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 TM 9-8000
the magnetism remaining in the pole pieces after the
field-magnetizing current has stopped. If there is no
residual magnetism in the pole pieces, there will be no
initial output of the generator, and it will not build up
voltage to push current. If the pole pieces lose residual
magnetism through long storage, or by being newly
rebuilt, subjected to extreme heat or cold, dropped,
vibrated, or struck by a sharp blow, it can be restored by
passing direct current through the field winding in the
proper direction. If current is passed through the field
windings in the reverse direction, the generator will be
polarized in reverse. This reverse polarity will cause the
generator to discharge the batteries instead of charging
them, and also could cause damage to some of the
vehicle accessories. Several conditions are necessary
for the generator to build up a voltage. Two of the most
important requirements are that the pole pieces have
residual magnetism as a foundation on which to build,
Figure 13-7. Generator Drive Systems.
and that the current in each field coil be in a direction
around the pole that it will produce magnetism to assist,
gears at one to one and one-half times the crankshaft
and not oppose, the residual magnetism. If the field
speed. The present trend is to have the water pump and
current opposes it, the voltage built up will not be higher
generator driven by a V-type drive belt from the pulley on
than that produced by the residual magnetism.
the forward end of the crankshaft. Pivoting the generator
on the generator mounting studs allows adjustment of
g. Construction. The armature core is made of sheets
the belt tension. A rotary fan usually is contained on the
of iron insulated from each other so that the magnetic
generator pulley to draw cooling air through the
field will not induce eddy currents in the core. Eddy
generator.
currents are currents that are induced within the core by
13-9.
Waterproofed Generator Systems.
The
the constant variation in the lines of force. Making the
generators, as well as other electrical components are
armature core in one piece would allow eddy currents to
made watertight on military vehicles that ford bodies of
become large enough to create a counter-voltage, which
water. This is done by completely sealing the generator
would result in a large portion of the generator's output to
so that water cannot enter. In addition, stainless steel
be converted to heat. The armature core is wound with
bearings are used to prevent corrosion.
The
coils of copper wire and mounted on a shaft with a
commutator-end bearing is packed with heat-resistant
commutator on one end. Field coils are made of many
grease on assembly, while the drive-end bearing is
coils of fine wire arranged for shunt connection. The
lubricated by a pickup gear rotating in an oil reservoir.
field frame, usually two or four poles with brushes, brush
Neither bearing requires attention or lubrication between
holders, and end housings with bearings, completes the
generator overhauls. Other types of sealed bearings
essential parts of the generator.
also may be used. The generator leads are carried in a
waterproof conduit that is connected by plug-ins to the
h. Generator Drives (Fig. 13-7). The method of
generator terminals. The conduit is attached to the
mounting and driving the generator depends to a large
generator terminal assembly by a waterproof coupling.
extent on the construction and design of the engine. It
usually is mounted on the side of the engine and driven
13-10. Circuit Breaker or Cutout Relay (Fig. 13-8).
by belts or
a. Purpose. The circuit breaker is simply an
TA233546
13-6
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