Virtual Space Vector-Based Direct Torque Control Schemes for Induction Motor Drives

This article presents new direct torque control (DTC) schemes for induction motor (IM) drives based on the concept of virtual voltage space vectors. In the first scheme presented in this article six virtual voltage space vectors alone are used for realizing a DTC with constant switching frequency. The second scheme utilizes both virtual voltage space vectors and the actual voltage space vectors for realizing a twelve-vector DTC for reducing the torque ripple. In the third scheme the number of vectors is increased to 18, by utilizing 12 virtual voltage space vectors and six actual vectors. The fourth scheme extends the concept of virtual vectors to dual inverter fed open end winding IM drives to eliminate the common mode voltage across the windings with increased number of vectors. The virtual voltage space vectors are obtained by switching the adjacent voltage space vectors of a three-phase, two-level inverter at constant switching frequency. Like the conventional DTC scheme these DTC schemes are simple as they are free from co-ordinate transformations and real time computations. These DTC schemes are experimentally verified on an IM fed by two-level voltage source inverters under transient as well as steady state operating conditions. A digital signal processor (TMS320F28335) is used for implementation of the control scheme.