TM 5-685/NAVFAC MO-912
GENERATORS AND EXCITERS
4-1. Electrical energy.
needed to direct the flow of current in one direction.
The generator rotating commutator provides the
Mechanical energy provided by a prime mover is
converted into electrical energy by the generator
(see fig 4-l). The prime mover rotates the generator
4-4. AC generators.
rotor causing magnetic lines of force to be cut by
a. AC generators are considered either brush or
electrical conductors. Electrical energy is thereby
brushless, based on the method used to transfer DC
produced by electromagnetic induction. The ratio of
exciting current to the generator field. In addition,
output energy generated by input energy is ex-
AC generators are classified as salient-pole or
pressed as a percentage and always shows a loss in
nonsalient-pole depending on the configuration of
the field poles. Projecting field poles are salient-pole
units and turbo-type (slotted) field poles are
4-2. Generator operation.
nonsalient-pole units. Typical AC generator
a. A generator consists of a number of conducting
armatures are shown in figures 4-3 and 4-4.
coils and a magnetic field. The coils are called the
b. Damper windings on the rotor stabilize the
armature. Relative motion between the coils and
speed of the AC generator to reduce hinting under
magnetic field induces voltage in the coils. This
changing loads. If the speed tends to increase,
action is called electromotive force (emf). A sche-
induction-generator action occurs in the damper
matic for a typical generator system is shown in
windings. This action places a load on the rotor,
tending to slow the machine down. If the speed
b. An alternating current (AC) generator needs a
tends to decrease, induction-motor action occurs in
separate direct current (DC) source to feed the mag-
the damper winding, tending to speed the machine
netic field. The required DC is provided by an exter-
up. The windings are copper bars located in the
nal source called an exciter. Usually, the exciter is a
faces of the rotor pole pieces. Mounted parallel to
small DC generator that is driven by the generator
the rotor axis, the bars are connected at each end by
rotor. The exciter may be mounted on the rotor shaft
a copper ring.
or rotated by belt-drive. Some generating systems
c. AC generators that operate at a speed that is
use a static, solid-state exciter to provide DC.
exactly proportional to the frequency of the output
c. A voltage regulator controls the induced volt-
voltage are synchronous generators. Synchronous
age by regulating the strength of the electromag-
generators are usually called alternators.
netic field established by the exciter. Frequency is
4-5. Alternator types.
controlled by the speed at which the prime mover
rotates the rotor.
Alternators are single-phase or polyphase. Varia-
tions include three-phase alternators used as
4-3. Types of generators.
single-phase units by insulating and not using one
Depending on the type of generating equipment em-
phase lead. Since the lead is unused, it is not
ployed, the electrical energy produced is either di-
brought out to a terminal. The kilowatt rating is
rect current ( D C ) or alternating current (AC).
reduced from that of the three-phase unit as limited
a. AC generators. AC generators are classified as
by the amount of current carried by a coil. An alter-
single-phase or polyphase. A single-phase generator
nator designed only for single-phase operation usu-
is usually limited to 25 kW or less and generates AC
ally does not have coils in all of the armature slots
power at a specific utilization voltage. Polyphase
because end coils contribute little to the output volt-
generators produce two or more alternating volt-
age and increase the coil impedance in the same
ages (usually two, three, or six phases).
proportion as any other coil.
b. DC generators. DC generators are classified as
(a) Single-phase alternators are usually used in
either shunt, series, or compound-wound. Most DC
smaller systems (limited to 25kW or less) and pro-
generators are the compound-wound type. Shunt
duce AC power at utilization voltages.
generators are usually used as battery chargers and
(1) Terminal voltage is usually 120 volts. The
as exciters for AC generators. Series generators are
electric load is connected across the terminals with
sometimes used for street lights. The emf induced in
a DC generator coil is alternating. Rectification is
measure the output in volts and amperes, respec-