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TM 5-685/NAVFAC MO-912
(g) Most parts of the world have standard-
end of each coil is connected to separate terminals.
ized on either 50 or 60 Hertz alternating current
Conductors attached to the four terminals carry the
power. Sixty Hertz power is commonly used in the
current to the system's switchgear and on to the
United States. Fifty Hertz power is used in many
countries outside the United States. The ratio be-
d. Collector slip rings. Slip rings are usually
tween the 60-50 Hertz frequencies is 6:5. Electrical
made of nonferrous metal (brass, bronze or copper);
energy received at one frequency can be converted
iron or steel is sometimes used. Slip rings usually
to a different frequency by using a frequency
do not require much servicing. The wearing of
changer. If a large power requirement exists, it may
grooves or ridges in the slip rings is retarded by
be more economical to use a special alternator to
designing the machine with limited endplay and by
produce power at the desired frequency The appli-
staggering the brushes. Surfaces of the slip rings
cable equation is:
should be bright and smooth, polishing can be per-
formed with fine sandpaper and honing stone. Elec-
trolytic action can occur at slip ring surfaces pro-
where V = generated voltage
ducing formation of verdigris. Verdigris is a
K = constant value number (speed)
= phase/phase angle
greenish coating that forms on nonferrous metals.
N = number of turns
Electrolytic deterioration can be prevented by re-
f  = line frequency
versing the polarity of the slip rings once or twice a
(h) The generated voltage is proportional to the
year. The stator of the three-wire, three-phase unit
strength of the magnetic field, phase, and number of
also has three sets of armature coils spaced 120
turns in series between terminals and the speed.
electrical degrees apart. The ends of the coils are
connected together in a delta configuration. Conduc-
4-6. Design.
tors are attached to the three connecting points.
a. Components. A typical AC generator consists of
a stationary stator and a rotor mounted within the
4-7. Characteristics of generators.
stator (see fig 4-l). The stator contains a specific
a. Voltage. Generated voltage is the emf denoting
number of coils, each with a specific number of
the electric pressure between phases in the arma-
windings. Similarly, the rotor consists of a specific
ture. The magnetic flux linking each armature coil
number of field poles, each with a specific number of
changes as the machine rotates. The change in flux
windings. In addition to the rotor and stator (refer
per turn occurs at the conductors in the armature
to paragraphs 4-6b and 4-6c, respectively), a gen-
slots. Each conductor is regarded separately as it
erator has a collector assembly (usually consisting
cuts the flux. At a specific rotating speed, instanta-
of collector slip rings, brushes, and brush holders).
neous volts per conductor are proportional to air
The slip rings are covered in paragraph 4-6d. DC
gap flux density at the conductor.
flows from the exciter, through the negative brush
and slip ring, to the rotor field poles. The return
b. Current. Current is the rate of transfer (flow)
path to the exciter is through the positive brush and
of electricity, expressed in amperes. Field current
slip ring.
required for a particular load condition, i s deter-
b. Rotor. The rotor contains magnetic fields which
mined by the magnetic circuit, in conjunction with
are established and fed by the exciter. When the
armature and field windings. Load current is equal
rotor is rotated, AC is induced in the stator. The
to the generated voltage divided by the impedance
changing polarity of the rotor produces the alternat-
of the load.
ing characteristics of the current. The generated
c. Speed. Normally, a generator operates at a con-
voltage is proportional to the strength of the mag-
stant speed corresponding to the frequency and
netic field, the number of coils (and number of wind-
number of poles. Variations may occur due to
ings of each coil), and the speed at which the rotor
changes in driving torque, load, field excitation, or
terminal voltage.
c. Stator. The frame assembly is the main compo-
d. Frequency. AC frequency is determined by the
nent of the stator. Insulated windings (or coils) are
rotating speed and number of poles of the generator.
placed in slots near an air gap in the stator core.
Frequency is usually expressed in Hertz, the fre-
There is a fixed relationship between the unit's
quency used most is 60 Hertz. A two-pole generator
number of phases and the way the coils are con-
must operate at 3600 rpm to maintain 60 Hertz.
nected. The stator in a four-wire, three-phase unit
Four-pole and six-pole units must operate at 1800
has three sets of armature coils which are spaced
rpm and 1200 rpm, respectively, to maintain 60
120 electrical degrees apart. One end of each coil is
Hertz. Frequency at 60 Hertz is expressed in the
connected to a common neutral terminal. The other
following equation:

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