Abstract
Loss minimization during transient operation is an often neglected in most induction machine applications. However, loss minimization can greatly improve efficiency in case of fast changing load torques or reference speed profiles. Optimal control theory has been successfully applied previously to develop control laws that minimize losses during transients. All previous works were based on the field oriented induction machine model. In this paper, however, an optimal control problem based on the non-field oriented induction machine model is developed and simulated. Additionally, a more comprehensive cost functional is developed and utilized that would guarantee a stable steady state operation of the machine. Offline optimal control histories are generated using the conjugate gradient method. The performance of the proposed control law is verified through simulation and the results are compared with those from an indirect field oriented controller.
I. INTRODUCTION
THE ubiquity of induction machines (IM) in industry has led to a large volume of research into improving their efficiency at various operating points. However, it becomes clear from the literature surveys on this topic that a majority of the work has focused on steady state loss minimization [1]. The most common approach to IM loss minimization is to optimize the rotor flux during steady state. But during operating point transitions (change in reference speed or load torque) the loss minimization algorithm is disabled and the flux is maintained at rated level so as to get a faster dynamic performance.
VII. CONCLUSION
Optimal control theory was applied to both non-FO and FO models of an IM to determine the necessary conditions for a control history that would minimize transient power losses and have an acceptable dynamic performance. Conjugate gradient method was used to obtain numerical solutions of the control law that satisfy the necessary conditions. Simulation results were presented that compared the performance of OC with that of IFOC. It was shown that a significant reduction of energy losses was possible during the transient period without undue sacrifice of the dynamic performance. Furthermore, it was show that OC based on non-FO models can give equivalent results compared to the existing OC approaches that exclusively use FO models. The work in this paper can be further extended by performing hardware experiments.
