Adaptive hysteresis band control for constant switching frequency in DTC induction machine drives

It is fairly well known that direct torque control (DTC) exhibits fast dynamic torque response, a property that originates in the fact that torque and flux is directly controlled by instantaneous space voltage vector, unlike field-oriented control. For this reason direct torque control gradually has been used in the field, eliciting quick response and high efficiency since its introduction in the mid-1980s. Conventional fixed band hysteresis control has several problems. One of these is variable switching frequency, which causes serious problems in DTC. In this paper, an adaptive hysteresis band control strategy is proposed, where the hysteresis band is controlled in real time as variation of applied voltage vectors, thereby reducing the torque ripple whilst maintaining a constant torque switching frequency. The proposed adaptive hysteresis band control technique is verified by simulations. The system is first simulated by using the program Matlab and tested by hardware in the loop. Then, it is implemented in hardware based on a TMS320C6711, 32-bit fixed-point digital signal processor. Experimental results prove the feasibility of the proposed strategy compared with conventional method.