ADAPTIVE BANDWIDTH APPROACH ON DTC CONTROLLED INDUCTION MOTOR

International Journal of Instrumentation and Control Systems (IJICS) Vol.5, No.2/3, July 2015
DOI : 10.5121/ijics.2015.5301 1
ADAPTIVE BANDWIDTH APPROACH ON
DTC CONTROLLED INDUCTION MOTOR
Yılmaz Korkmaz1
,Fatih Korkmaz2
,İsmail Topaloğlu2
and Hayati Mamur2
1
Faculty of Technology, Department of Electric and Electronic Engineering, Gazi
University, Ankara, Turkey
2
Department of Electric and Electronic Engineering, Çankırı Karatekin University,
Çankırı, Turkey
ABSTRACT
Induction motors are most commonly used motor type in industrial applications because of its well-known
advantages like robust structure, cheaper prices etc. Today, field oriented control (FOC) and direct torque
control (DTC) methods, also called vector control, are most famous control methods in high-performance
applications. The main structural and behavioural differences between the both methods can be
summarized as: the FOC has parameter dependence while the DTC has high torque ripples. In this study, a
new adaptive bandwidth approach was presented to reduce torque ripples in DTC controlled induction
motor drives. With the proposed method, instead of fixed bandwidth, adaptive bandwidth approach was
investigated in hysteresis controllers on the DTC method. Both the conventional DTC(C-DTC) method and
adaptive bandwidth DTC (AB-DTC) for induction motor were simulated in MATLAB/SIMULINK and the
results were presented and discussed to verify the proposed control. The comparisons shown that, torque
ripples were reduced remarkably with the proposed AB-DTC method.
KEYWORDS
Direct torque control, Adaptive hystresis controller, Induction motor control, Vector control
1. INTRODUCTION
Three-phase induction motors (IMs) are the most common motors used in industrial control
systems. This is because they have simple and rugged design, also low-cost, and low maintenance
beside direct connection to an AC power sources of IMs[1].
About 4-5 decades ago, IMs were used limited constant speed applications because of speed
adjustment on IMs were not only hard to realize but also need high costs. It means, DC motors
were the optimum option for the variable speed applications. But today, depending on
developments in power electronics and semiconductor technology, IMs also be controlled like the
DC motor through vector control method.
The vector control methods can basically be grouped under two headings: FOC and DTC. The
FOC was first introduced by Blaschke [2] in the 1970's. It was unrivalled in industrial induction
motor drivers until DTC was introduced by Takahashi [3] in the middle of the 1980's. It was a
good alternative to FOC due to some well-known advantages, such as simple control structure, no
need much motor parameters so independency of parameter changes, fast dynamic response.
Besides these advantages, DTC scheme still had some disadvantages like high torque and current
ripples, variable switching frequency behaviour and implementation difficulties owing to
necessity of low sampling time [4].

2
The DTC was originally developed for induction motors, but it has also been applied other motor
types like PMSM and BLDC [5-6].
In literature, many kind of studies can be found about improve performance of the DTC. In these
studies, researchers proposed different ways. Some studies suggest using different switching
techniques and inverter topologies [7-8], in another group of researchers, different observer
models have been suggested [9-10]. On the other hand, intelligent control methods like fuzzy
logic or neural networks have been explored by several researchers for its potential to improve the
speed regulation of the drive system. [11-12]
In this paper, a new hysteresis controller structure has presented to improve the dynamic torque
performance of the DTC controlled IM. To illustrate the effect of the proposed system,
conventional and proposed systems were simulated in Matlab/Simulink environment and results
were analyzed. Simulation studies were proved that this method reduces the torque ripple of the
DTC method.
2. BASICS OF DTC
In the DTC stator flux linkage and electromagnetic torque are directly controlled by inverter
voltage vectors, considering the motor, voltage source inverter, and the control strategy at the
system level. A relationship is established between the torque, the flux and the optimal inverter
switching so as to achieve a fast torque response. It exhibits better dynamic performance than
conventional control methods, such as vector control, is less sensitive to parameter variations, and
is simpler to implement [13].
The DTC bases on the selection of the optimum voltage vector which makes the flux vector rotate
and produce the demanded torque. In this rotation, hysteresis controllers keep the amplitude of
the flux and the torque errors within acceptable limits [14]. In Fig. 1., the rotation of the stator
flux vector and an example of the effects of the applied inverter switching vectors were given.
The DTC allows for very fast torque responses, and flexible control of the induction motor.
The flux and torque errors are kept within acceptable limits by hysteresis controllers [15].
Figure 1. The rotation of the stator flux vector and an example for the effects of the applied inverter
switching vectors [16].

3
The block diagram of the conventional DTC controlled motor is given in Fig. 2.
Figure 2. The block diagram of the conventional DTC
The DTC algorithm controls the stator flux and the torque by using measured currents and
voltages.
The instantaneous values of the flux and torque can be obtained by using the transformation of
the measured currents and the voltages of the motor. The stator flux is calculated as given in
Eq.1-3 in a stationary reference frame.
( )dtiRV s∫ −= αααλ
(1)
( )dtiRV s∫ −= βββλ
(2)
22
βα λλλ +=
(3)
Where, αλ - βλ are stator fluxes, αi - βi are stator currents, αV - βV are stator voltages, βα −
components and sR is the stator resistance. Motor torque can be calculated as given in Eq.4.
( )αββα λλ iipTe −=
2
3
(4)
Where, p is the motor pole pairs. The stator flux vector region is an important parameter for the
DTC, and it can be calculated as given in Eq.5:








= −
α
β
λ
λ
λ
θ 1
tan
(5)

The torque and flux errors, which are obtained by comparing the reference
are converted to control signals by hysteresis comparators. The switching table is used to
determine the optimum switching inverter states, and it determines the states by using the
hysteresis comparator outputs and the flux region
3. ADAPTIVE BANDWIDTH
Hysteresis control is one of PWM methods used for generating pulses to order the power switches
of the inverter. Among the various current control techniques,
response, simple implementation, negligible tracking error, inherent robustness to load parameter
variations and proper stability [18].
In DTC method, two different hysteresis controllers are used to determine the changes i
flux and electromagnetic torque. In constant bandwidth approach, small bandwidth values results
in a higher switching frequency. So, it results in low harmonic copper losses in
switching losses in the inverter are high.
losses decrease in the inverter while
selection of optimum amplitude of flux and torque hysteresis band is important for the drive but
there are no certainties to determine optimum amplitude of hysteresis bandwidth.
The main idea of adaptive hysteresis bandwidth approach as given in Fig.
regions.
Figure 3. Adaptive hysteresis bandwidth
In this approach hysteresis bandwidth is determined by error values in
flux and electromagnetic torque. So, for the next step of the control process,
is adapted with change in error. It means, if error on flux / torque is high,
will be extended, on the contrary, if error on flux / torque is low,
reduced. In this way, hysteresis
tries to minimize previous step errors at every step. In other words, flux and torque errors are
associated with each other with this approach
The torque and flux errors, which are obtained by comparing the reference and observed values,
hysteresis comparator outputs and the flux region data [17].
BANDWIDTH APPROACH
is one of PWM methods used for generating pulses to order the power switches
inverter. Among the various current control techniques, it is widely used due to the fast
[18].
In DTC method, two different hysteresis controllers are used to determine the changes i
in a higher switching frequency. So, it results in low harmonic copper losses in the
switching losses in the inverter are high. Conversely, in a large bandwidth values case, switching
losses decrease in the inverter while the harmonic copper losses increase in the motor [
to determine optimum amplitude of hysteresis bandwidth.
The main idea of adaptive hysteresis bandwidth approach as given in Fig. 3. as flux and torque
Figure 3. Adaptive hysteresis bandwidth
bandwidth is determined by error values in the previous step for stator
flux and electromagnetic torque. So, for the next step of the control process, hysteresis
adapted with change in error. It means, if error on flux / torque is high, hysteresis
will be extended, on the contrary, if error on flux / torque is low, hysteresis bandwidth will
bandwidth is designed to be flexible and the control algorithm
associated with each other with this approach [20].
4
and observed values,
is one of PWM methods used for generating pulses to order the power switches
is widely used due to the fast
In DTC method, two different hysteresis controllers are used to determine the changes in stator
the motor while
, in a large bandwidth values case, switching
motor [19]. So
as flux and torque
previous step for stator
hysteresis bandwidth
resis bandwidth
bandwidth will be
bandwidth is designed to be flexible and the control algorithm

4. SIMULATIONS AND
The C-DTC and the proposed AB
Matlab/Simulink and the AB-DTC system with
Figure 4. The proposed
The both control systems were simulated in orde
DTC system. The sampling interval
bands in the C-DTC were 0.1 Nm and 0.02 Wb, respectively. The nominal motor parameters
mentioned in Table.
Table. Specifications and parameters of the IM
Symbol
P
Vn
p
f
Rs
Lm
L
To investigate and compare the performances of the C
speed and the torque responses of the motor were obtained
In the speed tests, the motor was tested 1000 rpm and 2800 rpm working conditions. The
was unloaded along the speed tests. Detailed speed responses of the motor for both control
methods were given in Fig. 5–6.
AND RESULTS
d the proposed AB-DTC drive systems have been developed using
DTC system with the Simulink blocks is shown in Fig.
proposed AB-DTC system with the Simulink blocks
simulated in order to demonstrate the validity of the proposed AB
DTC system. The sampling interval was 50 µs. The magnitudes of the torque and flux hysteresis
e 0.1 Nm and 0.02 Wb, respectively. The nominal motor parameters
Table. Specifications and parameters of the IM
Symbol Quantity Value
Power (kW) 1.1
Line to Line voltage (V) 230
Number of poles 2
Frequency(Hz) 50
Stator resistance(Ω) 8
Mutual inductance (H) 0.55
Inductance (H) 0.018
investigate and compare the performances of the C-DTC and the AB-DTC algorithms,
speed and the torque responses of the motor were obtained for different references.
In the speed tests, the motor was tested 1000 rpm and 2800 rpm working conditions. The
5
DTC drive systems have been developed using
Simulink blocks is shown in Fig. 4.
r to demonstrate the validity of the proposed AB-
s 50 µs. The magnitudes of the torque and flux hysteresis
e 0.1 Nm and 0.02 Wb, respectively. The nominal motor parameters were
DTC algorithms, the
In the speed tests, the motor was tested 1000 rpm and 2800 rpm working conditions. The motor

6
a b
Figure 5. Speed responses of the motor at 1000 rpm reference
a) C-DTC b) AB-DTC
a b
Figure 6. Speed responses of the motor at 2800 rpm reference
a) C-DTC b) AB-DTC
As it can be seen in Fig. 5–6, the speed ripples of the motor were reduced for both working
conditions with the proposed AB-DTC method. It must be pointed out that, the response time of
the motor was almost same (about 0.38 sec. in 2800 rpm) for the both methods.
In the torque tests, unloaded and loaded (3 Nm) working conditions are employed. The speed
reference is set at 2800 rpm for both conditions. The electromagnetic torque responses of motor
for unloaded and loaded conditions are shown in Fig. 7 and Fig. 8, respectively.
As it can be seen if Fig. 7, for unloaded working conditions, the C-DTC torque ripple changes in
±1.5 Nm band while the AB-DTC changes in ±1 Nm. It means the motor torque ripples were
reduced about 65% with the proposed DTC method for unloaded working conditions.

Figure 7. Electromagnetic torque responses at unloaded working conditions
Figure 8. Electromagnetic torque responses at
If we need to investigate loaded working conditions, as it can be seen in
torque ripple band changes between about 4 Nm /1.75 Nm (bandwidth 2.25 Nm) and the AB
DTC torque ripple band changes between about 3.6 N
the motor torque ripple has been reduced about 60% with the proposed DTC method for loaded
working conditions. So, in general, it must be pointed out that the torque ripple of the motor has
been reduced about 60% with the proposed DTC approach.
5. CONCLUSIONS
The vector control methods have
induction motors. The DTC method is an
option is the FOC. The conventional DTC
dynamic performance and robust controller structure compared to the F
most faced problem in the conventional
control approach on hysteresis controllers which are directly effect on system performance
been presented. In this approach
limits are determined by error values in
. Electromagnetic torque responses at unloaded working conditions
. Electromagnetic torque responses at loaded working conditions
investigate loaded working conditions, as it can be seen in Fig. 8.
torque ripple band changes between about 4 Nm /1.75 Nm (bandwidth 2.25 Nm) and the AB
DTC torque ripple band changes between about 3.6 Nm/2.25 Nm (bandwidth 1.35 Nm). It means
h the proposed DTC approach.
methods have gained great importance because of the control abilities on the
induction motors. The DTC method is an option on the vector controlled drivers. The second
onventional DTC has several important advantages such as faster
dynamic performance and robust controller structure compared to the FOC for IMs.
onventional DTC is high torque ripples. In this paper,
on hysteresis controllers which are directly effect on system performance
. In this approach, hysteresis controller band limits are not constant and band
limits are determined by error values in the previous step for stator flux and electromagnetic
7
8., the C-DTC
torque ripple band changes between about 4 Nm /1.75 Nm (bandwidth 2.25 Nm) and the AB-
m/2.25 Nm (bandwidth 1.35 Nm). It means
gained great importance because of the control abilities on the
on the vector controlled drivers. The second
advantages such as faster
IMs. However,
In this paper, an adaptive
on hysteresis controllers which are directly effect on system performance has
controller band limits are not constant and band
x and electromagnetic

8
torque. The proposed approach is verified by the numerical simulations and the speed and the
torque responses obtained and compared for the proposed method. The obtained results shows
that the proposed DTC approach can reduce the torque ripple about 60% and improve the driver
performance.
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