Comparison of three-level torque hysteresis controllers for direct torque control

In this paper, a comparison is made between different implementations of the three-level torque comparator for a direct torque control (DTC) based induction motor drive. The DTC scheme controls stator flux and torque by means of hysteresis comparators, respectively a two-level control structure for the stator flux and a three-level comparator for the electromagnetic torque. The standard three-level hysteresis controller has a DC offset torque error. In this paper, an additive implementation is investigated in order to remove this DC offset error. The operational principles of the different implementations are compared based on continuous and discrete simulation results, which demonstrate the additional advantages of the additive structure in a discrete implementation of the control scheme.