Improvement of maximum efficiency and torque dynamics for direct torque control drive system

Abstract

In high-performance direct torque control (DTC) drive systems, the torque and flux components are always decoupled to establish a fast instantaneous torque response. Furthermore, the rated flux is always applied to ensure maximum torque capability during the torque dynamic even at full load conditions. However, most of the time, the drive normally operates at a lower than the full-load condition, and operating the drive at rated flux reduces its efficiency. To overcome this problem, an optimal efficiency DTC drive is normally employed whereby the flux is set to an optimal value that produces maximum efficiency whenever the drive is operated below its rated load conditions. However, to ensure fast dynamic torque, the rated flux will be set during transient states, and a new optimal flux has to be recalculated for the new operating condition once the steady-state speed is reached. Two problems are faced with this method: (i) it will take some time to calculate the new optimal flux after the transient states, and (ii) the dynamics torque will be sluggish due to the poor flux response during the step from the optimal value to the rated value. To overcome the first problem, a new method is introduced in this thesis that instantaneously calculates the reference flux to a value that is almost equal to the optimal flux, called the High-Efficiency Flux Reference (HEFR). The calculation of the HEFR is based on the load torque and is obtained almost immediately with no convergence time. The second problem is tackled by introducing a modified voltage vector constructed based on the initial and final values of the flux reference. The effectiveness of the proposed method is studied through simulation using MATLAB and verified experimentally. In the experiment, a 186 W induction motor is used, with the proposed algorithm implemented using dSPACE DS1104 controller board and Xilinx FPGA controller board. It is found that at steady-state, the drive efficiency using HEFR is almost similar to the conventional optimal efficiency DTC, which is 63% and 72% at the speed of 70 rad/s and 90 rad/s, respectively. However, with the HEFR, the drive efficiency during the transients is improved by 4%. The rise time of the torque with the modified voltage vectors was measured as 1.64 ms, and has improved to 1.2 ms when it is implemented with the HEFR.