Fast-transient current control strategy and other issues for vector controlled AC drives

The sensorless vector controlled drive system of AC motors has been examined by several researchers for over the past fifteen years. Most of this research has been dedicated to the design of flux and speed estimations using advanced control theories such as Sliding Mode Control, Model Reference Adaptive System, and Kalman filter. Only a few publications have focused on the current control strategies for the vector controlled drive of AC motors. Typically, the regular PI controller is chosen because of its ease of implementation. However, under a limited DC-bus voltage, the AC motor drives with PI controller cannot produce the fast current responses with a sudden change of speed reference or load torque, because the applied voltage becomes saturated during the occurrence of a current overshoot. Although the full DC-bus voltage is applied, this phenomenon can also happen when the machine is operated at very high speeds.; This dissertation includes: (1) an investigation and implementation of the generic flux and speed estimations that are applicable for both induction and permanent-magnet synchronous motors; (2) an analysis of the quantization errors inherent in a digital controller with fixed-point digital implementation; and (3) a proposed time-optimal control technique for fast transient current response in the sensorless vector controlled drive of AC motors. Based on the Pontryagin's maximum principle, it is guaranteed that the dq-axis current transition from one point to another point is within the minimum-time under a limited DC-bus voltage. Both computer simulation and experimental investigations have been carried out to substantiate the proposed technique. The overall system is implemented in a 32-bit fixed-point digital signal processor based controller (i.e., eZdsp2812 board). Based on the results, the proposed time-optimal controller can achieve the fast current responses without the current overshoots and voltage saturations under a constrained DC-bus voltage.