ARMY TM 9-4120-380-14
NAVY EE-000-BA-MMA-010/7053-AC
AIR FORCE TO 35E9-136-21
4.19 COOL FUNCTION. The principles of operation for the air conditioner, operating as the primary unit while in the
COOL or AUTO mode of Operation, are described in the following subparagraphs (see fig. 4-12 and fig FO-2.).
NOTE
The air conditioner utilizes a hot gas bypass system to maintain
temperature control instead of switching the compressor on and
off each time air temperature crosses the threshold at thermostat
(S2).
In AUTO mode of operation, all HEAT functions are also enabled under the control of the thermostat (S2).
a. The control circuit overload circuit breaker (CB 1) and the crankcase heater circuit breaker (CB2), accessible only by
disassembling the control panel, are automatic-reset type breakers. There are no manual controls for these circuit
breakers. Their protective functions are as described by their respective names.
b. The phase relay (K6) monitors the voltage levels of phases A, B, and C input power. If any phase voltage drops
below a preset value, the relay deenergizes and shuts down the air conditioner by removing voltage from the control
circuits.
c.
When MODE SELECT CT switch (S1) is placed in the AUTO or COOL position, four fault relays (K7, K8, K9 and
K10) are immediately energized to enable the fault circuits. (These four relays are also energized in VENT and
HEAT modes of operation.)
d. The evaporator fan motor contactor (K2) is energized to close the circuits to the evaporator fan motor (B2) and to
start airflow within the unit. The compressor and condenser fan motor time delay relays (TR1 and TR2) start their
timing cycles.
e. Input power is applied to the COOL indicator (DS2) and the CIRCULATE indicator (DS4), causing both to light. In
AUTO mode, the HEAT indicator (DS3) is also enabled Seven resistors (R1 through R7) provide voltage drop
regulate n for the power, mode selection, and fault indicating lights on the control panel.
f.
The solenoid valve (L1 which allows refrigerant flow to the primary expansion valve (V5) is energized (opened). The
solenoid valve (L2) which allows refrigerant flow to the liquid quench expansion valve (V6) is enabled for operation
as required by thermostat (S2) switch action.
g. After about 5 seconds, the condenser fen motor time delay relay (T R2) energizes, causing the condenser fen motor
contactor (K3) to energize and start the condenser fan motor (B3).
h. About 10 seconds after mode selection, the compressor time delay relay (TR1) energizes, causing the compressor
contactor (K1) to energize and start the compressor motor (B1).
i.
The remainder of the COOL mode control circuitry consists of the following fault detection equipment protection
circuits.
(1)
High Discharge Pressure. If the high refrigerant pressure switch (S5) located at the output port of the
compressor senses excessive pressure, it opens to deenergize the compressor contactor (K1) and shut down
the compressor (B 1), and it energizes the high pressure fault slave relay (K14).
This relay (K14) then acts to deenergize the high discharge pressure fault relay (K8), enabling the fault to be
reported at the remote status terminal board (TB 2), closes a circuit which causes the HIGH DISC
PRESSURE fault indicator (DS6) to light on the control panel; and energizes the latching relay (K11), which
opens the normally closed contacts in the input voltage circuit and shuts down the air conditioner.
The normally open contacts of this relay (Ku) will close across terminals 3 and 4 of the remote status
terminal board (TB2. If the air conditioner is installed in a dual configuration with a similar backup unit, this
contact closure {through site interconnecting cabling) will place phase C input voltage on terminal 1 of the
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