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For 25, 30, 50 and 60 ton units, the Saturated Condenser
OA Temp Suppression Setpoint (SCTOATSS) is a Human
Interface settable parameter (default: 40°F, range: 30 -
50°F, Disabled: above 50°F) which determines how many
condenser fan outputs will turn on.
Upon first compressor start on a circuit, all condenser fans
remain off until the SCT rises above the control band lower
limit. On 25, 30, 50 and 60 ton units, if the Outdoor Air
Temperature (OAT) is greater than or equal to the
SCTOATSS, or the SCTOATSS Function is disabled, all
condenser fan outputs will be energized. If the OAT is less
than the SCTOATSS, only Outputs B and C are energized.
On 20 and 40 ton units, all condenser fan outputs will be
energized.
When one or more condenser fan outputs are energized:
If the SCT rises above the SCTUL, for 20 and 40 ton units,
all condenser fan outputs are energized. For 25, 30, 50
and 60 ton units, if the OAT is less than the SCTOATSS
only one condenser fan stage will be added. If the OAT is
greater than or equal to SCTOATSS, or the SCTOATSS
Function is disabled, all condenser fan stages will be ener-
gized.
If the SCT falls below the SCTLL, for 20 and 40 ton units, if
both Outputs A and C are on, then Output A is de-ener-
gized. If only Output C was on, it remains on. For 25, 50
and 60 ton units, if both Outputs A and B are on, Output A is
de-energized.
(See Note 1)
If only Output B is on, Output B is de-
energized.
(See Note 2)
For 30 ton units, if both Outputs A and B
are on, Output A is de-energized.
is on, Output B is de-energized and Output A is energized.
(see Note 4)
If only Output A is on, Output A is de-energized.
Note 2)
If the SCT rises above the Saturated Condensing Tempera-
ture Efficiency Check Point (SCTECP), for 20 and 40 ton
units, no outputs are changed. For 25, 50 and 60 ton units,
if the OAT is less than the SCTOATSS, no outputs are
changed. If the OAT is greater than the SCTOATSS, or dis-
abled, if Output A and Output B are off, then Output B is en-
ergized. If Output B is on and Output A is off, no outputs
are changed. For 30 ton units, if the OAT is less than the
SCTOATSS, no outputs are changed. If the OAT is greater
than the SCTOATSS, or disabled, if Output A and Output B
are off, Output A is de-energized. If Output A is on and Out-
put B is off, Output B is energized and Output A is de-ener-
gized. If Output B is on and Output A is off, no outputs are
changed.
Notes:
1. If the SCT remains below the SCTLL for 30 seconds after
the stage change, then Output B will be de-energized. At
this time, only the fan (under the low ambient damper)
controlled by Output C will be on.
2. At this time, only the fan (under the low ambient damper)
controlled by Output C will be on.
3. If the SCT remains below the SCTLL for 30 seconds after
the stage change, then Output A will be energized and
Output B will be de-energized. If the SCT remains below
the SCTLL for an additional 30 seconds after the stage
change, then Output A will be de-energized. At this time,
only the fan (under the low ambient damper) controlled
by Output C will be on.
4. If the Sat Cond. Temp. remains below the SCTLL for an
additional 30 seconds after the stage change, then Out-
put A will be de-energized. At this time, only the fan (un-
der the low ambient damper) controlled by Output C will
be on.
To prevent rapid fan cycling, minimum condenser fan
stage on and off time is 5.2 seconds, except when all
compressor stages on a circuit are de-energized for
any reason, the condenser fan stages on that circuit
(see Note 3)
If only Output B
(see
General Information
are de-energized immediately without regard to the
condenser fan minimum on time. Also, if four fan
stage changes occur within a ten minute period, then
SCT is controlled to SCTLL minus "Saturated Con-
denser Temp Control Band Temporary Low Limit Sup-
pression" for a period of one hour.
In a low ambient condition, adding or subtracting a full fan
stage provides excess capacity. To provide head pressure
control in low ambient conditions, the UCM controls SCT to
the "Saturated Condensing Temperature Low Ambient Con-
trol Point" by modulating the capacity of a dedicated con-
denser fan stage between 0 and 100% via the Low Ambient
Fan Control Actuator output
Normally, the head pressure control allows unit operation
down to 30°F. With the addition of a low ambient damper,
This feature allows for low ambient compressor operation
down to 0°F.
The low ambient control algorithm is active on all units, re-
gardless of whether the unit is configured as having low
ambient damper option.
High Pressure Controls (Standard)
High Pressure controls are located on the discharge lines
near the scroll compressors. They are designed to open
when the discharge pressure approaches the unit specific
pressure setting. The controls reset automatically when the
discharge pressure decreases to the unit specific pressure
setting. However, the compressors on that circuit are locked
out and a manual reset diagnostic is initiated.
Low Ambient Control (Optional)
The low ambient modulating output on the compressor
module is functional on all units with or without the low am-
bient option.
Anytime the unit is powered-on, this output is operational,
regardless if the compressors are ON or OFF. Once the
condenser fan operation is started, the condenser fan lo-
cated under this damper will remain ON until all compres-
sors on that circuit are turned OFF. These dampers will
modulate to maintain the saturated condensing temperature
to the "Saturated Condensing Temperature Control Point"
and the "Low Ambient Control Deadband" which are located
under the Human Interface setup screens for Head Pres-
sure Control.
Status/Alarm Output (Standard)
Is an internal function within the CGM control module that
provides;
a. diagnostic signals to the Human Interface Alarm
LED.
b. control of the binary Alarm output.
c. control of the binary outputs on the GBAS module to
inform the customer of the operational status and/or
diagnostic conditions.
Compressor Circuit Breakers (Standard)
The Scroll Compressors are protected by circuit breakers
which interrupt the power supply to the compressors if the
current exceeds the breakers "must trip" value and opens a
set of auxiliary contacts in the control circuit.
When the Compressor Module (MCM) detects the open
auxiliary compressor contacts, it turns any operating
compressor(s) on that circuit "Off", locks out all compressor
operation for that circuit, and initiates a manual reset diag-
nostic.
7
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