Qualitative performance evaluation of torque control methods for industrial applications

摘要

Robust control of torque has been a motivation for research in the servo and electric drives industry. For industrial drives and servo applications AC machine drives are gaining prominence over the conventional DC-motor drives. Two methods, (a) field oriented control (FOC) and (b) the direct torque control (DTC) have been proposed to control an AC motor. FOC is a current control method and needs co-ordinated transformation. Torque dynamic depends upon the modulator, which also decides the steady state performance. DTC on the other hand is more robust and has physically the fastest torque dynamic. These methods have succeeded in achieving a fast and dynamically decoupled torque control but at what expense? Problems of FOC are saturation of current controllers, current control in the overmodulation region, and parameter sensitivity. Problems with DTC are variable switching frequency and sampling period dependence of hysteresis controllers, during digital implementation. This paper highlights the problems and explains a method of achieving direct control of torque using a method of stator flux vector control. The analysis carried out here is backed with appropriate simulation results.