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.INDEX TERMS Constant switching frequency torque controller, direct torque control, induction motor, three-level neutral-point clamped inverter. NOMENCLATURE KYO-BEUM LEE (S'02-M'04-SM'10) received the B.S. and M.S. degrees in electrical and elec-

Direct torque control (DTC) is considered one of the simplest and fastest control strategies used in motor drives. However, it produces large torque and flux ripples. Replacing the conventional two-level hysteresis torque controller (HTC) with a four-level HTC for a three-level neutral-point clamped (NPC) inverter can reduce the torque and flux ripples in interior permanent magnet synchronous motor (IPMSM) drives. However, the torque will not be controlled properly within the upper HTC bands when driving the IPMSM in the medium and high-speed regions. This problem causes the stator current to drop, resulting in poor torque control. To resolve this problem, a simple algorithm based on a torque error average calculation is proposed. Firstly, the proposed algorithm reads the information of the calculated torque and the corresponding torque reference to calculate the torque error. Secondly, the average value of torque error is calculated instantaneously as the reference torque changes. Finally, the average value of the torque error is used to indicate the operation of the proposed algorithm without the need for motor speed information. By using the proposed algorithm, the torque can be controlled well in all speed regions, and thus, a better stator current waveform can be obtained. Simulation and experimental results validate the effectiveness of the proposed method.

Classical direct torque control (DTC) is considered one of the simplest and fastest control algorithms in motor drives. However, it produces high torque and flux ripples due to the implementation of the three-level hysteresis torque regulator (HTR). Although, increasing the level of HTR and utilizing multilevel inverters has a great contribution in torque and flux ripples reduction, stator flux magnitude of induction motor (IM) droops at every switching sector transition, particularly at low-speed operation. This problem occurs due to the utilization of a zero voltage vector, where the domination of stator resistance is very high. A simple flux regulation strategy (SFRS) is applied for low-speed operation for DTC of IM. The proposed DTC-SFRS modifies the output status of the five-level HTR depending on the flux error, torque error, and stator flux position. This method regulates the stator flux for both forward and reverse rotational directions of IM with retaining the basic structure of classical DTC. By using the proposed algorithm, the stator flux is regulated, hence pure sinusoidal current waveform is achieved, which results in lower total harmonics distortion (THD). The effectiveness of the proposed DTC-SFRS is verified through simulation and experimental results.

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