214 research outputs found
Adaptive tuning of the stator inductance in a rotor - current - based MRAS observer for sensorless doubly fed induction machine drives
No full textThis paper deals with the adaptive tuning of the stator inductance in a rotor-current-based Model Reference Adaptive System observer for the sensorless control of doubly fed induction machines. At first, the effect of mismatched parameters in this observer is discussed in order to show the considerable influence of the stator inductance on the accuracy of the estimated rotor position. Then, an adaptive tuning of the stator inductance is proposed and a small signal model is deduced in order to design the tuning loop. Moreover, a theoretical sensitivity and stability analysis is performed. Finally, the performances of the proposed scheme are experimentally investigated and validated
On line tuning of the stator inductance in a MRAS observer for sensorless DFIM drives
No full textThis paper proposes a scheme for the self tuning of the stator inductance in a Rotor Current MRAS Observer for the sensorless control of Doubly Fed Induction Machines. The effect of mismatched parameters in this observer is briefly discussed and the strong dependence of the estimated rotor position on the stator inductance is highlighted. A self tuning scheme is formulated by using only quantities which are already exploited by the observer, so that only a marginally additional computational effort is required in the implementation. Some guidelines to design the tuning loop are given. Finally the performances of the proposed scheme are experimentally tested and discussed
Active Current Control of a Linear Generator for Energy Harvesting Applications
No full textThis paper presents an algorithm for automatic maximum power point tracking for linear generator drives used in Stirling or Thermoacoustic generators. The method adjusts the magnitude of the current command responsible for the active power set-point. The proposed approach combines a simple model-based feed-forward term with a slow but accurate correction based on the response of the output power to a lowfrequency modulation in the set-point current magnitude. The correction is derived from the phase of output power oscillations and used to adjust the power set-point and achieve activeload matching conditions. In the paper, the control principle is validated with simulations. As a basis for future experimental validation and to back part of the assumptions, the paper also includes experimental results on two-linear-machine test-rig showing the possibility to vary the internal equivalent damping of the prime-mover
Modulation and Feed-Forward Based Damping Matching Strategy for Linear Generators
No full textThis article presents an active damping matching control strategy to optimize the output power of linear generators in energy harvesting applications. The method adjusts the magnitude of the current command responsible for active power set-point. The proposed approach combines a simple model-based feed-forward term with a slow but accurate correction based on the response of output power to a low-frequency modulation in set-point current magnitude. The correction is derived from the phase of output power oscillations and used to adjust the power set-point to achieve active-load matching conditions. In order to remove the low-frequency oscillations in the power at steady state, the method automatically stops the current amplitude modulation when maximum output power is reached. In addition, the proposed control incorporates an overstroke protection, which increases the active current component thereby reducing the stroke rapidly to a safe limit. The overall control principle is validated experimentally
Overcurrent estimation in a doubly-fed induction generator-DC system during a voltage dip in the dc grid
No full textThis 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
Enhanced torque control in a DFIG connected to a DC grid by a diode rectifier
No full textThis paper considers the control of a doubly fed induction machine (DFIG) connected to a dc-bus or dc-grid by using a diode rectifier placed on the stator-side and a single PWM converter on the rotor. In this system, the torque and the frequency have to be regulated simultaneously, in order to make the prime mover operating at the maximum power point and to guarantee at the same time a flux level near the rated value. Since this system is based on a unique reduced-power controlled converter and a diode rectifier, a noticeable reduction of the costs of the power electronics can be achieved. However, the diode bridge causes stator current and voltage harmonics which produce a noticeable torque ripple. A regulation scheme able to reduce the torque ripple is proposed in this paper: resonant current controllers are implemented to track the sixth harmonic components in the reference rotor current commands, in such a way to kill the corresponding harmonic in the electromagnetic torque
Minimum Loss Conditions in a Salient-Pole Wound-Field Synchronous Machine Drive
No full textThe 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
Sensorless Control of a Linear Generator for Energy Harvesting Applications
No full textLinear generators are key components for enabling power conversion from reciprocating prime movers such as Stirling and Thermoacoustic engines and wave energy convertors. The control of a linear generator is usually synchronized with instantaneous speed which sets a reference for optimal current-usually in phase with speed. Measuring speed, however, is often challenging because the linear alternator is located in a sealed pressurized vessel or operates in a harsh environment. This paper proposes a simple sensorless control based on a linear observer which estimates instantaneous velocity of the plunger to be used as a phase reference to synchronise the control. The performance and sensitivity of the proposed observer is studied and verified by means of both simulation and experiment. For the particular example the estimated position (from estimated speed integration) is lagging 6.8° towards the measured position, and their amplitudes are different by less then 1%
On the Stability of a Minimum-Loss Controlled Dual-VSI DFIG-DC System
No full textRecently 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
Stability Study of a Minimum-Loss Strategy Controlled Dual-VSI DFIG DC System
No full textA minimum-loss control strategy for the Dual-VSI-DFIG system was presented recently. Based on analytic optimization, three rules were derived for the optimal stator frequency, stator/rotor magnetizing current split and airgap-flux magnitude. These optimal rules were implemented via vector control using airgap-flux orientation. In this paper, the synthesis of the Proportional Integral controllers based on the symmetrical optimum criteria and the resulting stability are analyzed
- …
