Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 192

Parameters Calculated from Input Data

Description Value Units Symbol

Design Wind Speed 10.50 m/s V_design

50 year extreme wind speed 52.50 m/s V_e50

50yr extreme tip speed ratio 1.93 n/a λ _e50

Design Tip Speed Ratio 6.77 n/a λ _design

Drive Train Efficiency 0.60 n/a H

Design Torque 11.32 Nm Q_design

Projected Area (turbine swept area) 2.96 m^2 A_proj

Design Rotational Speed of the Rotor 73.30 rad/s ω _n,design

Maximum Possible Rotor Speed 104.72 rad/s ω

_n,max

Max Yaw Rate 2.99 rad/s

ω _yaw,max

50yr extreme tip speed ratio 1.93 n/a λ _e50

Eccentricity of the Rotor Centre of Mass 4.85E-03 m e_r

Effective brake torque 0.00 Nm M_brake

Fig. 9.2 The second sheet of the SLM spreadsheet

The fifth chart in Fig. 9.5 gives the output of equivalent stresses. Figure 9.6
shows the sixth chart used to determine the material strengths for ultimate loading
and fatigue using the input information on material characterisation and number of
cycles to fatigue. It has been assumed that the materials for the blade and the main
shaft have not been properly characterised and so attract the highest partial load
factors from Tables 9.7 and 9.8 . The sixth chart completes the data entry.
The final chart in Fig. 9.7 shows the outcome of the SLM calculations in terms
of the safety of the relevant component (blade root or rotor shaft). The rotor shaft
fails load case B, whereas the shaft and the blades easily pass all other load cases.
A major part of the reason why the rotor shaft fails Case B is the inadequate
characterisation of its properties. The assumed yield stress of 250 MPa is much
lower than is usual for case-hardened steels often used for shafts. In any case, even
for this low value, being able to demonstrate proper characterisation would lower
the material safety factor from 3 to 1.1 and this alone would allow the shaft to pass.

9.7 Further Test Requirements

IEC 61400-2 mandates testing to determine or validate the design power, design
rotational speed, design shaft torque, and maximum rotational speed. In addition
the following are required:
• a blade static test, combining the highest centrifugal load with the largest
bending moment to test the integrity of the blade. The standard recommends
fatigue testing of the blade, but does not require it.