FORM 150.67-NM2 (209)
171
JOHNSON CONTROLS
8
General
T
he low ambient option consists of a VFD (Variable
Frequency Drive) for each system that controls the
speed of the
fi
rst 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. 31 and 32.
The VFD will control fan speed when only a single fan
is running on a system. As discharge pressure rises,
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. Whenever the
second fan is brought on, the inverter will already be
running the
fi
rst fan at full speed.
If discharge pressure falls, the chiller microprocessor
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 pressure.
Speed may drop to the point where the VFD turns the
fan completely off or virtually off with a continued drop
in pressure.
Single System VFD Enclosure Location
LD12080
FIG. 31 –
TYPICAL VFD ENCLOSURE LOCATIONS
Dual System VFD Enclosure Location
LD11298
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
ambient 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
discharge pressure setpoint and a differential control
range. When a compressor starts in a system, the
inverter 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 260 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 to potentially shift
-0 PSIG/+24 PSIG
.
YCAL0043 – 0066 LOW AMBIENT FAN CONTROL OPTION
Summary of Contents for YCAL0019
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