DIRECT POWER CONTROL OF INDUCTION MOTOR DRIVES

Abstract

Electric energy is the basic source of living, and the growth of energy demand is associated with prosperity around the world. At present, a major challenge is to meet the demand for energy in an environmentally friendly manner. Globally, electric motors are the largest consumers of electricity and more than 90 percent of them are induction motors. Generally, electric machines are designed to achieve maximum efficiency and high-power factor at and around the rated load. Therefore, improvements in efficiency and power factor reduce motor operating costs and reactive energy bills, respectively. To accommodate the excitation, the induction motor must either be fed through an inverter or redesigned with optimization algorithms. Unlike constant speed drives, variable speed drives provide precise speed control to match the load requirements. In previous numerous years, various control schemes have been developed for IM control, out of which field-oriented control (FOC) and direct torque control (DTC) schemes are most preferred. Even though FOC offers several advantages of decoupling but in high performance drives, where precise control is desirable, calculation of accurate flux, torque and flux vector angle is crucial, moreover it is motor parameter dependent. The Direct Torque Control (DTC) is another control method, which provides superior static and dynamic torque response by using a predestined switching table and hysteresis controllers. Recently the use of space vector modulation, Kalman filters and observers etc. with DTC, made it to tackle all those issues, but partial improvements increase complexity and sacrifices the ease of conventional DTC. In the starting of 20th century, a proposal of instantaneous power control (IPC) was presented, however the process was complex and most of the calculation in the proposed method was reliant on machine parameters. In many of the industry applications where power factor and efficiency have significant role, above discussed method cannot be functional, as in vector control even though flux is maintained constant, but both real and reactive power loss increase with loading and hence reduces the power factor and the efficiency. Consequently, an innovative approach for speed control of induction machine is required with the provision of improved efficiency and power factor etc. In this thesis, the importance of energy conservation on induction machine and its different applications is discussed. Along with that a brief literature review on variable speed drives and detailed review on speed control of induction machine drives with their contribution in energy saving technology is also discussed. The mathematical modelling, and an expression of electromagnetic torque for induction machine is established with a set of first order nonlinear equations. The mathematical modelling of three phase induction machine with its control systems is designed in MATLAB/ Simulink environment. Further in this thesis a novel flux states based nonlinear control method is presented which results in improved dynamic performance of induction motor drive. The proposed control method is simulated in the MATLAB/Simulink environment and experimentally validated on a 0.75kW IM. Further in this thesis, a novel high-performance direct power control (DPC) approach for induction motor (IM) drive is presented. The DPC clearly removes the certain disadvantages of conventional control schemes without integration of complex observers, filters, and reactive power compensators. The simulation and experimental studies are carried out to validate the proposed scheme and results are compared with the conventional indirect rotor FOC scheme. This thesis also contributes a fuzzy expert system based reactive power control of IM drive for EV application, where the flow of reactive power is governed by the reference speed and torque commands using fuzzy logic. Importantly this approach not only avoids the integration of complex flux estimators and observers, but remarkably the improved power factor operation is obtained throughout the EV cycle. To evaluate the performance and the effectiveness, MATLAB/Simulation and experimental study is carried out on a 0.75kW induction motor drive considering the wide-ranging EV driving cycle and compared with the widely used FOC scheme. The obtained results validate the proposed algorithm with noticeable improvements in power factor during wide-ranging EV driving cycle.