Predictive torque control for an induction machine with a virtual multilevel inverter

In this paper a predictive torque control for an inverter fed induction machine is presented. The proposed scheme assumes a virtual multilevel inverter and utilizes a conventional pulse width modulator (PWM). A first estimation of the voltage space phasor that has to be synthesized by the inverter is computed by using the equations of the mathematical model of the machine together with the measured currents, the shaft position and the reference value for the torque and the flux. The switching patterns are calculated by assuming that the “virtual” inverter has an arbitrary high number of levels. The cost function in a predictive control scheme is minimized by defining a mesh of voltage space phasors that correspond to the output states of the “virtual” multilevel inverter. The center of this mesh with a finite number of elements corresponds to the first calculated value of the reference voltage. For the minimization only the points belonging to the mesh are considered and the voltage space phasor chosen in this way is then used for the calculation of the switching signals in a conventional space phasor modulator and sent to the real two-level inverter. All delays of the digital control are compensated in order to get a suitable behavior of the control. The proposed scheme results in an easy implementation and delivers good dynamics in the torque behavior.