JOHNSON CONTROLS
112
FORM 150.62-NM6 (103)
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. Five-compressor units may be load limited to
40% (1 compressor per system runs) or 80% (up to 2
compressors per system) are permitted to run. No other
values of limiting are available.
There are two ways to load limit the unit. The rst is
through remote communication via an ISN.
A second way to load limit the unit is through closing
contacts connected to the Load Limit (CTB1 – Terminals
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 rst stage of limiting is
either 80%, 66% or 50%, depending on the number of
compressors on the unit. The second stage of limiting
is either 40% or 33% and is only available on 3, 5 & 6
compressor units. Table 32 shows the load limiting per-
mitted for the various number of compressors.
Simultaneous operation of Load Lim-
iting and EMS-PWM Temperature
Reset (described on following pages)
cannot occur.
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.
ALARM STATUS
System or unit shutdown is indicated by normally-open
alarm 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.
System 2 alarm contacts are located at CTB2 – terminals
31 to 32. The alarm contacts will close when conditions
allow the unit to operate.
Unit Controls
TABLE 32 – COMPRESSOR OPERATION – LOAD LIMITING
COMPRESSORS IN UNIT STAGE 1 STAGE 2
2 50% –
3 66% 33%
4 50% –
5 80% 40%
6 66% 33%
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