Carrier 69NT40-541-300, Операция и обслуживание, Страница: 43 / 152

T-318 3 − 11
3.3.20 Defrost Interval
There are two modes for defrost initiation, user-selected
timed intervals and automatic control.
The user-selected values are (OFF), 3, 6, 9, 12, 24
hours, AUTO, or PuLS; factory default is 3 hours.
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 set point (DTT closed). If the reading of DTS
rises above set point (DTT open) any time during the
timer count down, the interval is reset and the count-
down starts over.
3.3.21 Defrost Related Settings
If probe check (CnF31) is configured to SPECIAL, the
unit will proceed to the next operation (snap freeze or
terminate defrost). If CnF31 is configured to STAND-
ARD, the unit will perform a probe check. The probe
check is a test that compares temperature sensor read-
ings to determine if any sensors have failed.
If probe check fails, the system will run for eight minutes
to validate. At the end of eight minutes, probe alarms will
be set or cleared based on the current conditions.
When the return air temperature falls to 7
C (45 F), the
controller ensures that the defrost temperature sensor
(DTS) reading has dropped to 10 C or below. If it has
not it indicates a failed DTS, a DTS failure alarm is
triggered and the defrost mode is operated by the return
temperature sensor (RTS).
If 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 4 minutes at 100% capacity.
When the snap freeze is completed, defrost is termin-
ated.
If CnF23 is configured to “SAv” (save), then the value of
the defrost interval timer will be saved at power down
and restored at power up. This option prevents short
power interruptions from resetting an almost expired de-
frost interval, and possibly delaying a needed defrost
cycle.
CnF11 determines whether the operator will be allowed
to chose “OFF” as a defrost interval option.
CnF64 determines whether the operator will be allowed
to choose “PuLS” as a defrost interval option. For units
operating with “PuLS” selected, defrost interval is de-
termined by the unit temperature setpoint and the Evap-
orator Fan Pulsing Temperature Setting (Cd60). When
the unit temperature setpoint is equal to or less than the
Evaporator Fan Pulsing Temperature Setting, the de-
frost interval is set to 6 hours. Otherwise, the defrost in-
terval is determined using the Automatic Defrost Inter-
val Determination logic. In either case, “PuLS” remains
displayed in this function select code.
After a new Defrost Interval is selected, the previously
selected Interval is used until the next defrost termina-
tion, the next time the DTT contacts are OPEN, or the
next time power to the control is interrupted. If the previ-
ous value or the new value is “OFF”, the newly selected
value will be used immediately.
If any Auto Pretrip sequence is initiated, Cd27 will be set
to ’AUTO’ unless CnF49 (OEM Reset) is set to “Cus-
tom” AND CnF64 (Evaporator Fan Pulsing Logic) con-
figuration variable is set to IN, in which case Cd27 will be
set to “PuLS”.
3.4 PROTECTION MODES OF OPERATION
3.4.1 Evaporator Fan Operation
Opening of an evaporator fan internal protector will shut
down a unit with Normal Evaporator Fan Operation.
(CnF32 set to 2EFO). On units equipped with Single
Evaporator Fan Capability (CnF32 set to 1EFO),
additional relays are installed to allow the unit to
continue to operate on a single fan (refer to Table 3-4).
3.4.2 Failure Action
Function code Cd29 may be operator set to allow
continued operation in the event the control sensors are
reading out of range. The factory default is full system
shutdown (refer to Table 3-5).
3.4.3 Generator Protection
Function codes Cd31 and Cd32 may be operator set to
control start-up sequence of multiple units and
operating current draw. The factory default allows on
demand starting of units and full current draw (refer to
Table 3-5).
3.4.4 Condenser Pressure Control
When configuration variable CnF14 is set to “In,” the
condenser pressure control logic is activated to
maintain discharge pressures above 130psig in low
temperature ambients. The logic turns the condenser
fan on or off in accordance with the condenser pressure
transducer reading (refer to Table 3-4, page 3 − 19.) The
function is enabled when the following conditions are
met:
1. The ambient sensor reading is less than or equal to
27 C (80 F), and
2. Voltage/Frequency ratio is less than or equal to 8.38.
When the above conditions are met, either pressures or
timers may dictate a change of state from OFF to ON or
ON to OFF. If the condenser fan is OFF, it will be
energized if saturated condensing pressure is greater
than 200psig OR if the condenser fan has been OFF for
a variable time period of up to 60 seconds depending on
the ambient temperature. As the ambient temperature
increases, the amount of time that the condenser fan is
energized will correspondingly increase towards the
maximum.
If the condenser fan is ON, it will de-energize only if the
saturated condensing pressure is less than 130psig and
the condenser fan has been running for a minimum of
thirty seconds depending on the ambient temperature.