610 ROTATING MACHINES
13.5 Learning Objectives
Thelearning objectivesof this chapter are summarized here, so that the student can check whether
he or she has accomplished each of the following.- Principles of operation of elementary synchronous, induction, and dc machines.
- Modeling a polyphase induction machine and evaluating its steady-state performance.
- Torque–speed characteristics of three-phase induction motors.
- Basic notions of speed and torque control of polyphase induction motors.
- Starting methods for polyphase induction motors.
- Analysis of single-phase induction motors by revolving-field theory.
- Starting methods for single-phase induction motors.
- Basic ideas about applications for induction motors.
- Modeling a polyphase synchronous machine and evaluating its steady-state performance.
- Parallel operation of interconnected synchronous generators.
- Basic notions about applications for synchronous motors.
- Modeling a dc machine and evaluating its steady-state performance as a motor and generator.
- Basic ideas about speed control of dc motors, dc motor starting, and applications for dc
machines.
13.6 PRACTICAL APPLICATION: A CASE STUDY
Wind-Energy-Conversion Systems
It has been well recognized that renewable energy sources would have to play a key role in
solving the world energy problem. Wind energy with an estimated potential of 130 million MW
far exceeds the world’s hydraulic supply of about 3 million MW. Consequently, researchers have
been looking into the economic utilization of wind energy on a large scale by developing cost-
competitive and reliable wind-energy-conversion systems (WECSs) for various applications such
as electricity generation, agriculture, heating, and cooling.
The power coefficientCpof wind turbines varies with the tip-speed ratioλ, as shown in
Figure 13.6.1. Maximum power transfer is achieved by ensuing operation ofλopt, where the
turbine is most efficient. The mechanical powerPmavailable at the shaft of the wind turbine may
be expressed as a function of the wind speedvand the shaft speedw,
Pm(v, w)=C 1 vw^2 +C 2 v^2 w+C 3 v^3 (13.6.1)
whereC 1 ,C 2 , andC 3 are constants to be determined by curve-fitting techniques. Figure 13.6.2
depicts typical wind-turbine characteristics of mechanical power versus wind speed for different
values of shaft speed. The two nonzero roots of the curves represent a lower limit for the cut-in
wind speedvciand an upper limit for the cut-out wind speedvco. If the variable-speed operation is
opted and the resulting system is able to follow the wind-speed variations, the operation at optimum
tip-speed ratio can be ensured. In such a case, the power versus wind-speed characteristic will be
as shown by the dashed line in Figure 13.6.2.
WECSs developed for the generation of electricity are generally classified as: