46
The Loadshed Group Number (SH.NM) corresponds to the
loadshed supervisory device that resides elsewhere on the CCN
network and broadcasts loadshed and redline commands to its
associated equipment parts. The SH.NM variable will default to
zero which is an invalid group number. This allows the loadshed
function to be disabled until configured.
Upon reception of a redline command, the machine will be pre-
vented from starting if it is not running. If it is running, then
DEM.L is set equal to the current running cooling capacity (Run
Status
COOL
C.CAP).
Upon reception of a loadshed command, the DEM.L variable is
set to the current running cooling capacity (Run Status
COOL
C.CAP) minus the configured Loadshed Demand
Delta (SH.DL).
A redline command or loadshed command will stay in effect
until a Cancel redline or Cancel loadshed command is re-
ceived, or until the configurable Maximum Loadshed time
(SH.TM) has elapsed.
HEAD PRESSURE CONTROL
Head pressure refers to the refrigerant pressure at the discharge
side of the compressor. Thus it is sometimes refers to as “dis-
charge pressure.” Head pressure control for will be managed di-
rectly by the ComfortLink controls (no third party control).
The head pressure control stages fixed speed fans and modulating
fans, if available, to maintain the head pressures of circuit A and
circuit B within acceptable ranges. For controls purpose, the head
pressures are converted to saturated condensing temperatures
(SCTs) as the feedback information to the condenser fans (also re-
ferred to as “outdoor fans”). SCT.A is the saturated condensing
temperature for refrigeration Circuit A, and SCT.B is the saturated
condensing temperature for refrigeration Circuit B. There are a to-
tal of up to 6 condenser fans (depending on unit size and installed
options) for controlling the head pressures of the 2 refrigeration
circuits, of which up to 3 fans can be controlled by each VFD
(variable frequency drive) upon installation option.
The control described in this document is also referred to as
condenser fan control. Where Greenspeed
®
control is involved,
it may also be referred to as low ambient control.
The Greenspeed/low ambient or the factory-installed Motormaster
control will be directly implemented in the ComfortLink soft-
ware. It is not compatible with the field-installed Motormaster V
control as found in CESR131343-07-xx and earlier software ver-
sions that used an accessory with part numbers
CRLOWAMB018A00 through CRLOWAMB026A00. These
field-installed accessory Motormaster V speed controls are com-
pletely self-contained control units and are not controlled by the
unit’s ComfortLink controller. On 48/50A 060 RTPF units with 6
fan motors, the Motormaster control configuration (M.M.) must
be set to YES for this field-installed option.
Head Pressure Control Operation
Condenser head pressure control for the 48/50A Series roof-
tops is controlled discretely by the unit, except when the unit is
equipped and configured for Greenspeed/low ambient or facto-
ry-installed Motormaster Option control where head pressures
would be controlled via factory-installed VFDs. For a unit with
the factory-installed Motormaster option, the control would be
able to cycle up to three stages of outdoor fans (see Table 49)
to maintain acceptable head pressure.
For 48/50A units, fan stages react to discharge pressure transduc-
ers (DPT) (Pressures
REF.P
DP.A and DP.B) which are
connected to the compressor discharge piping in circuit A and B.
The control converts the pressures to the corresponding saturated
condensing temperatures (Temp eratures
REF.T
SCT.A and
SCT.B).
Unit size (Configuration
UNIT
SIZE), refrigerant type
(Configuration
UNIT
RFG.T), and condenser heat ex-
changer type (Configuration
UNIT
CND.T) are used to
determine if the second stage fans are configured to respond to
a particular refrigerant circuit (independent control) or both re-
frigerant circuits (common control). The 48/50A 060 units
with microchannel (MCHX) condenser heat exchangers are the
only units that utilize independent fan controls.
If the unit is equipped with the optional Greenspeed / low ambi-
ent control, the Outdoor VFD installed configuration (Configu-
ration
COOL
OV.EN) must be set to YES, to fully utilize
head pressure control function for optimal operations.
The SCT.A and SCT.B sensors, which are connected to the
condenser coils in circuit A and B, will be used to measure the
saturated condensing temperature and may be used to control
head pressure. The saturated condensing temperatures can be
viewed in the Temperatures
REF.T submenu. The equiva-
lent refrigerant pressure values, DP.A and DP.B, can be viewed
under the Pressures
REF.P submenu.
Head Pressure Configurations
There are two configurations provided for head pressure con-
trol that can be found at the local display based on the option
being installed in the factory:
Configuration
COOL
OV.EN (Outdoor VFD Enabled)
and Configuration
COOL
LASP for units with Green-
speed/low ambient option
Configuration
COOL
HPSP (Head Pressure Setpoint)
Head Pressure Outputs
There are two condenser fan relays used to control head pres-
sure for standard non-outdoor VFD option units:
Condenser Fan A (Outputs
FANS
CD.F.A)
Condenser Fan B (Outputs
FANS
CD.F.B)
For units with Greenspeed/low ambient option, the head pres-
sure would be controlled by the Outdoor Fan VFD A and B
(Outputs
FANS
OV.A / OV.B).
For units with factory installed Motormaster option, the head
pressure would be controlled by the Motormaster VFD:
MotorMastr Fan Circuit A (Outputs
FANS
MM.F.A)
MotorMastr Fan Circuit B (Outputs
FANS
MM.F.B)
MotorMastr Fan Command A (Outputs
FANS
A.VFD)
MotorMastr Fan Command A (Outputs
FANS
B.VFD)
Head Pressure Algorithm
The following logic will describe the head pressure control
routine when any compressor has been commanded on:
• CD.F.A = ON
• If the highest active circuit SCT is above the HPSP:
CD.F.B = ON
• If OAT is above 75°F: CD.F.A = ON and CD.F.B = ON
(until OAT temperature drops below 73°F or the compres-
sors are turned off)
• If the SCT on an active circuit drops 20°F below the HPSP
for 2 minutes: CD.FB = OFF*
* For 60 ton size units not configured for Greenspeed/low
ambient control or factory-installed Motormaster option,
the control stages down differently than the other units.
Because the condenser fan relays each turn on a different
number of outdoor fans, the control, when staging down
will first turn off condenser fan relay A and then in 2 more
minutes will turn off relay B and turn back on relay A.
The details of fan staging are summarized in Fig. 8 and 9 for
each scenario.
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