Estudo de observadores de estado combinados para acionamento sensorless de motores de indução trifásicos

Abstract

State observers are dynamic models whose state variables are estimates of variables in a system. In a dynamic system not all states may be available for measurement. The cost of adding the necessary sensors may make the project unfeasible or it may be physically impossible to measure all the state variables. In this perspective, this work deals with the study and implementation of speed and flux observers applied to a three-phase induction motor with a 0.25 hp squirrel cage rotor (IM), aiming at speed sensorless control. The IM is driven by using the Indirect Field Oriented Control (IFOC) technique. Controllers are proportional-integral (PI) type tunned by pole allocation. Six distinct observers are implemented, three of which are classical observers in the literature, namely: MRAS; SMO; and EKF. The other three observers are obtained through combinations between the classical observers mentioned. The first of the combinations consists of using an SMO as a reference model for a classic MRAS observer. For the second combination, it is proposed to use an EKF observer combined with an MRAS observer. This combination results in excellent flux and speed responses, in addition to the simplification and elimination of an adaptation mechanism commonly found in MRAS topologies. For the third combination, it is proposed to use an EKF observer combined with an SMO, which shares the same advantages of the previous combination. The simulation results are auspicious, taking into account that the estimate resulting from the application of the proposed observers presents a negligible percentage error. The analysis of the quality of the output signal of the six observers is carried out by decomposing the signals into their frequency components (Fourier Transform). The observers proposed in this work have a low frequency content in the velocity variable. As a result, the robustness of the system becomes high, enabling the use of simpler controllers. The experimental results corroborate the analysis of the simulated results. The proposed observers (EKF combined with MRAS and EKF combined with SMO) present a reliable speed response to the identified system, in addition to better estimates of magnetic flux. The voltage sensor, necessary for the observers implementation, is replaced by the reconstructed voltages through the switching signal sent to the frequency inverter, thus achieving the elimination of the encoder and the voltage sensor.