Subharmonic content in Finite-State Model Predictive Current Control of IM

The interest in model based current control has been steadily growing during the last years. In particular, predictive techniques using a model and an exhaustive optimizer have been used, with many variations, for current control of IM using a VSI. In this paper it is shown that predictive controllers using the standard quadratic cost function seem to be prone to produce currents with subharmonic and non-integer harmonic content. This subject is rarely encountered in the predictive literature. The study shows that the harmonic content these controllers depend heavily on the precision of the predictive model. The relationship between model accuracy and harmonic distribution is illustrated using simulations of a three-phase induction machine. The results unveil complex relationships among factors that are analyzed providing guidelines that help in the practical design of predictive controllers.