Control System Design Of Asynchronous Motor In Field Weakening Region

High speed CNC is the technological base of equipment manufacturing industry and isone of its developing directions. The performance of high speed CNC depends on theperformance of high speed spindle, and nowadays there still exist a big gap between ourcountry and foreign advanced level in high speed drive technology. Asynchronous motor iswell suited for field weakening because of its structure, and it has been the main spindlemotor for a long time. For these reason, doing research on field weakening technology ofasynchronous motor is of certain practical significance.Decoupling between torque current component and flux current component can beachieved under the vector control based on rotor field orientation, which is the basis of fieldweakening control. In this thesis, first the dynamic equation of asynchronous motor isdeduced from the definition of space vector. Then the principles of rotor field orientation andindirect vector control are elaborated. In the vector control system of AC motor, SpaceVector Pulse Width Modulation (SVPWM) is the most popular generation method of pulsedrive signals. The fundamental of SVPWM and its implementation steps are introduced, andbased on Fourier expansion an over modulation method with linear gain is proposed, whichreduces the output voltage’s distortion and brings full modulation index at the same time.The operation process of asynchronous motor is limited by the maximum input voltageand current and the maximum slip. On the basis of analyzing these limited condition, currentvector locus with maximum torque is obtain, in line with which a basic field weakeningstrategy based on ideal motor model is derived. Considering the shortages of the basic fieldweakening strategy, an improved method with voltage feedback is proposed. Besides,complex vector PI controller is constructed to cancel the coupling between control quantitieswhich intensifies in high speed region, and a simple rotor time constant on-line adjustingalgorithm based on reactive power is designed, which aims at weakening the impact to field orientation brought by parameter inaccuracy.Finally, the whole field weakening control system of asynchronous motor is simulated inMatlab, and the effectiveness of proposed strategy and algorithm are verified.