System Description of Inverter

TM 11-6130-377-14

protecting the system from prolonged overloads on the

TB1 (+) and (-). In the 1973 model, power is also applied

input. Fuse (Fl) is also blown by diode (CR5) if the DC

to shunt regulator (Q1) through door interlock switch (S1)

input voltage leads are reversed.

contacts 1 and 2, providing +10 to +12 vdc to inverter

drive board A1A2.

(5) If the door of the cabinet is opened,

contacts 4 and 6 of switch S1 (door interlock) close,

(2) When DC circuit breaker CB1 is first

turning off the system by energizing the shunt trip coil of

positioned to ON, capacitor C1 (C2) charges through

circuit breaker CB1. Switch S1 is positioned to enable the

circuit breaker CB1, DC CURRENT meter M2, and

system either by closing the door or by pulling out the

inductor L1 (also F1 and R6 providing a soft start for the

switch plunger. The shunt trip coil of circuit breaker CB1

system) After a power on delay, the inverter drive circuit

is also energized by the sensors on inverter drive circuit

board energizes SCR Q1 and Q2 (In the 1973 model, the

board A1A2 upon detection of a voltage fault (dc input

inverter drive circuit board energizes SCR Q2 and relay

voltage outside its upper or lower limits). In the 1973

K3; SCR Q2 is turned on by passing R6 and closing the

model, this is accomplished through relay (K1) which is

contacts of relay K3; closing the system output circuit

closed by the inverter drive circuit board upon detection of

breaker connects output terminals X1 and X2 (TB3).) The

a voltage fault.

system is energized as indicated by dc ON indicator DS1.

Input filter L1 and C1 (C2) prevents the inverter from

(6) Thyristors (SCR's) Q1 (Q3) and Q2 (Q4)

introducing noise back onto the input DC line.

are connected to output transformer T1. These SCR's are

alternately triggered into conduction by the inverter drive

(3) In the 1974 model, contacts 1 and 2 of

circuit board to produce an alternating current in the

A1CB1 are held open while the inverter is operating.

primary winding of the output transformer.

Capacitor A1A2C25 and resistor A1A2R28 (on the

inverter circuit board) prevent arcing across the contacts

(7) The SCR's are commutated by capacitor

when circuit breaker ALCB1 is first turned on. When the

C2 (C3), which is connected between the anodes of Q1

inverter is turned off, the contacts relax and discharge

(Q3) and Q2 (Q4) through L3 (L5). The flow of current

capacitor A1C1. The capacitors are discharged through

through the circuit can be traced by assuming that initially

resistor A1A2R28 and circuit breaker A1CB1, contacts 1

SCR Q2 (Q4) is conducting and Q1 (Q3) is non-

and 2, in less than 5 seconds. Circuit breaker A1CB1

conducting, and that the common cathode connection of

provides normal overload protection as well as input

the SCR's is the reference point. For this condition, the

overvoltage and undervoltage protection for the system.

voltage at the anode (220 volts) of SCR Q1 (Q3) is twice

In the 1973 model, relay (A1K2) is energized by circuit

the voltage of the DC input voltage. The load current

breaker (A1CB1) and is held open while the inverter is

flows from the positive terminal of the DC input through

operating. Capacitor (A1C1) and resistor (A1R3) prevent

circuit breaker CB1, DC CURRENT meter M2, inductor

arcing across the contacts of relay (A1K2) when circuit

L1, one half of the primary winding of transformer T1,

breaker (A1CB1) is first turned on. When the inverter is

SCR Q2 (Q4), and inductor L2 to the negative terminal of

turned off (either by positioning circuit breaker (A1CB1) to

the DC input. When the firing current is applied to the

OFF or by removing input power), the contacts of relay

gate of Q1 (Q3), this SCR turns ON and conducts.

(A1K2) relax and discharge capacitors (A1C1) and (A1C2)

(8) While SCR Q2 (Q4) is still on, capacitor C2

and all the capacitors connected to connector J1, pin 2 on

(C3) begins to discharge through SCR's Q1 (Q3) and Q2

the inverter drive circuit board. The capacitors are

(Q4). The discharge current through SCR Q2 (Q4) flows

discharged through resistor (A1R3) and relay (A1K2),

in a reverse direction, and after the carriers of Q2 (Q4)

contacts 2 and 9, in less than 5 seconds.

are swept out (recombined), Q2 (Q4) is switched to the off

(4) In the 1973 model, circuit breaker (A1CB1)

state. At this time, the voltage across capacitor C2 (C3),

provides normal overload protection as well as input

which is approximately 220 volts, appears across SCR Q2

overvoltage and undervoltage protection for the system.

(Q4) as reverse voltage. This voltage remains long

A small input overload (400 to 800 amperes) may take

enough to allow Q2 to recover for forward blocking.

more time to trip the circuit breaker than a large overload.

Simultaneously during this interval, the conducting SCR

An input overload that takes more than 15 milliseconds to

Q1 (Q3) establishes another discharge path for capacitor

trip circuit breaker (CB1) causes fuse (F1) to blow,

C2 (C3)

Change 1

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