Direct Torque Control for sensorless induction motor drives using an improved H-Bridge multilevel inverter

This paper presents the application of a sensorless direct torque control (DTC) for an induction motor, using an improved H-Bridge multilevel inverter with 27-Levels. The inverter topology reduces the power sources from nine to only four active sources and three ultracapacitors. The power sources are unidirectional and non-redundant; scaled in power of three to optimize the number of voltage levels with a minimum of semiconductors and power sources. For the improved inverter topology, two additional control strategies are introduced; 1) an inhibit negative currents (INC) controller, which solves the regeneration problem when unidirectional sources are used; and a 2) proportional-integral (PI) controller, which keeps the ultracapacitor voltages at the reference value. Both controls work on a pulse width modulation (PWM) signal, where the INC control decides the levels among the PWM operates, and the PI controller changes the duty-cycle. A closed-loop estimator called model reference adaptive system (MRAS) was used for the speed estimation, with the advantage of using the stator voltages and currents already obtained for the DTC. The application of the system was simulated and implemented in Matlab®/Simulink® software using the industrial controller AC800PEC from ABB, obtaining satisfactory results. The multilevel inverter was specially designed and built for this application.