Three-Level Inverter-Fed Direct Torque Control of the Synchronous Motor

Abstract

Synchronous motors and back-to-back converters with IGCT three-level NPC active front end (AFE) rectifiers are widely used in up-to-date large adjustable speed drives. Thanks to the development of the microprocessor technology and power electronics, Direct Torque Control (DTC) became possible and available. The structure and the formation of the switching table for switching devices of the inverter are of a great importance for the DTC system. The quality of the transient, particularly, fast response and precision of the motor variables control, depends largely on the table structure and formation. The paper describes the main principles of the inverter control. Twenty-four voltage vectors were divided into seven different variants. These variants can be represented in the form of two adjacent groups; each group contains two variants of short vectors, one variant of long ones and middle vectors are common for the both groups. As a result of the conducted research, a switching table was formed where the switchover from one variant of voltage vectors to the other one is provided by the switching of one switching device only. The simulation results obtained in the Matlab Simulink environment confirmed the efficiency and acceptable characteristics of the transients quality of the formed switching table. During the steady state operation the main control variables, such as torque and stator flux, are maintained with an error which does not exceed 5 %.