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Thursday, 13 March 2014
11:15 - 12:45 Innovative concepts for drive train components
Science & Research  


Room: Ponent
Session description

New developments in wind turbines need innovation and advances in technology in the field of wind turbine drive trains. This session focuses on topics related to transmissions and generators

Lead Session Chair:
Emilio Gomez-Lazaro, Universidad de Castilla-La Mancha. Renewable Energy Research Institute, Spain
Georgios Messinis National Technical University of Athens (NTUA), Greece
Co-authors:
Georgios Messinis (1) F P Konstantinos Latoufis (1) Nikos Hatziargyriou (1)
(1) National Technical University of Athens (NTUA), Athens, Greece

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Giorgis M. Messinis was born in Athens in 1987. He received the Diploma degree in electrical and computer engineering from the National Technical University of Athens (NTUA), Athens, Greece, in 2011. He is currently a PhD student at the Electrical Systems Laboratory, Department of Electrical and Computer Engineering, NTUA. His current fields of interest are wind power generation, axial flux permanent magnet machines, microgrids and smart grids.

Abstract

Design aspects of coreless axial flux permanent magnet generators for low cost small wind turbine applications

Introduction

This paper provides a guide on design aspects of coreless Axial Flux Permanent Magnet generators for small wind turbines in rural electrification applications. Specific design parameters are studied w.r.t. their influence on overall cost, weight and volume of the generator using simulations in Finite Element Analysis software. In addition, their influence on power quality and electricity production of the generator is considered, in terms of efficiency and total harmonic distortion of the induced EMF waveform. A prototype 3kW generator for grid connection is designed, simulated and tested using an experimental setup, for demonstration and validation purposes.

Approach

In order to attain the aspects of low cost AFPM generator design, a complete dimensioning of the small wind turbine needs be conducted, with respect to the rotor to be used, a process which is described in detail in the paper, according to relevant literature. Initially and in order to dimension the small wind turbine generator in terms of outer radius, the nominal wind speed, the nominal frequency and the number of poles are decided upon. The above modeling is developed in MATLAB. The efficiency of the generator as well as the total harmonic distortion of the induced EMF waveform are significant aspects that are studied in this paper and are calculated through finite element simulations in FEMM.

As a result of the dimensioning mentioned, specific design parameters can be calculated and varied accordingly in order to observe certain design trends and aspects. Specifically, the Pole Arc to Pole Pitch ratio ai, the Inner to Outer Radius ratio kd and the winding Fill Factor kf, are studied. Their effects on important generator characteristics are observed such as the outer radius and mass of the generator, the generator's total cost, its power losses and efficiency, and the quality of the induced EMF waveform. After varying over a wide range of values for ai, kd, and kf, design trends are observed focusing on low cost designs.

Based on this analysis, a low cost 3kW small wind turbine generator for grid connection is designed, and locally manufactured. Its operation is bench tested in the NTUA laboratories for validation of the simulated results.

Main body of abstract

The generator under study is a double-rotor-single-stator coreless machine. In the stator, the absence of an iron core and the choice of a double layer non overlapping winding, simplify the construction process. Graphical representations of the variations of the three important design parameters mentioned earlier, are presented in this paper with respect to overall cost, mass, volume, losses (copper, eddy and rotational), efficiency and total harmonic distortion of the output waveform. A brief compilation of the results included in this paper are presented below.

Specifically, for the pole arc to pole pitch ratio ai, a range of values between 0.2 and 0.9 have been studied and a significant influence on the total cost of the generator can be observed (Fig. 1). This ratio can acquire different values with respect to the present cost of materials. A selection of the optimal value of ai for low cost designs is challenging, since ai also influences the mass of the generator. Thus, a low value of ai should be chosen if low weight is the main design criterion. When high efficiency (Fig.2) is sought for, ai should be set at higher values. According to simulation results which are supported by relevant literature, a value of 2/π is suitable if a sinusoidal EMF waveform is required. Further on, it was observed that by increasing the pole arc to pole pitch ratio, for constant generator nominal power and magnet thickness, an increase in magnetic material occurs and thus an increase in the magnets' active surface, which results in decreasing the generator's outer diameter and thus its volume. The same does not apply to weight. By increasing ai and thus increasing the active surface of the magnets, the attractive forces between the rotor discs are increased, making mandatory the increase of the rotor’s back iron thickness. This increases the mass and results in a heavier generator.





Specifically, for the inner to outer radius ratio kd, a range of values between 0.5 and 0.9 have been studied and a significant influence on the total mass of the generator can be observed(Fig. 3). It was observed that for constant generator nominal power and magnet thickness, a reduction in kd, increased the active length of the generator and thus increased the volume of magnetic material. This in turn increased the attractive forces between the magnet plates and so the back iron thickness needed to be increased in order to avoid deflection thus making the generator heavier. At the same time, the outer radius and thus the volume of the generator were decreased by the increase in magnetic material mass. According to the simulation results, high values of up to 0.8 are recommended if a lightweight construction is required reducing at the same time the total cost of the generator. A choice between 0.7 and 0.8 is acceptable for low cost applications, where the generator efficiency is not of primary importance compared to cost and mass. According to simulation results and relevant literature, when high power density is desirable kd should be set at 1/√3.



Finally, the effect of the coil fill factor kf is investigated for a range of values between 0.4 and 0.75. The value of the fill factor cannot be chosen precisely by the designer, though it can be regulated to some extent by the shape of the coil, the number of turns and conductor cross-section, and the winding type. Maximization of the fill factor affects the dimensions of the stator coils. This can influence the coreless stator operation temperature, which is of great importance due to the maximum temperature limit of the polyester resin. The maximization of this factor is desirable in order to minimize the cost, mass and volume of the machine. This may create a slight increase in copper and eddy current losses (Fig. 4), and a subsequent efficiency degradation, due to the decrease of coil surface area and thus a degradation of the induced EMF, which is accounted for with an increase in the number of turns of the winding.




Conclusion

Environmental and social issues, such as the lack of access to electrical energy by a third of the world’s population and the increasing adverse effects on the environment by the use of conventional energy sources has raised interest in the development of small wind turbines for rural electrification applications, especially in developing countries. Such turbine sizes have the ability to be locally manufactured using simple construction techniques, while requiring a low cost design. These characteristics are crucial in providing energy access, income generation and environmental benefits to local communities, while local small scale industry can be boosted. An axial flux permanent magnet (AFPM) coreless generator is the most suitable choice for such applications because it can be easily manufactured locally, either for direct battery charging applications or for grid connection, as microsources in community microgrids.

The findings of this paper have all been materialized in the manufacturing of a low cost prototype (Fig. 5), specifically a 3kW small wind turbine generator for grid connection, in order to experimentally validate the simulations. The prototype generator had outer radius of 238mm, pole arc to pole pitch ratio ai of 0.66 and an inner to outer radius ratio kd of 0.87 for a low cost design. The fill factor of the windings was measured to be 0.65. The total cost of materials was calculated to be 588 € resulting in a value of 196 €/kW. The mass of the generator was measured to be 30 kg. All of the above where in accordance to the simulation results as well as the performance of the generator (Fig. 6), with a maximum error in power production of 4%.





It is thus concluded that the design aspects of coreless AFPM generators that are presented in this paper, supported by simulation and experimental results, are an essential tool for small wind turbine designers and also enrich current scientific literature.


Learning objectives
The small wind turbine industry is still an emerging market with more companies starting up each year. Yet research in small wind turbine technology has not been following this trend and engineers mostly use design approaches that are more suitable for larger wind turbines. As stated, this paper will enable small wind turbine designers to better understand the design aspects of AFPM generators, which are an important generator technology, especially for businesses in developing countries.


References
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