Carrier 69NT40-541-505, Operation And Service Manual, Page: 40 / 143

T-368 4–10
a. Automatic defrost starts with an initial defrost at
three hours and then adjusts the interval to the
next defrost based on the accumulation of ice
on the evaporator coil. Following a start-up or
after termination of defrost, the time will not
begin counting down until the DTS reading falls
below 10°C (50°F). If the reading of DTS rises
above termination setting any time during the
timer count down, the interval is reset and the
countdown starts over. The Auto defrost time is
reset to three hours start time after every PTI ini-
tiation or trip start interval.
b. Fan Pulsing Logic is used to help prevent ice
formation in the drain gutter and drain cup and
ice buildup in supply air channel by using the
evaporator fans to blow the warm air onto these
areas during unit defrost. When cooling at lower
setpoints, evaporator fan pulsing can be used
during Defrost/De-ice when the “PuLS” option is
selected in the Defrost Interval function select
code. When enabled, evaporator fan pulsing will
occur based on the unit temperature setpoint
and the Evaporator Fan Pulsing Temperature
Setting (Cd60). QUEST II also pulses the evap-
orator fans during Defrost/De-ice within a nar-
row perishable setpoint range. The logic for
each evaporator fan pulsing feature is described
below.
c. After a new Defrost Interval is selected, the pre-
viously selected Interval is used until the next
defrost termination, the next time the DTS con-
tacts are OPEN, or the next time power to the
control is interrupted. If the previous value or the
new value is “OFF”, the newly selected value
will be used immediately.
3. If defrost is initiated due to Probe Check imme-
diately following the defrost cycle the evapora-
tion fans are started and run for eight minutes to
stabilize the temperature throughout the con-
tainer. A probe check comparison is carried out
at the end of the eight minute period if any sen-
sor is found out of calibration. At this time it is no
longer used for control/reorder purposes.
4. Probe Check Logic: The logic determines that a
Probe Check is necessary based on tempera-
ture values currently reported by the supply and
return probes.
5. Remote: An Initiate Defrost command is sent via
communications.
6. Delta T Logic: If the difference between return
and supply air temperature (Delta T) becomes
too great indicating possible reduced airflow
over the evaporator coil caused by ice buildup
requiring a defrost.
a. In Perishable Pull Down - Delta T increases to
greater than 12°C, and 90 minutes of compres-
sor run time have been recorded.
b. In Perishable Steady State – A baseline Delta T
is recorded following the first defrost cycle after
steady state conditions are reached, (the unit is
cooling, and the evaporator fans and heaters
must remain in a stable state for a period of five
minutes). Defrost will be initiated if Delta T
increases to greater than 4°C above the base-
line, and 90 minutes of compressor run time
have been recorded.
c. In Frozen Mode - Defrost will be initiated if Delta
T increases to greater than 16°C and 90 min-
utes of com presso r run time have been
recorded.
NOTICE
When defrost is initiated, the controller
closes the SMV, opens contacts TC, TN
and TE (or TV) to de-energize the com-
pressor, condenser fan and evaporator
fans.
The controller then closes contacts TH to supply power
to the heaters. The orange DEFROST light and heat
light are illuminated and the COOL light is also de-
energized.
Figure 4.10 Defrost
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
SIGNAL TO
CONTROLLER
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