A new direct torque control for AC motors with over modulation ability

Comparison of various motor drives has revealed that direct torque control (DTC) drive has several advantages and a much faster torque response compared to other AC or DC drives. However, the conventional DTC cannot operate in the six-step mode, which is required in high-speed applications. Therefore, an accurate method is essential in order to achieve a seamless transformation between conventional DTC and six-step operation. In the present paper, a single-mode seamless control of AC motors is achieved by proposing a new direct torque control. Since seamless control is not possible using the conventional DTC, we develop a new DTC in which the flux trajectory is modified. In the proposed method, the motor flux trajectory changes continuously between a circular shape and a hexagonal shape, and thus the inverter voltage changes from high-frequency switching to a six-step square wave with only one control mode. Therefore, in the proposed method, which uses only one mode, torque discontinuity does not occur because this discontinuity usually occurs during switching between modes. In addition, since the proposed method is a single-mode control, this method is much simpler than existing technologies, which are based on multi-mode controllers.