TM 11-6125-240-34
field. When transistor Q6 is off, total dc voltage ap-
transformer of a series LC circuit, tuned to approx-
pears across the transistor Q6.
imately 570 Hz. The LC circuit operates at 400 Hz on
b. The sensing voltage is taken from the output ter-
the low side of the resonance curve. The value of the ac
minal of the generator and applied to the primary of
potential developed is proportional to the frequency
the sensing transformer. The stepped-down ac voltage
and will continue to be constant for any one frequency
across the secondary is rectified by the full-wave,
so long as the values of L and C remain constant. This
center-tap arrangement of the transformer, and
potential is then stepped down, rectified, and compared
filtered to a dc level of approximately 15 volts. The
with another potential obtained by rectification of the
differential or de sensing voltage and a 12-volt Zener
ac sensing voltage. The two are approximately 180 out
voltage from the voltage sensing circuit is fed to the
of phase and are rectified, one positive and one nega-
base of a driver transistor on the amplifier circuit
tive. Thus, the difference between these potentials
which determines the length of on-or-off-time of power
becomes a representation of the frequency the level of
transistor Q6. Any increase in differential voltage will
which is independent of any change in ac voltage level;
cause a decrease in on-time of the power transistor
frequency control is independent of voltage control.
which will result in a lower field current and a re-
c. The signal obtained by comparison of the two
resultant decrease in output voltage. Any decrease in
above mentioned potentials is fed to the base of a driver
differential voltage will increase the on-time with a
transistor which is part of the amplifier circuit. The
corresponding increase in output voltage.
amount of the differential voltage fed to the base of the
c. The regulator reacts instantaneously to any
drive transistor (located in the amplifier circuit) estab-
change in output voltage and maintains it constantly
lishes the length of on-time or off-time of power tran-
at a preset value. Free-wheeling diode CR8 is used to
sistor Q3. Any increase in frequency will cause a
provide a path for the field current during the off-time
decrease in the differential voltage that will increase
of power transistor Q6.
the on-time of power transistor Q3, resulting in a high-
er field current and a consequent reduction in frequen-
2-6. Frequency Regulation (CSV1186-1)
cy.
d. The regulator reacts instantaneously to any
a. One side of the motor shunt field is connected to
change in output frequency and maintains it con-
negative dc and the other side is connected to the col-
stantly at a preset value.
lector of power transistor Q3. When the power transis-
tor is on, practically total dc voltage appears across the
e. Free-wheeling diode CR9 is used to provide a path
field. When the power transistor is off, total dc voltage
for the field current during the off-time of power tran-
appears across transistor Q3.
sistor Q3. Anticipation to changes in input voltage and
b. Frequency regulation is accomplished by sensing
electric load are coupled into the speed control
the ac potential developed across a reactance
amplifier circuit with an ac network.
2-4
