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International Journal of Advanced Research in Engineering and Technology
(IJARET)
Volume 7, Issue 2, March-April 2016, pp. 78–90, Article ID: IJARET_07_02_008
Available online at
http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=7&IType=2
Journal Impact Factor (2016): 8.8297 (Calculated by GISI) www.jifactor.com
ISSN Print: 0976-6480 and ISSN Online: 0976-6499
© IAEME Publication
___________________________________________________________________________
SPEED CONTROL OF INDUCTION
MACHINE WITH REDUCTION IN TORQUE
RIPPLE USING ROBUST SPACE-VECTOR
MODULATION DTC SCHEME
Yeshoda Harish Kumar
M.Tech, Power Electronics, Manipal University Jaipur, India
Vishnu Goyal
Assistant Professor, EEE Department,
Manipal University Jaipur, India
ABSTRACT
In this paper a novel and simple algorithm for three-phase induction
motor(IM) under Direct Torque Control (DTC) scheme using Classic DTC
switching table for dynamic torque ripple reduction and space-vector
modulation scheme for steady state torque and flux control is proposed. The
proposed scheme having the advantages of low torque ripples as well as
constant switching frequency.
Simulation results are given to prove the ability of the proposed method
obtaining good speed control bandwidth while overcoming classic DTC and
DTC-SVM drawbacks.
Key words: Direct Torque Control, Induction Motor, Space-Vector
Modulation, Torque Ripple Minimization.
Cite this Article: Yeshoda Harish Kumar and Vishnu Goyal. Speed Control
of Induction Machine with Reduction In Torque Ripple Using Robust SpaceVector Modulation DTC Scheme. International Journal of Advanced
Research in Engineering and Technology, 7(2), 2016, pp. 78–90.
http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=7&IType=2
1. INTRODUCTION
The basic concept of DTC of AC motor drives is to control both stator flux linkage
and the electromagnetic torque of the machine simultaneously. Since a DTC-based
drive system select the inverter switching states using switching table, neither current
controllers nor pulse-width modulation (PWM) modulator is required, as shown in
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Speed Control of Induction Machine with Reduction In Torque Ripple Using Robust SpaceVector Modulation DTC Scheme
Figure.1, thereby introducing fast torque dynamics response in comparison with the
field oriented vector control technique. However, this conventional DTC approach has
some disadvantages such as; high torque ripple and variable switching frequency,
which is varying with speed, load torque and the selected hysteresis bands. On the
other hand to reduce the torque ripple; the hysteresis torque and flux controller bands
must be reduced to match the required torque performance, which requires reduction
of the system sampling time and it is necessary to use a very fast processing
controller. Although the system sampling frequency can be increased in the
conventional DTC the inverter switching frequency is still low, approximately less
than one third of the sampling frequency [1, 2]. The inverter switching frequency can
be increased using a dithering signal, by adding a limited amplitude high frequency
signal to the torque and flux error signals [3]. Although the switching frequency is
increased it is still variable for small error bands. Other research concerns with these
disadvantages using multilevel inverter, there are more voltage space vectors
available to control the flux and torque [4]. However, these approaches require more
power devices, which increase the cost of the system and make it more complex. In
[5, 6] discrete space vector modulation DTC approach is used to reduce the torque
ripple. However, there is a complexity of selecting the additional hysteresis
controllers. On the other hand, space vector modulation (SVM) modulator is
incorporated with direct torque control for Induction Motor drives to provide a
constant inverter switching frequency and low torque ripple, a predictive PI controller
is used to calculate the command voltage vector [7].
In this paper, a new and simple DTC algorithm with fixed switching frequency for
Induction Motor is proposed to reduce the torque ripples. The well-developed SVM
technique is applied to inverter control in the proposed DTC-based Induction Motor
drive system, thereby dramatically reducing the torque ripples. The simulation results
are carried out by modelling the drive systems using a Matlab/Simulink.
2. CLASSICAL
OPERATION.
DIRECT
TORQUE
CONTROL
(DTC)
The block diagram of classical DTC proposed by I. Takahashi and T. Nogouchi is
presented in Figure.1.
Figure 1 Block Diagram of Classical-DTC
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The stator flux amplitude
and the electromagnetic torque
are the reference
signals which are compared with the estimated
values respectively. The
flux and torque errors are delivered to the hysteresis controllers. The digitized output
variables and the stator flux position sector selects the appropriate voltage vector from
the switching table. Thus, the selection table generates pulses to control the power
switches in the inverter.
The stator flux vector and the torque produced by the motor can be estimated
respectively. These equations only require the knowledge of the previously applied
voltage vector Vs, measured stator current Is, stator resistance Rs, and the motor poles
number p [5].
The components of stator flux are given by:
(1)
(2)
The magnitude of the stator flux can be estimated by:
(3)
The flux vector zone can be obtained using the stator flux components. The
electromagnetic torque can be calculated by:
(4)
For the flux is defined two-level hysteresis controller and for the torque threelevel hysteresis controller is used.
The output signals are defined as:
Where
is the total hysteresis bandwidth of the controller. The actual stator
flux is constrained within the hysteresis band and tracks the command flux. The
torque control loop has three levels of digital output represented by the following
conditions.
The above considerations allow construction of the selection table as presented in
below Table-1.
Table I Switching Scheme of Basic-DTC
S(1)
1
-1
S(2)
S(3)
S(4)
S(5)
S(6)
1
0
-1
1
0
-1
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Speed Control of Induction Machine with Reduction In Torque Ripple Using Robust SpaceVector Modulation DTC Scheme
The classical DTC method can be characterized as follows:
Advantages: simple structure, no coordinate transformation, no separate voltage
modulation block, no current control loops, very good flux and torque dynamic
performance,
Disadvantages: Variable switching frequency, problems during starting and low
speed operation, high torque ripples, flux and current distortion caused by stator flux
vector sector position change and high sampling frequency is required for digital
implementation.
3. DTC-SVM SCHEME
In the DTC-SVM system, the same flux and torque estimator which is used for the
conventional DTC is still used in the proposed DTC scheme as shown in Figure.2.
Instead of the switching table and hysteresis controllers, an optimal voltage vector
calculator is used to calculate the reference voltage vector as a function of the torque
and flux errors. The applied voltage vectors and their duration times are selected and
calculated using the SVM modulator. From the block diagram of the DTC-SVM
shown in Fig.4, it is seen that this scheme have the most important advantage over
conventional DTC, such as at steady state time of the torque response the SVM is
applied to insure a ripple reduction occurring at the steady state torque response[7].
Figure 2 Block Diagram of DTC-SVM Scheme
The procedure for implementing a two-level space vector PWM can be summarized
as follows:
1. Calculate the angle
and reference voltage vector
based on the input voltage
components.
2. Find the sector in which
lies and the adjacent space vectors of Vk and Vk + 1
based on the sector angle .
3. Find the time intervals
, and the angle .
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3.1. Voltage Vector Calculation
In this modulation technique the three phase quantities can be transformed to their
equivalent 2-phase quantity in stationary frame. A reference voltage vector V that
rotates with angular speed in the
plane represents three sinusoidal waveforms
with angular frequency in the abc coordinate system. The magnitude of reference
vector used for modulating inverter output is,
(5)
The phase angle is evaluated from,
(6)
3.2. Sector Determination
It is necessary to know in which sector the reference output lies in order to determine
the switching time and sequence. The identification of the sector where the reference
vector is located is straightforward. The phase voltages correspond to eight switching
states: six non-zero vectors and two zero vectors at the origin. Depending on the
reference voltages
and
, the angle of the reference vector can be used to
determine the sector as per Table.2.
Table II Voltage Vector Sector Determination
Sector
1
2
3
4
5
6
Angle
(Degrees)
3.3. Time Durations T0, T1, T2
Times T1 and T2 denote the required on-time of the active-state vectors
during each sample period, and k is the sector number denoting the reference location.
The calculated times T1 and T2 are applied to the switches to produce space vector
PWM switching patterns based on each sector. The switching time is arranged
according to the first half of the switching period while the other half is a reflection
forming a symmetrical pattern. T0 denote time of the null state vectors.
Assuming that the reference voltage and the voltage vectors
and
are
constant during each pulse-width modulation period Ts and splitting the reference
voltage
into its real and imaginary components (V or and V) gives the following
result:
(7)
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Speed Control of Induction Machine with Reduction In Torque Ripple Using Robust SpaceVector Modulation DTC Scheme
The inverse matrix of above is used to calculate
as
(8)
4. PROPOSED DTC BY MEANS OF BOTH SWITHCHING
SCHEME AND SVM
From the block diagram of the proposed DTC shown in Figure.3, it is seen that the
proposed scheme have the most important advantage of the conventional DTC, such
as fast torque dynamics, by applying the conventional DTC during the step change in
the torque command; then at steady state time of the torque response the SVM is
applied to insure a ripple reduction occurring at the steady state torque response.
The proposed DTC scheme can be further explained as follows; at starting up or at
the instance of the load change, almost when the torque error is large, the switching
state selector selects the switching states of the conventional DTC switching table to
get a fast torque response at starting and at a step change in the load torque command.
When the torque error is within the hysteresis band limits, the switching state selector,
select the switching states generated by the SVM which guaranty that the switching
frequency is fixed and the ripples at the steady state torque response are reduced.
Figure.3 Block Diagram of Proposed-DTC Scheme
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5. SIMULATION RESULTS
The system performance under the proposed DTC strategy is evaluated, and the
results are compared with the results obtained by the conventional DTC and DTCSVM schemes. MATLAB/SIMULINK models were developed to examine the
conventional and the proposed DTC algorithms. In simulation, the sampling time is
20 µs for both the conventional and proposed DTC schemes and the switching time
for the proposed DTC is 100 µs. The switching table used in [1] is employed for the
conventional DTC. The switching delays and the forward drop of the power switches,
the dead time of the inverter and the non-ideal effects of the Induction motor are
neglected in the models. The torque dynamics performances of the conventional and
proposed DTC schemes at 700 rpm are compared as shown in Figure.5 under the
same operation conditions. It is seen that the torque ripple is reduced by more than 50
% at no-load and load conditions in the proposed DTC.
The speed and torque response for the classical DTC algorithm is given in Fig. 4
(b) and the Fig. 4 (c) show the torque response for the DTC algorithm with DTCSVM Scheme, and finally the proposed DTC algorithm torque response is given in
Fig. 4 (d). It can be seen from the figure that the proposed DTC has achieved a fast
speed response than that for the conventional DTC. The speed dynamic response and
their corresponding torque response for three algorithms more declaration is shown in
Figure.5 and Figure.7. It is seen that the torque ripple reduction is still valid at low
and high speed ranges. A speed reverse dynamics is carried out at two different speed
references, +700 rpm and -700 rpm, to indicate the validity of the controller operation
at both speed directions. The simulation results for the speed reversing dynamics are
shown in Figure.7.
Also a comparative simulation results between the proposed DTC algorithm and
other algorithms are shown in Figure.5 and Figure.6, where a reference speed of 700
rpm is applied to the motor then, a load torque of 7 Nm is applied at t = 0.75 sec.
4(a) Reference Speed and Load Torque
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Speed Control of Induction Machine with Reduction In Torque Ripple Using Robust SpaceVector Modulation DTC Scheme
4(b) Classical-DTC Scheme
4(c) DTC-SVM Scheme
4(d) Proposed DTC Scheme
Figure.4 Speed and Torque Responses for Three Different DTC Schemes
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5(a) Classical-DTC scheme
5(b) DTC-SVM Scheme
5(c) Proposed-DTC Scheme
Figure 5 Speed-Torque Dynamics with Change in Speed (0 to 700 rpm)
6(a) Classical-DTC Scheme
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6(b) DTC-SVM Scheme
6(c) Proposed-DTC Scheme
Figure 6 Speed-Torque Response with Applied Load (7N-m)
7(a) Classic-DTC Scheme
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7(b) DTC-SVM Scheme
7(c) Proposed-DTC Scheme
Figure 7 Speed-Torque Dynamics with Speed Reversal (0 to -700rpm)
The Stator-Flux trajectory of Classic-DTC scheme is shown in Figure.8 (a) and
Figure.8 (b) shows the Stator-Flux trajectory of DTC-SVM scheme, finally StatorFlux trajectory of proposed-DTC scheme is shown in Figure.8 (c). It clearly shows
that the Proposed-DTC has better Flux control over Classical and SVM DTC
schemes.
8(a) Classic-DTC
8(b) DTC-SVM
8 (c) Proposed-DTC
Figure 8 Stator-Flux Circular Trajectory of Three DTC Schemes
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The Stator-Current characteristic with change in load (7 Nm to -7Nm) is shown in
Figure.9. It clearly shows that the Proposed-DTC scheme has fast and better stator
current response due to change in load.
Figure 9 Stator Current Characteristics with Load Reversal (7Nm to-7Nm)
6. CONCLUSION
In this paper a new DTC algorithm for the 3-Phase Induction Machine has been
developed. The effectiveness of the proposed DTC algorithm in reducing the torque
ripples was verified by computer simulations on a 3-phase Induction Motor rated at 4
kW. The presented results have demonstrated that the problems which were
associated with conventional DTC schemes such as, high torque ripples and variable
switching frequency have been completely avoided. It is also interesting that the
proposed torque controller has been shown to be robust to control the machine flux,
allowing satisfactory overall performance of the machine under study. The developed
algorithm also offers the prospect for optimizing the machine performance in a
manner similar to conventional vector controllers with better toque dynamics.
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APPENDIX
Parameters of the used Induction Machine are given in below Table.3.
Table III Induction Motor Parameters
Nominal Power
Frequency
Number of Pole Pairs
Stator Resistance
Rotor Resistance
Stator Inductance
Rotor Inductance
Mutual Inductance
4000W
50Hz
2
1.405ohm
1.395ohm
5.839mH
5.839mH
172.2mH
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