**1. Introduction**

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When designing a wind turbine for power generation there are two methods of linking the wind sensor to the generator, as seen in figure 1. *Julien.jac@erneo.fr* Nicolas Ziegler, *Société ERNEO SAS Cap Alpha - Avenue de l'Europe - 34830 Clapiers – France Nicolas.ziegler@erneo.fr*

**Speed Multiplier** 

Daniel MATT, *Institut d'Electronique du Sud - Université Montpellier 2 Place Eugène Bataillon - 34095 Montpellier Cedex 5 – France matt@univ-montp2.fr* Julien JAC, *Société ERNEO SAS Cap Alpha - Avenue de l'Europe - 34830 Clapiers – France* 

Figure 3. Elementary domain; tooth coupling

A

low speed rotor stator permanent magnet magnetic slot x y Dr Ds <sup>r</sup> s <sup>M</sup> Ns magnetic slots s The first, the most common, links the turbine to the generator via a mechanical speed multi‐ plier. In this configuration, the mechanical power is transmitted at high speed to the electri‐ cal machine. The size of the latter may then be easily reduced. This method has the major advantage of allowing the use of simply designed synchronous or asynchronous generators, which are readily available and inexpensive.

rs 2.Nr permanent magnets have s l1 dimensions of the pattern adimensional parameters This first method is mainly used for high power wind turbines (above a few tens of kilo‐ watts, to establish an order of magnitude) because at this power level, the large size of the generator becomes a problem, it becomes difficult to do without the speed multiplier.

> sl1L el L e a l © 2012 Matt et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Matt et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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The second, more recent, alternative is to link the generator directly to the turbine without a mechanical intermediary [8]. This method is known as "direct drive" and has become eco‐ nomically viable in recent years thanks to the progress made on permanent magnets. The cost of permanent magnets has dropped significantly while their performance has continued to improve. They have enabled the design of high performance, high power density, syn‐ chronous machines, well suited to the low speed operation imposed by the wind sensor, at reasonable cost.

The direct-drive method is attractive because it eliminates the weak element of the conver‐ sion chain: the speed multiplier gearbox. This is indeed a frequent source of failure, an addi‐ tional noise source and may also require regular maintenance, resulting in high operating costs [8,9]. Finally, the multiplier can be the source of chemical pollution due to the lubricant oil. This explains why the latter option is widely preferred in the installation of small and medium size wind turbines for domestic applications, which are intended for operation over a long time without maintenance.

Above a certain power level, typically 10 kW, both methods become competitive in terms of cost; only a fine techno-economic study would tip the balance one way or the other.

As part of a medium-power design, there is a third method which offers an interesting alter‐ native to the mechanical speed multiplier: the use of a magnetic gear [3-7]. This chapter, then, is devoted to the description of this device and shows the utility and feasibility of such a wind conversion chain. The different magnetic multiplier structures are presented and the design of this device will allow comparison with traditional solutions.

The advantage of the magnetic speed multiplier over its mechanical counterpart is clearly the contact-free force transmission that enables operation without any maintenance. We also show that the size and efficiency of magnetic speed multiplier are not prohibitive for the in‐ tended application.
