Thesis Abstract

Abstract
This research project proposes a novel approach for controlling a squirrel cage induction motor drive using a current source inverter (CSI) with constant air gap flux control. The conventional voltage source inverter (VSI) fed induction motor drives have limitations in achieving precise control over the motor's torque and flux due to their inherent voltage control characteristics. In contrast, the proposed CSI-fed drive offers improved performance by directly regulating the motor current, resulting in better control of the air gap flux and torque. The control strategy implemented in this project involves maintaining a constant air gap flux by adjusting the amplitude and frequency of the current injected by the CSI. This control scheme ensures that the motor operates at the desired flux level under varying load conditions, thereby enhancing the motor's efficiency and performance. Additionally, the use of a CSI allows for bidirectional power flow, enabling regenerative braking and improved dynamic response during rapid changes in the motor speed. Simulation studies were conducted using MATLAB/Simulink to validate the proposed control strategy and evaluate the performance of the CSI-fed drive system. The simulation results demonstrate the effectiveness of the constant air gap flux control in regulating the motor's speed and torque with high accuracy. The system's robustness and stability were also assessed under various operating conditions, including sudden load changes and speed references. Furthermore, the proposed drive system offers advantages in terms of reduced harmonics and improved power quality compared to VSI-fed drives. The CSI's ability to deliver controlled current waveforms results in lower total harmonic distortion (THD) and improved power factor, making it suitable for applications requiring high-quality power supply. Overall, the research findings highlight the potential of utilizing a CSI-fed constant air gap flux control strategy for enhancing the performance of squirrel cage induction motor drives. The proposed approach offers superior control over the motor's flux and torque characteristics, leading to improved efficiency, reliability, and dynamic response. Future research directions may focus on experimental validation of the proposed control strategy and exploring its application in real-world industrial systems.

Thesis Overview

In this thesis, a current source inverter fed squirrel cage induction motor drive at constant air gap flux is presented. The drive scheme is conceived to take advantage of the short circuit withstand capability of the current source inverter and the ruggedness (even under harsh conditions) of the squirrel cage induction motor. The variable inverter input dc link current is derivable from either a controlled ac to dc converter or a controlled dc to dc converter. The constant motor air gap flux control drive scheme has inner dc link current control loop and an outer motor speed control loop that maintains the motor slip frequency constant for a given motor load torque. The closed loop control parameters are selected such that negligible torque pulsation and relatively fast motor speed response are obtained without the need for an inverter output capacitor filter. A 400V, 10hp, 1440rpm, 50Hz squirrel cage induction motor is used to simulate the motor drive scheme in Matlab-Simulink environment. For step changes in demand (reference) speed at no load torque and at load torque equals to or less than the motor rated torque, the drive response has relatively fast settling time and acceptably low overshoot/undershoot over the motor speeds not exceeding the rated value.