Study and Analysis of Controls for Induction Motor Drives for High-Power Applications

Induction motors have become the backbone of modern industrial systems, owing to their unparalleled efficiency, robustness, and low maintenance requirements. Their simplicity in design, coupled with high reliability and cost-effectiveness, has made them the preferred choice in various applications, ranging from manufacturing and transportation to renewable energy generation. Furthermore, induction motors offer seamless speed control and can operate across a wide range of environmental conditions, making them indispensable in various industrial settings. In recent years, rapid advancements in technology have led to the development of sophisticated control strategies and innovative materials, enhancing the performance and versatility of induction motors. Researchers and engineers have focused their efforts on optimizing motor designs, improving energy efficiency, and reducing carbon emissions. This study focuses on two of the main control strategies for robust variable-frequency drives of induction motors, V/Hz and Field Oriented Control (FOC). The first part of the thesis will concentrate on the open loop V/Hz control, one of the widely recognized speed control techniques. Within this control method, two modulation techniques are examined: asynchronous modulation and synchronous modulation. The asynchronous modulation technique introduces subharmonics, which can be problematic. To address this issue, the synchronous modulation technique is proposed and implemented alongside the open-loop V/Hz control strategy. Simulation tests are conducted to validate the elimination of subharmonics. Another challenge associated with the open-loop V/Hz control strategy is the presence of significant current and torque oscillations at low to medium frequencies due to nonlinear interactions between the electrical and mechanical subsystems. To mitigate these undesired oscillations, a stability analysis of the open-loop V/Hz control is performed, identifying a region of instability. Two mitigation techniques are presented in this thesis: the varying slope V/Hz control strategy and the active damping control strategy. These techniques are verified and validated through simulation tests on a 7 MW medium voltage (MV) induction motor using MATLAB/Simulink and a low voltage (LV) induction motor in a laboratory setup without a mechanical load. Furthermore, the thesis explores the impact of considering the magnetic saturation of the motor, which leads to more stable operations. Simulations confirm that considering magnetic saturation allows for the use of higher flux values, resulting in more stable machine operations and greater torque. Experimental tests on a 10-kW induction motor validate these findings. Closed-loop control methods, such as Field Oriented Control (FOC) and Direct Torque Control (DTC) are mostly preferred compared to V/Hz in order to achieve high dynamic performances. In the second part of the thesis a submarine application of a sensorless induction motor drive where a FOC algorithm is adopted has been considered. The electric motor serves as the prime mover for propelling a gasoline dispensing apparatus situated at a substantial distance from the offshore petroleum extraction platform. Consequently, the motor-pump assembly is significantly removed from the primary processing facility. The system analyzed consists of an inverter that links to an induction machine via a filter, as well as a lengthy submarine cable. The inverter and filter are situated on the offshore oil platform, while the motor and pump are located in the subsea oil extraction area, with a cable spanning a distance of 19.74 kilometers connecting the two. This arrangement induces a voltage drop that necessitates compensation. After presenting the system and its features, such as the rotor flux Luenberger Observer, and the speed estimation algorithm, the compensation strategy for the filter and cable has been investigated by implementing the entire system in the MATLAB/Simulink environment and validated through simulation results.