Evaluation of Direct Torque Control with a Constant-Frequency Torque Regulator under Various Discrete Interleaving Carriers

Abstract

Constant-frequency torque regulator-based direct torque control (CFTR-DTC) provides an attractive and powerful control strategy for induction and permanent-magnet motors. However, this scheme has two major issues: A sector-flux droop at low speed and poor torque dynamic performance. To improve the performance of this control method, interleaving triangular carriers are used to replace the single carrier in the CFTR controller to increase the duty voltage cycles and reduce the flux droop. However, this method causes an increase in the motor torque ripple. Hence, in this work, different discrete steps when generating the interleaving carriers in CFTR-DTC of an induction machine are compared. The comparison involves the investigation of the torque dynamic performance and torque and stator flux ripples. The effectiveness of the proposed CFTR-DTC with various discrete interleaving-carriers is validated through simulation and experimental results.Electronics 2019, 8, 820 2 of 15 and constraints. Generally, PTC strategies, although helping to address the problem, are associated with some issues such as the increase in total harmonic distortion and a high computational burden [11].Lately, interest has increased in using DTC with a constant-frequency torque regulator (CFTR-DTC) in AC motor drives [18][19][20][21][22][23]. The DTC-CFTR was initially created for achieving a constant switching frequency of inverters and a reduction of torque and flux ripples [18]. Unlike the DTC-SVM and MPC methods, there is a possibility to solve the stated DTC problems by applying CFTR in the basic configuration of classical DTC while maintaining the simple DTC algorithm. To date, only a handful of studies have been reported to address these problems in the CFTR. Some studies of DTC-CFTR have applied it in multilevel-based DTC (ML-DTC), such as three-level [19] and five-level [20] systems. They can provide a higher degree of freedom in control to obtain smoother torque and flux responses and to lead to a torque ripple reduction. In previous work [21], an over modulation method was proposed to enhance CFTR-DTC by retaining a single active-voltage vector during the torque transient. In other work [22], the digital signal processor (DSP) board was extended using a field programmable gate array controller to generate a higher carrier frequency, thus achieving a further reduction of the torque ripple. Nevertheless, this work requires extending extra software and an external controller to the drive system. In other research [23], CFTR-DTC has been investigated for low-speed operation showing satisfactory performance in terms of flux regulation improvement. Nevertheless, it still suffers from sector-flux droop and poor torque dynamic performance. In [24], CFTR-DTC with interleaving carriers was reported to improve the CFTR at the low-speed range. The configuration of the controller is like the conventional CFTR; yet, the method needs two carrier waveforms with a 90 • phase shift for the upper and lower sides. However, the CFTR rules in [24] were ...

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