Speed sensorless direct torque control of 3-level inverter-fed induction motor drive based on optimized switching table

Three-level neutral-point-clamped (NPC) inverters are very appropriate for high-power adjustable-speed drive applications. Direct torque control is characterized by the merits of fast response, simple structure and strong robustness to motor parameters variations. This paper presents a 3-level inverter-fed direct torque controlled (DTC) induction motor (IM) drive based on optimized switching table. The problems of high voltage jump, smooth vector switching and neutral point balance are investigated and solved by introducing appropriate intermediate vectors. Furthermore, a speed-adaptive flux observer is employed to enhance the performance of the system by estimating the state variables including the stator flux and state current. Large starting current, which can occur in conventional DTC drives, is controlled effectively by introducing pre-excitation of the stator flux. Both simulation and experimental results are presented to verify the effectiveness of the proposed schemes introduced in this paper.