Direct Torque Control of IM Fed by 3L-NPC Inverter with Simple Flux Regulation Technique

Abstract

Classical direct torque control (DTC) is considered one of the simplest and fastest control algorithm in motor drives. However, it produces high torque and flux ripples. Moreover, it loses its robustness when driving induction motor (IM) at very low-speed region. Replacing the conventional three-level hysteresis torque regulator (HTR) by five-level HTR for three-level neutral-point clamped (3L-NPC) inverter reduces the torque and flux ripples in DTC drives. Nevertheless, the stator flux is unregulated at very low-speed of IM where the domination of stator resistance is high. This problem causes the stator current to be highly distorted. To alleviate this problem, a simple flux regulation method is applied for low-speed operation of IM fed by 3L-NPC inverter. The proposed flux regulation algorithm modifies the output status of the five-level HTR depending on flux error, torque error and stator flux position. It first detects the direction of stator flux vector and which sector has been affected by flux-droop, and then it modifies the five-level HTR status to eliminate the usage of zero voltage vector for that particular effected sector only. This method stabilizes stator flux for both forward and reverse rotational directions of IM with retaining the basic structure of classical DTC. By using the proposed algorithm, the stator flux is regulated hence pure sinusoidal current waveform is achieved which results in lower total harmonics distortion (THD). Simulation results show the effectiveness of the proposed flux regulation method.