MAINTENANCE MANUAL OF ENGINE
127
tion power generation. This is the main cause that
the alternator has excellent low-rate charging per-
formance. Usually, the separate excitation is
switched to self-excitation after the alternator
speed is up to 1000r/min.
Self-excitation
As the alternator speed increases, the electromo-
tive force of the alternator gradually increases and
can be output (usually at the time when the engine
speed is up to idling speed). When the output
voltage "ub" of the alternator is more than the
battery voltage, the alternator can supply electric-
ity; and when the alternator can deliver electricity,
it can supply electricity produced by itself to its
field winding for electricity generation. So, this
method by which the alternator delivers electricity
to the field is called self-excitation.
Excitation process of the alternator (the sepa-
rate excitation occurs before the self-excitation)
When the engine speed reaches normal idling
speed, the output voltage of the alternator is nor-
mally 1
～
2v higher than the battery voltage, so as
to charge the battery; at this moment, the alterna-
tor starts self-excitation power generation.
Excitation circuit of the alternator (see the
right figure)
Excitation
circuit
with
excitation
di-
ode-Nine-diode alternator Circuit characteris-
tics:
Two full-wave rectification circuits are formed by
9 rectifier diodes. Among them, the three-phase
full-wave bridge rectifier circuit formed by 6 big
power rectifier diodes is designed to deliver elec-
tricity to the external loads, and the three-phase
full-wave bridge rectifier circuit by three small
power diodes and three big power negative diodes
is designed to deliver electricity to the alternator
field. So, these three small power diodes are
called excitation diode. The charging circuit is
usually installed with a charging indicator lamp,
which cannot only indicate operating condition of
the charging system through controlling the
charging indicator lamp, but also can prevent de-
livery of electricity by the battery to the excitation
winding through the regulator for a long period
due to the reason that the operator forgets turning
off the ignition switch.
Summary of Contents for KM/4L22
Page 6: ......
