Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 23

Table 1.6 Typical small turbine operating parameters

Category P (kW) R (m) Max. X

(rpm)

Typical uses Generator type(s)

Micro B0.5 1.5 700 Electric fences yachts Permanent magnet

(PMG)

Mid-range 0.5-5 2.5 400 Remote power systems,

single-user grid connections

PMG or induction

Mini 20-50+ 5 200 Mini grids, remote

communities

PMG or induction

Adapted from Clausen and Wood [ 14 ]

Fig. 1.13 Variation of blade
mass with radius for the LM
range. Data downloaded from
www.lmglassfiber.com
(accessed 19 May 2010)

10 4

mass ~ 2R 2.2

10 3

10 1 10 2

Blade radius, R (m)

Because of their high operational speed and low torque from Table 1.7 , micro-
turbines have the poorest starting performance which is often exacerbated by PMG
cogging torque. A related and size-dependent issue is the generator inertia, J.It
will be shown in Chap. 6 that generator inertia is important for two reasons. First,
if it and the cogging torque can be neglected, then turbine starting becomes
independent of the number of blades. Secondly, a generator with inertia large
compared to that of the blades it will also impair starting. Figure 1.14 shows some
data on J for small generators plus a line of fit that suggests J * P 3/2 * R 3 . The
IG data does not include gearbox inertia which should also scale on P. This neglect
is partly mitigated by showing J only for 8-pole IGs which would require a smaller
gearbox than the lower-pole IGs in Table 1.4 . Since blade inertia scales on a larger
power of R, it can be concluded that, if the generator inertia of micro-turbines can
be neglected—as will be demonstrated in Chap. 6 —then it is safe to exclude
generator inertia from starting considerations for turbines of all sizes.
A further interesting fact of the sub-division of small turbine sizes is that micro-
turbines are by far the most common small turbine. They are mass produced in
China, Britain, and the USA, and the number sold vastly outstrips the number of
the mid-range and mini turbines, only some of which are mass produced.
The remaining issue of turbine size to be discussed is the often critical one of
cost. Large wind turbines cost typically around $US 2 per watt of installed