State Variables Estimation in Vector Controlled Induction Machines

Abstract

The paper analyzes the structures of vector control of an asynchronous motor based on stator flux linkage and rotor flux linkage. Appropriate simulation programs have been developed for these control structures. The influence of the error in the estimation of the motor inductance has been taken into account. The influence of the saturation effect in iron on the static and dynamic characteristics of an asynchronous motor at constant stator flux linkage and constant rotor flux linkage has been analyzed. A comparison of the structures of vector control of an asynchronous motor based on stator magnetic flux and rotor magnetic flux has been made. Step changes of the electromagnetic torque reference have been simulated with and without the influence of the error in the estimation of the leakage inductances and inter-inductances. The state variables, electromagnetic torque and power have been estimated on a laboratory model of a motor in an open control loop using digitally recorded stator voltages and currents. To make this procedure feasible, a programming algorithm was created in C language for the numerical integration of digitally recorded voltages and stator currents. Appropriate digital filters were created using the MATLAB software package with the aim of removing noise and parasitic effects. The paper also contains a brief overview of the theory of digital filters and the elements of digital filter selection. The state variable estimation procedure was also carried out using appropriate analog circuits. Mathematical procedures of summation, integration and multiplication were performed using circuits with operational amplifiers and analog multipliers, and analog filtering of the phase voltages of the asynchronous motor stator was performed with a second-order analog filter. In this paper, it was shown that the analog procedure for estimating the state variables of an asynchronous motor is recommended only for stationary modes of operation and for frequencies of the fundamental harmonic of the supply voltage that are less than 10 Hz. The need to estimate state variables at lower supply voltage frequencies (when the influence of higher harmonic components in the supply voltage becomes significant) can be solved digitally by filtering the stator voltage vector components with an analog filter, and then digitally integrating, adding, subtracting and multiplying them. In the sixth chapter, one of the vector control structures for an asynchronous motor without a speed measuring term is analyzed. The proposed vector control structure is based on the theory of adaptive control and observer theory with simultaneous identification of the stator ohmic resistance. This vector control structure differs from the conventional control structure based on a reference model with an adaptive system (MRAS) with respect to the definition of the reference and adaptive systems, and the choice of the rotational speed of the coordinate system in which the mathematical model is described. In a conventional control structure based on a reference model with an adaptive system, the so-called voltage model for estimating the rotor magnetic flux is usually used as the reference model, and the current model as the adaptive model. In this paper, the voltage model was considered as adaptive, and the current model as reference. In this work, a suitable program was created for the simulation of start-up and braking of an asynchronous motor with simultaneous evaluation of the rotation speed and identification of the ohmic resistance of the stator.

Dinko Vukadinović

Full Professor | Department of Power Electronics and Control

Full professor at the Faculty of Electrical Engineering, Mechanical Engineering, and Naval Architecture in Split, specialized in modern control systems for power electronic converters, electric motors, and generators. At the Power Electronics Research Laboratory, he leads experimental projects and develops advanced methods for regulating electrical machines and converters, while supervising doctoral research in these areas.