Filters

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