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FORM 150.73-NM2
ISSUE DATE: 09/07/2020
LOAD LIMITING
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. 2 and 4 compres-
sor units can be load limited to 50%. This would allow
only 1 compressor per system to run. 3 and 6 compres-
sor 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 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 (XTBC1 – ter-
minals 13-21) and PWM inputs (XTBC1 – 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%, de-
pending 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
25 shows the load limiting permitted for the various
numbers of compressors.
TABLE 23 - COMPRESSOR OPERATION – LOAD
LIMITING
COMPRESSORS
IN UNIT
2
3
4
5
6
JOHNSON CONTROLS
STAGE 1
STAGE 2
50%
-
66%
33%
50%
-
80%
40%
66%
33%
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 XTBC2 – terminals 25 to 26 for system 1 and
XTBC2 – 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 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 XTBC2 - terminals 29
to 30. System 2 alarm contacts are located at XTBC2 -
terminals 31 to 32. The alarm contacts will close when
conditions allow the unit to operate.
SECTION 8 – UNIT OPERATION
133
8
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