PUBLICATIONS

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.