Product data

Page 18
Installation, Operation & Maintenance Manual # 036-1009(0608)
HIGH PRESSURE LIMIT SWITCH
During cooling operation, if a high pressure limit switch opens,
the UCB will de-energize the associated compressor, initiate the
ASCD (anti short cycle delay), and, if the other compressor is
idle, stop the condenser fan. If the call for cooling is still present
at the conclusion of the ASCD, the UCB will re-energize the
halted compressor.
Should a high pressure switch open three times within two hours
of operation, the UCB will lock out the associated compressor
and flash a code (see Table 10). If the other compressor is
inactive, the condenser fan will be de-energized.
LOW PRESSURE LIMIT SWITCH
initial 30 seconds of a cooling system’s operation. For the
following 30 seconds, the UCB will monitor the low pressure
switch to ensure it closes. If the low pressure switch fails to close
after the 30 second monitoring phase, the UCB will de-energize
the associated compressor, initiate the ASCD, and, if the other
compressor is idle, stop the condenser fan.
Once the low pressure switch has been proven (closed during
the 30 second monitor period described above), the UCB will
monitor the low pressure limit switch for any openings. If the low
pressure switch opens for greater than 5 seconds, the UCB will
de-energize the associated compressor, initiate the ASCD, and,
if the other compressor is idle, stop the condenser fan. If the call
for cooling is still present at the conclusion of the ASCD, the UCB
will re-energize the halted compressor. Should a low pressure
switch open three times within one hour of operation, the UCB
will lock out the associated compressor and flash a code (table
10). If the other compressor is inactive, the condenser fan will be
de-energized.
FREEZESTAT
During cooling operation, if a freezestat opens, the UCB will de-
energize the associated compressor, initiate the ASCD, and, if the
other compressor is idle, stop the condenser fan. If the call for
cooling is still present at the conclusion of the ASCD, the UCB will
reenergize the halted compressor. Should a freezestat open three
times within two hours of operation, the UCB will lock out the
associated compressor and flash a code (table 10). If the other
compressor is inactive, the condenser fan will be de-energized.
LOW AMBIENT COOLING (OPTIONAL)
To determine when to operate in low ambient mode, the UCB
has a pair of terminals connected to a temperature activated
switch set at 50°F. When the low ambient switch is closed and
the thermostat is calling for cooling, the UCB will operate in the
low ambient mode.
Low ambient mode operates the compressors in this manner
: 10 minutes on, 5 minutes off. The indoor blower is operated
throughout the cycle. The 5 minute off period is necessary to
defrost the indoor coil. Low ambient mode always begins with
compressor operation. Compressor minimum run time may
extend the minutes of compressor operation.
The defrost cycle will begin immediately following the elapse of
the minimum run time. When operating in low ambient mode,
the UCB will not lockout the compressors due to a freezestat
trip. However, a freezestat trip will de-energize the associated
compressor. If the call for cooling is still present at the end of
the ASCD and the freezestat has closed, the unit will resume
operation.
SAFETY CONTROLS
The unit control board monitors the following inputs for each
cooling system :
1. A suction line freezestat to protect against low evaporator
temperatures due to a low air flow or a low Installation,
return air temperature, (opens at 26 ± 5°F and resets at 38 ±
5°F).
2. A high pressure switch to protect against excessive
discharge pressures due to a blocked condenser coil or a
condenser motor failure, (opens at 415 ± 15 psig and resets
at 280 ± 15 psig).
3. A low pressure switch to protect against loss of refrigerant
charge, (opens at 35 ± 5 psig and resets at 60 ± 5 psig).
The above pressure switches are hard soldered to the unit.
The refrigeration systems are independently monitored and
controlled. On any fault, only the associated system will be
affected by any safety / preventive action.
The other refrigerant system will continue in operation unless it
is affected by the fault as well. The unit control board monitors
the temperature limit switch of electric heat units.
COMPRESSOR PROTECTION
The compressors also have inherent (internal) protection. If there
is an abnormal temperature rise in a compressor, the protector
will open to shut down the compressor. The UCB incorporates
features to minimize compressor wear and damage.
An anti short cycle delay (ASCD) is utilized to prevent operation
of a compressor too soon after its previous run. Additionally,
a minimum run time is imposed any time a compressor is
energized.
The ASCD is initiated on unit start up and on any compressor
reset or lock out.
NOTE : 2nd stage of cooling is available only in DMS 150.
FLASH CODES
The UCB will initiate a flash code associated with errors within
the system. Refer to UNIT CONTROL BOARD FLASH CODES Table
10.