**1. Introduction**

166 MATLAB – A Fundamental Tool for Scientific Computing and Engineering Applications – Volume 1

Vol. 1, pp. 240-245.

Research Review

and Systems, Vol.53, No. 4

Wang, T.G., Zhou, X., & Lee, F.C., (1997), *A Low Voltage High Efficiency and High Power Density DC/DC Converter*, 28th Annual IEEE Power Electronics Specialists Conference,

Xiong, Y., Sun, S., Jia, H., Shea, P., & Shen, Z.J., (2009), *New Physical Insights on Power MOSFET Switching Losses*, IEEE Transactions on Power Electronics, Vol. 24, No. 2 Zhou, S., (2003). *Fully Integrated Power-Saving Solutions for DC-DC Converters Targeted for the Mobile, Battery-Powered Applications*, Georgia Tech Analog Consortium Industry

Zhou, S. & Rincon-Mora, G.A., (2006),*A high Efficiency, Soft Switching DC-DC Converter with Adaptive Current-Ripple Control for Portable Applications*, IEEE Transactions on Circuits

> Permanent Magnet Synchronous Motors (PMSM's) are widely used in high-performance drives such as industrial robots, automotive hybrid drive trains and machine tools thanks to their advantages as: high efficiency, high power density, high torque/inertia ratio, and free maintenance. In the recent years, the magnetic and thermal capabilities of the Permanent Magnet (PM) have been considerably increased by employing the high-coercive permanent magnet material.

> Direct Torque Control (DTC) method has been first proposed and applied for induction machines in the mid-1980s, by Takahachi and Noguchi, for low and medium power applications. This concept can also be applied to synchronous drives. Indeed, PMSM DTC has appeared in the late 1990s. However, for some applications, the DTC has become unusable although it significantly improves the dynamic performance (fast torque and flux responses) of the drive and is less dependent on the motor parameters variations compared to the classical 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. Also this frequency varies with speed, load torque and hysteresis bands selected. In addition, a high sampling frequency is needed for digital implementation of hysteresis comparators and a current and torque distortion is caused by sectors changes.

> In the last decade, several contributions have been proposed to overcome these problems by using the three level or the multilevel inverter: more voltage space vectors are available to control the flux and torque. However, more power switches are needed to achieve a lower

© 2012 Chikh et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Chikh et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ripple and almost fixed switching frequency, which increases the system cost and complexity. In order to improve the DTC performance and overcome the above cited problems, another solution combines basic DTC and fuzzy logic control advantages in one control strategy, named Fuzzy Direct Torque Control (FDTC). In this technique, the hysteresis comparator and the switching table used in basic DTC are replaced by a fuzzy logic switcher, which decides directly on the switches states of the Voltage Source Inverter (VSI). In addition, it's known that fuzzy control works as well for complex non-linear multidimensional system, system with parameter variation problem and/or cases where the sensor signals are not precise. The fuzzy control is basically nonlinear and adaptive in nature, giving robust performance under parameter variation and load disturbance effect. For all these reasons, a Fuzzy Logic Controller can be used instead of the speed PI controller in FDTC in order to achieve a complete fuzzy control for the PMSM. Also, much interest has been focused on the use of modified DTC structures to improve basic DTC performances by replacing the hysteresis controllers and the switching table by a PI regulator, predictive controller and Space Vector Modulation (SVM). Indeed, under DTC-SVM strategy, both torque and flux linkage ripples are greatly reduced when compared with those of the basic DTC, because the application of SVM guarantees lower harmonics current by eliminating the distortion caused by sector changes in case of DTC switching table and by fixing the switching frequency.

Moreover, the design of the speed controller used in basic DTC or in modified DTC strategies, greatly affects the performance of the drive. The PI controllers have a simple structure and can offer satisfactory performances over a wide range of operations. However, due to the uncertainties, the variations in the plant parameters and the nonlinear operating conditions, the fixed gains of the PI controller may become unable to provide the required control performance. In order, to realize a good dynamic behaviour of the PMSM, a perfect speed tracking with no overshoot and a good rejection of impact loads disturbance, the speed PI controller can be replaced by a PI-Fuzzy speed controller.

This chapter is organized as follows: PMSM modelling and simulation results of the basic DTC by using Matlab/Simulink environment with a simple speed PI corrector, will be presented and discussed in Section 2 and 3, respectively. Whereas in Section 4 a complete Fuzzy Direct Torque Control (FDTC), which uses a fuzzy switching table and a PI-Fuzzy speed controller, for PMSM is proposed to reduce the torque and flux ripples. In addition, the simulation results show the effectiveness of this strategy when compared with the basic DTC and a classical speed PI controller. Section 5 and 6 are devoted to presenting a fixed switching-frequency DTC with two approaches: Sinusoidal Pulse With Modulation (SPWM) and Space Vector Modulation (SVM). The objective, of these two strategies, is reducing the flux and torque ripples and fixing the switching-frequency. Of course, the simulation results of these two approaches will be discussed and compared with those of basic DTC and FDTC, in section 7. Section 8 is devoted to study DTC performances under PMSM parameters variation, also a solutions have been proposed to overcome this issue. Eventually, conclusion and simulation results interpretations are included in Section 9.
