the field pole pieces have retained a slight amount
of permanent magnetism called RESIDUAL MAG-
NETISM. When the generator is put in motion, the
FULL-WAVE RECTIFIED
weak residual magnetism causes a small voltage to
AC WAVEFORM
be generated in the armature. This small voltage
+
applied to the field coils causes a small field
current. Although small, this field current
OUTPUT
strengthens the magnetic field and allows the
VOLTAGE
armature to generate a higher voltage. The higher
voltage increases the field strength, and so on. This
0
process continues until the output voltage reaches
the rated output of the generator.
_
TIME
Classification of Self-Excited Generators
ASf07024
Self-excited generators are classified according to
Figure 7-24.--Graph of the output of a simple dc generator.
the type of field connection they use. There are three
commutator segments are used. The coils are insulated
g e n e r a l t y p e s o f fi e l d c o n n e c t i o n s -- S E R I E S -
from each other, the same as the commutator segments.
W O U N D , S H U N T- W O U N D ( p a r a l l e l ) , a n d
Figure 7-25 shows the output of a two-coil generator.
COMPOUND-WOUND. Compound generators are
This output is the combination of the two individual
further classified as cumulative-compound and
outputs produced by the individual coils. Note that the
differential-compound; however, these last two
waveform is smoother but still fluctuates. To get the
classifications are not discussed in this course.
waveform still smoother, more coils must be added. In
SERIES-WOUND GENERATORS.--In the
some cases filters are used to smooth out the voltage.
series-wound generator, view A of figure 7-26, the
These include capacitors and inductors.
field windings are connected in series with the
armature. Current that flows in the armature flows
Field Excitation
through the external circuit and through the field
windings. The external circuit connected to the
When a dc voltage is applied to the field windings
generator is called the load circuit.
of a dc generator, current flows through the windings
and sets up a steady magnetic field. This is called
A series-wound generator uses very
FIELD EXCITATION.
low-resistance field coils, which consist of a few turns
of large diameter wire.
This excitation voltage can be produced by the
generator itself or it can be supplied by an outside
The voltage output increases as the load circuit
source, such as a battery. A generator that supplies
starts drawing more current. Under low-load current
its own field excitation is called a SELF-EXCITED
conditions, the current that flows in the load and
through the generator is small. Since small current
GENERATOR. Self-excitation is possible only if
0
90
180
270
360
RESULTANT OUTPUT *
VOLTAGE WAVEFORM
+
OUTPUT
VOLTAGE
0
TIME
*COIL 1 AND 2 VOLTAGES
COIL1
COIL 2
VOLTAGE
VOLTAGE
ADDED TOGETHER
ASf07025
Figure 7-25.--Results of additional coil.
7-16
