Sensitivity Analysis of Flux Estimation Methods for Field Oriented Control of Induction Machines

Sammanfattning

Field-oriented control of induction machines requires precise knowledge of the rotor flux linkage angle. It is difficult to directly measure it, most often it is estimated using the induction machine model equations. However, this assumes good knowledge of the machine parameters, including resistances and inductances, which will vary during operation. Errors in the machine parameters cause errors in the field orientation, which ultimately lead to errors in the currents and produced torque. This thesis investigates and evaluates different flux estimation methods for speed-sensored control of induction machines over a wide speed range. The work includes a literature study of existing methods, from which three methods are developed and evaluated. First, the basic current model, which estimates the rotor flux linkage by simulating the rotor circuit, is modified by embedding it in a phase-locked loop with an estimate of the back-EMF to improve the performance at high speeds. Second, the basic voltage model, which estimates the rotor flux linkage by simulating the stator circuit, is implemented with a modified integration algorithm to solve issues associated with open-loop integration. Third, a closed-loop full-order observer is implemented. The possibility of real-time resistance estimation with the full-order observer is also explored. To evaluate each method, a model of an induction machine drive is constructed in Simulink. This includes a discrete-time controller, a voltage source converter with switching dead times, and an induction machine plant with non-linear inductances. Each flux estimation method is implemented in the controller and the field orientation error, torque error, and flux magnitude error are quantified for varying parameter errors and voltage errors. The results show that different methods are robust to different parameters, and general recommendations are given for suitable methods depending on the application requirements.