Abstract
Direct torque control (DTC) is known for its fast control in AC motor drives due to its simple control structure that directly controls the torque without the need for modulation blocks or frame transformations. However, when used in induction motor (IM) drives it has three main drawbacks: large torque ripples, variable switching frequency, and sector flux-droop at the low-speed region due to the employment of the torque hysteresis controller (THC). Torque ripple minimization can be achieved in DTC drives by replacing the originally proposed two-level inverter with a three-level neutral-point clamped (3L-NPC) inverter. Nevertheless, the switching frequency remains variable and low, which produces an elevated torque ripple and asymmetrical switching signals for the inverter. In addition, sector flux-droop resulting from driving the IM at the low-speed region produces a high current distortion that consequently eliminates the robustness of DTC. To alleviate these problems, an interleaving constant switching frequency torque controller-based DTC (CSFTC-DTC) was implemented. It improves the operation of the IM at the low-speed region by increasing the duty-cycle of the applied active voltage vector and reducing the duration of the applied zero vectors. Although the conventional CSFTC-DTC regulates the stator flux of the IM at the low-speed region and minimizes the total harmonic distortion (THD) of the stator current, it produces a high torque ripple-one of the main disadvantages of classical DTC. In this paper, an interleaving CSFTC-DTC is proposed to subdivide the duty cycle of the applied vectors of the 3L-NPC inverter to limit the influence of the large duty-cycle of the applied vectors on flux-regulation and torque ripples. The simulation and experimental results presented validate the effectiveness of the proposed method over the conventional method.
Highlights
Classical direct torque control (DTC) has been the focus of numerous studies during the past four decades for its excellent control features compared to field orientated control (FOC) [1], [2]
Despite its fast dynamic response, there are some issues associated with the use of a torque hysteresis controller-based DTC (THC-DTC), such as high torque ripples and variable switching frequency
The proposed interleaving constant switching frequency torque controller (CSFTC) has excellent control in medium and high-speed operation of the induction motor (IM) drives fed by 3L-NPC inverter in reducing the torque ripples compared to the five-level THC-DTC and conventional interleaving CSFTC methods
Summary
Classical direct torque control (DTC) has been the focus of numerous studies during the past four decades for its excellent control features compared to field orientated control (FOC) [1], [2]. The authors in [25], [30] proposed a 3L-NPC inverter for CSFTC-DTC to further reduce both flux and torque ripples and to obtain smoother responses Good performance, such as fixed switching frequency and minimized torque and flux ripples was achieved in most of these previous studies, none have improved the critical low-speed operation of IMs that causes stator flux drops due to the domination of the stator resistor and the selection of a zero voltage vector [4], [5]. In [32], [33] the sector flux-droop has been corrected by increasing the duty cycle of the applied active vector using interleaving carriers This technique results in large flux and torque ripples, which are comparable to those produced by classical THC-DTC.
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