Improved Performance of CFTC-based Direct Torque Control of Induction Machines by Increasing Torque Loop Bandwidth

Abstract

Constant frequency torque controller-based direct torque control (CFTC-DTC) of AC motor drives is known to have limitation with torque loop bandwidth, since it uses sampling frequency available from a digital signal processor for generating triangular carrier-based waveform. The switching frequency of the control system is determined by the carrier frequency of CFTC. The worst case of CFTC-DTC drive exists at very low speed under light load where the zero voltage vectors are dominant, and the radial component of active voltage vectors are very short leading to flux droop at sector transitions. This results in the distortion of motor phase current and aggravation of speed waveform. This paper proposes a modification on the frequency carriers of CFTC to increase the torque loop bandwidth by replacing the triangular carrier-based with ramp carrier-based waveform. The proposed method is proven by the simulation results showing excellent performance of motor flux, phase current, and speed at low speed.