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enter defrost if the Defrost Temperature Sensor is at or
below 25.6°C(78°F). Defrost will terminate when the
Defrost Sensor Temperature reading rises above the
CnF41 setting.
When the unit enters defrost, the controller opens
contacts TC, TN and TE (or TV) to de-energize the
compressor, condenser fan and evaporator fans. (See
Figure 4-8.) The COOL light is also de--energized.
The controller then closes TH to supply power to the
heaters. The defrost light is illuminated.
When the defrost temperature sensor reading rises to
the
applicable
"opening" point the de--icing operation is terminated.
If defrost does not terminate correctly and temperature
reaches the set point of the heat termination thermostat
(HTT) the thermostat will open to de--energize the
heaters. If termination does not occur within 2.0 hours,
the controller will terminate defrost. An alarm will be
given of a possible DTS failure.
If probe check (controller function code CnF31) is
configured to special, the unit will proceed to the next
operation (snap freeze or terminate defrost). If the code
is configured to standard, the unit will perform a probe
check. The purpose of the probe check is to detect
malfunctions or drift in the sensed temperature that is
too small to be detected by the normal sensor out of
range tests. The system will run for eight minutes in this
condition. At the end of the eight minutes, probe alarms
will be set or cleared based on the conditions seen.
When the return air falls to 7_C (45_F), the controller
checks to ensure the defrost temperature sensor (DTS)
reading has dropped to 10_C or below. If it has not, a
DTS failure alarm is given and the defrost mode is
operated by the return temperature sensor (RTS).
If controller function code CnF33 is configured to snap
freeze, the controller will sequence to this operation.
The snap freeze consists of running the compressor
without the evaporator fans in operation for a period of
four minutes with the suction modulation valve at
maximum allowed opening. When the snap freeze is
completed, defrost is formally terminated.
T-296
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4.11 Arctic Mode
With arctic mode enabled, if the ambient is colder than
--10.0_C there is a 30 minute time delay at startup for
any of the components in the system, except for the
controller and the compressor crankcase heater (CCH),
which should be active at this point. In arctic mode, the
CCH is energized for 30 minutes to warm the oil in the
compressor, and boil off any liquid refrigerant that may
be present in the crankcase.
If Pre-Trip is initiated during the 30 minute time period,
Pre-Trip will be allowed to run normally. Once Pre-Trip is
over, the controller will revert to its normal control mode
logic.
If ambient is warmer than --10.0_C, the system will run
its normal startup logic.
Arctic mode is configurable by using the configuration
variable #29, refer to Table 3--1.
4-8
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