WIND ENERGY SYSTEMS - Cd3wd

Chapter 4—Wind Turbine Power 4–47
T L = Kn 2 N · m/rad (52)
The constant K is selected so the load torque curve passes through the peaks of the curves
for turbine torque at each wind speed.
Figure 28: Load with square-law torque variation connected to Sandia 17-m turbine.
In order for the turbine to operate at steady state or at a constant speed, the dω m /dt
term of Eq. 51 must be zero, and the load torque must be equal to the shaft torque. Suppose
that we have a steady wind of 6 m/s and that the shaft torque and load torque have reached
equilibrium at point a in Fig. 28. Now suppose that the wind speed suddenly increases to
8 m/s and remains constant at that speed. The shaft torque T m increases to the value at
a ′ before the rotational speed has time to change. The load is still requiring the torque at
point a. Since the shaft torque is larger than the load torque, the turbine rotational speed
will increase until point b is reached, at which time the two torques are equal and steady state
has again been reached.
If the turbine is operating at point b and the wind speed suddenly decreases to 6 m/s,
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001