Simulation for Ordinary
Direct Torque Control
TABLE ĉ The Switcher Table of Direct Torque Control in
Simplified Brushless DC Motor
Section
6
6
6
6
6
6
9
9
9
9
9
9
9
9
9
9
9
9
V.
SIMULATION AND EXPERIMENT RESULT
Establish simulation model in Matlab and Simulink as
indicated in Figure. 3. The model mainly consists of Brushless
DC Motor, speed controller, torque controller, 3-phase
inverter and current detection. The main parameters: rated
voltage 220V, resistance R 0.2 , inductance L 8.5 mH ,
moment of inertia J 0.089kg m2 , pole-pairs number p 2,
speed reference
excitation
flux f 0.175 Wb ,
Figure 3. The Speed and Torque Simulation for Simplified
Direct Torque Control
n 2000r / min , loaded torque starts from TL 8 N m and
changes to TL 12 N m in 0.6s.
VI.
CONCLUSION
Due to the advantage of high efficiency and excellent
power density, the Brushless DC Motor is widely used in the
industrial field. However, significant torque pulsation caused
by its working principles and manufacturing artwork has
limited its application in the occasions with high accuracy and
high stability requirements. This paper puts forward simplified
direct torque control method to omit rotator flux closed loop
control for the sake of simplifying the control system. In
Figure 2 indicates the speed and torque response speed of
ordinary direct torque control for Brushless DC Motor and
Figure 3 indicate the speed and torque response speed of
simplified direct torque control for Brushless DC Motor.
Judged from comparison between Figure 2 and Figure 3, the
simplified direct torque control has better speed dynamic
response with less torque pulsation (less than 0. .2N·m), while
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addition, this paper, by adopting simulation experiment, proves
the correctness and practicality of this method and proves that
this method can bring rapid torque response, reduce torque
pulsation of Brushless DC Motor effectively and improve
motor’s operation performance. Therefore, it’s of high
application value.
ACKNOWLEDGMENT
The authors are grateful to the financial support from the
Natural Science Foundation of Zhejiang, China (No.
Y1110686).
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