Operating instructions

TM 11574A-OI
2-57
The processing of a defrost request is controlled by the defrost termination thermostat. The
defrost termination thermostat is not a physical component. It is a software point that acts as a
thermostat, allowing defrost when it is considered “closed” and preventing or terminating defrost
when it is considered “open.” The actual temperatures used to make the “open” or “closed”
determinations are dependent on the type of defrost request made and the operator setting of
configuration variable CnF41. Configuration variable CnF41 may be factory set at the default
value of 78 F (25.6 C) or a lower value of 64 F (18 C).
When a request for defrost is made by the use of the Manual Defrost Switch, the unit will enter
defrost when the reading at the defrost temperature sensor is at or below the CnF41 setting.
Defrost will terminate when the defrost sensor temperature reading rises above the CnF41
setting.
When a request for defrost is made by probe check, the unit will enter defrost when the defrost
temperature sensor reading is at or below 78 F (25.6 C). The unit will terminate defrost when the
Defrost Temperature Sensor reading rises above 78 F (25.6 C).
When a request for defrost is made by demand defrost, the unit will enter defrost when the
reading at the defrost temperature sensor is at or below 64.4 F (18 C). Defrost will terminate
when the defrost sensor temperature reading rises above the CnF41 setting.
When a defrost has terminated, the defrost interval timer will begin counting when the reading at
the defrost temperature sensor is at or below 50 F (10 C). Once the timer has counted the
required time, the unit will enter defrost if the Defrost Temperature Sensor is at or below 78 F
(25.6 C). 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 4-13). 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 defrost termination
thermostat “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 two 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.