Vector control of AC induction motors

Alternating current (AC) induction machines are frequently used in variable speed drives with applications ranging from computer peripherals, robotics, and machine tools to railway traction, ship propulsion and rolling mills. High performance motion control with AC motors is achieved through the so-called vector control. Vector control means, in general, decoupled flux linkage and torque control yielding fast torque response and, through adequate reference flux linkage-torque relationship, high energy conversion rates. The first part of this thesis is to investigate different vector control algorithms, understand the working principles and establish a dedicated simulation package based on MATLAB. By using this software, we can evaluate the performance of different control algorithms easily. Furthermore, an advanced control method, H∞ loop shaping was also implemented and evaluated in this package. The second part of this research project is to build a hardware test bed so that we can verify the simulation results by the hardware experiment. The test bed includes four building blocks, namely, PWM (pulse width modulation) driver, current and velocity sensing units, low cost DSP (digital signal processing) implementation of advanced speed controller and vector control algorithm. Both analog and digital circuit design techniques were needed in each block of this mixed-signal system. Six IGBTs (Insulated Gate Bipolar Transistor) were employed in the PWM driver in order to convert a rectified 500V DC into a variable frequency variable amplitude three phase AC source. A hall effect sensor and an optical encoder were used in the current and velocity sensing units respectively. Complex mathematical functions such as the coordinate transformation were implemented by writing assembly language of a low cost DSP fromAnalog Device Inc. With the available test bed, we compared the theoretical and the experiment results and found out that they matched well with each other. Moreover, the output response of the vector controlled AC induction motor resembles a DC (direct current) motor, which means that we can emulate a DC motor by an inexpensive, rugged and maintenance free AC induction motor