The brushless Doubly-Fed Induction Machine (DFIM) provides an interesting alternative to the commonly applied conventional DFIM in modern multi-MW wind turbines. This paper presents a literature survey on the brushless DFIM. The evolution of the cascaded induction machine into the modern brushless DFIM is discussed, as well as several theories explaining this machines complex operating principles. Nowadays, brushless DFIM research is mainly focussed on the development of a commercially competitive brushless DFIM based drive for multi-MW wind turbine applications. An overview of the latest developments in the fields of brushless DFIM modeling, design, and control is provided. Based on this, a qualitative comparison is presented of the brushless DFIM and other well-established generator systems for wind turbines and the challenges for future research are explored.@en

Brushless doubly-fed induction machines (DFIMs) have great potential as variablespeed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple to two stator windings with different pole-pair numbers and different frequencies. These undesired space harmonics could lead to noise, vibrations and low power quality. Applying skew benefits to overcome these drawbacks. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of skew which was time consuming. In the stage of initial design, it is not efficient to use such a model to predict how much the average torque and the torque ripple would be reduced by skewing slots. This paper makes use of normal 2D FE results and applies skew factors in post-processing to investigate the influence of rotor skew. It also proves that to apply skew factors appropriately, not only the space-harmonic order needs to be considered, but also the time-harmonic order. The proposed method can give an approximate prediction of skew effects with limited computing time. The results also indicate that skewing the rotor slot over one stator slot pitch could be a good choice to minimize the torque ripple in a brushless DFIM with a nested-loop rotor.@en

The brushless Doubly-Fed Induction Machine (DFIM) is a special machine type that is of interest as a generator in wind turbine drive-trains. The brushless DFIM has a significant space-harmonic content compared to conventional machine types, due to its construction. This results in additional harmonic distortions, such as torque-ripple and time-harmonics. This paper studies the causes and origins of harmonic distortions in the brushless DFIM. Using time- and space-harmonic analysis an analytical evaluation method is derived to predict torque ripple frequencies and time-harmonic frequencies in stator and rotor voltages and currents. The evaluation method is applied to a prototype brushless DFIM to demonstrate the severity of harmonic related distortions that can be present in brushless DFIMs. The evaluation is validated by additional FE analysis and measurements. Furthermore, measures are proposed to reduce harmonic distortions. The insight into harmonic distortions in the brushless DFIM and the ability to predict and prevent them must lead to improved brushless DFIM designs in the future.@en

The brushless Doubly-Fed Induction Machine, without brushes and slip-rings, is regarded as an attractive alternative to the conventional Doubly-Fed Induction Machine in terms of reliability. This paper presents a sensorless field oriented control strategy for the Brushless-DFIM, which further increases its attractiveness as a drive system.With the use of a straightforward and alternative time-dynamic Brushless-DFIM model, a complete Brushless-DFIM based drive is modelled and used for the development of the sensorless control strategy. The developed sensorless control strategy is implemented in an experimental set-up. Simulation and measurement results demonstrate that the developed sensorless control strategy can control the Brushless-DFIM in a stable and responsive manner over its full operating speed range. In this way, a fully functional sensorless control strategy for the Brushless-DFIM is demonstrated for the first time in literature.

@en

The air-gap magnetic fields of brushless doubly fed induction machines (DFIMs) are complicated because of the crosscoupling between two stator fields via a special nested-loop rotor. Comparedwith classical analyticalmodels, transient finite-element (FE) modeling is easier to evaluate the machine performance taking saturation into account. However, it is not efficient to evaluate lots of candidates in a large design space using transient FE models considering the time cost. In the transient simulation, the induced rotor currents are calculated by solving several time differential equations using the backward differentiation formula. This paper presents a computationally efficient FE analysis for brushless DFIMs. The induced rotor currents can be calculated using a single magnetostatic FE simulation. The average torque, losses, and efficiency can also be predicted using one magnetostatic solution. One candidate design can be evaluated within 1 or 2 min on a personal workstation. The efficient analysis is validated by the transient FE results. The presented model is applied to the optimization of a prototype. The influence of two construction variables, namely, polepair combinations and the number of loops per nest, is studied. One pole-pair combination is selected for manufacturing a prototype. @en

The brushless doubly fed induction machine (DFIM) shows great potential as a generator in large-scale wind turbines. The motion of the magnetic field in this machine is not a simple rotation, which makes it not so straight forward to understand its operating principles. This paper develops an analytical magnetic field model for the brushless DFIM that includes the effects of rotor time harmonics and space harmonics due to the winding distribution and slotting. Using a case study machine, the developed analytical model is then validated by comparison with finite-element (FE) calculations. In addition, a 2-D spectral analysis is applied to the FE derived radial air-gap magnetic field as a function of time. This analysis verifies the space–time relations of the rotating magnetic field components in the airgap of the brushless DFIM. Finally, the developed analytical magnetic field model is used to analyze the brushless DFIM operating principles. The interaction of the stator magnetic field with the rotor nested loops is explained, as well as the development of electromagnetic torque.@en

In this paper, the brushless DFIM based wind turbine drive-train topology is compared to the DFIG based and PM generator based drive-train topologies, that are most commonly applied in modern wind turbines. The comparison will be based on a 3:2MW case study wind turbine. By using FE based multi-objective optimization, optimized generator designs for the dierent topologies are generated. Then the capital expenditures of the resulting drive-train topologies are calculated and compared. Additionally, wind turbine drive-train congurations with 1, 2 and 3 stage gearboxes as well as a direct-drive conguration are taken into account. The resulting comparison shows that the brushless DFIM based drive-train with a 2 stage gearbox conguration provides a feasible alternative in commercial wind turbine drive-train applications. @en

Brushless doubly-fed induction machines (DFIMs) can be potential variable-speed generators for off-shore wind turbine applications. This special machine has two stator
windings with different pole-pairs. There is no direct magnetic coupling between two stator windings. A unique nested-loop rotor is designed to couple both stator windings. However, it results in lots of undesired space-harmonics in the airgap fields compared to conventional induction machines. There are few papers working on the design of nested-loop rotors. To design a better rotor, it is important to understand the complicated magnetic coupling in brushless DFIMs. This paper analyzes the space-harmonics and time-harmonics to provide a better understanding of the magnetic cross-coupling through the nested-loop rotor. The harmonic analysis is validated by FEM simulation and by taking measurements on a prototype in simple induction operation mode.@en

The brushless Doubly-Fed Induction Machine (DFIM) has many advantages over the conventional DFIG commonly applied in wind turbines. However, due to the complex motion of the magnetic field in this machine type, the inclusion of nonlinear iron saturation in brushless DFIM models has been proven to be challenging. This paper combines a brushless DFIM Electric Equivalent Circuit (EEC) model with an analytical derived magnetic field model. The saturated magnetic field is iteratively obtained using the secant method. Saturation is included in the EEC model by introducing complex saturation factors derived from the magnetic field. This results in an EEC model that is able to accurately determine brushless DFIM operating characteristics. The model is validated by application to a case study machine and comparing the results with those derived from a finite element model.@en

Brushless doubly-fed induction machines have great potential as variable-speed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple both stator windings which with different
pole-pair numbers and different frequencies. These undesired space harmonics lead to a bigger torque ripple compared with conventional induction machines. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of rotor skew on torque responses in brushless DFIMs. It results in a significant computing time because several 2D FE slices are coupled and calculated simultaneously in one model. It is not efficient to use such a model to predict how much average torques and torque ripples would be reduced by
applying skewed slots at the beginning of design. This paper makes use of normal 2D FEM results and applies skew factors in post-processing to investigate the influence of rotor skew on the torque responses. The proposed method can give an approximate prediction of skew effects on torque responses with limited computing time.@en