Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 55

Two-Switch Demand Limiting (DM.L.S = 1)
This type of demand limiting utilizes 2 discrete inputs:
• Demand Limit Switch 1 Setpoint (D.L.S1) — Dmd Limit
Switch Setpoint 1 (0 to 100% total capacity)
• Demand Limit 2 Setpoint (D.L.S2) — Dmd Limit Switch
Setpoint 2 (0 to 100% total capacity)
The state of the discrete switch inputs can be found at the local
display:
Inputs  GEN.I  DL.S1
Inputs  GEN.I  DL.S2
The following table illustrates the demand limiting (Run Sta-
tus  COOL  DEM.L) that will be in effect based on the logic
of the applied switches:
Switch Status
 
DL.S1 = OFF
Inputs GEN.I
 
Inputs GEN.I DL.S2 = OFF
 
Inputs GEN.I DL.S1 = ON
 
DL.S2 = OFF
Inputs GEN.I
 
DL.S1 = ON
Inputs GEN.I
 
DL.S2 = ON
Inputs GEN.I
 
Inputs GEN.I DL.S1 = OFF
 
Inputs GEN.I DL.S2 = ON
4-20 mA Demand Limiting (DM.L.S = 2) — If the unit has
been configured for 4 to 20 mA demand limiting, then the In-
puts  4-20  DML.M value is used to determine the amount of
demand limiting in effect (Run Status  COOL  DEM.L).
The Demand Limit at 20 mA (D.L.20) configuration must be
set. This is the configured demand limit corresponding to a 20
mA input (0 to 100%).
The value of percentage reset is determined by a linear interpo-
lation from 0% to "D.L.20"% based on the Inputs  4-
20  DML.M input value.
The following examples illustrate the demand limiting (Run
Status  COOL  DEM.L) that will be in effect based on
amount of current seen at the 4 to 20 mA input, DML.M.
D.L.20 = 80% D.L.20 = 80%
DML.M = 4 mA DML.M = 12 mA
DEM.L = 100% DEM.L = 90%
CCN Loadshed Demand Limiting (DM.L.S = 3) — If the unit
has been configured for CCN Loadshed Demand Limiting,
then the demand limiting variable (Run Status  COOL 
DEM.L) is controlled via CCN commands.
The relevant configurations for this type of demand limiting are:
Loadshed Group Number (SH.NM) — CCN Loadshed Group
number
Loadshed Demand Delta (SH.DL) — CCN Loadshed Demand
Delta
Maximum Loadshed Time (SH.TM) — CCN Maximum Load-
shed time
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).
 
Run Status COOL DEM.L = 1
100%
 
Configuration DMD.L D.L.S1
 
Configuration DMD.L D.L.S2
 
Configuration DMD.L D.L.S2
D.L.20 = 80%
DML.M = 20 mA
DEM.L = 80%
A redline command or loadshed command will stay in effect
until a Cancel redline or Cancel loadshed command is received
or until the configurable Maximum Loadshed Time (SH.TM)
has elapsed.
HEAD PRESSURE CONTROL
Condenser head pressure for the 48/50N Series is managed di-
rectly by the ComfortLink controls. The controls are able to cy-
cle up to 9 stages of outdoor fans to maintain acceptable head
pressure. Fan stages will be turned on or off in reaction to dis-
charge pressure sensors with the pressure converted to the cor-
responding saturated condensing temperature.
An option to allow fan speed control (Motormaster
first stage is configured
 COOL  M.M = Yes.
There are 5 configurations provided for head pressure control
that can be found at the local display:
Configuration  COOL  M.M (Motormaster enable)
Configuration  M.PID  SCT.H (Maximum Condensing Temp)
Configuration  M.PID  SCT.L (Minimum Condensing Temp)
There are up to 5 outputs provided to control head pressure:
Outputs  FANS  CDF.1 — Condenser Fan Output 1
Outputs  FANS  CDF.2 — Condenser Fan Output 2
Outputs  FANS  CDF.3 — Condenser Fan Output 3
Outputs  FANS  CDF.4 — Condenser Fan Output 4
Outputs  FANS  CDF.5 — Condenser Fan Output 5
The specific staging sequence for a unit depends on 3 factors:
the unit size (tonnage), which refrigeration circuits are current-
ly operating, and whether or not Motormaster control is en-
abled. See Fig. 6-8 for fan staging sequencing.
The condenser fan output controls outdoor fan contactors and
outdoor fans for each unit tonnage as shown in Fig. 6. Each
stage of fans is also shown. The ComfortLink controller adds
or subtracts stages of fans based on SCT.H and SCT.L. When
the SCT rises above SCT.H, a fan stage will be added. The
ComfortLink controller will continue to add a fan stage every
10 seconds thereafter if the SCT remains above SCT.H. If SCT
rises above 130°F, the controller will turn on the maximum fan
stages for the unit. When the SCT drops below the SCT.L, a
fan stage will be subtracted. The ComfortLink controller will
continue to drop a fan stage every 2 minutes thereafter if the
SCT remains below SCT.L.
When a condenser fan output is common to both refrigeration
circuits, in other words, when the fan(s) will affect both Circuit
A and Circuit B, the following logic is used: in order to add a
fan stage, the SCT of either circuit must be above SCT.H for
30 seconds and in order to subtract a stage, the SCT of both cir-
cuits must be below SCT.L for 30 seconds.
Whenever the outdoor ambient temperature (OAT) is above
70°F, the maximum stage will always be on when the compres-
sors are on.
On the initial start-up of a circuit, the condenser fans will start
5 seconds prior to the compressor starting in order to ensure
proper head pressure of the compressor immediately at start-
up. After the compressor starts, the normal head pressure rou-
tine will begin 30 seconds after the condenser fan pre-start.
What stage fans starts depends on the outdoor ambient tem-
perature. The 3 situations are:
OAT < 50°F
50 F < OAT < 70°F
OAT > 70°F
See Fig. 6-8 for what stage of fans starts for each scenario.
55
) on the
®
by setting Configuration