TM 5-685/NAVFAC MO-912
insulated from, the generator shaft as well as from
one to support each end of the armature shaft. On
some generators, one end of the shaft is supported
g. Static exciters contain no moving parts. A por-
by the coupling to the prime mover and one bearing
tion of the AC from each phase of generator output
is used at the other end. The selections of bearing
is fed back to the field windings, as DC excitations,
type and lubrication are based on generator size,
through a system of transformers, rectifiers, and
type of coupling to prime mover, and expected us-
reactors. An external source of DC is necessary for
age. A generator is usually equipped with either
initial excitation of the field windings. On engine-
sleeve or ball bearings which are mounted in end
driven generators, the initial excitation may be ob-
shields attached to the generator frame.
tained from the storage batteries used to start the
b. Sleeve bearings. Sleeve bearings are usually
engine or from control voltage at the switchgear.
bronze and are lubricated with oil.
(1) Most u nit s with sleeve-type bearings have a
4-9. Characteristics of exciters.
reservoir for the oil and a sight gauge to verify oil
a. Voltage. Exciter voltages in common use in-
level. Bearings and the reservoir are fully enclosed.
clude 63 and 125 volts for small units and 250, 375,
(2) Distribution of oil to shaft and bearings
or 500 volts for large units. Exciters with normal
from the reservoir is by an oil-slinger ring mounted
self-excitation are usually rated at about 135 per-
on the generator shaft. Rotation of the slinger ring
cent of rated voltage and a rate buildup of about 125
throws the oil to the top of the bearing. Holes in the
volts per second. Working range is between 75 and
bearing admit oil for lubrication.
125 percent of rated exciter voltage.
(3) Some units with sleeve-type bearings have
b. Current. An exciter provides direct current to
an absorbent fiber packing, saturated with oil,
energize the magnetic field of an AC generator. Any
which surrounds the bearing. Holes in the bearing
DC generator or storage battery may be used as a
admit oil for lubrication.
field current source.
c. Ball bearings. Ball bearings (or roller-type
c. Speed. Speed, in rotating exciters, is related to
bearings) are fully enclosed and lubricated with
generator output voltage. Usually, if magnetic field
intensity is increased (by higher rotating speed),
(1) Most units with ball or roller-type bearings
output voltage of the generator is also increased.
are equipped with a fitting at each bearing to apply
d. Power. Exciter voltage to the magnetic field of
fresh grease. Old grease is emitted from a hoie (nor-
an AC generator is usually set at a predetermined
mally closed by a plug or screw) in the bearing
value. A voltage regulator controls the generator
voltage by regulating the strength of the magnetic
(2) Some units are equipped with prepacked,
field produced in the exciter.
lifetime lubricated bearings.
d. Bearing wear. Noise during generator opera-
4-10. Field flashing.
tion may indicate worn bearings. If source of noise
a. Field flashing is required when generator volt-
is the generator bearing, replacement of the worn
age does not build up and the generating system
bearing is recommended.
(including the voltage regulator) does not have field-
e. Service practices. Service practices for genera-
flash capability. This condition is usually caused by
tors and exciters consist of a complete maintenance
insufficient residual magnetism in the exciter and
program that is built around records and observa-
generator fields. In some cases, a generator that has
tions. The program is described in the manufactur-
been out-of-service for an extended period may lose
er's literature furnished with the component. It in-
its residual magnetism and require flashing. Re-
cludes appropriate analysis of these records.
sidual magnetism can be restored by flashing the
f. Record keeping. Generator system log sheets
field thereby causing a current surge in the genera-
are an important part of record keeping. The sheets
tor. Refer to the voltage regulator manufacturer's
must be developed to suit individual applications
literature for procedural instructions.
(i.e., auxiliary use).
b. Solid-state components may be included in the
g. Log sheet data. Log sheets should include sys-
voltage regulator. Perform field flashing according
tem starts and stops and a cumulative record of
to the manufacturer's instructions to avoid equip-
typical equipment operational items as follows:
(1) Hours of operation since last bearing lubri-
4-11. Bearings and lubrication.
(2) Hours of operation since last brush and
a. Location. Several types of bearings, each with
spring inspection or servicing.
specific lubrication requirements, are used on the
(3) Days since last ventilating and cooling
generators. Usually, a generator has two bearings,
screen and duct cleaning.