Abstract Dynamical and structural systems are susceptible to sudden excitations and loadings such as wind gusts, blasts, earthquakes, and others which may cause destructive vibration amplitudes and lead to catastrophic impact on human lives and economy. Therefore, various vibration absorbers of linear and nonlinear coupling dynamics have been widely studied in plenty of publications where some have been applied in real-world practical applications. Firstly, the tuned-mass-damper (TMD), the first well-known linear vibration absorber that has been well-studied in the literature and applied with various structural and dynamical systems, is discussed. The linear vibration absorbers such as TMDs are widely used in real-life small- and large-scale structures due to their robust performance in vibration suppression of the low natural frequency structural modes. However, the TMD performs efficiently at narrowband frequency range where its performance is deteriorated by any changes in the frequency content in the structure and the TMD itself. Therefore, the targeted-energy-transfer mechanism which is found to be achieved by nonlinear energy sinks (NESs) has ignited the interest in passive nonlinear vibration suppression. Unlike TMDs, the NESs are dynamical vibration absorbers that achieve vibration suppression for wide range of frequency-energy levels. Given the very rapid growth in this field and the extensive research studies supporting the robustness of the NESs, this paper presents the different types of NESs and their applications with main emphasis on the rotary-based and impact-based NESs since they are of high impact in the literature due to their strong nonlinear dynamical behavior and robust targeted energy transfer. 

https://link.springer.com/content/pdf/10.1007/s11071-022-08094-y.pdf

A review on nonlinear energy sinks: designs, analysis and applications of impact and rotary types

The brushless doubly-fed machine(BDFM) is a family of multiport electric machines with two ac electrical ports and a common mechanical port. Different from the conventional singly-fed machines whose synchronous speed is solely determined by a single supply frequency and the actual pole pair number, the BDFM has two supply frequencies and two different pole pair numbers to control the rotor speed. By the two accessible electrical ports, all BDFMs are endowed with more degrees of freedom for speed and power control, inherent fault-tolerant capability and high reliability. The BDFM in its broad sense has been extensively investigated as a promising alternative to the conventional slip-ring doubly-fed induction machine(DFIM) during the past decades, for both limited and wide speed range applications. This paper presents a new theoretical framework of the BDFM within which all topological variants can be closely linked by the similarities in working principle. The individualities of each machine topology are presented first, followed by the commonalities such as the modeling techniques, modes of operation, design considerations and control strategies. The challenges are identified and highlighted based on recent developments and possible opportunities are predicted considering the unique nature of this special AC machine type.

/pdf/brushless-doubly-fed-machines-opportunities-and-challenges-5e4civ1v0h.pdf

Brushless doubly-fed machines : opportunities and challenges

A multi-stable ultra-low frequency energy harvester using a nonlinear pendulum and piezoelectric transduction for self-powered sensing

As well known, the vibration reduction efficiency of the nonlinear energy sink (NES) is poor under strong excitation. In this paper, a tristable NES (TNES) is proposed. The TNES can degenerate into bistable and mono-stable NES by adjusting the geometric parameters of the springs. The governing equations of a linear oscillator coupled with the TNES under harmonic excitation are derived. The approximate analytical solution of the coupled system is obtained by using the harmonic balance method and verified numerically. The vibration suppression efficiency of TNES and NES is compared. The dynamic behavior of TNES under strong excitation is demonstrated. The results show that the nonlinear restoring force is softened due to the wide distribution of the three stable points of TNES. Therefore, compared with NES, the proposed TNES can suppress stronger excitation vibration. In addition, the low side barrier depth is conducive to TNES to perform chaotic inter-well oscillation, which can effectively suppress the strong excitation vibration and obtain good vibration reduction performance. As a result, the proposed TNES can eliminate the detached resonance curve and enlarge the effective range of the NES. Under relatively weak excitation, the vibration suppression efficiency of TNES is slightly lower than that of NES, although the TNES is also relatively significant. Therefore, this paper reveals the vibration suppression mechanism of TNES, and provides a way to effectively solve the problem of low vibration reduction efficiency of NES under strong excitation. 

A tristable nonlinear energy sink to suppress strong excitation vibration

Nonlinear energy sink (NES) is a type of vibration absorbers that does not have linear stiffness. Through establishing strongly nonlinear coupling between a primary system and a NES, the targeted energy transfer can be achieved from the primary system to the NES, thereby realizing vibration absorption in a wide frequency range. A significant amount of research work has been conducted on developing the NES for unidirectional vibration energy transfer over the last decade. More research is expected to develop further NESs to address various engineering vibration problems. Meanwhile, the question of whether NES can be practically applied to engineering is always being asked. The main objective of this paper is to review the research progress on dynamic design of NESs to promote the application of NESs to reduce engineering structure vibration. To do so, this paper first summarizes the characteristics of NESs, including vibration absorption mechanism, the threshold of targeted energy transfer, and strong nonlinearity characteristics. Then, dynamic designs of the NESs proposed in the literation are reviewed in terms of nonlinear stiffness design, mass design and damping design. Special attention is placed on the nonlinear stiffness design for NESs, including design principle, multi-stability design, track design, and magnetic design. The gaps between these design approaches and applications are explained. NES cells and their distributed vibration control strategy are also introduced. The research progress on the NES optimization design is also briefly discussed. Following the extensive review on NES dynamic design research, future promising research topics are recommended with an attempt to advancing the engineering application of NES. It is expected that this paper would help readers to understand the progress of NES dynamic design research and the future NES development for more practical applications. 

A state-of-the-art review on the dynamic design of nonlinear energy sinks

The brushless doubly-fed motor (BDFM) is versatile and can perform under several modes of operation. Other than the commonly used doubly-fed mode, BDFM can also operate in an asynchronous mode wherein the control winding of the motor is short circuited. The proposed study first analysed the mechanical characteristics of BDFM operation under this mode, whereby it was noted that there exist two induction torque components corresponding to different pole pairs within the motor. The motor, therefore, behaved as a conventional induction machine. Based on the above analysis and by introducing the concept of equivalent resistance of the machine-side inverter, an asynchronous soft-starting method for BDFM is proposed herein. By switching one side of the inverter bridge arms to the off state and adjusting the duty cycle on the other side, a controllable equivalent resistance is obtained and inserted into the control winding of BDFM. Consequently, the starting characteristic of BDFM can be improved to achieve soft start-up of the motor. Finally, experiments were conducted on a 30 kW BDFM prototype, of which, the results verify the accuracy of the torque analysis and improved performance of the proposed soft-starting method.

Asynchronous operation characteristics and soft-starting method for the brushless doubly-fed motor

An inerter nonlinear energy sink for torsional vibration suppression of the rotor system

The brushless doubly-fed generator (BDFG) is a kind of novel machine, which can be regarded as a combination of conventional synchronous and asynchronous motors. Due to its absence of brush and slip rings, BDFG has potential in hydropower generation. The paper first evaluates the feasibility of BDFG for hydropower generation system and verify the proposed control strategy, based on a practical application in Daliang hydropower station.

Application of Brushless Doubly-Fed Machine System in Hydropower Generation

The brushless doubly fed machine (BDFM) drive system based on a fractional unidirectional converter is a promising low-cost variable-speed drive system, which shows great potential in applications of driving fans and pumps. However, the harmonic current generated by a diode rectifier can flow into the machine and the grid under a weak grid, which will cause a 6th-order frequency torque ripple and lead to voltage distortion. A steady equivalent circuit considering the uncontrolled rectifier and the grid impedance is built firstly to study the harmonic distribution characteristics. To eliminate the influence of harmonic currents, the harmonic equivalent impedance of the machine system should be regulated to change the harmonic distribution characteristics. This paper improves the conventional control method through adding a harmonic control loop to prevent harmonic currents from being injected into the machine or the grid, which is then applied in the fundamental synchronous frame. Two indirect parameters are selected to realize the two control targets. Afterwards, the influence of the control system on the harmonic equivalent impedance of the machine system under the conventional method and the proposed method are compared. Finally, experimental results obtained from a 30 kW BDFM prototype verify the proposed method.

/pdf/bidirectional-harmonic-current-control-of-brushless-doubly-4k25sumjbh.pdf

Bidirectional Harmonic Current Control of Brushless Doubly Fed Motor Drive System Based on a Fractional Unidirectional Converter Under a Weak Grid

The Brushless Doubly-fed Induction Generator (BDFIG) has potential in Wind Power generation due to its absence of brush and slip rings compared to the Doubly-Fed Induction Generator (DFIG). To promote its commercialization process, the BDFIG is required to be able to ride through the grid voltage dips. An improved control strategy using virtual inductance is proposed to reduce the overcurrent caused by grid faults. Theoretical analysis is given to calculate the value of virtual inductance and the utility of the proposed control scheme has been verified by experiment.

Virtual Inductance Control Strategy for Brushless Doubly-Fed Induction Generator During Grid Voltage Dips

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