Analyzing the DTC System Structure with the Help of Modeling

Abstract

Direct torque control is a vector-control method that is widely used in variable-frequency drives. This system consists of specific control units. Interacting, they form the control mode. DTC has the analog speed controlled and relay torque and flux linkage controllers. The system core is made up of units handling discrete signals. The objective is to discover relations between system elements, analyze how signals are formed within the system, study the system behavior in different modes of operation and under different controller settings. For this purpose, the structural modeling method is used, implemented with SIMULINK in MATLAB. As a system element, the frequency changer is a three-phase DC-powered voltage inverter. The structural circuit of the asynchronous motor is presented in the framework of a static coordinate system. Relay controllers are three-position, with variable hysteresis loop and deadzone widths. The method involves using models of the phase sector identification unit, electric drive coordinate positioning units, switching table execution unit, and switching table string signal forming unit. Configuration of the control system consisted of setting up the speed controller and properties of relay torque and flux linkage controllers. Parameters of the speed controller and relay torque and flux linkage controllers were found empirically. As discovered, the system is highly sensitive to the parameters and properties of controllers. The developed model was used to analyze how control signals are formed at different times, depending on values of torque and magnitude of the flux linkage vector of the asynchronous motor. Such analysis helps realize the way system units interact within any specific period. System operation was analyzed for two cases: using (1) a proportional and (2) a proportional-integral speed controller. It was discovered that the motor torque rapidly responds to changes in control signals. However, in view of the relay nature of control, torque and speed fluctuate in the drive system even at the constant set speed.