Feedback of the Input Voltage in FDTC Control Using a Three-Level NPC-VSI

Abstract

A new control structure for sensorless induction machine dedicated to electrical drives using a three-level voltage source inverter VSI-NPC is completely analysed. The amplitude and the rotating velocity of the flux vector could be controlled freely. Both fast torque and optimal switching logic could be achieved; the selection is based on the value of the stator flux and the torque. A novel DTC scheme of induction motors is proposed in order to develop a suitable dynamic. We proposed a DTC approach based in fuzzy logic, in witch the response of torque and flux is enhanced trough optimal switching strategies and torque ripple minimisation is achieved. However, the middle point voltage of the input DC voltages of the three-level NPC voltage source inverter presented serious problems caused by a fluctuation of the DC voltage sources U_CU, U_CL. As consequence to these problems, we obtained an output voltage of the inverter, which is asymmetric and with an average value different from zero. We presented one solution to minimise this fluctuation. This solution used a feedback to regulate the input voltages of a three-level inverter VSI NPC. A scheme of a fuzzy direct torque control "FDTC" with complete cascade is simulated for an induction motor. The results obtained indicate superior performance over the FOC one without need to any mechanical sensor.