JOHNSON CONTROLS
145
SECTION 8 – UNIT OPERATION
Form 150.63-NM9
Issue date: 5/20/2021
8
Load limiting
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. Two and four com-
pressor units can be load limited to 50%. This would
allow only one compressor beforesystem to run. three
and six compressor units can be load limited to 33%
or 66%. The 66% limit would allow up to two com-
pressors beforesystem to run, and the 33% limit would
allow only one compressor beforesystem to run. Five
compressor units may be load limited to 40% (one
compressor beforesystem runs) or 80% (up to two
compressors beforesystem) are permitted 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 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 first 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 and
6 compressor units. Table 23 shows the load limiting
permitted for the various numbers of compressors.
Simultaneous operation of Remote Load
Limiting 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 TB1 – terminals 25 to 26 for system 1 and TB1
– terminals 27 to 28 for system 2. The respective con-
tact 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 compres-
sors.
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 TB1 - terminals 29 to 30.
System 2 alarm contacts are located at TB1 - terminals
31 to 32. The alarm contacts will close when condi-
tions allow the unit to operate.
Table 23 - Compressor Operation – Load Limiting
COMPRESSORS IN UNIT STAGE 1 STAGE 2
2 50% -
3 66% 33%
4 50% -
5 80% 40%
6 66% 33%
To Next Page
Loading...
