Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 20

PMGs have the advantages of:

• High efficiency at low rpm and no need for a gearbox
• Simple design
• Ease of control

This is balanced partly by their relatively high cost and the often possibility that
the permanent magnet material may lose some of its magnetism over the standard
turbine lifetime of say 20 years. Induction generators (IGs) are often common
induction motors running ''backwards''. They have the advantages of:

• Inexpensive due to mass production
• Robust and easy to replace

but are often less efficient especially at small size—see Table 1.4 —and part-
load. Some of the relative inefficiency is related to the fact that induction gener-
ators are not self-exciting whereas PMGs are. The former must be supplied with
excitation capacitors which are not guaranteed to work in all situations.
For example, if a stationary turbine experiences a very rapid increase in wind-
speed, the blade acceleration may be too rapid for excitation to occur. Further-
more, the capacitance may need to be adjusted for changes in cable length between
the generator and the controller. Induction generators should be fitted with
independent speed encoders for protection and this adds to their cost.
As explained in Chap. 11 , basic generator function is independent of size and
there are only two important aspects for small wind turbines. The first is
the generator inertia which is considered in the next section. Figure 1.12 highlights
the second: the ''cogging torque'' for small PMGs as a fraction of the rated torque
(torque at rated power). Cogging torque is required to force the rotor through
the stator's permanent magnetic field, e.g. Jahns and Soong [ 13 ], and so must be
overcome by the blades before they begin to rotate. It depends on the azimuthal
angle between the rotor and the stator and its magnitude is independent of the
direction of rotation. The data in Fig. 1.12 , obtained from the sources in Table 1.5 ,
are the maximum magnitudes of the ''cogging torque'' and the term will be used
with that implication throughout this topic.
Note that the cogging torque ratio tends to increase with decreasing PMG
output power. Tudorache et al. [ 14 ] state that cogging torque must be in the range
of 1.5-2.5% of the rated torque for a wind turbine to start at 2.5-3 m/s. Chapters 6

Table 1.4 Typical efficiencies of small induction motors

Motor
power
(kW)

Number of
poles (N P )

Motor
efficiency
(%)

Number of
poles (N P )

Motor
efficiency
(%)

Number of
poles (N P )

Motor
efficiency
(%)

1.1 4 77.8 6 754 8 72.8

1.5 4 79.2 6 77.5 8 76.5

2.2 4 81.0 6 79.1 8 79.6

3.0 4 82.6 6 81.4 8 82.9

Data from http://www.westernelectric-motors.com/ (accessed 9 Sep 2010)