Tim D. Strous

Brushless Doubly-Fed Induction Machines: Magnetic Field Modelling

The brushless DFIM is a complex machine type. Though never commercially exploited, it has a great potential as generator system in large-scale wind turbines. This paper develops an analytical magnetic field model for the brushless DFIM. Since the brushless DFIM rotor construction has a rich space harmonic spectrum, attention is paid to the effects of winding space-harmonics. The developed model is validated by FE calculations. With use of the developed model, the brushless DFIM operating principles are considered from the point of view of the magnetic field.

Finite element modeling of brushless doubly-fed induction machine based on magneto-static simulation

The brushless doubly-fed induction machine (BD-FIM) is a potentially attractive choice for a variable-speed wind generator. Using classical analytical models is not so straightforward, because the motion of the magnetic field in BDFIM is not a simple rotation. Finite element (FE) modeling makes it easier to evaluate the machine performance considering the saturation. However, for the purpose of a better convergence and an accurate calculation of the magnetic field equation and the coupled circuit equation, small time steps are utilized in the adaptive time-step solver of the transient FE model. A long computing time makes FE model be difficult to combine with an optimization program. This paper presents one possible alternative method to predict the BDFIM performance by using the magneto-static FE solutions and the space-time transformation. The simulation time then reduces significantly making it possible to search in a large design space for the optimization purpose.

Modeling and Optimization of Brushless Doubly-Fed Induction Machines Using Computationally Efficient Finite-Element Analysis

The air-gap magnetic fields of brushless doubly fed induction machines (DFIMs) are complicated because of the cross-coupling between two stator fields via a special nested-loop rotor. Compared with classical analytical models, 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, pole-pair combinations and the number of loops per nest, is studied. One pole-pair combination is selected for manufacturing a prototype.

A Novel Analytical Approach and Finite Element Modelling of a BDFIM

Nowadays many wind turbines use a doubly fed induction machine (DFIM) as a generator (DFIG). The brushless DFIM (BDFIM), however, has increased fault handling capabilities and suffers less from reliability issues. This paper presents a different method of analytically evaluating the BDFIM. This is done by analyzing the flux density produced by the two stator windings and the interaction with the nested loops of the rotor. Subsequently FEM is used to verify the analytical findings and analyze the behavior of the machine. Additionally several design parameters of the BDFIM such as the pole pair combinations and the number of loops per nest are evaluated. General rules of thumb regarding these design parameters are presented.

LVRT performance of brushless doubly fed induction machines — A comparison

The Brushless Doubly Fed Induction Machine (B-DFIM) shows promise for use in wind turbine drivetrains. This paper discusses the performance of this machine under symmetric low voltage dips and compares this with the performance of two other machines - the Permanent Magnet Synchronous Machine (PMSM) and the Doubly-Fed Induction Generator (DFIG). Attention is paid to the controller for the B-DFIM and protection methods for improved Low Voltage Ride Through (LVRT) performance are discussed. It is shown that the B-DFIM has an improved LVRT performance when compared with the DFIG, moreover, the B-DFIM does not require an external circuit for protection from low voltage events.

Brushless Doubly Fed Induction Machines: Magnetic Field Analysis

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.

Finite element based multi-objective optimization of a brushless Doubly-Fed Induction Machine

Although the brushless Doubly-Fed Induction Machine (DFIM) has great potential as generator system in large-scale wind turbines, its complexity has so far retained a commercial breakthrough. This paper contributes by combining brushless DFIM Finite Element (FE) modelling with multi-objective optimization. A static brushless DFIM FE model is applied with the NSGA-II multi-objective optimization algorithm. The result is an accurate and fast brushless DFIM design optimization tool. This tool is then used to generate a machine design with optimized performance, that fits a fixed volume frame size D180. Optimization results of different brushless DFIM construction variations are compared. The best designs of each construction variation are then analysed in more detail using a time-stepping brushless DFIM FE model. This provides good insight in the effects of different construction variations on torque ripple and induced time-harmonics. The optimized design of the best construction variation will be selected to be manufactured as prototype machine.

Inductance calculations for PM machines with concentrated windings

The usage of three phase permanent magnet (PM) machines with concentrated coil fractional pitch double layer windings, proofs to be very cost-effective for range extenders in the automotive sector. However, the number of possible slot pole combinations for these machine types is countless. This paper presents an analytical method for calculating the inductance components of these electrical machine types. This method can be used for calculations of all possible slot pole combinations. It does so by first deriving the machine optimal winding configuration. The machine winding configuration is then used to set up an armature reaction flux model. From the armature reaction flux the different inductance components, including the self-inductance with its main and leakage component and the mutual-inductance component can be determined. The analytical model is used for the inductance calculations of two prototype generators for the Peec-Power range extender. The results are compared to FEM calculations. An accurate analytical model is the result.

Brushless doubly-fed induction machines: Torque ripple

The brushless Doubly-Fed Induction Machine (DFIM) without its brush-gear and slip-rings seems interesting as successor of the normal DFIG in wind turbine drivetrains. However, the brushless DFIM magnetic field has a rich space-harmonic spectrum, which causes additional torque ripple. This paper focuses on torque ripple in the brushless DFIM. The causes and origin of torque ripple are discussed and an analytical model is developed that derives the torque ripple from the air-gap magnetic field distribution. Further, a finite element method for torque ripple calculation is presented. Both methods are used to calculate the torque response of a case study machine. Results are compared and the accuracy of the analytical model is validated with good result. The analytical model is then used to further analyse the torque ripple, resulting in exact torque ripple frequencies and additional insight in the cause of the torque ripple.

Effects of rotor skew on the performance of brushless doubly-fed induction machine

Brushless doubly-fed induction machines (BDFIM) have great potential as variable-speed generators in larger-scale wind turbines. Undesired space harmonics exist because the special rotor needs to couple both two stator windings which with different pole-pair numbers and different frequencies. These rich space harmonics lead to a bigger torque ripple comparing to the normal induction machine. This paper makes use of 2D multi-slice finite element (FE) method to study the effect of the rotor skew on the performance of the BDFIM in the cases without and with saturation. The results show the torque ripple is reduced significantly by skewing the rotor both in those two situations. But the rotor skew has negligible influence on the harmonics due to the saturation effect. Furthermore, the rotor skew has little influence on the total amount of losses. However, based on the simulation results, it redistributes the core losses along the axial direction in the BDFIM.