Fundamental Engineering Concepts - Mathematics and Statistics for the Environment - page 288

most part, homemade apparatus. Indeed, the innovators of electricity had to fabricate nearly all
of the laboratory equipment used in their experiments. At the time, the only convenient source
of electrical energy available to these early scientists was the voltaic cell, invented some years
earlier. Because cells and batteries were the only sources of power available, some of the early
electrical devices were designed to operate from direct current (DC). For this reason, initially
direct current was used extensively; however, when the use of electricity became widespread,
certain disadvantages in the use of direct current became apparent. In a DC system, the sup-
ply voltage must be generated at the level required by the load. To operate a 240-volt lamp, for
example, the generator must deliver 240 volts. A 120-volt lamp could not be operated from this
generator by any convenient means. A resistor could be placed in series with the 120-volt lamp to
drop the extra 120 volts, but the resistor would waste an amount of power equal to that consumed
by the lamp.
Another disadvantage of DC systems is the large amount of power lost due to the resistance
of the transmission wires used to carry current from the generating station to the consumer. This
loss could be greatly reduced by operating the transmission line at very high voltage and low cur-
rent. This is not a practical solution in a DC system, however, because the load would also have to
operate at high voltage. Because of the difficulties encountered with direct current, practically all
modern power distribution systems use alternating current, including water/wastewater treatment
plants.
Unlike DC voltage, AC voltage can be stepped up or down by a device called a transformer .
Transformers permit the transmission lines to be operated at high voltage and low current for max-
imum efficiency. At the consumer end, the voltage is stepped down to whatever value the load
requires by using a transformer. Due to its inherent advantages and versatility, alternating current
has replaced direct current in all but a few commercial power distribution systems.
11.7.11.1 Basic AC Generator
As shown in Figure 11.53, an AC voltage and current can be produced when a conductor loop
rotates through a magnetic field and cuts lines of force to generate an induced AC voltage across
its terminals. This describes the basic principle of operation of an alternating current generator, or
alternator. An alternator converts mechanical energy into electrical energy. It does this by utilizing
the principle of electromagnetic induction. The basic components of an alternator are an armature,
about which many turns of conductor are wound and which rotates in a magnetic field, and some
means of delivering the resulting alternating current to an external circuit.
11.7.11.2 Cycle
An AC voltage is one that continually changes in magnitude and periodically reverses in polarity
(see Figure 11.54). The zero axis is a horizontal line across the center. The vertical variations on the
voltage wave show the changes in magnitude. The voltages above the horizontal axis have positive
(+) polarity, and voltages below the horizontal axis have negative (-) polarity.
Figure 11.54 shows a suspended loop of wire (conductor or armature) being rotated (moved) in
a counterclockwise direction through the magnetic field between the poles of a permanent magnet.
For ease of explanation, the loop has been divided into a thick and thin half. Notice that in part A
the thick half is moving along (parallel to) the lines of force; consequently, it is cutting none of these
lines. The same is true of the thin half, moving in the opposite direction. Because the conductors are
not cutting any lines of force, no emf is induced. As the loop rotates toward the position shown in
part B, it cuts more and more lines of force per second because it is cutting more directly across the
field (lines of force) as it approaches the position shown in part B. At position B, the induced voltage
is greatest because the conductor is cutting directly across the field.
As the loop continues to be rotated toward the position shown in part C, it cuts fewer and fewer
lines of force per second. The induced voltage decreases from its peak value. Eventually, the loop is
once again moving in a plane parallel to the magnetic field, and no voltage (zero voltage) is induced.