Photovoltaic system applications should operate under good conditions. The maximum power point depends on the sunlight angle on the panel surface. In this chapter, an induction motor (IM) controlled with a direct torque control (DTC) is used to control the photovoltaic panel position. The conventional DTC is chosen thanks to its capability to develop the maximum of torque when the motor is standstill. However, the DTC produces a torque with high ripples and it is suffer from the flux demagnetization phenomenon, especially at low speed. To overcome these problems, two DTC approaches are proposed in this chapter: (1) the DTC based on the fuzzy logic and (2) the DTC based on space vector modulation (SVM) and proportional integral (PI) controllers (DTC-SVM-PI). The suggested approaches are implemented on a field programmable gate array (FPGA) Virtex 5 circuit in order to reduce the sampling period of the system and the delay in the control loop. The simulation and hardware implementation results demonstrate that the DTC-SVM-PI offers best the results in terms of ripples.
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The induction motor drive is widely used in several industrial applications, such as the elevators, pumps, electric vehicle, steel and cement mills, chip propulsion, fans, and subway transportation, etc., thanks to its low cost, mechanical robustness and low rate of maintenance relative to the DC motors, several information about the induction motor can be found in Bimal (2002) and Enany et al. (2014). The DC motor disadvantages are the high cost, the important rate of maintenance due to the existence of the brushes and commutators, see Bimal (2002). However, from control point of view, the induction motor is considered as the one of the most challenging topics. The control of the induction motor is complex because its dynamic is nonlinear, multivariable, and highly coupled. Recently, several control strategies are utilized to control the induction motor drive, like the scalar control, Field Oriented Control (FOC) and the Direct Torque Control (DTC). The scalar control has been widely used to industry thanks to its easy implementation. The major drawback of the scalar control is the poor performance during transients. However, the importance of this control approach is diminished recently because of the superior performance offered by the FOC approach, as given in Bimal (2002). According to the Takahashi & Noguchi (1986), Baader et al. (1992) the DTC on AC drives was developed by Takahashi in 1986 and then by Depenbrock in 1992. The DTC is featured by its simple structure relative to the FOC, as given in Ai et al. (2010) and Casadei et al. (2002). The DTC provides a high dynamic speed response and a good tracking control in the torque and the flux; see Lekhchine et al. (2013). With these strong points, the DTC can be considered as an alternative solution to replace the FOC in several industrial applications.
Traditionally, position control applications are based on a Permanent-Magnet Synchronous Motor (PMSM), as given in Vittek et al. (2005), Vittek et al. (2008), and Shin et al. (2012), thanks to its high efficiency, high torque and low inertia, as given in Tomohiro et al. (2014). The disadvantage of the PMSM is its high cost relative to the Induction Motor (IM). Thanks to the DTC, it is possible to use the IMs in position control applications. According to Rakesh (2003), The IM is more popular thanks to its robust performance, low maintenance and low cost. Utilizing an IM is equivalent to minimizing the cost of a control system.
Nowadays, the DTC has become the best candidate in several industrial applications especially where a high torque at a low speed range is required, in particular position control applications that require a maximum of torque when the motor is off, which favors the use of the DTC approach. Nevertheless, the basic DTC suffers from several drawbacks, like torque and flux ripples, stator current distortions, demagnetization problem at low speed, and commutation losses in the inverter. Referring to the researches works presented in Sivaprakasam & Manigandan (2013), Pham et al. (2016), Bhim et al. (2008), Atia (2009), and Rashag et al. (2013), these drawbacks are caused by the hysteresis controllers and the switching table used in its structure as well as the variation of the switching frequency. In fact, the conventional DTC uses two hysteresis controllers to control the torque and the stator flux. These controllers generate two states, which produce the same results for the small and the big error of the torque and the stator flux. As consequence, the generated switching states of the inverter cannot produce the best voltages vectors to make the flux and the torque errors both zero. This leads in ripples of torque and flux, and variations of the switching frequency as given in Kaboli et al. (2003), Sanila, (2012), Mohanty (2009), and Idris & Yatim (2004). Kang & Sul (2001) present an analysis of the switching frequency variation which is caused by the hysteresis band, the motor speed and torque slope. This leads in additional switching losses in the inverter.