WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–31
declines rather rapidly toward zero at n = n s . Meanwhile the torque required by the load
is increasing with speed. The two torques are equal and steady state operation is reached
at point a in Fig. 18. Rated torque is usually reached at a speed about 3 percent less than
synchronous speed. A four pole induction motor will therefore deliver rated torque at about
1740 r/min, as compared with the synchronous speed of 1800 r/min. The no load speed will
be less than synchronous speed by a few revolutions per minute. The reason for this is that
at synchronous speed the rotor conductors turn in unison with the stator field, which means
there is no time changing magnetic field passing through these conductors to induce a voltage.
Without a voltage there will be no rotor current I 2 , and there is no torque without a current.
Figure 18: Variation of shaft torque with speed for a three-phase induction machine.
If synchronous speed is exceeded, s, T m ,andP m all become negative, indicating that the
mechanical load has become a prime mover and the motor is now acting as a generator. This
means that an induction machine can be connected across a three-phase line, used as a motor
to start a wind turbine such as a Darrieus, and become a generator when the wind starts
to turn the Darrieus. The Darrieus has no pitch control, the induction machine has no field
control, and synchronization is unnecessary, so equipment costs are significantly reduced from
those of the system using a synchronous generator.
The circuit of the induction generator is identical to that of the induction motor, except
that we sometimes draw it reversed, with reversed conventions for I 1 and I 2 as shown in
Fig. 19. The resistance R 2 (1 − s)/s is negative for negative slip, and this negative resistance
can be thought of as a source of power.
The induction generator requires reactive power for excitation. It cannot operate without
this reactive power, so when the connection to the utility is broken in Fig. 19, the induction
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001