Carrier 38APS025 - 4 To 20 Ma Demand Limit, Demand Limit Select DMDC=2; 4-20 Ma Controlled Demand Limit; Configuring 4 to 20 Ma Demand Limit

116
4-20 mA Controlled Demand Limit
— To configure demand
limit for 4 to 20 mA control, set the Demand Limit Select
(Configuration
o
RSET
o
DMDC) to 2. Then configure the
Demand Limit at 20 mA (Configuration
o
RSET
o
DM20) to
the maximum loadshed value desired. Connect the output from
an externally powered 4 to 20 mA signal to terminal block
LVT strip terminals 7 (+) and 8 (-). The external signal can be
monitored at the scrolling marquee, 4-20 mA Demand Signal
(Inputs
o
4-20
o
DMND). Refer to the specific control type
wiring diagrams or unit wiring diagram for these connections.
The control will reduce allowable capacity to this user-pro-
grammed level for the 20 mA signal. See Table 42 and
Fig. 145.
The 4-20 mA Input Demand Limit example (Fig. 145) will
allow the unit to operate at full capacity if required with
DMND =4 mA or 100% Demand Limit. At DMND=20 mA
the unit is not allowed to operate, with a Demand Limit of 0%.
Between DMND=4 mA and DMND=20 mA a proportional
demand limit amount will be applied to the machine.
Table 42 — Configuring 4 to 20 mA Demand Limit
CAUTION
To avoid unit damage, care should be taken when interfac-
ing with other manufacturer’s control systems due to
power supply differences, full wave bridge versus half
wave rectification. The two different power supplies cannot
be mixed. ComfortLink controls use half wave rectifica-
tion. A signal isolation device should be utilized if a full
wave bridge signal generating device is used.
CONFIGURATION
o
RSET
ITEM DISPLAY COMMENTS EXAMPLE
DMDC Demand Limit Select
0=None (Default)
1=Switch
2=4 to 20 mA Input
3=CCN Loadshed
2
DM20 Demand Limit at 20 mA
Range: 0 to 100%
Default: 100%
0%
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
DM20 = 0%
Demand Limit at 20 mA
Unit allowed to
operate at full capacity
with a 4 mA signal
Unit allowed to operate
at 50% capacity with a
12 mA signal
DEMAND LIMIT RESPONSE (%)
4 TO 20 mA DEMAND LIMIT INPUT SIGNAL, DMND
A38-7288
Fig. 145 — 4 to 20 mA Demand Limit, Demand Limit Select DMDC=2