Robustness analysis of a discretized current controller of induction motor drive

The robustness of the discretized current controller for an IM drive that has been developed by using the theory of variable structure systems and Lyapunov stability theory is discussed and the guidelines for the selection of controller parameters are given based on the root-locus method. For that purpose the discrete transfer function of a closed current control loop has been derived and its characteristic polynomial has been written in a form suitable for the plot of root-loci. The controller parameters have been chosen to assure a fast dynamic response in the transient states and the minimal current variations for the sinusoidal form in the steady state. Obtained plots of root-loci confirm the robustness of a closed current control loop under the variation of induction motor rotor time constant in a wide range. All theoretical issues have been verified through simulation and experimental results.