York MILLENNIUM YCUL0016SC, Assembly, Installation, Operation & Maintenance

Page: 127 / 172

YORK INTERNATIONAL
FORM 150.63-NM1
127
PUMPDOWN (LLSV) CONTROL
Each system has a Pumpdown feature upon shut-off.
On a non-safety, non-unit switch shutdown, all com-
pressors but one in the system will be shut off. The
LLSV will also be turned off. The final compressor will
be allowed to run until the suction pressure falls below
the cutout, or for 180 seconds, whichever comes first.
Manual pumpdown from the keypad is not possible.
LOAD LIMITING
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. 2 and 4 compressor
units can be load limited to 50%. This would allow only
1 compressor per system to run. 3 and 6 compressor units
can be load limited to 33% or 66%. The 66% limit would
allow up to 2 compressors per system to run, and the
33% limit would allow only 1 compressor per system to
run. No other values of limiting are available.
There are two ways to load limit the unit. The first is
through remote communication via the ISN. Refer to
the section on ISN control for more information.
A second way to load limit the unit is through closing
contacts connected to the Load Limit (CTB1-Termi-
nals 13-21) and PWM inputs (CTB1-Terminals 13-20).
Stage 1 of load limiting involves closing the Load Limit
input. Stage 2 of load limiting involves closing both
the Load Limit and PWM inputs. The first stage of lim-
iting is either 66% or 50%, depending on the number
of compressors on the unit. The second stage of limit-
ing is 33% and is only available on 3 and 6 compressor
units. Table 25 shows the load limiting permitted for
the various number of compressors.
COMPRESSOR RUN STATUS
Compressor run status is indicated by closure of con-
tacts at CTB2 – terminals 25 to 26 for system 1 and
CTB2 – terminals 27 to 28 for system 2. The respec-
tive contact will close anytime a compressor is running
in that particular system.
The compressor Run Status contacts can also be used to
cycle the evaporator fan contactor with the compressors.
ALARM STATUS
System or unit shutdown is indicated by normally-open
contacts opening whenever the unit shuts down on a
unit fault, or locks out on a system fault. System 1 alarm
contacts are located at CTB2 - terminals 29 to 30. Sys-
tem 2 alarm contacts are located at CTB2 - terminals
31 to 32. The alarm contacts will close when condi-
tions allow the unit to operate.
COMPRESSOR SEQUENCING
The unit control will attempt to equalize the total run
hours on individual compressors within a system. When
a system is about to start, the compressor with the least
run time in that system will be the first to start. When
the system has to load, the next compressor to start will
be the one with the least run time that is currently not
running in that system.
TABLE 25 – COMPRESSOR OPERATION – LOAD LIMITING
COMPRESSORS IN UNIT STAGE 1 STAGE 2
2 50% –
3 66% 33%
4 50% –
6 66% 33%
3