Constant frequency torque controller-based direct torque control (CFTC-DTC) is emerging as a powerful control strategy for a high-performance control of alternating-current motor drives. Compared to DTC, the CFTC-DTC has less torque and flux ripples with a fixed switching frequency. Nevertheless, it has a limitation of the torque-loop bandwidth, as it uses the sampling frequency from a digital signal processor to generate a triangular-based carrier. The switching frequency of the control system is determined by the carrier frequency of the CFTC. The frequency limitation of CFTC leads to sector-flux droop, which causes a distortion in the motor phase current and aggravation in the torque and speed waveforms at low speed regions. In this article, we propose a modification of the frequency carriers of the CFTC-DTC of induction motors to increase the switching frequency and torque loop bandwidth by replacing the triangular-carrier-based waveform with a ramp-carrier-based waveform. The proposed method is verified by simulation and experimental results showing its excellent performances in steady-and transient-state operations.
Manuscript received April 19, 2019; revised July 2, 2019, September 22, 2019, and November 6, 2019; accepted November 27, 2019. Date of publication December 18, 2019; date of current version August 18, 2020. This work was supported in part by Korea Electric Power Corporation under Grant R19XO01-20 and in part by the Railroad Technology Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government under Grant 19RTRP-B146008-02. (Corresponding author: Kyo-Beum Lee.) The authors are with the Department of Electrical and Computer Engineering, Ajou University, Suwon 16499, South Korea (e-mail: alsofyani2002@yahoo.com; kyl@ajou.ac.kr).
