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network functions to reduce voltage transients in the
through primary winding 3-4 of output transformer T1
inverter 3-phase output.
(through Q1, L1, and the battery), inducing a voltage in
secondary 6-7 in one direction. By autotransformer
Drive Switch
action, a voltage is also induced in winding 4-5. This
voltage charges capacitor C1 through Q1, CR 1, and R1.
The drive switch (fig. 1-37, S-1) has three positions:
When the commutating SCR (Q3) is triggered on by the
driver, the positive voltage from the right plate of Cl is
OFF, START, and RUN. In the OFF position, power is
applied through Q3 (Q3 conducting) to the Q3-CR3
supplied to the standby indicator light to indicate the
inverter is in the standby mode. In the standby mode, a
junction. This applies a reverse negative anode to
+30-volt dc signal is supplied to the synchronizing
positive cathode voltage to Q1, causing Q1 to stop
conducting. Capacitor C 1 discharges through L1, CR3,
stage. Also, in the OFF position, the input dc voltage is
and Q3. With Q1 off, the current in winding 3-4 of T1
connected as a source of power for the control circuit
gradually drops to zero; and slightly later when the 3-4
+30-volt dc power supply.
current ceases, the voltage between secondary terminals
In the START position, power is removed from the
6-7 drops to zero. The voltage between terminals 4-5
indicator light. Also, the +30-volt dc signal to the
also drops to zero. When C1 discharges to zero, Q3 stops
synchronizing stage is removed, allowing signals to pass
conducting. Because of the gradual drop of current in
to start the inverter properly.
the 3-4 winding, the voltage induced in the 6-7 winding
When the drive switch is switched to the RUN
is of reversed polarity and low amplitude.
position, the input dc voltage for the control circuit
On the other side of the power stage, power SCR
+30-volt dc power supply is disconnected and a bridge
Q2 is then triggered on by the driver output, and
rectified output from phase CA of the inverter is used.
capacitor C2 charges in the same manner as C1 charged.
The operation of this side of the power stage is the same
Power Supplies
as the side just discussed. However, the polarity of the
output is reversed, completing the square-wave output
The power supplies in the inverter are the control
on the secondary of T1.
circuit +30-volt dc power supply and the drive circuit
+30-volt dc power supply. The control circuit +30-volt
Filters
dc power supply provides power for all control circuits
except the drivers and under-over voltage circuits. These
The filters (fig. 1-38) convert the square-wave out-
two circuits are supplied by the drive circuit +30-volt dc
puts of power stages 1 and 2 to sine waves. Each filter
power supply.
consists of one series and four shunt LC filters. The
series filter provides a low-impedance path for the
The input power for the control circuit +30 volt
400-Hz fundamental frequency and a high-impedance
power supply comes from two sources. During the
path for the odd harmonics in the output. The pre-
START mode, power is obtained from the inverter input
dominate odd harmonics are filtered out by individual
dc source. In the RUN mode, the control circuit +30-volt
shunt filters. A shunt filter is provided for the third, fifth,
dc power supply receives its input from phase CA of the
seventh, and ninth harmonic. Even harmonics are
inverter output.
negligible due to the balanced design of the push-pull
The drive circuit +30-volt dc power supply provides
power stage.
power for the drivers and the under-over voltage
circuits. This power is obtained from the inverter input
Scott "T" Transformer
dc voltage.
The Scott "T" transformer is a center-tapped
Overload Circuit
autotransformer. The output voltages from the main and
secondary inverter fibers combine in the Scott "T'
The overload circuit turns the inverter off in case of
transformer to produce a 120-volt, 3-phase output.
overload. An overload signal from the current-sensing
circuit produces a dc signal of sufficient amplitude to
Clipper
trigger a unijunction transistor that, in turn, triggers a
bistable multivibrator. The bistable multivibrator output
The 3-phase clipper network, consisting of
is fed to the binary circuit in the SYNC stage, which
capacitors, resistors, and diodes, is connected across the
switches the inverter off.
3-phase output of the Scott `T' transform. The clipper
1-33

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