Environmental Engineering Reference
In-Depth Information
Figure 4.5
The stator of a large induction generator, with the armature removed, showing the complex
wiring and magnetic circuits needed for high efficiency.
size effect is related to the lower rotational speed of larger machines and the necessarily more
complex structure of the magnetic and electric circuits. Figure 4.5 shows the stator of a large
induction generator and the complex electric and magnetic circuit features that are needed to
achieve high efficiency.
To achieve good efficiency and meet the practical limits on magnetic field strength, the power
per unit volume or mass of a motor or generator is limited, being of the order of 1E(5) W/m
3
and
30 W/kg. Because of the similarity of the structure of these devices, the material amounts per unit
volume are approximately independent of size and hence power output.
4.3
ELECTRIC POWER TRANSMISSION
The vast growth of electric power production and consumption in the twentieth century was a
consequence of the ability to transmit electric power from the source of production, the central
power plant, to the many consumers in residential, commercial, and industrial locations far removed
from the site of the power plant. Even though there is little energy storage in the transmission and
distribution system and the power plant must produce sufficient power to meet the instantaneous
demands of the many consumers, this system works remarkably well, especially when many power
plants are connected in a network that permits sharing the power demand in an economical and
reliable manner.
The components of an electric power system are sketched in Figure 4.6. A gas or steam turbine
powers an electric generator supplying alternating current to a transformer that steps up the line
voltage to a high value in the transmission line, usually 60-500 kilovolts. After transmission from
the power plant to the vicinity of load centers, a step-down transformer reduces the voltage to a
lower value, 12-35 kilovolts, for the distribution system that delivers power to the end-users. For
residential use, a further reduction to 120-240 volts is required.
