The oscillating water column is one of the most tested and used wave energy converter types. It requires the use of an air turbine connected to an electrical generator. The deep understanding of all physical quantities of the energy chain components is based on transducers. The electrical generator efficiency is a key input to wave-to-wire numerical models for wave energy conversion to enable the industry to embark on projects with reliable data. Assembly of a standard torque meter is not feasible in prototypes following an industrial design. Therefore, the turbine shaft power must be calculated from the electrical generator dry-testing results at various rotational speeds and loads. This paper presents normalised data for estimating generator power output and turbine torque based on generator efficiency measurements. Experimental results show the electrical generator efficiency dependency on the rotational speed and load. A new kind of filter to post-process the raw data was used. The results will be used as a reference for wave-to-wire numerical models of oscillating water columns and post-processing the data from prototypes operating in real sea state conditions.

This article presents a new minimum-loss control strategy for vector-controlled dual-voltage source inverter (VSI) doubly-fed induction generator (DFIG)-dc systems. The optimization is based on a simplified formalism using Lagrange multipliers to determine the optimal stator frequency, stator/rotor magnetizing current split, and the magnetizing flux magnitude as a function of the machine operating conditions, namely speed and torque. Compared with existing minimum-loss control schemes, the proposed approach provides an explicit solution for all three optimal quantities, which makes the implementation and control design easier, thus avoiding the use of an additional proportional–integral (PI) controller to solve the flux implicit optimality condition. The theoretical framework also includes a sensitivity analysis against DFIG parameter variations, to appraise the robustness of the proposed control scheme. The devised approach and related control implementation scheme are validated with the simulation and experimental results.

Recently a minimum-loss control strategy for the Dual-VSI-DFIG system was proposed. It is implemented using three rules for the determination of the optimal stator frequency, of the stator/rotor magnetizing current split and of the airgap-flux magnitude. The method uses airgap-flux orientation with direct airgap-flux and rotor-current control. This paper presents a stability analysis for the Dual-VSI-DFIG system considering two syntheses of the Proportional Integral controllers based respectively on the symmetrical optimum and on the ITAE criteria. The stability is analyzed by computing the eigenvalues of the closed-loop model. Although both tuning criteria ensure close-loop stability, the system can become unstable in case of controllergain mismatch compared to the theoretical values.

This article addresses the minimum-loss control of the dual voltage-source inverter (VSI) and doubly fed induction generator (DFIG) system connected to a dc link. The minimum-loss operating conditions for field-oriented control based on the airgap flux are obtained analytically using Lagrange multipliers and validated with numerical optimization. As the main contribution of this article, the analysis accounts for core and VSI losses, providing the optimal stator frequency law and rotor/stator d-axis current split ratio, and an implicit expression for the optimal flux trajectory formulated as equality between suitable d-axis and q-axis loss functions. In the proposed implementation, this implicit condition is enforced by using a proportional-integral controller and avoiding look-up tables. Furthermore, the stator and rotor VSI controls are implemented in two independent digital signal processors with no communication, which may ease the use of off-the-shelf VSI units. The optimal conditions and control strategy are fully validated by simulations and experiments on a prototype. The main scope of application is wind-energy dc-grid technology.

The conditions for minimum losses in a salient-pole wound-field synchronous machine (WFSM) drive are studied in this paper. The drive comprises a WFSM energized by a stator inverter and excited by a dc-dc converter both tied to a DC link. The minimum-loss operation is formulated as a nonlinear constrained optimization problem with equality constraints (e.g, torque command), and inequality constraints (flux, voltage and current limits). Lagrange multipliers are applied to solve this problem analytically. At low load, the torque demand can be met using different values for two independent electric variables (e.g. stator flux and field current magnitude). These can be optimized, thereby leading to two optimal implicit conditions. At higher load, when the stator flux reaches the maximum value, the free variables reduce to one and yield a single implicit optimal condition. For these two scenarios, the paper presents analytical derivations of the optimal conditions and numerical validation using MatLab. These conditions can be used to devise a control system optimizing the drive operation.

Inaccurate machine parameters can cause orientation errors and instability in field-oriented control schemes relying on model-based estimations. This paper analyzes the accuracy and stability of two field-oriented control schemes for a stand-alone doubly fed induction generator (DFIG), where the field orientation is affected by the stator inductance and stator/mutual inductance ratio. After deriving a reduced-order model accounting for parameter mismatch, the paper deduces a formula to calculate the field orientation error as a function of the inductance mismatch, revealing parameter sensitivities. A stability analysis is then carried out proving that overestimating the stator/magnetizing inductance ratio may trigger instabilities in case of a high load level, whereas underestimation allows always stable operation. The theoretical insight is supported with simulation and test results on a laboratory rig.

This paper describes an innovative solution for the power supply of a fast field cycling (FFC) nuclear magnetic resonance (NMR) spectrometer considering its low power consumption, portability and low cost. In FFC cores, the magnetic flux density must be controlled in order to perform magnetic flux density cycles with short transients, while maintaining the magnetic flux density levels with high accuracy and homogeneity. Typical solutions in the FFC NMR literature use current control to get the required magnetic flux density cycles, which correspond to an indirect magnetic flux density control. The main feature of this new relaxometer is the direct control of the magnetic flux density instead of the magnet current, in contrast with other equipment available in the market. This feature is a great progress because it improves the performance. With this solution it is possible to compensate magnetic field disturbances and parasitic magnetic fields guaranteeing, among other possibilities, a field control below the earth magnetic field. Experimental results validating the developed solution and illustrating the real operation of this type of equipment are shown.

The doubly fed induction machine has been traditionally adopted in adjustable-speed ac power generation drives in order to take advantage of the reduced rating for the power electronic interface. Aside this well-established application where the doubly fed induction generator (DFIG) is controlled by a back-to-back converter, recent literature records a growing interest toward un-conventional DFIG drives for dc power generation, combining DFIG high control freedom with simplified power electronic interfaces to achieve an overall cheap and fully controllable system. Despite several concepts have been demonstrated on small-scale rigs, there is a lack of systematic comparison among different topologies and control solutions. This paper bridges this gap by providing a review of recent topologies, their control, design and performance, and operation issues. As major novelties, this paper includes off-spec performance comparison of different torque-ripple mitigation strategies, discussion of sizing requirements for generator and power electronics, fundamental aspects of the behavior under voltage dips, and priorities and challenges for future research on the subject.

The severe torque ripple normally occurring in the doubly fed induction generator dc (DFIG-dc) system can cause premature failure of mechanical components and shorten the life of the drive train. This paper addresses the torque ripple issue by proposing a predictive direct torque control strategy, which delivers at the same time torque ripple suppression and minimization of losses. The existing control algorithms for torque ripple mitigation are mostly based on resonant controllers and repetitive control forcing the compensation signal either through the current chain or directly into the rotor voltage commands. All these techniques lead to structures with multiple controllers whose tuning is not straightforward. Furthermore, they are very sensitive to the operating frequency, making optimized operation with variable frequency highly challenging. Conversely, the proposed algorithm predicts directly the best rotor voltage space vector to minimize torque ripple and track a prescribed rotor flux amplitude to minimize losses, with no current control chain. As confirmed by simulations and experiments, the strategy allows large stator frequency variations as required by the optimal flux command for minimum losses, whilst ensuring effective torque ripple compensation.

Torque ripple caused by stator current and flux harmonics is one of the main issues in the doubly fed induction generator (DFIG)-dc system, which inherently has to operate with distorted waveforms produced by the diode commutation. This paper proposes a torque-ripple mitigation strategy based on a predictive estimation of the reciprocal of flux linkage. The predictive estimation compensates for the intrinsic delay in the actuation of the torque-ripple rejection signal through the rotor current control loops. Unlike other approaches relying on complex current regulators with selective harmonic tracking, this strategy is based on well-established proportional-integral (PI) controllers for the rotor currents. PI current controllers can then still have bandwidth values typical of usual DFIG systems. Simulations and experiments on a test-rig show that the compensation strategy achieves a strong torque ripple reduction and is very robust against stator frequency variations.

This paper proposes a new application for the rotary VCM.In developing a low cost ultrasound scanner for the developing world an oscillating transducer is required to sweep over the skin.The ultrasound scanner must operate from a USB power supply in remote locations.The application requires a 3.3N force on the coils of the motor to overcome the inertia of the skin.A proof of concept prototype motor with electronics has been designed, simulated and tested.The VCM optimisation is discussed in detail with the unique separation of the magnets being critical to reduce the axial bearing forces for this application.

The field-weakening operation of a doubly fed induction generator (DFIG) connected to a dc-link is analyzed in this paper, in order to optimize the efficiency. In the considered DFIG-dc system, the stator feeds a constant-voltage dc link by a diode bridge, and the rotor current is controlled using a voltage-source inverter connected to the same dc link. Since the stator voltage amplitude is imposed by the dc-link, a variation in stator flux magnitude results in a frequency change. However, in this system, a stator frequency variation over a wide range can be accepted, if the rated flux is not exceeded. Thus, the stator flux amplitude can be adjusted through the magnetization current component by the voltage-source inverter and according to the load level, in order to reduce losses in the machine and in the inverter. This paper presents an optimization analysis and simplified formulae determining the optimal reference magnetization current in the control of the system. Conversely to field weakening in conventional drives, in this case, the enabling of field weakening control is not dependent on rotor speed, but depends on the reference torque. The proposed optimal control is validated through simulation and experimental results.

This study analyses the voltage dip behaviour of the doubly-fed induction generator (DFIG) connected to a common dc link on both the stator and rotor sides, via a diode bridge and a voltage source inverter (VSI), respectively. After a voltage dip in the dc grid, the rotor VSI can transiently lose control so that rotor currents circulate through the VSI free-wheeling diodes. However, during the uncontrolled period, both stator and rotor voltages are clamped to the same dc-link voltage and no rotor overvoltage occurs. In order to analyse the behaviour of the system during the first periods of the uncontrolled current transients, this study presents a simplified average model which is validated through simulations. Voltage dips are classified into three types depending on their severity. A simple analytical expression is derived for the maximum amplitude of the current during the voltage dip, and an overcurrent chart in the plane ‘rotor speed – dip amplitude’ is deduced. These results provide deep insight into the DFIG-DC system behaviour and are useful for design purpose.

This paper analyses the operation of a variable-speed woundfield synchronous generator (WFSG) connected to a constant-voltage dc-system. The challenges for the operation under variable speed and constant voltage and the need for a dc-dc converter to adjust the stator side voltage are pointed out. The boost converter introduces a degree of freedom which can be used to optimize the WFSG efficiency. An optimization analysis is carried out with a simplified model. Optimal rectifier-side dc-voltage and field current values are found as a function of speed, torque and WFSG parameters. The analysis is validated by simulations.

A sensorless stand-alone control scheme of a doubly fed induction generator (DFIG)-DC system is investigated in this paper. In this layout, the stator voltage is rectified by a diode bridge that is directly connected to a dc bus. The rotor-side voltage source inverter is the only controlled converter required in this system and is directly powered by the same dc bus created by the stator-side rectifier. DC voltage and stator frequency are regulated by two independent proportional-integral regulators that give the references for inner current controllers implementing field-oriented control. As it is capable of creating a stable and regulated dc bus, this system can be conveniently adopted to supply dc loads or to form a dc grid. Due to the constraint imposed by the stator diode bridge, the DFIG has to operate under a constant stator voltage, and the conventional stator field-oriented control implemented in stand-alone ac DFIG must be modified. This paper presents the control structure and the theoretical framework for the controller synthesis. Simulation and experimental validations on a small-scale rig are included.

A DFIG connected to a dc bus by a diode rectifier and a unique reduced-power PWM converter is considered in this paper. With respect to the traditional ac-grid connected DFIG, such a layout avoids the grid-side PWM converter and is an interesting solution to integrate the DFIG in a dc microgrid together with other generating units, loads as well as storages. The peculiarity of the DFIG, which allows the control of the rotor current space vector independently of the mechanical position, offers two important benefits when the stator is connected to a constant voltage dc grid by a diode bridge: it avoids the need to boost the flux amplitude at low speed, and it allows to considerably reduce the torque ripple due to the diode commutation. This last issue is developed in this paper by using a field-oriented control scheme based on proportional-integral and resonant Controllers. The proposed control is validated through simulation and experiments.

A current error space vector based hysteresis controller for two-level voltage source inverter fed Induction Motor drives is proposed in this paper. All inherent advantages of conventional hysteresis controllers like fast dynamic response, simple implementation, switching in one phase at a time etc. are also retained in the proposed controller . The harmonic spectrum of phase voltage obtained using proposed controller is similar to that of constant switching frequency bus clamping space vector pulse width modulated voltage source inverter. In this proposed controller hysteresis boundary is computed online using estimated stator voltages along α and β axes. The current errors along alpha and beta axes and steady state model of induction motor are used for the estimation of stator voltages along α and β axes. Hysteresis boundary is computed and using that the vector selection logic is deduced in such a way that the vector switching pattern is exactly similar to that of bus clamping SVPWM technique. Extensive simulation of the proposed hysteresis controller based inverter fed drive scheme is carried out using SIMULINK toolbox of MATLAB for steady state performance.

This book is very well-written and provides in-depth coverage of the analysis, modeling, testing, and control of DFIM-based wind power systems. Topics such as offline estimation of DFIM model parameters are uniquely treated, while the DFIM control methods are thoroughly covered, including predictive control at constant switching frequency. The book can be considered a reference on DFIM-based wind energy electrical generators. The text introduces the wind energy electrical generation system, where the DFIM is used as an adjustable-speed electrical generator. Practical and commercial illustrative information is given. The modeling of the DFIG and power electronic converter operation is then presented, including the power electronics converters with appropriate control and block diagrams for the offline model parameter estimation. The DFIM analysis under voltage dips, including the solutions for this issue, is also avidly explained. Some particular aspects as the starting process and standalone operations, are described. As a final point, new trends for wind power energy innovation are discussed.

This paper presents a self-sensing technique for the field-oriented control and frequency regulation of a doubly fed induction generator connected to a dc-link. In this system, the stator circuits are connected to a dc-link through a diode bridge and the rotor circuits are controlled by a voltage source inverter connected to the same dc-link. As the diode bridge maintains the fundamental harmonics of the stator current and voltage approximately in phase, an almost zero average $d\mbox{-axis}$ stator current results in the stator flux reference frame. This property is used to estimate the slip angle, required to implement the field orientation in the self-sensing technique in loaded conditions. At no load the system is controlled using a different methodology. The analysis and synthesis of the control chains are presented by analytic relations. The sensitivity study shows that the method exhibits reduced sensitivity to the parameter mismatch, resulting in a small orientation error. Experimental results confirm the good performance of the proposed method.