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The multi-level inverter (MLI) is more efficient due to its ability to eliminate low-order harmonics. Thus, it is widely used in renewable energy generation, electric vehicles (EVs), etc. The choice of the neutral-point clamped (NPC) converter topology as a research object is of great significance in practical engineering. However, complex modulation algorithms and neutral-point voltage unbalance are two inherent problems for such NPC inverters. In this paper, an improved multi-voltage vector model predictive control (MVV-MPC) strategy based on neighboring four voltage vector synthesis is presented for three-phase NPC inverters. The duty cycle calculation and derivation methods for the proposed method are very simple, and they were obtained through the action time of each voltage vector being inversely proportional to the value of the corresponding cost function. Moreover, the fixed switching frequency of this strategy is maintained and the current peak ripple is reduced, guaranteeing the preferred total harmonic distortion of the output waveform. Furthermore, a voltage balancing method based on zero-sequence voltage injection is utilized to achieve dynamic balancing of the midpoint voltage difference and to limit the oscillation magnitude of the midpoint voltage difference to within ± 1 V. Finally, MATLAB simulations and an HIL experimental platform are built to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract In operation, blisks are subject to dynamic loading, which result in large response amplitudes near resonance. To reduce the response level and prevent damages caused by high cycle fatigue, friction damping sources are introduced to dissipate vibrational energy. The effectiveness of friction dampers depends not only on the mode shape of the vibrational mode being excited, but also on the damper material and geometrical properties. To create a robust design, it is necessary to parametrize the damper geometry, and to study the effect of geometric parameters on energy dissipation. The effectiveness of the damper is determined by evaluating the reduction in forced responses due to friction damping. To compute the nonlinear forced responses of a blisk–damper system, a reduced-order model is constructed with the method of coherent ring dampers (CoRiD). The equations of the reduced-order model are efficiently solved by the harmonic balance method (HBM). A parametric study on the effectiveness of a V-shaped friction ring damper is presented, focusing on the sensitivity of the damper effectiveness to variations of the dimensions of the damper geometry. Highlights • Specialized friction dampers for turbomachinery blisks are investigated. • The geometry of ring dampers is parametrized for enhancing damper effectiveness. • The damper geometry is optimized for effectiveness and robustness. • A novel design method for ring dampers is proposed, namely the frequency split. • A reduced-order modelling method (coherent ring damper, CoRID) is used. [ABSTRACT FROM AUTHOR]

In turbomachinery applications, rotating bladed disks (blisks) are often subject to high levels of dynamic loading, such as traveling wave excitations, which result in large response amplitudes at resonance. To prevent premature high cycle fatigue, various dry friction dampers are designed for blisk systems to reduce the forced responses. Ring dampers are located in the disk, underneath the blades, and are held in contact with the blisk by centrifugal loading. Energy is dissipated by nonlinear friction forces when relative motions between the ring damper and the blisk take place. To investigate the dynamic responses of blisk–damper systems in the presence of the nonlinear frictional contacts, conventional methods based on numerical time integration are not suitable since they are computationally expensive. This paper presents a reduced-order modeling technique to efficiently capture the nonlinear dynamic responses of the blisk–damper systems. Craig–Bampton component mode synthesis (CB-CMS) serves as the first model reduction step. A novel mode basis that mimics the contact behavior under sliding and sticking conditions is developed to further reduce the CB-CMS model while maintaining its accuracy. The resulting reduced nonlinear equations of motion are solved by a hybrid frequency/time domain (HFT) method. In the HFT method, the contact status and friction forces are determined in the time domain by a three-dimensional contact model at each contact point, whereas the reduced equations of motion are solved in the frequency domain according to a harmonic balance formulation. Moreover, to investigate the effects of blade mistuning, which can lead to drastic increase of forced responses, an extension of the reduced-order models (ROMs) is developed based on component mode mistuning. Forced responses computed by the proposed ROMs are validated for both tuned and mistuned systems. A statistical analysis is performed to study the effectiveness of ring dampers under random blade mistuning patterns. [ABSTRACT FROM AUTHOR]

Small deviations of the structural properties of individual sectors of blisks, referred to as mistuning, can lead to localization of vibration energy and drastically increased forced responses. Similar phenomena are observed in blisks with large damages or repair blends. Such deviations are best studied statistically because they are random. In the absence of cyclic symmetry, the computational cost to predict the vibration behavior of blisks becomes prohibitively high. That has lead to the development of various reduced-order models (ROMs). Existing approaches are either for small mistuning, or are computationally expensive and thus not effective for statistical analysis. This paper discusses a reduced-order modeling method for blisks with both large and small mistuning, which requires low computational effort. This method utilizes the pristine, rogue and interface modal expansion (PRIME) method to model large blends. PRIME uses only sector-level cyclic modes strategically combined together to create a reduction basis which yields ROMs that efficiently and accurately model large mistuning. To model small mistuning, nodal energy weighted transformation (NEWT) is integrated with PRIME, resulting in N-PRIME, which requires only sector-level calculations to create a ROM which captures both small and large mistuning with minimized computational effort. The combined effects of large blends and small mistuning are studied using N-PRIME for a dual flow path system and for a conventional blisk. The accuracy of the N-PRIME method is validated against full-order finite element analyses for both natural and forced response computations, including displacement amplitudes and surface stresses. Results reveal that N-PRIME is capable of accurately predicting the dynamics of a blisk with severely large mistuning, along with small random mistuning throughout each sector. Also, N-PRIME can accurately capture modes with highly localized motions. A statistical analysis is performed to study the effects of random mistuning on both natural frequencies and forced responses and reveals that 1) new clusters of mistuned natural frequencies exist and cannot be treated as small deviations from nominal frequencies, and 2) unexpectedly high amplifications of forced responses exist, showing 3–5 times higher responses compared to the nominal system. [ABSTRACT FROM AUTHOR]

Summary With the characteristics of high efficiency and high power density, the four switch buck–boost (FSBB) converter is widely used in electric vehicles, direct current microgrids, and so forth. However, how to achieve smooth switching between different modes of FSBB has always been a hot issue in the theoretical and engineering fields. In this paper, considering the dynamic performance and output voltage or inductance current deviation, a control algorithm with integrated natural trajectory method is designed to realize the switching of different modes of FSBB converter. The proposed control algorithm is based on the theory of large signal natural locus and average natural locus. The normalized phase locus of inductance current and output voltage divides the phase plane into five regions and performs large signal or average switching operations in different regions. Theoretical analysis shows that the proposed control algorithm can achieve the balance between the dynamic performance and output voltage or inductance current deviation of FSBB converter in different mode switching. Finally, the performance of the proposed control algorithm is verified by experiments results. [ABSTRACT FROM AUTHOR]

The size of the V-defects in the GaN/InGaN-based quantum wells blue light-emitting diode (LED) was intentionally modified from 50 nm to 300 nm. High resolution photoluminescence and electroluminescence of a single large V-defect were investigated by near-field scanning optical microscopy. The current distribution along the {10-11} facets of the large defect was measured by conductive atomic force microscopy. Nearly 20 times the current injection and dominant emission from bottom quantum wells were found in the V-defect compared to its vicinity. Such enhanced current injection into the bottom part of quantum wells through V-defect results in higher light output power. Reduced external quantum efficiency droops were achieved due to more uniform carrier distribution. The un-encapsulated fabricated chip shows light output power of 172.5 mW and 201.7 mW at 400 mA, and external quantum efficiency drop of 22.3% and 15.4% for the sample without and with large V-defects, respectively. Modified V-defects provide a simple and effective approach to suppress the efficiency droop problem that occurs at high current injection, while improving overall quantum efficiency. [ABSTRACT FROM AUTHOR]