Carrier 48LC Series Controls, Start-Up, Operation And Troubleshooting page 20

will remove cooling stages. If at any time the Supply-Air Tem-
perature (SUPPLY AIR TEMP) falls below the Minimum Sup-
ply Air Temperature Upper Level (UPPER MIN SAT), the re-
quested stages will not be allowed to increase. If at any time the
SAT falls below the Minimum Supply Air Temperature Lower
Level (LOWER MIN SAT), the requested stages will be reduced
by one. If these SAT limits are configured so that they are too
close together, the last stage might cycle rapidly, slowed only by
its minimum on and off-time requirements.
Adaptive Control
Stage timers and Supply air trend apply when determining the
request for stages. The first request (REQ. COOL STAGES =1)
comes immediately when starting the staging process. The Cool
Stage Increase Time (COOL STAGEUP TIME) has to expire
and the Supply-Air Trend (SUPPLY AIR TREND) has to be
above the cooling supply air trend level (COOL SATTREND
LEV) before another stage can be added. Requested stages will
only be allowed to increase as the actual system demand allows
(DEMAND). A "LOW COOL" demand will only allow one re-
quested stage, "MED COOL" two stages, and "HIGH COOL" 3
stages. The requested stages will be reduced if the cooling de-
mand is lowered or dropped completely, or if the supply air falls
below the lower level (LOWER MIN SAT).
Traditional Thermostat Control
Stage timers and Supply air trend do not apply when determin-
ing the request for stages. Request staging will follow the ther-
mostat inputs directly. "LOW COOL" will request one stage.
"MED COOL" will request two stages. "HIGH COOL" will
request 3 stages.
COMPRESSOR CONTROL
The compressor control works hand in hand with the staging con-
trol. As the staging control request stages, the compressor control
determines what is available or running and tries to provide stages
for what is requested. The availability of the compressors depends
on time guards, circuit diagnostics, and outdoor temperature. The
low cooling compressor (LOW COOL COMP) informs the con-
trol which compressor is desired for a low cooling demand.
IMPORTANT: When LOW COOL COMP is set to 2, the
unit will operate as a 2 stage unit where the larger compres-
sor is stage one and both compressors are stage 2.
There are time guards to protect the compressor: Compressor
Min On Time (COMP MIN ON TIME) and Compressor Min
Off Time (COMP MIN OFF TIME) apply before the com-
pressors can be turned back on or turned off. Timeguard A1
(COMP A1 TIMEGUARD) and Timeguard A2 (COMP A2
TIMEGUARD) display the time the compressors have before
they can transition state.
Circuit diagnostic tests are performed during operation which
may or may not allow the compressors to be used. The availabil-
ity of the compressors is shown as Compressor A1 Available
(COMP A1 AVAILABLE) and Compressor A2 Available
(COMP A2 AVAILABLE). The lockout status of the compres-
sors is shown as Compressor A1 Lockout (COMP A1 LOCK-
OUT) and Compressor A2 Lockout (COMP A2 LOCKOUT).
The actual stages running at any given time is displayed as
Actual Cooling Stages (ACTIVE COOL STAGE):0 (Off), 1
(Compressor A1 On only), 2 (Compressor A2 On only), and 3
(both compressors are on). Individual compressor output state is
shown as (COMPRESSOR A1) and (COMPRESSOR A2).
Any time the outdoor ambient falls below the low cooling mini-
mum outdoor temperature (LOW COOL MIN OAT), the low
cooling lockout will be active (LOW COOL LOCKOUT),
preventing compressor A1 from running by itself. Any time the
outdoor ambient falls below the medium cooling minimum out-
door temperature (MED COOL MIN OAT), the medium cooling
lockout will be active (MED COOL LOCKOUT) preventing
compressor A1 and compressor A2 from running by themselves.
OUTDOOR FAN CONTROL
Outdoor fans can be controlled by one of two methods: normal
operation of discrete speed based on the cooling being per-
formed, or low ambient operation that varies the outdoor airflow
to control saturated discharge temperature within an acceptable
range. This is implemented using multi-speed motors. The sys-
tem outdoor fan speed (COMMANDED ODF SPD) represents
the commanded speed of all outdoor fan motors as a complete
system. The number of outdoor fans in the system is determined
by the number of outdoor fan outputs (ODF SIGNAL QTY).
IMPORTANT: The number of outdoor fans will not always
match the number of outdoor fan outputs (ODF SIGNAL
QTY). Fig. 12 shows how the outdoor fans are mapped with
the outdoor fan outputs.
NOTE: Factory default configurations account for these model
differences and should not be changed. The default configurations
have been qualified over a large range of conditions and are pro-
vided in case a field replacement of a control board occurs and the
settings need to be checked or manually configured. Outdoor fan
operation is further described below to assist in troubleshooting.
Typical Operation
When OAT is above low ambient temperature (LOW AMBI-
ENT TEMP), the ODFs will run at 4 discrete speeds: Off, Low
Cool Speed (ODF LOW COOL SPD), Medium Cool Speed
(ODF MED COOL SPD), and High Cool Speed (ODF HIGH
COOL SPD), corresponding to the 4 discrete cooling stage of
the compressors (ACTIVE COOL STAGE): 0 (Off), 1 (Com-
pressor A1 On only), 2 (Compressor A2 On only), and 3 (both
compressors are on).
Low Ambient Operation
Low ambient operation will be used when either of the 2 condi-
tions is met: 1) OAT is less than low ambient temperature (LOW
AMBIENT TEMP) – 5°F or 2) OAT is less than low ambient
temperature (LOW AMBIENT TEMP) – 2.5°F and the Saturated
Discharge Temperature (CIR.A DIS. TEMP) is less than 92°F.
The low ambient ODF control will manipulate ODF speed to keep
the discharge temp above 95°F. As OAT continues to drop, ODF
speed will continue to decrease. At a certain point, the low ambi-
ent control may decide to turn off ODFs and only control the
speed of the rest of the outdoor fans. If the discharge temp keeps
on decreasing, it may reach a point that all ODF fans will be
turned off. The ODFs will be turned on starting from the highest
number first, meaning the ODF1 will be the last one to shut off.
The ODFs are protected with a 45 second run time and a 60 sec-
ond off time. See Fig. 12 for the ODF arrangement. The lowest
speed allowed to run any ODF is determined by the outdoor fan
Minimum Speed Configuration (ODF MINIMUM SPEED).
NOTE: During ODF cycling all fans that are commanded on will
be at the same speed and the off ones will be at zero speed. The
motors will not be allowed to run different speeds at the same
time.
When OAT rises above low ambient temperature (LOW AM-
BIENT TEMP), ODF control will transition from low ambient
to normal operation. A period (5 min) is allowed for the ODF
speed to ramp from current position to one of the 4 discrete
speed settings.
IMPORTANT: The low ambient temperature (LOW
AMBIENT TEMP) is default to 66 degrees and should not
be changed unless directed by authorized Carrier personnel.
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