**2. Modelling of the VSI and the PMSM**

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

switching frequency.

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

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

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.

speed PI controller can be replaced by a PI-Fuzzy speed controller.
