Switching-Table-Based Direct Torque Control of Six-Phase Drives With $x-y$ Current Regulation

Abstract

High harmonic contents of the stator currents due to uncontrolled x − y subspace dramatically overshadow the performance of the switching table-based direct torque control (ST-DTC) strategy for multi-phase drives. The concept of ST-DTC based on virtual voltage vectors (VVs) has been frequently developed to alleviate this problem. However, VVs with fixed duty ratios are incapable of compensating inherent machine/converter asymmetries and dead time harmonics, which are mapped into the x − y subspace. To solve this problem, this paper develops a dynamic duty-ratio-based DTC (DDR-DTC) technique for six-phase induction machine (6PIM) drives, where the duty ratios of the selected VVs are not constant anymore. These duty ratios are updated based on the x − y voltage commands arising from the closed-loop x−y current controllers over every sampling period. The attained merits over and above the conventional fixed duty-ratio-based DTC (FDR-DTC) schemes include effective x − y current cancellation without increase in average switching frequency and decrease in DC-link utilization. Experimental results are presented to validate the effectiveness of the proposed control technique.

Five-Dimensional Switching-Table-Based Direct Torque Control of Six-Phase Drives

“…3) Virtual Voltage Vectors: As reported in [25], [28], synthesizing the VVs effectively alleviates the problem of large stator current harmonics within classical ST-DTC, where the maximum amplitude of synthesized VVs in the α−β subspace is 0.5977V dc with zero resultant vector in the x − y subspace. Therefore, a feasible maximum rate of dc-link utilization can be calculated as 92.82% [28], which is 7.18% less than full utilization rate of the dc supply, when actual large voltage vector vectors are used.…”

“…Therefore, a feasible maximum rate of dc-link utilization can be calculated as 92.82% [28], which is 7.18% less than full utilization rate of the dc supply, when actual large voltage vector vectors are used. The VVs with maximum voltage level can be synthesized in two ways, by large and medium large voltage vectors (hereinafter labeled 2D-VVs) or by three consecutive actual large voltage vectors (hereinafter labeled 3D-VVs) [25]. In this paper, 3D-VVs are employed due to their flexibility to compensate for x − y components, which will be discussed further in the next section.…”

“…In this paper, 3D-VVs are employed due to their flexibility to compensate for x − y components, which will be discussed further in the next section. Details of 3D-VVs construction can be found in [25], [28].…”

“…It is worth mentioning here that the idea of null voltage vectors in the x − y subspace cannot guarantee that there will be no x − y currents in real implementation, because there is no control over x − y currents yet. Therefore, devising an ST-DTC method to suppress x − y currents is still a challenging task and requires more investigation, which has been studied to a limited extent until now [20], [25]. [20] has proposed a modified ST-DTC scheme with closed-loop current regulation based on proportional-integral-resonant (PIR) controller for dual three-phase PMSMs, where this scheme poses the risk of increasing the average switching frequency and complexity of the ST-DTC method.…”

“…[20] has proposed a modified ST-DTC scheme with closed-loop current regulation based on proportional-integral-resonant (PIR) controller for dual three-phase PMSMs, where this scheme poses the risk of increasing the average switching frequency and complexity of the ST-DTC method. In [25], a dynamic duty-ratio-based DTC technique has been proposed, which maintains the average switching frequency at the same level of conventional ST-DTC method [17]. However, it may be still overshadowed by the complexity due to extra PI regulators.…”