JOHNSON CONTROLS
168
FORM 150.67-NM2
ISSUE DATE: 01/31/2019
SECTION 8 – UNIT OPERATION
YCAL0033 Low Ambient Fan Control Option
General
The low ambient option consists of a VFD (Variable
Frequency Drive) that controls the speed of the îš¿rst
fan (Fan 1) in the fan staging sequence. The VFD is
located on the top of chiller above the compressor sec-
tion. Examples of the typical VFD location and enclo-
sure mountings are shown in Figure 49 on page 168
and Figure 50 on page 168.
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. Whenev-
er the second fan is brought on, the inverter will already
be running the first fan at full speed. When discharge
pressure falls, the chiller microprocessor will turn the
second fan off by de-energizing the fan contactor.
FIGURE 49 - TYPICAL VFD ENCLOSURE LOCATIONS
VFD Enclosure Location
LD12080
VFD Enclosure
LD12081
FIGURE 50 - TYPICAL VFD ENCLOSURE
CONFIGURATIONS
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 com-
pletely 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 and the VFD. The VFD controls the fan speed
based on discharge pressure.
The VFD will control the fan speed not only in low
ambient conditions, but in all ambients based on dis-
charge pressure. Speed control of the respective system
will occur whenever high voltage power is applied to
the VFD power inputs through the 7M contactor. The
chiller microprocessor will energize the 7M and 10M
contactors whenever the 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 in-
verter is activated through the 7M 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 all but 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 poten-
tially shift -0 PSIG/+24 PSIG.
The VFD will ramp up the speed of the fan as pres-
sure rises above the low end of the speed control range.
Throughout the pressure control range, the VFD con-
trols the speed of the fan based on a discharge pres-
sure in the range of approx 260-292 PSIG. At pressures
above 292 PSIG, the VFD will run the system fan at
full speed.
As pressure drops below 292 PSIG, the VFD will
slow the speed of the fan to try to maintain discharge
pressure within the control range. The VFD will try
to maintain pressure in the range of 260-292 PSIG by
raising and lowering the speed of the fan. If pressure
drops below 260 PSIG, the VFD will virtually turn the
system fan completely off. Some slight fan movement
or very slow rotation may be noted, although the fan
may appear to stop completely.
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