16,798 research outputs found
Streaks to Rings to Vortex Grids: Generic Patterns in Transient Convective Spin-Up
We observe the transient formation of a ringed pattern state during spin-up
of an evaporating fluid on a time scale of order a few Ekman spin-up times. The
ringed state is probed using infrared thermometry and particle image
velocimetry and it is demonstrated to be a consequence of the transient balance
between Coriolis and viscous forces which dominate inertia, each of which are
extracted from the measured velocity field. The breakdown of the ringed state
is quantified in terms of the antiphasing of these force components which
drives a Kelvin-Helmholtz instability and we show that the resulting vortex
grid spacing scales with the ring wavelength. This is the fundamental route to
quasi-two dimensional turbulent vortex flow and thus may have implications in
astrophysics and geophysics wherein rotating convection is ubiquitous. sics.Comment: 4 pages, 5 figure
Nonlinear Control of Single-Phase PWM Rectifiers with Inherent Current-Limiting Capability
In this paper, a nonlinear controller with a currentlimiting
property is proposed to guarantee accurate dc output
voltage regulation and unity power factor operation for singlephase
PWM rectifiers without the need of a phase-locked-loop
(PLL). The proposed current-limiting controller is fully independent
of the system parameters and can guarantee asymptotic
stability and convergence to a unique solution for the closedloop
system using nonlinear control theory. Without requiring
the instantaneous measurement of the grid voltage, a PLL,
an external limiter or a saturation unit, the proposed strategy
guarantees that the input current of the rectifier will always
remain below a given value. An analytic framework for selecting
the controller parameters is also presented to provide a complete
controller design procedure and it is also proven that the currentlimiting
property is maintained even when the grid voltage
drops. Extensive experimental results are presented to verify
the proposed controller when the load changes, the reference
dc output voltage changes and the grid voltage drops
Structural Resemblance Between Droop Controllers and Phase-Locked Loops
It is well known that droop control is fundamental to the operation of power systems and now the parallel operation of inverters while phase-locked loops (PLL) are widely adopted in modern electrical engineering. In this paper, it is shown at first that droop control and PLLs structurally resemble each other. This bridges the gap between the two communities working on droop control and PLLs. As a result, droop controllers and PLLs can be improved and further developed via adopting the advancements in the other field. This finding is then applied to operate the conventional droop controller for inverters with inductive output impedance to achieve the function of PLLs, without having a dedicated synchronization unit. Extensive experimental results are provided to validate the theoretical analysis
Nonlinear control of dc/dc power converters with inherent current and power limitation
A nonlinear controller with an inherent current-limiting capability is presented in this paper for different types of dc/dc power converters (boost, buck-boost). The proposed controller is based on the idea of applying a dynamic virtual resistance in series with the inductor of the converter, which varies according to a nonlinear dynamical system. It is shown that the proposed approach acts independently from the converter parameters (inductance, capacitance) or the load and has a generic structure that can be used to achieve different regulation scenarios, e.g. voltage, current or power regulation. Based on the nonlinear model of the boost and the buck-boost converter, it is analytically proven that the inductor current remains always bounded below a given maximum value using input-to-state stability theory under a suitable choice of the controller parameters. Hence, the proposed control strategy offers an inherent protection property since the power of the converter is limited below a given value during transients or unrealistic power demands. Simulation results for both types of dc/dc converters are presented to verify the desired controller performance
Current-limiting DC/DC power converters
A new nonlinear control framework that guarantees the desired regulation (voltage, current, or power) with an inherent current-limiting capability for different types of dc/dc power converters is presented in this brief. This framework is based on the idea of applying a virtual resistance in series with the inductor of the converter, which changes according to nonlinear dynamics that depend on the control task. Without requiring any knowledge of the converter inductance, capacitance, or the load, the controller structure is appropriately formulated for each power electronic system based on the nonlinear model of the converter. Using input-to-state stability theory, it is proven that the inductor current remains below a maximum value at all times, even during transients, independently of load and input voltage variations. This offers an inherent current-limiting property of the converter under faults, input voltage sags, and unrealistic power demands without the need of external protection mechanisms, saturation units, or current limiters. Extensive simulation and experimental results validate the effectiveness of the proposed control scheme and its current-limiting property, with comparison to traditional control strategies
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