The modeling, modulation and control of the three-phase four-switch (TPFS) PWM rectifier are investigated in this paper. Three space vector pulse width modulation methods using different equivalent zero vectors are developed, where sector identification and the trigonometric function are not required. Then, the high frequency model for the current ripple analysis is proposed, and the effects of three SVM approaches on the AC current ripple are investigated. According to the analytical results, the method introducing the smallest current ripple is selected. With the optimized SVM approach, a control-oriented model, considering the capacitor voltage oscillation and deviation, is built in the d-q synchronous frame to facilitate the controller design. Furthermore, a control strategy implementing the proportional controller is developed to eliminate the capacitor voltage deviation.
Meanwhile, the dual-loop control of the TPFS is not affected by the proposed strategy as the capacitor voltage deviation is eliminated. Finally, a novel linear modulation index function is defined to reject the low frequency harmonic current introduced by the overmodulation. Experimental results demonstrate that excellent current performance is achieved with comprehensive considerations of the modeling, modulation and control strategy.Index Terms-AC-DC power rectifiers, space vector modulation (SVM), three-phase four-switch (TPFS), current ripple root mean square (RMS), linear modulation.
0885-8993 (c)
In this paper, a two-step segmentation method is developed for segmenting the hematoma area from brain CT images. The volume of hematoma area is calculated after the segmentation. During the second segmentation process, the method of two-dimensional entropy is introduced to separate hematoma. In using the method of two-dimensional entropy, most important is to find the optional threshold which can be achieved by an improved genetic algorithm (GA) i.e. hierarchical genetic algorithm (HGA). HGA is more efficient than simple GA in overcoming the shortcoming of standard GA in local optimal solution and low precision convergence. An experiment is designed to test the effectiveness of automatic segmentation. The results prove that the precision of automatic segmentation is better than artificial segmentation, and the clinical needs are met.
The adjustable speed range for three-phase four-switch inverter (TPFSI) fed permanent magnet synchronous motor (PMSM) drives is seriously limited by the DC-link capacitor voltage offset in TPFSI. For the traditional offset suppression schemes, the low-pass filters or notch filters are necessary to extract the capacitor voltage offset, which will deteriorate the control performance of the offset suppression schemes severely, especially for the low-speed range. Thus, a novel offset suppression scheme with no filters is proposed to improve the situation. For the proposed scheme, to extract the offset with no filters, a new static coordinate is defined, the relationships between the fundamental/offset components of capacitor voltages and the load currents are revealed, based on which a simple algorithm is developed. Then, the desired compensatory current is obtained by a closed-loop control and injected into the stator current control loop. Experimental results have validated the proposed scheme.
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