Service manual

T -318
4-- 7
4.10 SEQUENCE OF OPERATION
General operation sequences for cooling, heating, and
defrost are provided in the following subparagraphs.
Schematic representation of controller action are
provided in Figure 4-3 and Figure 4-4. Refer to Section
3 for detailed descriptions of special events and timers
that are incorporated by the controller in specific modes
of operation.
4.10.1 Sequence of Operation -- Perishable Mode
Cooling
NOTE
In the conventional Perishable Mode of
Operation, the evaporator motors run in high
speed. In the economy Perishable Mode, the
fan speed is varied.
NOTE
In low temperature ambients, the condenser
fan will be cycled by the controller to maintain
proper condensing pressure.
a. With supply air temperature above set point and de-
creasing, the unit will be cooling with the condenser
fan motor (CF), compressor motor (CH), evaporator
fan motors (EF) energized, and the COOL light illumi-
nated (see Figure 4-5).
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-- ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
NOTE: HIGH SPEED EVAPORATOR FAN SHOWN. FOR LOW SPEED
CONTACT TE IS DE-ENERGIZED AND CONTACT TV IS ENERGIZED
Figure 4-5 Perishable Mode Cooling
b. When the air temperature decreases to a predeter-
mined tolerance above set point, the in-range light is
illuminated.
c. As the air temperature continues to fall, modulating
cooling starts at approximately 2.5 degrees C (4.5
degrees F) above set point (see Figure 4-3).
d. The controller monitors the supply air. Once the sup-
ply air falls below set point and 0% SMV position is
reached, the controller periodically records the sup-
ply air temperature, set point, and time. A calculation
is then performed by subtracting the set point reading
from the supply air and multiplying the result by the
time reading. The result is negative number.
e. When the calculation reaches -250, contacts TC and
TN are opened to de-energize compressor and con-
denser fan motors. The cool light is also de-ener-
gized.
f. The evaporator fan motors continue to run to circulate
air throughout the container. The in-range light re-
mains illuminated as long as the supply air is within
tolerance of set point.
g. When the supply air temperature increases to 0.2 de-
grees C (0.4 degrees F) above set point and the three
minute off time has elapsed, relays TC and TN are en-
ergizes to restart the compressor and condenser fan
motors. The cool light is also illuminated.
4.10.2 Sequence of Operation --
Perishable Mode Heating
NOTE
The unit will heat only when in the Perishable
Mode, relay TH is electronically locked out
when in the Frozen Mode.
a. If the air temperature decreases 0.5 degrees C (0.9
degrees F) below set point, the system enters the
heating mode (see Figure 4-3). The controller closes
contacts TH (see Figure 4-6) to allow power flow
through the heat termination thermostat (HTT) to en-
ergize the heaters (HR). The HEAT light is also illumi-
nated. The evaporator fans continue to run to circu-
late air throughout the container .
b. When the temperature rises to 0.2 degrees C (0.4 de-
grees F) below set point, contact TH opens to de-en-
ergize the heaters. The HEAT light is also de-ener-
gized. The evaporator fans continue to run to
circulate air throughout the container .
c. A safety heater termination thermostat (HTT), at-
tached to an evaporator coil support, will open the
heating circuit if overheating occurs.
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
Figure 4-6 Perishable Mode Heating