Alternating Current Power Systems 147
small power ratings can be used with the rotating-armature type of alter-
nator. The major disadvantage of this method is that the AC voltage is ex-
tracted from a slip ring/brush assembly (see Figure 6-5A). A high voltage
could produce tremendous sparking or arc-over between the brushes and
the slip rings. The maintenance involved in replacing brushes and repair-
ing the slip-ring commutator assembly would be very time-consuming
and expensive. Therefore, this method is used only for alternators with
low power ratings.Figure 6-6. Energy conversion by generatorsRotating-field Method
The rotating-field method, shown in Figure 6-5B, is used for alternators
capable of producing larger amounts of power. The direct current (DC)
excitation voltage, which develops the magnetic field, is applied to the ro-
tating portion of the machine. The AC voltage is induced into the station-
ary conductors of the machine. Since the DC-excitation voltage is a much
lower value than the AC voltage that is produced, maintenance problems
associated with the slip ring/brush assembly are minimized. In addition,
the conductors of the stationary portion of the machine may be larger, so
as to handle more current, since they do not rotate.THREE-PHASE AC GENERATORS
The vast majority of electrical power produced in the United States
is ee-phasethr power. Because of their large power ratings, three-phase gen-
erators utilize the rotating field method. A typical three-phase generator
in a power plant might have 250 volts DC excitation applied to the rotat-