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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. [ABSTRACT FROM AUTHOR]

This paper evaluates the parasitic currents that can occur in the structural part of some types of yokeless and segmented armature axial flux permanent magnet machines, where the frame-supporting stator floating teeth can originate stray paths. In this paper, these paths are defined and modeled, and the consequent effects (induced electro motive forces and currents) are estimated. The amount of these effects depends on the machine geometry, the magnetic circuit structure, and the winding layout. In this paper, the analysis is performed on two different rating machines, comparing their results. The model is validated by measurements on the corresponding prototypes. [ABSTRACT FROM AUTHOR]

In some topologies of yokeless and segmented armature (YASA) axial flux permanent-magnet (AFPM) machines, the particular frame structure required to fasten floating teeth can originate stray paths where parasitic currents may circulate. This paper investigates how the amplitude of these currents and the related additional Joule losses depend on the machine size. Reference is made to wind energy generators, rated from some tens of kilowatts up to a few megawatts. The developed model is mainly based on wind turbine basic data of vendor data sheets, and on the quantities evaluated and tested in two YASA AFPM prototypes, rated 50 and 200 kW. [ABSTRACT FROM PUBLISHER]

This paper investigates the optimal arrangement of the coils in a wound-type interphase reactor to avoid the decreased inductance due to local saturation in the core when operated with high dc current. In theory, the direct currents and the related magnetomotive forces in the two ways of an interphase reactor are equal, and this compensation leads one to conclude that the global flux in the yokes is zero. However, the dc currents in the coils produce a significant leakage flux that flows locally in the core. If the coils are not suitably arranged and if the dc current is high, the leakage flux causes local saturation, a drop in the inductance, and noticeable circulation of a triple harmonic current through the reactor. Such a phenomenon is investigated by means of both a simplified field model and three-dimensional magnetostatic simulations. A number of criteria for the arrangement of the coils to improve operation are derived and applied to a real rectification system used in an electrolysis plant. Some in situ operational measurements of both the original and the improved interphase reactor are reported. [ABSTRACT FROM PUBLISHER]

Due to the high torque capability, axial flux (AF) permanent-magnet (PM) machines are an interesting solution for direct driven wind energy conversion systems and low-speed motors. However, the structure of such machines makes them sensitive to geometrical imperfections due to manufacturing process. This paper analyzes the effects that various types of geometrical imperfections produce in concentrated coil AF PM generators. In particular, a case study is presented, where the mechanical stresses, the resulting axial force, and the bending torques on the shaft, as well as the circulation current in the winding parallel paths, are calculated. The results obtained by means of an analytical approach are compared with those based on finite element method simulations and are validated by some experimental results on a prototype. [ABSTRACT FROM PUBLISHER]

This paper focuses on the improvement of an air-gap-power-vector-based observer for sensorless doubly fed induction generators. After showing how a mismatch in the machine inductances can worsen the performance of the basic observer scheme, an adaptive tuning of the stator inductance is proposed. The adaptation is achieved by minimizing the error between the squared modulus of two suitable air-gap complex power space vectors. A criterion to settle the dynamics of the adaptation mechanism is provided, and a sensitivity analysis in the steady-state operation is performed. Some experimental results are reported and discussed in order to validate the soundness of the proposed adaptive tuning algorithm. [ABSTRACT FROM PUBLISHER]

This paper proposes an observer for the sensorless control of doubly fed induction machines which is based on the effect of a natural signal injection in the stator windings due to the fifth harmonic of the line voltage. A method to extract the rotor position and speed only from the rotor current and stator voltage measurements is developed: As a novelty, the stator current measurement and the anisotropies in the machine design are not required. A phase-locked loop is used in order to track the harmonic current component which is induced in the rotor by the fifth line voltage harmonic and whose speed and position are directly related to the rotor speed and position. The tuning of the observer parameters is discussed together with the effect of the other line voltage harmonics. This paper is completed by the experimental validation of the observer performances. [ABSTRACT FROM PUBLISHER]

An analytical method is presented to evaluate the field and global quantities for the operation and design analysis of axial flux permanent-magnet (PM) machines. The method, of general applicability, is based on the definition and superposition of suited field functions, able to accurately model the different singularities of the studied device: PM field distribution, effects of slotting, and radial end effects. Selected explorative FEM analyses are used in order to investigate the field functions shape and to help in identifying their structure and parameters. Then, the flux density distribution is obtained, as a function of the rotor position, taking into account some manufacture dissymmetry effects, such as air-gap offsets and PM rotor disk inclinations. This approach allows to analyze the effect of dissymmetry on the circulation of current among winding parallel paths in no-load operation. Moreover, by using the Maxwell stress tensor, the stress distribution is evaluated, as well as the resultant axial forces and the bending torques on the shaft due to the rotor disk tilts. Several FEM simulations show the soundness of the proposed method, which exhibits high accuracy and great evaluation speed. [ABSTRACT FROM PUBLISHER]

This paper deals with the performance of some numerical integration (NI) methods for ordinary differential equations in real-time applications involving stiffness. After summarizing the main results about accuracy and stability of some well-known schemes, the point of view of the control is emphasized, by showing the impact of the NI on the dynamics of the discretized model. Moreover, some peculiarities of the NI in hybrid systems are touched by introducing a simple hybrid model for testing the performances of some NI schemes. Finally, some experimental tests are shown, dealing with a control of a sensorless drive whose model is characterized by stiffness. [ABSTRACT FROM AUTHOR]

This paper deals with the ac–dc interface conversion in grid-connected wind energy conversion systems based on modular permanent-magnet synchronous generators. In particular, some solutions based on diode rectifiers are extensively analyzed as for the waveforms, the efficiency, and the torque ripple, by considering several connections, which are allowed by the inherent modularity of the considered generators. Rectifier schemes with a high number of pulses can be easily realized without using special shifter transformers so that these solutions can be attractive for low- and medium-power applications. [ABSTRACT FROM PUBLISHER]

This 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. [ABSTRACT FROM AUTHOR]

Some design and operation aspects of axial flux permanent magnet synchronous machines, wound with concentrated coils, are presented. Due to their high number of poles, compactness, and excellent waveform quality and efficiency, these machines show satisfactory operation at low speeds, both as direct drive generators and as motors. In this paper, after a general analysis of the model and design features of this kind of machine, the attention is focused on wind power generation: The main sizing equations are defined, and the most relevant figures of merit are examined by means of a suitable parametric analysis. Some experimental results obtained by testing a three-phase, 50-kW, and 70-rpm prototype are presented and discussed, validating the modeling theory and the design procedure. [ABSTRACT FROM AUTHOR]