8
COOLING SYSTEM OPTIONS (CONT”D)
Head Pressure Control
Low ambient control. Cycling the condenser fan or fans in response to compressor discharge pressure will permit stable
operation in ambients down to 32F. The operating pressure switch is adjustable to match customer needs; it is factory set
to re-energize the fans when discharge pressure drops to 295 psig and energize them when pressure increases to 410
psig.
Variable Speed Control
The VARISPEED™ Fan Control System controls the compressor discharge pressure. The speed control module
responds to discharge pressure; it speeds the condenser fan up as pressure rises and slows the fan down as pressure
falls due to load conditions or as outdoor ambient temperature falls.
Head pressure control is accomplished with one or two variable speed condenser fan drives and controller, factory set to
begin fan rotation at 311 psig and be at full fan speed at 410 psig.
Dual compressor, dual fan units have one variable speed motor and controller on each circuit.
Single or tandem compressor, four fan units have one variable speed motor and control and three 3 phase constant
speed motors. Two adjustable pressure controls are used on the 3 fans, the first operating one fan between 443 psig and
358 psig, the second operating two fans between 460 psig and 375 psig.
CAUTION: Pressure settings on the constant speed fans must NOT be set so as to permit operation below 358 psig or
the variable speed motor may stall and overheat.
Single compressor, single fan units will have one variable speed motor and control.
At low ambient, the variable speed fan operates, increasing in speed until maximum RPM is achieved at or around 45°F
ambient. An adjustable pressure switch operates the constant speed three-phase fan set to energize the motor at 443
psig and de-energize at 358 psig. In the ambient temperature span of approximately 50°F to 53°F, the variable speed fan
will ramp between maximum and minimum speed while the constant speed fan cycles. The start-stop cycle varies from
45 seconds to 2½ minutes during this period.
At 53°F, both fans are operating; the variable speed at minimum RPM and the constant speed at full RPM. As the ambient
continues to rise, the variable speed motor increases to full speed and remains there.
If the application calls for a closer setting between maximum and minimum pressure settings on the constant speed fan,
for example 443 psig on, 388 psig off, the effect will be to lengthen the temperature span during which the cycling takes
place, for example 50°F to 57°F.
Adjustable High and Low Pressure Switches
Standard cooling units are equipped with non-adjustable pressure switches. The low pressure switch is set to open at
135± psig and is an automatic reset switch closing at 99± psig. The high pressure switch is set to open at 640± psig and
is a manual reset switch set to close at 595± psig.
Units can be equipped with adjustable high and low pressure switches for those installations that require finer settings
than the non-adjustable switches provide. Low pressure switches have both set point and differential adjustments and are
automatic reset. High pressure switches must not be set above 640 psig and are manual reset. Set point and differential
are adjustable.
MECHANICAL ADJUSTMENTS
SET FAN RPM.
All evaporator motor sheaves are set when tested and shipped from the factory. Actual rpm must be set and verified with
a tachometer. Refer to the following Blower Performance Chart for basic unit fan rpm.
With disconnect switch open, place a jumper wire across Terminals R and G at TS1 Terminal Block. Close disconnect
switch; evaporator fan motor will operate so rpm can be checked.
FAN ROTATION CHECK
Check that fan rotates clockwise when viewed from the drive side of unit and in accordance with rotation arrow shown on
blower housing. If it does not, reverse two incoming power cables at TB Terminal Block.
Do not attempt to change load side wiring. Internal wiring assures all motors will rotate in correct direction once
evaporator fan motor rotation check has been made.
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