Field-Weakening Methods for Torque-Flux Direct Field-Oriented Control of Induction Motors

The torque and flux direct field-oriented control systems of induction motors based on the proposed open-loop and closed-loop field-weakening methods are presented. Field-weakening operation regimes are designed to take into account physical limitations of the motor and inverter. The developed solutions guarantee torque-flux reference trajectories tracking in all speed regulation regions of induction electric drive: constant torque, constant power and constant slip frequency. Tracking of torque and rotor flux vector module is ensured by special simultaneous generation of the torque and flux reference functions. The closed-loop method has an additional non-linear feedback channel of inverter voltage module which adjusts the flux reference based on the measured (estimated) stator voltage module in order to maintain it at the maximum level. It is shown by simulations that the closed-loop algorithm with a voltage controller provides more complete utilization of motor power compared to the open-loop method. It increases the dynamic power in the supply voltage limitation conditions. The proposed control systems have the typical structure of the modern vector-controlled induction drives and can be used to improve dynamic properties and efficiency in the field-weakening operation mode.