Direct torque and flux regulation in sensorless control of an induction motor

A novel scheme for direct torque and flux control of induction motors (IM) is presented. The scheme does not use a transformation to the rotor flux related frame and it guarantees the asymptotic stability of the torque and flux tracking. To perform the controller design a torque-flux dynamic model with stator voltage as an input is obtained and separation of the torque and flux regulation is achieved. The controller is elaborated as a part of the sensorless control scheme of IM and it shows robustness with respect to rotor time constant changes. For slow reference commands a control scheme based on the steady state solution of the torque-flux model is also analyzed. It is significantly simpler than the dynamic scheme and has better robustness properties with respect to the current noise. The controllers can also be applied if the rotor speed signal is available. The analysis of the stator current and voltage constraints determines the allowable set of the torque and flux reference commands. Simulation results are presented to verify the proposed approach