Optimal rotor flux control for induction motors

The root loci of induction drives show weakly damped roots for the motor running at low speeds in accelerating mode. Thus, the torque will oscillate during speed-up. We show in this paper, that the stationary and dynamic behavior of the motor is improved, if the rotor flux peak is regulated to a constant reference. Thus, the stator voltage and stator flux peak are decoupled from the torque. Dynamically, the weakly damped dominant root pair at low speeds is converted into a fast and a slow root giving a low pass torque behavior. Two new control laws are presented. The former controls only the stationary behavior of the rotor flux peak and leaves the dynamics unchanged. Real measurements show that, whenever enough friction of the appended mechanics is present, this controller is a valuable solution. The latter is a dynamic extension of the first one and controls the rotor flux peak at all times to a constant reference using an observer based LQR-designed state feedback. The root locus of the controlled induction drive shows that this controller is a good choice, if mechanics with low friction are used with the motor. Such a result is confirmed by simulations. The main advantage of both controllers stands in their simplicity and modest requirements on hardware, i.e. voltage inverter, measurement, computational power