Carrier 48AJ Operating And Service Manual page 39

Reverse Rotation Verified? (REV.R) — If this configuration
is set to NO, then after a power up, in the normal run mode, the
control will check the suction pressure on the first circuit that is
energized after 5 seconds of run time. If the control does not
see a sufficient decrease in suction pressure over the first 5 sec-
onds, mechanical cooling will be shut down, and an alarm will
be generated (A140). This alarm requires a manual reset.
If the unit is in the Service Test mode, the test will be
performed any time a compressor is energized.
Once it has been verified that power to the rooftop and
compressors has been applied correctly and the compressors
start up normally, this configuration can be set to YES in order
to prevent the reverse rotation check from occurring.
High SST Alert Delay Time (H.SST) — This option allows
the low saturated suction temperature alert timing delay to be
adjusted.
COMPRESSOR SAFETIES — The 48/50A Series units with
the ComfortLink™ controls include a compressor protection
board (CSB) that protects the operation of each of the compres-
sors. These boards sense the presence or absence of current to
each compressor.
If there is a command for a compressor to run and there is
no current, then one of the following safeties or conditions
have turned the compressor off:
• Compressor overcurrent — Smaller compressors have
internal line breaks and larger compressors have a dedicated
circuit breaker for over current protection.
• Compressor short circuit — If the compressor circuit
breaker that provides short circuit protection has tripped
then there will not be current.
• Compressor motor over temperature — If the internal line-
break or over temperature switch has opened.
• High-pressure switch trip — High-pressure switch has
opened.
Alarms will also occur if the current sensor board malfunc-
tions or is not properly connected to its assigned digital input. If
the compressor is commanded OFF and the Current Sensor
reads ON, an alert is generated. This will indicate that a com-
pressor contactor has failed closed. In this case, a special mode
"Compressor Stuck on Control" will be enabled and all other
compressors will be turned off and an alarm enabled to indicate
that service is required. Any time the alert occurs, a strike is
called out on the affected compressor. If three successive strikes
occur the compressor will be locked out requiring a manual
reset or power reset of the circuit board. The clearing of strikes
during compressor operation is a combination of 3 complete
cycles or 15 continuous minutes of run time operation. If there
are one or two strikes on the compressor and three short cycles
(ON-OFF, ON-OFF, ON-OFF) less than 15 minutes each occur,
the strikes are reset to zero for the affected compressor. If the
compressor turns on and runs for 15 minutes straight with no
compressor failure, the compressor strikes are cleared.
COMPRESSOR TIME GUARDS — The control will not al-
low any output relay to come on within 3 seconds of any other
output relay. For outputs connected to the compressors, the con-
trol will use a Compressor Minimum OFF Time of 2 minutes, a
Compressor Minimum ON Time of 3 minutes and a Minimum
Delay before turning on another compressor of 10 seconds.
COOL MODE SELECTION PROCESS — The A Series
ComfortLink™ control offers three distinct methods by which
it may select a cooling mode.
1. Thermostat (C.TYP=3 and 4): The thermostat does not
depend upon the state of occupancy or temperature and
the modes are called out directly by the discrete inputs
→ →
(Inputs STAT Y1 and Y2).
2. Occupied VAV cooling types (C.TYP=1 and 2) are called
out in the occupied period (Operating Modes
→
MODE OCC=ON).
3. Occupied VAV cooling types (C.TYP=1 and 2) are called
out in the unoccupied period (Operating Modes
→
MODE
sensor control types (C.TYP=5 and 6) in both the occu-
pied and unoccupied periods.
This section is devoted to the process of cooling mode
determination for the three types outlined above.
VAV Cool Mode Selection during the Occupied Period
(C.TYP = 1,2 and Operating Modes
— There is no difference in the selection of a cooling mode for
either VAV-RAT or VAV-SPT in the occupied period. The actual
selection of a cool mode, for both control types, is based upon
the controlling return-air temperature (Temperatures
→ →
CTRL R.TMP). Typically this is the same as the return air
temperature thermistor (Temperatures
when under CCN Linkage.
VAV Occupied Cool Mode Evaluation Configuration — There
are VAV occupied cooling offsets under Setpoints.
ITEM EXPANSION
V.C.ON VAV Occ.
Cool On Delta
V.C.OF VAV Occ.
Cool Off Delta
Cool Mode Determination — If the machine control type
(Configuration
SPT) and the control is occupied (Operating Modes
→ →
MODE OCC=ON), then the unit will not follow the occu-
pied cooling set point (OCSP). Instead, the control will follow
two offsets in the determination of an occupied VAV cooling
mode (Setpoints
them to the low-heat off trip point and comparing the resulting
temperature to the return-air temperature.
The Setpoints
→
Setpoints V.C.OF (VAV cool mode off offset) offsets are
used in conjunction with the low heat mode off trip point to
determine when to bring cooling on and off and in enforcing a
true "vent" mode between heating and cooling. See Fig. 5. The
occupied cooling set point is not used in the determination of
the cool mode. The occupied cooling set point is used for
supply air reset only.
The advantage of this offset technique is that the control can
safely enforce a vent mode without worrying about crossing set
points. Even more importantly, under CCN linkage, the
occupied heating set point may drift up and down and as such
this technique of using offsets ensures a guaranteed separation
in degrees Fahrenheit between the calling out of a heating or
cooling mode at all times.
NOTE: There is a sub-menu at the local display (Run Status
→
TRIP) that allows the user to see the exact trip points for
both the heating and cooling modes without having to calcu-
late them. Refer to the Cool Mode Diagnostic Help section on
page 43 for more information.
To enter into a VAV Occupied Cool mode, the controlling
temperature must rise above [OHSP minus L.H.ON plus
L.H.OF plus V.C.ON].
To exit out of a VAV Occupied Cool Mode, the controlling
temperature must fall below [OHSP minus L.H.ON plus
L.H.OF plus V.C.ON minus V.C.OF].
NOTE: With vent mode, it is possible to exit out of a cooling
mode during the occupied period if the return-air temperature
drops low enough. When supply-air temperature reset is not
OHSP
L.H.ON
→
Fig. 5 — VAV Occupied Period Trip Logic
39
OCC=OFF). They are also used for space
→
MODE
→
AIR.T
RANGE UNITS CCN POINT
0-25 deltaF VAVOCON
1-25 deltaF VAVOCOFF
→ →
UNIT C.TYP) = 1 (VAV-RAT) or 2 (VAV-
→ →
V.C.ON and Setpoints
→
V.C.ON (VAV cool mode on offset) and
V.C. ON
L.H.OF
→
→
OCC =ON)
→
AIR.T
→
RAT) except
DEFAULT
3.5
2
V.C.OF), applying
V.C. OF
A48-7700