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ABSTRACT
A modified Direct Torque Control (DTC) by using Space Vector
Modulation (DTC-SVM) for permanent magnet synchronous
machine (PMSM) drive is proposed in this paper. DTC-SVM
technique improves the basic DTC performances, which features
low torque and flux ripple and also fixed switching frequency.
The computer simulation results, in Matlab/Simulink,
demonstrate the effectiveness of the proposed control scheme
which improves the performance of the PMSM.
1. INTRODUCTION
Direct Torque Control (DTC) method has been first proposed
and applied for induction machines in the mid- 1980’s as
reported in [1]. This concept can also be applied to synchronous
drives [2]. Indeed, in the late 1990s, DTC techniques for the
interior permanent magnet synchronous machine appeared, as
reported in [1].Permanent magnet (PM) synchronous motors are
widely used in high-performance drives such as industrial robots
and machine tools to their advantages as: high efficiency, high
power density, high torque/inertia ratio, and free maintenance.
In recent years, the magnetic and thermal capabilities of the PM
have been considerably increased by employing the high
coercive PM material [2]. For some applications, the DTC
becomes unusable, despite it significantly improves the dynamic
performance of the drive compared to the vector control due to
torque and flux ripples. Indeed, hysteresis controllers used in the
conventional structure of the DTC generates a variable
switching frequency, causing electromagnetic torque oscillations
[4], this frequency is also varying with speed, load torque and
hysteresis bands selected [1]. In addition, a high sampling
frequency needed for digital implementation of hysteresis
comparators and a current and torque distortion caused by
sectors changes [2]. Several contributions have been proposed to
overcome these problems, by using a multilevel inverter: more
voltage space vectors available to control the flux and torque.
However, more power switches are needed to achieve a lower
ripple and almost fixed switching frequency, which increases the
system cost and complexity [2]-[5].
In [1] and [2], two structures of modified DTC have been
proposed to improve classical DTC performances by replacing
the hysteresis controllers and the commutation table by a PI regulator, predictive controller and Space Vector Modulation
(SVM). In this paper, a modified DTC algorithm with fixed
switching frequency for PMSM is proposed to reduce the flux
and torque ripples. It is an extension of the modified DTC
scheme for the PMSM proposed by the authors in [1] and [2].
The performance of the basic DTC and the proposed DTC
scheme is analyzed by modeling and simulation using
MATLAB.
2. DTC AND DTC-SVM STRUCTURES
Figures 1 and 2 represents two system configuration of DTC
controlled PMSM drive respectively; both of them use the same
flux vector and torque estimators. However, torque and flux
hysteresis controllers and the switching table are replaced by a
PI torque controller and a predictive calculator of vector voltage
reference to be applied to stator coils of the PMSM.
SIMULATION RESULTS
The simulation results of DTC-SVM are presented in Figures
13 to 17, respectively. In the beginning the machine starts
under a speed set-point of 1000 rpm at no load. Indeed, it’s
seen in the Simulation results, that the flux and torque ripples
are greatly reduced under the modified DTC. Figure 15 shows
the steady-state currents under modified DTC PMSM,
respectively.
This is mainly because in SVM algorithm, contrary to
hysteresis controller and PI controller, the switching
frequency is constant and also, in SVM, many vectors (IGBT
states) are selected to adjust the torque and flux ripple in each
sample time, whereas in basic DTC just one vector is selected
to adjust ripple inside hysteresis bands of torque and flux
regulators. Note that the sampling frequency of the modified
DTC is only half of that of the DTC. The reason for the high
distortion in the DTC is mainly due to the fact that the
switching function of the inverter is only updated at the
sampling instant and also the number of vectors applied to
adjust the torque and flux ripple.
Although the switching frequency of the basic DTC (varying
from 3.5 to 5 kHz) is lower than that of the DTCSVM (10
kHz), which means a lower switching loss, however, the
distortion of the basic DTC is too high. From the simulation
results, it is observed that the steady-state performance of
DTC-SVM is much better than the basic DTC.
CONCLUSION
In this paper it was presented a method of utilization of Space
Vector Modulation for the Direct Torque control of a PMSM.
To determine the reference voltage, a simple algorithm was
proposed, based on the torque error and the flux phase angle.
The results show that a smooth steady state operation was
obtained when using the proposed method. Moreover, a
constant inverter switching frequency is ensured by using
SVM topology. A modified direct torque control (DTC-SVM)
with speed loop has been proposed and validated in
Matlab/Simulink. The torque and flux ripples are agree with
the simulation results, additionally the implementation shows
that the speed and torque dynamic state is the same as the
simulation results. Basic and modified DTC assure very good
decoupling in torque and stator flux control.