Deadband Multiplier; Demand Limit; 2-Stage Switch Controlled Demand Limit; Configuring 2-Stage Demand Limit - Carrier 38APS025 Manual

DEADBAND MULTIPLIER — The
Deadband Multiplier (Configuration SLCT Z.GN) has a
default value of 1.0. The range is 1.0 to 4.0. When set to a
value other than 1.0, this factor is applied to the Capacity
Load/Unload Factor (Run Status VIEW LOD.F). The
configuration affects the cycling rate of the cooling stages by
raising or lowering the threshold that capacity Load/Unload
Factor must build to in order to add or subtract a stage of cool-
ing. The larger this value is set, the longer the control will
delay between adding or removing stages of capacity.
Normally this configuration should not require any tuning
or adjustment. If there is an application where the unit may be
significantly oversized and there are indications of high com-
pressor cycles, then the Deadband Multiplier (Z.GN) can be
used to adjust the overall logic gain. As the value of (Z.GN) is
increased, the cycling of cooling stages will be slowed.
DEMAND LIMIT — Demand limit is a feature that allows
the unit capacity to be limited during periods of peak energy
usage. Depending on the load profile for the space and the
demand limit placed on the machine, the unit may not be able
to satisfy the cooling requirements while demand limit is
active.
For units with a digital compressor, digital operation is ig-
nored when determining capacity limit of the machine. Since
Demand Limit controls the number of compressors operating,
the requested demand limit must allow for the corresponding
capacity of the full digital compressor capacity plus any re-
maining compressors. For example, a 38APS040 unit with a
digital compressor will require a demand limit of at least 33%
for the first compressor to be energized. No compressor opera-
tion will be allowed prior to this demand limit level. Digital op-
eration 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 allow 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. For other unit capacity steps, see
Tables 39 and 40.
If Demand Limit is active, the control will indicate an Oper-
ating Mode of Demand Limited, Operating Modes
MODE MD15=YES.
Four 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
signal (no reduction) down to the user configurable level at a
ITEM
C.TYP Machine Control Type
DMDC Demand Limit Select
DLS1 Demand Limit Switch 1
DLS2 Demand Limit Switch 2
user configurable
Table 41 — Configuring 2-Stage Demand Limit
CONFIGURATION OPT2
EXPANSION
Range: 1 to 9
1 (VAV)
3 (Tstat Multi)
4 (Tstat 2stg)
5 (SPT Multi)
7 (PCT Cap)
8 (Dual Tstat)
9 (VAV Setpoint)
Default: 4
Value must not be set to 8.
CONFIGURATION RSET
0=None (Default)
1=Switch
2=4 to 20 mA Input
3=CCN Loadshed
Range: 0 to 100%
Default: 80%
Range: 0 to 100%
Default: 50%
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. The
fourth type utilizes communications to write directly to the de-
mand limit point.
NOTE: The 2-stage switch control and 4 to 20 mA input signal
types of demand limiting require the energy management mod-
ule (EMM).
To use demand limit, select the type of demand limiting to
use. Then configure the demand limit set points based on the
type selected.

2-Stage Switch Controlled Demand Limit

NOTE: Single or 2-Stage Switch Controlled Demand Limit is
not available when using Dual Thermostat Control Type,
C.TYP=8.
To configure 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 (Con-
figuration RSET DLS1 and DLS2) to the desired capacity
limit. See Table 41. Capacity steps are controlled by 2 relay
switch inputs field-wired to low voltage terminal (LVT) strip
terminal 3-6. Refer to the specific control type wiring diagrams
or unit wiring diagram for these connections. Demand Limit
Switch status can be monitored at the unit's scrolling marquee,
Demand Limit Switch 1 and Demand Limit Switch 2 (In-
puts GEN.I DLS1 and DLS2) respectively.
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. Closing contacts
on the second demand limit switch prevents the unit from ex-
ceeding the capacity entered as Demand Limit Switch 2 set
point. The demand limit stage that is set to the lowest demand
takes priority if both demand limit inputs are closed. If the de-
mand limit percentage does not match unit staging, the unit
will limit capacity to the closest capacity stage.
As an example, 2-stage demand limit is planned with De-
mand Limit Switch 1 to allow the unit to operate to 80% capac-
ity. Demand Limit Switch 2 is to allow the unit to operate to
50% capacity. See Table 41 for programming requirements.
When Demand Limit Switch 1 closes, the unit's capacity will
not exceed 80%. If Demand Limit Switch 2 closes, the unit's
capacity will not exceed 50%. The Operating Mode, Demand
Limited (MD15) will remain active as long as either Demand
Limit Switch 1 or Demand Limit Switch 2 is closed.
COMMENTS
115
EXAMPLE VALUE
1, 3, 4, 5, 7, or 9
1
80%
50%