Design and development of small wind turbines - Wind Power Generation and Wind Turbine Design - page 267

effi ciency at lower wind speeds can be signifi cant in terms of energy production,
as a typical small turbine spends much of its life operating at low wind speeds.
On the smaller turbines, it is also possible to have more than three blades with
relatively little impact on cost. Multiple blades allow higher starting torques, and
lower operational speed (and therefore lower noise). This must be traded off
against higher thrust loads and the slightly higher cost. On some of the smallest
machines there is no rotor overspeed control at all, i.e. the machine is simply
designed to survive the high rotational speed and thrust load of a “runaway” condi-
tion. In this case having multiple blades (e.g. the well-proven Rutland 913, which
has six blades) limits the rotational speed to some extent.
The hub is part of the rotor, and small wind turbines typically have very simple
hubs, as the blade pitch is typically fi xed. On some rotors blade pitch is not adjust-
able, other rotors use shims to set the pitch, while others use a rotary adjustment at
the blade root that is locked in place after fi nal adjustment. Some small wind tur-
bines have more complex hubs, consisting of springs and hinges (e.g. Proven wind
turbines, which pitch the blades to stall for overspeed control). None of these
features are typical of large wind turbines.

1.3 System design

1.3.1 DC systems
Traditionally small wind turbines use DC generators. The DC generator is now
usually in fact a permanent magnet three-phase synchronous AC generator (alter-
nator), with a diode rectifi er either located up in the turbine (with two wires com-
ing down the tower) or at the control panel (with three smaller wires coming down
the tower). The rotor mounts directly onto the alternator shaft, and no gearbox is
required. This remains the most common approach used by small wind turbine
designers. With the advent of grid-tie inverters (see below), it is a solution that
makes small wind turbines suitable for battery charging as well as grid-connected
applications.
In the battery charging mode, DC systems operate at fairly constant speed.
Figure 13 shows the simplifi ed equivalent circuit of such a system. The voltage pro-
duced by the generator is proportional to rotational speed. If the sum of the circuit
resistances (generator winding resistance, cable resistance and battery resistance) is
relatively small, then V gen

V batt , i.e. the battery voltage effectively regulates the
generator voltage, and therefore the generator speed, to be relatively constant.
In real applications the generator rotational speed increases noticeably with
power output, as suggested by Fig. 13. When current is high, then voltage drop
across the resistances is signifi cant, and V gen rises (and therefore generator rota-
tional speed) with power output. This impacts aerodynamic performance and
design. For example, if a stalling airfoil is being relied upon to limit power output,
the power output at which stall occurs is a function of rotational speed as sug-
gested by Fig. 14. When batteries reach a fully charged condition, the charge con-
troller disconnects the wind turbine and the wind turbine freewheels, held back
only by the overspeed control system.

≅