Direct Torque Control Of Induction Mortor Fed By Three-Level Inverter

Multilevel inverters are very suitable for high-voltage and high-power conversion applications, for they have the advantages such as low voltage stresses for power devices and reduced harmonic content of the output voltage waveform. At the same time, the direct torque control (DTC) of induction motors has been widely used in recent years because of its simple structure and high stable/transient performance.In this paper, two direct torque control algorithms for induction motors fed by three-level diode-clamped inverter (neutral-point clamped, NFC) are presented.On the basis of the concept of space vector, the basic theory of DTC is analyzed. Also a representative DTC strategy improving operation performance of induction motors, which is called DSVM, is introduced. The performances of the two strategies are analyzed with the simulation results.For three-level NPC inverter, to keep the neutral-point-voltage balanced and simplify the complexity of vector selection, a virtual-vector method is applied.To avoid the stator-flux drooping phenomenon provoked by nonzero-voltage vector at low-speed operations six-sector plane is subdivided into a 12-sector plane.The phenomenon that a given voltage vector has a different effect on the drive behavior at high and low speed is analyzed. Taking these into account, a DTC algorithm for three-level NPC inverter, which is based on multilevel hysteresis comparator, is presented. To reduce torque ripple, a DTC based on torque prediction is proposed for induction motors fed by three-level NPC inverter. Simulation results show the effectiveness of the two algorithms.What is more, a 3kVA three-level NPC inverter is designed to verify the proposed methodologies.