Page 380 - Wind Energy Handbook

354 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

Figure 6.10 Blade Pitching System Using a Separate Electric Motor for Each Blade. (A
pinion, driven by the motor via a planetary gearbox, engages with gear teeth on the inside of
the inner ring of the pitch bearing, to which the blade is bolted. The blade is not attached to
the bearing in this photograph, so the fixing holes are visible).

hoses or power cables for pitch actuation together with signal cables for pitch angle
sensing. In addition, appropriate slip rings are required at the rear end of the shaft.
Methods of providing back-up power supplies to ensure blade feathering in the
event of grid-loss are considered in Section 8.5.
Although full-span pitch control is the option favoured by the overwhelming
majority of manufacturers, power control can still be fully effective even if only the
outer 15 percent of the blade is pitched. The principal benefits are that the duty of
the pitch actuators is significantly reduced, and that the inboard portion of the
blade remains in stall, significantly reducing the blade load fluctuations. On the
other hand partial-span pitch control has several disadvantages as follows:

• the introduction of extra weight near the tip,

• the difficulty of physically accommodating the actuator within the blade profile,
• the high bending moments to be carried by the tip-blade shaft,
• the need to design the equipment for the high centrifugal loadings found at large
radii,
• the difficulty of access for maintenance.

It should be apparent from the above brief survey of pitch actuation systems that
the design of the hardware required for pitch-regulation is a significant task.