178
JOHNSON CONTROLS
FORM 201.21-NM4 (616)
178
JOHNSON CONTROLS
OPERATION
LOAD LIMITING CONTROL
LOAD LIMITING
The Load Limiting Controls are intended to prevent
a system from reaching a safety trip level. Load
limiting controls prevent loading or unload compressors
to prevent tripping on a safety. Limiting controls operate
for Motor Current %FLA, Suction Pressure, Discharge
Pressure, VSD Baseplate Temperature, and VSD Internal
Ambient Temperature.
All running system’s load limit control values are
checked every 2 seconds. Load limiting prevents a
system from loading (no increase even though cooling
demand requires loading) when the specific operating
parameter is within a specific range of values. If the
value is above the range where loading is inhibited,
the logic will unload the chiller based on the amount
(%) the limit has been exceeded. Load limiting affects
all compressors, even though only one system may be
affected.
If more than one operating parameter is exceeding
the value where unloading is required, the value with
the highest amount of unloading will determine the
unloading. All load limiting controls are active at start-
up except suction pressure limiting.
Motor Current Load Limiting/Unloading
Motor current load limiting helps prevent the system
from tripping on the motor overload safety. The motor
“Current Limit Setpoint” is based on %FLA motor cur-
rent and is programmable under the Program key or may
be set by a remote device. Motor current load limiting
prevents the system from loading even though increased
loading may be required when the current is between the
“Current Limit Setpoint – 2%” and the “Current Limit
setpoint”. Between the “Current Limit Setpoint” and the
“Current Limit Se 5%”, the system will unload
every 2 seconds according to the amount current is ex-
ceeding the “Current Limit Setpoint”. At the “Current
limit Setpoint”, 0 Hz reduction in speed will take place
and at the “Current Limit Se 5%”, a 10 Hz speed
reduction will take place. Between the “Current Limit
Setpoint” and “Current Limit Se 5%”, unloading
will occur according to the TABLE 6:
TABLE 6 - CURRENT LIMIT
LOAD LIMITING/UNLOADING
CURRENT LIMIT SETPOINT
UNLOADING
Current Limit Setpoint
-2% to +0%
0 Hz
Current Limit Se1%
2 Hz
Current Limit Se2%
4 Hz
Current Limit Se3%
6 Hz
Current Limit Se4%
8 Hz
Current Limit Se5%
10 Hz
Discharge Pressure Load Limiting/Unloading
Discharge pressure load limiting protects the condenser
from experiencing dangerously high pressures. A system
is permitted to load normally as long as the discharge
pressure is below the High Discharge Pressure Cutout
– 20 PSIG. Between Cutout – 20PSIG and Cutout
– 15 PSIG loading is inhibited even though increased
loading may be required. Between Cutout –15 PSIG
and the Discharge Pressure Cutout, forced unloading
is performed every 2 seconds according to TABLE 7
below. The discharge pressure unload point is fixed at
255 PSIG.
TABLE 7 - DISCHARGE PRESSURE
LOAD LIMITING/UNLOADING
DISCHARGE PRESSURE
UN-
LOADING
Discharge Pressure Cutout- 20 PSIG
&
Discharge Pressure Cutout- 15 PSIG
0 Hz
Discharge Pressure Cutout- 13.5 PSIG
1 Hz
Discharge Pressure Cutout- 12 PSIG
2 Hz
Discharge Pressure Cutout- 10.5 PSIG
3 Hz
Discharge Pressure Cutout- 9 PSIG
4 Hz
Discharge Pressure Cutout- 7.5 PSIG
5 Hz
Discharge Pressure Cutout- 6 PSIG
6 Hz
Discharge Pressure Cutout- 4.5 PSIG
7 Hz
Discharge Pressure Cutout- 3 PSIG
8 Hz
Discharge Pressure Cutout- 1.5 PSIG
9 Hz
Discharge Pressure Cutout- 0 PSIG
10 Hz
