FORM 150.67-NM2 (209)
165
JOHNSON CONTROLS
8
General
The low ambient option consists of a single phase
Variable Frequency Drive (VFD) that controls the speed
of the
fi
rst fan (Fan 1) in the fan staging sequence. The
VFD is located in the control panel. An example of the
VFD location is shown in Fig. 22.
FIG. 22 –
TYPICAL VFD LOCATION
The VFD will control fan speed based on the liquid
temperature of the system. As liquid line temperature
rises and falls, the fan speed will operate between
minimum and full speed RPM.
As liquid temperature rises, the VFD will ramp the speed
of the fan from a minimum speed of about 200 Hz to
maximum RPM while attempting to control liquid line
temperature between 65-75° F. If the liquid temperature
is below 65-75° F, the VFD will shut off the motor even
though the VFD is powered by 7M.
The VFD control signal is sent from a liquid temperature
sensor connected to a condenser coil return bend. The
sensor is connected to S1 and COM terminals of the
VFD in the control panel. The sensor must always be
insulated. The location of the sensor is shown in Fig.
23.
50093b
BULB
FIG. 23 –
LIQUID LINE SENSING BULB LOCATION
50094
The VFD will not only control fan speed in low ambient
conditions, but in all ambients based on the liquid
line temperatute. Speed control of the fan will occur
whenever the liquid line solenoid is energized.
Potentiometer Confi guration
The VFD is pre-con
fi
gured from the factory prior to
shipping and should be ready for operation when the
chiller arrives on site. Potentiometers allow adjustment of
the minimum speed and for selection of the temperature
control range. A quick check of the potentiometer setting
is recommended. The potentiometer settings should be
in the position shown in Fig. 24.
FIG. 24 –
POTENTIOMETER SETTINGS
50095
YCAL0019-0028 LOW AMBIENT FAN CONTROL OPTION
VFD
Summary of Contents for YCAL0019
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