York YCAV0157 Installation Operation & Maintenance page 183

Suction Pressure Load Limiting/Unloading
Suction pressure load limiting helps to protect the
evaporator from freezing. A system is permitted to load
normally as long as the Suction Pressure is above the
Suction Pressure Cutout + 2 PSIG. Between Cutout +
2 PSIG and the Cutout, loading is inhibited, even though
increased loading is required. Between the Suction
pressure Cutout and Suction Pressure Cutout – 10 PSIG,
forced unloading is performed every 2 seconds accord-
ing to TABLE 8 below. This situation would occur if
the suction pressure cutout transient override control is
in effect (See Low Suction Pressure Cutout, Page 217).
The suction pressure cutout is programmed under the
Program key. The default Suction Pressure Cutout is
set at 24.0 PSIG.
TABLE 8 - SUCTION PRESSURE
LOAD LIMITING/UNLOADING
SUCTION PRESSURE
Suction Pressure is between
Cutout +2 PSIG
&
Suction Pressure Cutout
Suction Pressure Cutout- 1 PSIG
Suction Pressure Cutout- 2 PSIG
Suction Pressure Cutout- 3 PSIG
Suction Pressure Cutout- 4 PSIG
Suction Pressure Cutout- 5 PSIG
Suction Pressure Cutout- 6 PSIG
Suction Pressure Cutout- 7 PSIG
Suction Pressure Cutout- 8 PSIG
Suction Pressure Cutout- 9 PSIG
Suction Pressure Cutout- 10 PSIG
Suction pressure load limiting is active at start-up, to
only prevent loading of the compressors. Suction pres-
sure limit unloading will not occur until the system run
time reaches 5 minutes of operation to allow the system
to stabilize.
JOHNSON CONTROLS
LOAD LIMITING CONTROL (CON'T)
UN-
LOADING
0 Hz
1 Hz
2 Hz
3 Hz
4 Hz
5 Hz
6 Hz
7 Hz
8 Hz
9 Hz
10 Hz
VSD Internal Ambient Temperature
Load Limiting
VSD Internal Ambient temperature limiting helps pre-
vent the unit from tripping on the high internal cabinet
temperature safety. A system is permitted to load nor-
mally as long as the VSD Internal Ambient is below the
VSD Internal Ambient Cutout – 3°F. Between VSD
Internal Ambient Cutout – 3°F and the VSD Internal
Ambient Cutout – 2°F, loading is inhibited, even though
increased loading is required. Between the VSD Internal
Ambient Cutout – 2°F and the VSD Internal Ambient
Cutout, forced unloading is performed every 2 seconds
according to TABLE 9 below. The VSD Internal Ambi-
ent Safety Cutout is 158°F.
TABLE 9 - VSD INTERNAL AMBIENT
LOAD LIMITING/UNLOADING
VSD INTERNAL AMBIENT
TEMPERATURE
Internal Ambient Temp. is between
Cutout- 3ºF
&
Internal Ambient Cutout- 2ºF
Internal Ambient Cutout- 1.8ºF
Internal Ambient Cutout- 1.6ºF
Internal Ambient Cutout- 1.4ºF
Internal Ambient Cutout- 1.2ºF
Internal Ambient Cutout- 0ºF
Internal Ambient Cutout- 0.8ºF
Internal Ambient Cutout- 0.6ºF
Internal Ambient Cutout- 0.4ºF
Internal Ambient Cutout- 0.2ºF
Internal Ambient Cutout
FORM 201.21-NM1 (1223)
7
UN-
LOADING
0 Hz
1 Hz
2 Hz
3 Hz
4 Hz
5 Hz
6 Hz
7 Hz
8 Hz
9 Hz
10 Hz
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