core with the armature windings embedded in this
Rotating-Field Generators
core, which is secured to the stator frame.
The rotating-field generator has a stationary
Rotating field generators are used in
armature winding and a rotating-field winding, as
power-generating support equipment. The rotating
shown in view B of figure 7-28. The advantage of
field ac generators used in support equipment are
having a stationary armature winding is that the
actually two generators mounted on a single shaft.
generated voltage can be connected directly to the
Figure 7-29 shows a pictorial of an ac generator. The
load. That is, a rotating armature requires slip rings and
only purpose for the dc exciter generator is to supply
brushes to conduct the current from the armature to the
the direct current required to maintain the ac
load. The armature, brushes, and slip rings are difficult
generator's magnetic field.
to insulate; arc-overs and short circuits can result at
Refer to view A of figure 7-30 during the following
high voltages. For this reason, high-voltage generators
explanation. The exciter is a dc, shunt-wound,
are usually of the rotating-field type. Since the voltage
self-excited generator. The exciter shunt field creates
applied to the rotating field is low-voltage dc, the
an area of intense magnetic flux between its poles.
problem of high-voltage arc-over at the slip rings does
When you rotate the exciter armature in the
not exist.
exciter-field flux, voltage is induced in the exciter
The stationary armature, or stator, of this type of
armature windings. The output from the exciter
generator holds the windings that are cut by the
commutator is connected through brushes and slip
rotating magnetic field. The voltage generated in the
rings to the generator field. Since this is direct current,
armature as a result of this cutting action is the ac
already converted by the exciter commutator, the
power that is applied to the load.
current always flows in one direction through the
The stators of all rotating-field generators are
generating field. Thus, a fixed-polarity magnetic field
is maintained at all times in the generating field
about the same. The stator consists of a laminated iron
A-C FIELD
A-C FIELD
WINDINGS
INPUT
(ROTOR)
SLIPRING
EXCITER
OUTPUT
COMMUTATOR
EXCITER
GENERATOR
ARMATURE
ROTOR
DRIVE
A-C POWER OUTPUT
EXCITER
SHAFT
TERMINALS
CONTROL
TERMINALS
A-C GENERATOR
SECTION
COMMUTATOR
& SLIPRING
SECTION
EXCITER D-C
DRIVE
GENERATOR
SHAFT
SECTION
ASf07029
Figure 7-29.--An ac generator pictorial.
7-20
