Torque-maximizing Field-weakening Control Of Induction Motors

The induction motor plays an important role in variable drive systems,which owes to its simple construction,convenient control and ruggedness.Operation at field weakening is required for applications like machine tools,spindle drives and traction drives.So it's necessary for induction motor to obtain high torque and achieve high performance in all ranges.Frequent start-stop,accelerate-decelerate and many other complicated situations often occur in electric vehicle,which calls for higher demands to the control system.We should consider both the steady-state and transient requirement.The paper focuses on the key control techniques of high drives for induction motors.Firstly,the paper discusses the basic principles of vector control for induction motor.In A-B-C coordinate system,the motor model equations are series of multi-variable,nonlinear,strong-coupling,high-order,and time-varying differential equations.So we transform the three-phase static coordinate system to d-q coordinate system,which realizes the decoupling between the flux linkage and torque,brings convenience to the analysis and control of the system.Then,the paper discusses the limiting methods when over-current,over-voltage,over-load occurs.The current and voltage limiting conditions permit defining three different operating regions.Based on the different characters of three regions,we present the control scheme.The proposed method fully exploit the machine and converter capabilities without the use of d-axis current controller.Maximum torque production in a higher speed range of field weakening is achieved by stabilizing the operating point at the pullout point through the q-axis component of the current applied across the stator.Operation at the voltage limit to dynamic changes,a temporary voltage margin is therefore created in a dynamic condition by deviating the trajectory of stator voltage vector which improves the transient response of the system.Iron losses,which are traditionally ignored in the vector control scheme,increase rapidly in the high speed ranges and affect the magnitude of rotor flux and output torque.The compensation of iron losses is necessary to ensure the torque maximized and promote the work efficiency of induction motor.Lastly,the paper builds up the model in MATLAB/Simulink and tests on the LEVDEO 72V/5KW induction motor.Simulation and experiments results prove that the proposed scheme provides good performance on the steady-state and transient,get the torque maximized in the field weakening ranges.