Low Ambient Fan Control Option; General; Fig. 13 - Typical Inverter Location; Fig. 14 - Inverter Enclosure - York YCAL Series Installation Operation & Maintenance

FORM 150.66-NM3
ISSUE DATE: 09/25/2020

LOW AMBIENT FAN CONTROL OPTION

General

The low ambient option consists of a VFD (Variable
Frequency Drive) for each system that controls the
speed of the first fan (Fan 1, Sys #1 or Fan 2, Sys # 2)
in the fan staging sequence. The VFD's are located in
an enclosure in the compressor/piping/heat exchanger
section on (2) compressor chillers. System # 1 VFD
is located on the bottom and System # 2 is on the top.
Single system chillers have the VFD installed on the
top of chiller above the compressor section. Examples
of the typical VFD location and enclosure mountings
are shown in Figs. 13 and 14.
FIG. 13 – TYPICAL INVERTER LOCATION
FIG. 14 – INVERTER ENCLOSURE
JOHNSON CONTROLS
The VFD will control fan speed when only a single fan
is running on a system. As discharge pressure rises and
falls, the fan speed will be increased from zero RPM to
full speed. As discharge pressure continues to rise, the
VFD will operate the fan at full speed and the second
fan will be brought on in a system, if needed. When-
ever the second fan is brought on, the inverter will al-
ready be running the first fan at full speed.
If discharge pressure falls, the chiller microproces-
sor will turn the second fan off by de-energizing the
fan contactor. If pressure continues to fall, VFD speed
will decrease in an effort to maintain discharge pres-
sure. Speed may drop to the point where the VFD turns
the fan completely off or virtually off with a continued
drop in pressure.
The VFD control input signal is from the discharge
pressure transducer in the respective system. The
transducer signal feeds both the chiller microprocessor
board input and the VFD.
The VFD will control the fan speed not only in low am-
bient conditions, but in all ambients based on discharge
pressure. Speed control of the respective system will
occur whenever high voltage power is applied to the
VFD power inputs through the 7M (Sys 1) or 10M (Sys
2) contactors. The chiller microprocessor will energize
the 7M and 10M contactors whenever the respective
system liquid line solenoid is energized.
The VFD controls the speed of the fan based on a
LD11298
discharge pressure setpoint and a differential control
range. When a compressor starts in a system, the in-
verter is activated through the 7M (Sys 1) or 10 M (Sys
2) contactor, which is controlled from the respective
liquid line solenoid valve control signal. At discharge
pressures below 160 psig, the VFD will turn the fan
off or speed will be reduced to small movements in fan
rotation.
The pressures indicated in this section
describing the VFD control will vary
from VFD to VFD. Expect tolerances
for the entire pressure range of control
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to potentially shift ­0 psig/+24 psig.
SECTION 8 - UNIT OPERATION
137
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