Ibrahim Mohd Alsofyani, Yeongsu Bak, and Kyo-Beum Lee, "Fast Torque Control and Minimized Sector-Flux Droop for Constant Frequency Torque Controller Based DTC of Induction Machines," IEEE Transactions on Power Electronics, vol. 34, no. 12, pp. 12141–12153 > Paper

In this paper, two simple control methods are proposed for improving direct torque control with a constant frequency

torque controller (CFTC-DTC) in induction machines. The CFTC-DTC was initially introduced to reduce torque ripple and achieve

constant switching frequency in inverters. However, when compared to the original direct torque control, the CFTC-DTC algorithm

suffers from slow torque dynamic response owing to the selection of zero-voltage vectors during torque transient. In addition, when

the motor operates at low speed, flux droop occurs at sector transitions, which leads to undesirable current distortions. The problem

of flux droop occurs because of shortduty voltage cycles and small radial voltage components at low speeds. This paper has two

main contributions: to eliminate zerovoltage vectors during torque dynamics in order to establish a fast torque response in the

transient state, and to minimize the flux droop at the sector transitions by applying another triangular carrier and modifying the

torque output in order to increase the duty voltage cycles at low motor speeds. A detailed analysis of the problems and proposed

methods is presented. Experimental results are provided to validate the effectiveness of both proposed methods.