A Mixed Control Strategy For Induction Motor Drives

Field oriented control (FOC) which is based on rotor flux orientation and current loop control and direct torque control (DTC) which is based on stator flux orientation and Bang-Bang control are the two major control methods for high performance induction motion drives. This paper overviews the present state and perspectives of these two methods, and illustrates how they are applied to the practical motor drives by analyzing their theories and algorithms. Researches and practical projects show that these two schemes each have their own advantages and disadvantages. Various improvements have been proposed for these two control methods by many researchers.An innovative control strategy, which is an organic combination of the two schemes, is presented in this paper to adopt the other's strong points to offset one's weakness, with the purpose of realizing more flexible and high performance motion control and providing users with more options in various applications. Because FOC and DTC differ a lot in their control theories, control objects and controllers, there is some fluctuation in the switching moment, which is the key problem need to be solved in the mixed control scheme. For smooth switching between DTC and FOC, a new "resetable" PI controller is designed to replace the conventional PI controller. At the meantime, these modifications do not change the original control schemes, so the good control performance characteristics of DTC and FOC can be maintained. This paper develops a set of real time motor control prototype based on a squirrel-cage motor. Simulation and experiments show that the new "resetable" PI controller can remarkably reduce the fluctuation of mixed control system, and the smooth transition is guaranteed no matter in steady state or transient state, while maintaining the control performance of DTC and FOC. Experiment and simulation results demonstrate the validity of mixed control strategy proposed in this paper.