Control of AC Motor Drives: Performance Evaluation of Industrial State of Art and New Technique

Achieving fast, smooth and robust control of torque has been a motivation for research in the AC drives industry. The challenge is to replace the maintenance prone dc motor drives with more rugged and efficient motor drives. To this end, two methods, (a) Field oriented Control (FOC) and (b)the Direct torque control (DTC) have gained prominence. FOC is a current control method and needs coordinate transformation. Torque dynamic depends upon various factors which also decide the steady state performance. Classical DTC on the other hand is more robust and has physically the fastest possible torque dynamic. These methods have succeeded to achieve a fast and dynamically decoupled torque control but at what expense? Problems of FOC are, saturation of current controllers, current control in the over modulation range and parameter sensitivity. Problems with DTC are, variable switching frequency and sampling period dependance of hysteresis controllers, during digital implementation . This paper highlights these problems and shows how to solve them by exploiting the advantages of both methods. Dynamic and steady state analysis has been carried out for validation of the concepts with requisite simulation and experimental results.