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Small Wind Energy Systems 165

A generator with ferrite magnets must be overdesigned to comply with the specifications of lower
flux level of the ferrites. Therefore, the losses are more important for ferrite design than for NdFedB
magnet design. This is the reason why most of the PMSGs in the market use rare-earth magnets.
Actually, the optimal range for ferrite magnets is smaller and the captured wind energy decreases.
If only technical considerations are used, it is easy to disregard completely ferrite-based magnets
and just use rare-earth magnets instead. Only future high price of rare-earth magnets and their low
availability make ferrite-based PM machines to be implemented. Ferrites have an advantage: they
can be made anywhere as long as iron and ceramics are available, and the knowledge of making PM
ferrites is accessible to anyone in the world, making them a “sustainable generator” option forever.
The energy-captured area can be plotted to show the power that can be extracted from the turbine
P ) and by the losses, which
shaft. This area is bounded by the maximum power from the turbine ( max
can be calculated by
v design
E ca ( v min , v max) =  ∫  P v () − () − ()   (7.10)
P v
P vdv
i
c
v min
If the wind generator is overdesigned considering a high loss level, despite a large power range, the
energy-captured area is smaller. Consequently, maximizing the power range area is not recommend-
able; several scenarios might support a study in order to have a compromise between an acceptable
loss level and the power range. The overall efficiency and cost should be considered too. Table 7.5
shows the optimization results for the three machines designed for the same magnetic flux levels [13].
The flux density is lower for a ferrite magnet generator, so the total mass should be increased in
order to obtain the same performance. In this case, the captured energy is less than that in the other
cases. The best set of characteristics in terms of mass, energy stored, and consequently the mass–
energy ratios are reached for the sintered NdFeB (1.2 T) magnet due to the high efficiency of such
PM. The cost–energy ratio is computed using price data for all components (iron, copper, and PM).
For the cost–energy ratio criterion, the ferrite magnet configuration has the smallest ratio. Although
the mass is the highest one, the price of a ferrite magnet is about twenty times less than that of an
NdFeB magnet. Thus, the total cost is smaller than the NdFeB magnet. Despite the ratio of mass to
energy being the smallest, the ferrite magnet is a good alternative when compared to the rare-earth
PM design option. Table 7.6 shows a comparison of the machine with a bounded NdFeB magnet
designed and a high-torque off-the-shelf motor [13].
The PMSG offers many advantages as it is the most efficient of all electric machines since it
has a movable magnetic source inside itself. The use of PMs for the excitation consumes no extra

TABLE 7.5
Comparison of Three Designed Permanent Magnet Machines
Magnet
Type Ferrite Bounded NdFeB Sintered NdFeB
Iron mass 67 kg 37 kg 32 kg
Copper mass 38 kg 22 kg 20 kg
Magnet mass 27 kg 23 kg 21 kg
Total mass 132 kg 82 kg 73 kg
Energy 1.7 kWh 2.1 kWh 2.3 kWh
Mass/energy 80 kg/kWh 38 kg/kWh 32 kg/kWh
Cost/energy 0.6 k€/kWh 2.2 k€/kWh 1.9 k€/kWh

Source: Ojeda, J. et al., IEEE Trans. Ind. Appl., 48(6), 1808–1816, 2012.