Digital deadbeat controller based direct torque control of an induction motor

Abstract

In order to improve the transient and steady-state torque response of a direct torque controlled induction motor drive, a deadbeat controller has been introduced in this study. A deadbeat controller can exhibit the deadbeat response and zero steady-state error even with a very low switching frequency and can be easily designed and implemented according to the choice of the overall closed-loop pulse transfer function in z-plane. In this paper, deadbeat flux and torque controllers have been designed with only the stator dynamics to take advantage of the direct torque control strategy. Whereas the flux controller has been easily designed by employing high controller gain, the torque controller has not been easily designed because of the speed voltage. Consequently, the effect of the speed voltage has been compensated at the command side in this torque control system. Although a deadbeat control scheme has been applied, large torque overshoot has been caused while the torque command has been well followed in the steady-state with the aid of constant flux magnitude. This is why controllers have been designed with only the stator dynamics like in conventional controllers. Thus, another controller gain has been added to the proposed torque controller, so that a deadbeat torque response could be established by considering the rotor dynamics. Furthermore, characteristics and performance of the proposed control system has been analyzed with approximated full-order model of an induction motor. That is, the stability condition of the proposed control system, pole positions of the closed-loop pulse transfer function, and the final value of the steady-state error have been examined in the z-plane. Also, the influences of the internal and external disturbances such as the slip frequency, load torque, and the mechanical friction have been investigated. In accordance with the results of analysis, controller and compensation gains for a deadbeat torque response have been determi...