34
Demand Limit — Demand limit is a feature that allows
the unit capacity to be limited during periods of peak energy us-
age. Three types of demand limiting can be configured. The
first type is through 2-stage switch control, which will reduce
the maximum capacity to 2 user-configurable percentages. The
second type is by 4 to 20 mA signal input which will reduce the
maximum capacity linearly between 100% at a 4 mA input sig-
nal (no reduction) down to the user-configurable level at a
20 mA input signal. The third type uses the CCN Loadshed
module and has the ability to limit the current operating capaci-
ty to maximum and further reduce the capacity if required.
NOTE: The 2-stage switch control and 4 to 20-mA input sig-
nal types of demand limiting require the energy management
module (EMM).
For units with the digital compressor option, digital opera-
tion is ignored when determining capacity limit of the machine.
Since Demand Limit controls the number of compressors oper-
ating, the requested demand limit must allow for the corre-
sponding capacity of the full digital compressor capacity plus
any remaining compressors. For example, a 30MP045 unit
with a digital compressor will require a demand limit of at least
33% for the first compressor to be energized. No compressor
operation will be allowed prior to this demand limit level. Dig-
ital operation below 33% will require a demand limit of at least
33% to allow a compressor to start. Digital operation between
33 and 67% will require a demand limit of at least 67% to al-
low 2 compressors to be operating. Finally, for digital operation
above 67%, demand limit must be at 100% to allow for all
compressors to be operating.
To use demand limit, select the type of demand limiting to
use. Then configure the demand limit set points based on the
type selected.
DEMAND LIMIT (2-Stage Switch Controlled) — To con-
figure demand limit for 2-stage switch control, set the Demand
Limit Select (Configuration
RSET
DMDC) to 1. Then
configure the 2 Demand Limit Switch points (Configura-
tion
RSET
DLS1) and (Configuration
RSET
DLS2)
to the desired capacity limit. See Table 31. Capacity steps are
controlled by 2 relay switch inputs field wired to LVT as shown
in Fig. 4.
For demand limit by 2-stage switch control, closing the first
stage demand limit contact will put the unit on the first demand
limit level. The unit will not exceed the percentage of capacity
entered as Demand Limit Switch 1 set point (DLS1). Closing
contacts on the second demand limit switch prevents the unit
from exceeding the capacity entered as Demand Limit Switch
2 set point. The demand limit stage that is set to the lowest de-
mand takes priority if both demand limit inputs are closed. If
the demand limit percentage does not match unit staging, the
unit will limit capacity to the closest capacity stage.
To disable demand limit, configure DMDC to 0. See
Table 31.
EXTERNALLY POWERED DEMAND LIMIT (4 to
20 mA Controlled) — To configure demand limit for 4 to 20
mA control, set the Demand Limit Select (Configura-
tion
RSET
DMDC) to 2. Then configure the Demand
Limit at 20 mA (Configuration
RSET
DM20) to the
maximum loadshed value desired. Connect the output from an
externally powered 4 to 20 mA signal to terminal block LVT,
terminals 7 and 8 (+,–). Refer to the unit wiring diagram for
these connections to the optional/accessory energy manage-
ment module and terminal block. The control will reduce al-
lowable capacity to this level for the 20 mA signal. See
Table 31 and Fig. 23.
DEMAND LIMIT (CCN Loadshed Controlled) — To con-
figure Demand Limit for CCN Loadshed control set the De-
mand Limit Select (Configuration
RSET
DMDC) to 3.
Then configure the Loadshed Group Number (Configura-
tion
RSET
SHNM), Loadshed Demand Delta (Configu-
ration
RSET
SHDL), and Maximum Loadshed Time
(Configuration
RSET
SHTM). See Table 31.
The Loadshed Group number is established by the CCN
system designer. The ComfortLink controls will respond to a
Redline command from the Loadshed control. When the
Redline command is received, the current stage of capacity is
set to the maximum stages available. Should the loadshed con-
trol send a Loadshed command, the ComfortLink controls will
reduce the current stages by the value entered for Loadshed
Demand delta. The Maximum Loadshed Time is the maximum
length of time that a loadshed condition is allowed to exist. The
control will disable the Redline/Loadshed command if no
Cancel command has been received within the configured
maximum loadshed time limit.
Cooling Set Point (4 to 20 mA) — A field supplied
and generated, externally powered 4 to 20 mA signal can be
used to provide the leaving fluid temperature set point. Connect
the signal to LVT-10,8 (+,–). See Table 31 for instructions to
enable the function. Figure 24 shows how the 4 to 20 mA sig-
nal is linearly calculated on an overall 10 F to 80 F range for
fluid types (Configuration
OPT1
FLUD) 1 or 2. The set
point will be limited by the fluid (FLUD) type. Be sure that the
chilled water loop is protected at the lowest temperature.
CAUTION
Care should be taken when interfacing with other manufac-
turer’s control systems due to possible power supply
differences, full wave bridge versus half wave rectification.
The two different power supplies cannot be mixed.
ComfortLink controls use half wave rectification. A signal
isolation device should be utilized if a full wave bridge
signal generating device is used.
50% CAPACITY AT 20 mA
75% CAPACITY AT 12 mA
100% CAPACITY AT 4 mA
0
2
4
6
8
10
12
14
16 18
20
DEMAND LIMIT SIGNAL – 4 - 20 mA INPUT
100
80
60
40
20
0
MAX. ALLOWABLE LOAD (%)
Fig. 23 — 4 to 20-mA Demand Limiting
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