Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 173

blades and shaft is to use the EDC with an aero-elastic analysis rather than the
SLM.

9.2.1 Load Case A: Normal Operation

The first load case covers fatigue behaviour during normal operation of the turbine
and is the only fatigue load case. It is implicitly assumed that abnormal operation
occurs sufficiently seldom to contribute to the fatigue life. All the equations in this
sub-section define peak-to-peak loads generated by the assumed cycling between
1.5 times and 0.5 times the design values of the angular velocity and torque. The
first is the centrifugal load 6 :

DF zB ¼ 2m B R cog X 2 ð 9 : 4 Þ

where R cog is the radius of the centre of gravity (mass) of the blade. The next two
equations give, in order, the lead-lag (in the direction of rotation) and flapwise
moment (in the direction of the wind; alternatively, out of the plane of rotation) 7 , 8 :
DM xB ¼ Q design N þ 2m B gR cog ð 9 : 5 Þ

DM yB ¼ k design Q design N ð 9 : 6 Þ

These moments are to be applied to that part of the blade root with the lowest
ultimate strength. Note that most turbine blades are very stiff in the lead-lag
direction, so that the use of the term ''bending'' in conjunction with ''moment'' can
be taken to imply bending in the flapwise direction only.
The next three equations give the peak to peak fatigue loads on the turbine
shaft. This load is assumed to occur at the first shaft bearing (nearest to the
rotor) 9 , 10 , 11 :

DF x shaft ¼ 3k design Q design 2 ðÞ

9 : 7 Þ

DM x shaft ¼ Q design þ 2m r ge r ð 9 : 8 Þ

DM shaft ¼ 2m r gL rb þ R

6 DF x shaft

ð 9 : 9 Þ

6

(IEC 21).

7

(IEC 22).

8

(IEC 23).

9

(IEC 24).

10

(IEC 25).

11

(IEC 26).