Wind Power - Understanding Renewable Energy Systems - page 224

Figure 5.32 Speed-torque Characteristics for an Asynchronous Machine

and , (5.111)

the so-called Kloss formula can be obtained:

(5.112)

If the slip s is equal to the value s B , the second term becomes unity. The internal
torque M i reaches a maximum at this point. This maximum value is also called
the breakdown torque M iB . The slip at this torque is called the breakdown
slip s B .
Figure 5.32 shows the speed-torque characteristics for an asynchronous
machine. When starting-up, an asynchronous generator operates in motor
mode starting at the slip s = 1. After passing the synchronous operating point
of s = 0, the generator reaches the normal operating range. If the load at the
generator becomes higher than the breakdown torque M B , the speed increases
rapidly. Then, the generator can overspeed and destroy the system
mechanically. The normal operating range of an asynchronous generator is
between 0 and - s B ,. The speed is a little higher than the synchronous speed n S .
Table 5.8 shows data for a real asynchronous generator used in a wind
generator. The rated slip (that is, the relative speed difference between rotor
and stator) is -0.05 = -5 per cent. The normal speed range is between 1515
min -1 and 1650 min -1 , which can be obtained from the pole pair number p
= 2 and the frequency of the mains f 1 by:

(5.113)