Direct torque control for stacked multicell (SMC) VSI fed induction machine

This paper deals with the control of SMC converters for an induction machine drive. Different degrees of freedom offered by this topology are addressed and used to develop strategies aiming to control electromagnetic variables of the machine, to balance three phase inverter switchings and inverter flying capacitor voltages. All of these techniques are designed to be used with an SMC converter with any number of levels. For the control of the electromagnetic state of the machine, a "direct control" based strategy is used. This technique is well-known for both torque and flux reduced ripple and high dynamics. These performances obviously increase with the number of levels of the inverter. Another degree of freedom of the SMC inverter is used to control the switching frequency. Two techniques are presented: the first aims to regulate the switching frequency and the second proposes to minimize the global number of switchings of the converter. Finally, the flying capacitor voltages are balanced using a third degree of freedom of the SMC converter. It is shown that the algorithm regulates the cell voltages, which implies a very good control of the capacitor voltages. Simulation results are shown for 7-, 9- and 11-level SMC inverter.