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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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