QUICK DYNAMIC TORQUE CONTROL IN DTC-HYSTERESIS-BASED INDUCTION MOTOR BY USING NEW OPTIMIZED SWITCHING STRATEGY

NOVATEUR PUBLICATIONS
INTERNATIONAL JOURNAL OF INNOVATIONS IN ENGINEERING RESEARCH AND TECHNOLOGY [IJIERT]
ISSN: 2394-3696
VOLUME 2, ISSUE 7, JULY-2015
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QUICK DYNAMIC TORQUE CONTROL IN DTC-HYSTERESIS-BASED
INDUCTION MOTOR BY USING NEW OPTIMIZED SWITCHING STRATEGY
Madhuri D. Kulkarni
ME Student, Department of Electrical Engineering, ZES Zeal College of Engineering and Research,
Narhe, Pune, India
Vivek D. Bavdhane
Assistant Professor, Department of Electrical Engineering, ZES Zeal College of Engineering and
Research, Narhe, Pune, India
ABSTRACT
Induction machines are widely employed in industries due to their rugged structure, high maintainability and
economy than DC motors. There has been constant development in the induction motor drive system and
their implementation in industrial applications. The improvement of switching speed of power electronic
devices has enabled control techniques which possess high switching frequency and feasibility of high
efficiency drive systems. In this pretext, Direct Torque Control (DTC) was introduced to obtain quick and
better dynamic torque response. The DTC scheme in its basic configuration comprises torque and flux
estimator DTC controller, stator voltage vector selector and voltage source inverter. Direct Torque Control of
induction motor has increasingly become the best alternative to Field- Oriented Control methods. The
performance of an induction motor under the classical Direct Torque Control method and improved scheme
have been studied and confirmed by simulation using MATLAB.
Keywords: Direct Torque Control (DTC), Dynamic Over-modulation, Hysteresis Controller, Induction
Motor, Torque Controller.
INTRODUCTION
Industrial loads require operation at wide range of speeds. Such loads are generally termed as variable speed
drives. These drives demand precise adjustment of speed in a steeples manner over the complete speed range
required. The loads may be constant torque or a function of speed. These loads are driven by hydraulic,
pneumatic or electric motors. An industrial drive has some special features when driven by electric motors.
Induction machines have provided the most common form of electromechanical drive for industrial,
commercial and domestic applications that can operate at essentially constant speed. Induction machines
have simpler and more rugged structure, higher maintainability and economy than dc motors. They are also
robust and immune to heavy loading. The possible forms of drive motors are dc drives, ac drives. DC motors
are versatile for the purpose of speed control but they suffer from the disadvantage imposed by the
commentator. On the other hand ac drives are viable competitors with the advent of thruster power converter
technology. The evolution of ac variable speed drive technology has been partly driven by the desire to
emulate the performance of dc drive such as fast torque response and speed accuracy, while utilising the
advantages offered by standard ac motor. The Field Oriented Control (FOC) and the Direct Torque Control
(DTC) are two types of drives employed for high performance applications. Direct Torque Control was
introduced in Japan by Takahashi (1984) and Depenbrock (1985). Vector controlled induction motors are
employed in high performance drives having precise speed control and good static as well as dynamic
response. Direct Torque Controlled drives have increasingly become the best alternative to Field-Oriented
Control methods [10], [2]. Modern control methods use state space techniques. The methods of stabilizing
the drives and improvement in their transient responses have been realized by modern power electronic
devices [3]. The block diagram of Direct Torque Control for an induction motor is as shown in Fig. 1. The

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DTC scheme comprises torque and flux estimator, hysteresis controllers for flux and torque and a switching
table.
Fig. 1 Structure of basic DTC-hysteresis-based induction machine
INDUCTION MOTOR MODEL
The main objective of DTC is to control the induction motor. The per-phase equivalent circuit of an
induction motor is valid only in steady-state condition. In an adjustable speed drive like the DTC drive, the
machine normally constitutes an element within a feedback loop and hence its transient behavior has to be
taken into consideration [4]. The induction motor can be considered to be a transformer with short circuited
and moving secondary. The coupling coefficients between the stator and rotor phases change continuously in
the course of rotation of rotor [6], [5]. Hence the machine model can be described by differential equations
with time varying mutual inductances.
DIRECT TORQUE CONTROL CONCEPT
In recent years “induction motor control techniques” have been the field of interest of many researchers to
find out different solutions for induction motor control having the features of precise and quick torque
response, and reduction of the complexity of field oriented control [3]-[7]. The Direct torque control (DTC)
technique has been recognized as the simple and viable solution to achieve this requirements.DTC is one of
the most excellent and efficient control strategies of induction motor. This technique is based on decoupled
control of torque and stator flux and today it is one of the most actively researched control techniques where
the aim is to control effectively the torque and flux.
CONVENTIONAL DTC SCHEME
The conventional DTC scheme is a closed loop control scheme, the important elements of the control
structure being: the power supply circuit, a three phase voltage source inverter, the induction motor, the
speed controller to generate the torque command and the DTC controller. The DTC controller again consists
of torque and flux estimation block, two hysteresis controllers and sector selection block, the output of the
DTC controller is the gating pulses for the inverter.
The DTC scheme does not require coordinate transformation as all the control procedures are carried out in
stationary frame of reference. So this scheme does not suffer from parameter variations to the extent that
other control techniques do. Also there is no feedback current control loop due to which the control actions
do not suffer from the delays inherent in the current controllers, no pulse width modulator, no PI controllers,
and no rotor speed or position Sensor So it is a sensorless control technique which operates the motor

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without requiring a shaft mounted mechanical sensor. Here on-line torque and flux estimators are used for
closing the loop. Here the torque and stator flux are controlled directly by using hysteresis comparators. Fig.1
shows the basic block diagram of conventional DTC scheme [8]-[10].
PRINCIPLE OF DTC SCHEME
The basic principle of DTC is to directly select stator voltage vectors according to the torque and flux errors
which are the differences between the references of torque and stator flux linkage and their actual values. The
governing equation for torque for this scheme is due to the interaction of stator and rotor fields. Torque and
stator flux linkage are computed from measured motor terminal quantities i.e. stator voltages and current. An
optimal voltage vector for the switching of VSI is selected among the six nonzero voltage vectors and two
zero voltage vectors by the hysteresis control of stator flux and torque.
Fig. 2 Simplified 3- VSI
A three-phase VSI has eight possible combinations of six switching devices which is shown in fig.2. The six
switches have a well defined state: ON or OFF in each configuration. So all the possible configurations can
be identified with three bits (Sa, Sb, Sc), one for each inverter leg [17]. The bit is set to 1 if the top switch is
closed and to 0 when the bottom switch is closed. In order to prevent short circuit of the supply, the state of
the upper switch is always opposite to that of the lower one. The stator voltage space vector is,
DIRECT FLUX CONTROL
In stationary reference frame the stator flux equation can be written as:
If the stator resistance drop is neglected for simplicity, the stator flux varies along the direction of applied
voltage vector and the equation will be reduced to
Which means, by applying stator voltage vector for a time increment ∆t,Ψ̅s can be changed
incrementally. The command value of the stator flux vector Ψs̅ * follows a circular trajectory, the plane of
stator flux is divided into six sectors as shown in fig.3. Each sector has a different set of voltage vector to
increase or decrease the stator flux. The command flux vector rotates in anticlockwise direction in a circular
path and the actual stator flux vector Ψs̅ tracks the command flux in a zigzag path but constrained to the
hysteresis band which is shown in fig.3.

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Fig. 3 Circular Trajectory of stator flux
In general the active forward voltage vectors (Vs,k+1 and Vs,k+2) are applied to increase or decrease the stator
flux respectively when the stator flux lies in sector k. The radial voltage vectors (Vs,k and Vs,k+3) which
quickly affect the flux are generally avoided. The active reverse voltage vectors (Vs,k-1 and Vs,k-2) are used to
increase or decrease the stator flux in reverse direction.
The stator flux vector change due to stator voltage vector is quick whereas change rotor flux is sluggish
because of its large time constant Tr. That is why Ψ̅s movement is jerky and Ψr̅ moves uniformly at
frequency ωe as it is more filtered. However the average speed of both remains the same in steady state
condition.
DIRECT TORQUE CONTROL
The electromagnetic torque produced due to interaction of stator and rotor flux is given by the following
equation:
Fig. 4 Stator flux, rotor flux and stator current vectors in ds-qs reference plane
From the above it is clear that torque varies directly as angle between stator flux and rotor flux i.e. γ. So in
order to obtain high dynamic performance it is required to vary γ quickly. Assuming the rotor is rotating in
anticlockwise direction continuously and stator flux lies in sector k, the active forward voltage vectors (Vs,k+1
and Vs,k+2) are applied to increase γ so as the torque Te. The radial voltage vectors (Vs,k and Vs,k+3) are used to

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decrease γ and Te. By applying the reverse active voltage vectors (Vs,k-1 and Vs,k-2) torque can be decreased
rapidly. The two zero voltage vectors (Vs,0 and Vs,7) are applied to maintain the flux constant ideally and to
decrease the torque slightly.
SWITCHING SELECTION
A high performance torque control can be established due to the decoupled control of stator flux and torque
in DTC. Optimum switching vector selection table given by table 3.1 shows the optimum selection of the
switching vectors in all sectors of the stator flux plane. This table is based on the value of stator flux error
status, torque error status and orientation of stator flux for counterclockwise rotation of the shaft [8].
Table 1 Lookup Table (voltage vector selector)
Stator Flux
error status
Torque
error status
Sector
I
Sector
II
Sector
III
Sector
IV
Sector
V
Sector
VI
1
1 V2
(100)
V3
(110)
V4
(010)
V5
(011)
V6
(001)
V1
(101)
0 V0 V7
(111)
V0
(000)
V7
(111)
V0
(000)
V7
(111)
-1 V6
(001)
V1
(101)
V2
(100)
V3
(110)
V4
(010)
V5
(011)
0
1 V3
(110)
V4
(010)
V5
(011)
V6
(001)
V1
(101)
V2
(100)
0 V7
(111)
V0
(000)
V7
(111)
V0
(000)
V7
(111)
V0
(000)
-1 V5
(011)
V6
(001)
V1
(101)
V2
(100)
V3
(110)
V4
(010)
STATOR FLUX ESTIMATION
For exact calculation of stator flux and torque errors, an accurate estimator of stator flux is necessary. There
are commonly used methods of estimation of flux namely stator voltage model and current model.
HYSTERESIS CONTROLLER
DTC of induction motor drives requires two hysteresis controllers. The drive performance is influenced by
the width of the hysteresis bands in terms of flux and torque ripples, current harmonics and switching
frequency of power electronics devices. Current distortion is reduced by small flux hysteresis band and
torque ripple is reduced by small torque hysteresis bands. In each sampling time, the switching state of the
inverter is updated. The inverter state remains constant, until the output states of the hysteresis controller
change within a sampling interval. If the hysteresis band is fixed, the switching frequency totally depends on
the rate of change of torque and flux.
(a) (b)
Fig.5 Hysteresis Band (a) the stator flux, (b) electromagnetic torque

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PROPOSED DYNAMIC TORQUE CONTROL
In the proposed dynamic over modulation method, the most optimized voltage vector that produces the
largest tangential to the circular flux locus is switched and held (instead of selecting two active voltage
vectors) during torque dynamic to achieve the fastest dynamic torque control. As discussed in the previous
section, if sector k is considered, this would be vector vk+1 in subsector (i) and vector vk+2 in subsector (ii).
Fig. 6 shows the structure of DTC-hysteresis-based induction machine with the proposed modification of
flux error status.
Fig. 6 Structure of DTC-hysteresis-based induction machine with the proposed
Modification of flux error status block
Notice that all components of the DTC-hysteresis-based scheme are retained, except for the inclusion of the
“modification of flux error status” block which is responsible to perform the dynamic over modulation mode.
The selection of the optimized voltage vector to give the fastest torque response can be simply done by
modifying the flux error status ( ) to a new flux status ( ) before it is being fed to the lookup table. The
“modification of flux error status” block, and, hence, the proposed dynamic over modulation, is activated
when the torque error ETe is greater than twice of the hysteresis band of torque controller H BTe. When the
“modification of flux error status” block is activated, the output of this block depends on the position of
the flux position within a sector. If it is in subsector (i) = 0, hence vk+1 is selected, and if it is in subsector
(ii) = 1, vk+2 is selected. The border of the sectors and subsectors can be easily calculated using the
threshold values of denoted as and which can be calculated as
SIMULATION MODEL AND RESULTS
To study on the effectiveness of the proposed dynamic over modulation method, a simulation of the DTC-
hysteresis-based induction motor drive is performed using a Matlab/Simulink simulation package.
A 1.5-kW four pole squirrel-cage induction motor is used for simulation. The simulation results are carried
out for conventional and modified DTC of induction machine.

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SIMULATION MODEL OF DTC:
Fig.7. Simulation model of DTC
1. DTC Block for conventional Method
2. DTC Block for Modified Method
SIMULATION RESULTS:
Simulation results with conventional and Modified DTC are as shown below:
Simulation Results of Conventional DTC:

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Simulation Results of Modified DTC:
Stator Flux Locus of Conventional DTC:
Flux Loci for All Six Sectors with modified Method:

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(a) (b)
(c) (d)
(e) (f)
CONCLUSION
For any IM drives, direct torque control is one of the best controllers proposed so far. It allows decoupled
control of motor stator flux and electromagnetic torque. From the analysis it is proved that, this strategy of
IM control is simpler to implement than other vector control methods as it does not require pulse width
modulator and co-ordinate transformations.
The estimation accuracy of stator flux is very much essential which mostly depends on stator resistance
because an error in stator flux estimation will affect the behavior of both torque and flux control loops. The
torque and current ripple can be minimized by employing space vector modulation technique. A new strategy
is introduced further to get quick response by introducing one “modified flux error block” and proved by
MATLAB/SIMULINK model.

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An optimized voltage vector that produces the largest tangential to the circular flux locus is switched and
held so that a fast rate of change of angle ∆δsr is achieved. The selection of the optimized voltage vector is
simply obtained by modifying the flux error status before it is being fed to the lookup table. The main benefit
of the proposed method is its simplicity and, at the same time, be able to produce the fastest dynamic torque
response with a six-step mode.
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QUICK DYNAMIC TORQUE CONTROL IN DTC-HYSTERESIS-BASED INDUCTION MOTOR BY USING NEW OPTIMIZED SWITCHING STRATEGY

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QUICK DYNAMIC TORQUE CONTROL IN DTC-HYSTERESIS-BASED INDUCTION MOTOR BY USING NEW OPTIMIZED SWITCHING STRATEGY