attached to each end of the rotor. The segment varies
the reluctance in the magnetic circuit as it rotates. As a
result, the fixed stator poles experience a variation in
magnetic strength or coupling and produce a resulting
output voltage in the stator coils. In contrast to other
reversal. Consequently, there is only a 50-percent
utilization of the iron in the stator.
illustrates typical construction of the inductor alternator.
The advantages of an inductor alternator are easier
winding construction for field and stator coils; simplified
cooling; it is brushless; and it has an integral solid rotor
without windings that permits high-speed operation. The
disadvantages of an inductor alternator are that it has
less than 50 percent use of iron, resulting in a heavier
unit and the increased total air gap in the magnetic circuit
requires more excitation.
e. Brushless-Rotating Rectifier. Another means for
eliminating brushes and sliprlngs is found in the rotating
Figure 13-29. Rotor-to-Stator Relationship.
rectifier type of alternator. The machine consists of five
main functional elements. These include a stator-
that the rotor contains no windings. Excitation is induced
mounted exciter field, the exciter armature, a main
in the rotor poles by stationary field coils located at the
rotating field, the main output stator windings, and the
ends of the rotor. This results in the elimination of
output rectifier assembly.
sliprings and rotating windings. Further advantages can
be obtained by casting a nonmagnetic material around
The exciter field induces alternating current in the
the pole fingers, thus producing a smooth rotor with low
rotating armature and the output is rectified and directly
windage losses and high speed capability. An inherent
coupled to the rotating main field, which excites the
design requirement of this stationary field arrangement is
stator-mounted output windings. With this arrangement,
the inclusion of an auxiliary air gap in the magnetic
a small amount of exciter field excitation can be
circuit. This requires greater field current for excitation.
amplified in the exciter stage to supply a high level of
Figure 13-30 illustrates construction features. There are
main field current. A diagram of elements is shown in
a variety of advantages to a Lundell inductor. There are
figure 13-32, along with a cross section through the
no slipring wear or contamination problems, and the unit
alternator. The advantages of the brushless rotating
is inherently explosion proof. The rotor can be solid and
rectifier are that it is brushless and a low exciter field
permanently balanced. All windings are stationary and
current permits a low-level regulator. However, the
readily accessible for cooling. The low rotor mass
disadvantages of the brushless-rotating rectifier are that
reduces bearing loads and permits rapid acceleration.
a wound rotor limits top speed, multiple windings
The bearing center-to-center distance is minimized by
contribute to complexity and cost, a large number of
the elimination of sliprings, and this, combined with a
heat-producing rotating elements increases cooling
large shaft diameter, permits high-speed operation. The
requirements, and a large magnetic circuit limits
field windings are simple, bobbin-wound coils permitting
short mean turn length. The only disadvantage is that
extra air gaps in the magnetic circuit require increased
13-25. Cooling Generators. The common methods
used for cooling generators use heat transfer by airflow
or oil circulation. Each has its particular application
d. Inductor Alternator. An inductor alternator employs
based on their advantages and disadvantages.
a fixed, non-rotating field coil that induces excitation in
the central portion of the rotor as if it were a solenoid.
Each end of the rotor assumes a polarity. A multilobed