ITEM
DMD.L
DM.L.S
D.L.20
SH.NM
SH.DL
SH. TM
D.L.S1
D.L.S2
Table 63 -- Demand Limit Configuration
EXPANSION
DEMAND LIMIT CONFIG.
Demand Limit Select
Demand Limit at 20 ma
Loadshed Group Number
Loadshed Demand Delta
Maximum Loadshed Time
Demand Limit Sw.1 Setpt.
Demand Limit Sw.2 Setpt.
I RANGE I CCN POINTI UNITS
0-3 %
O- 100
0 - 99
0 - 60 %.
O- 120 I mm
0 - 100 '/o
0 - 100 %
I DEFAULT
DMD CTRL i
DMT2-OMA O0
SHED NUM
SHED DEL
SHED TIM _
DLSWSP1 80
DLSWSP2
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
Loadshed 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
cognnands to its associated equipment parts. The SH.NM
variable will default to zero which is an invalid group num-
ber. This allows the loadshed function to be disabled until
configured.
Upon reception of a redline cognnand, the machine will be
prevented from starting if it is not running. If it is running,
then DEM.L is set equal to the current running cooling capac-
ity (Run Status _COOL _C CAP).
Upon reception of a loadshed colmnand, the DEM.L vari-
able is set to the current running cooling capacity (Run Status
_COOL_(LCAP) minus the configured Loadshed Demand
Delta (SH.DL).
A redline colmnand or loadshed colmnand will stay in
effect until a Cancel redline or Cancel loadshed colmnand is
received, or until the configurable Maximum Loadshed time
(SH. TM) has elapsed.
HEAD PRESSURE CONTROL -- Condenser head pressure
control for the 48/50A series rooftops is controlled directly by
the unit, except when the unit is equipped and configured for
Motormaster(R) V control. The control is able to cycle up to
three stages of outdoor fans (see Table 64) to maintain accept-
able head pressure.
For 48/50AJ,AK,AW, AY units, fan stages will react to sam-
rated condensing temperature (SCT) sensors (Tempera-
tures--+REET--+SCT.A and SCT.B) which are connected to
the condenser coils in circuit A and B. The control converts the
temperatures to the corresponding refrigerant pressures (Pres-
sures--cREEP -+DRA and DRB).
For 48/50A2,A3,A4,A5 units, fan stages react to discharge
pressure transducers (DPT) (Pressures--cREEP-+DRA and
DRB) which are connected to the compressor discharge piping
in circuit A and B. The control converts the pressures to the
corresponding saturated condensing temperatures (Tempera-
tures--+REE T--+SCT.A and SCT.B).
Unit size (Configuration-cUnit-cSize), refrigerant type
(Configuration-+Unit-+RFG1), and condenser heat ex-
changer type (Configuration-+Unit-+(ND. 1) are used to de-
termine if the second stage fans are configured to respond to a
particular refrigerant circuit (independent control) or both re-
frigerant circuits (colmnon control). The 48/50A2,A3,
A4,A5060 units with microchannel (MCHX) condenser heat
exchangers are the only units that utilize independent fan
controls.
If the unit is equipped with the accessory Motormaster V
control, the Motormaster installed configuration
((bnfiguration--cCOOL--_M.M.) must be set to YES if the
unit size ((bnfiguration--+Unit--cSize) is 60 tons and the con-
denser heat exchanger type ((bnfiguration--cUnit--cCND.1)
is RTPF (round robe plate fin). This is because the condenser
fan relay A (MBB Relay 6) output must be energized to enable
the Motormaster V control and must not be mined off by the
head pressure control algorithin. The size 60 ton unit with
RTPF condenser heat exchangers offers 3 stages of head pres-
sure control and is the one case where condenser fan relay A
may be requested off during head pressure control operation.
By configuring M.M. to YES, the control is instructed not to
turn off the relay to attempt 3 stages of head pressure control.
There are two configurations provided for head pressure
control that can be found at the local display:
• (bnfiguration _COOL--_M.M. - Motor Master (bntrol?
• (bnfiguration-+COOL--cHPSP- Head Pressure Set-
point
There are two outputs (MBB Relays) provided to control
head pressure:
• Outputs-cFANS-cCD.EA - Condenser Fan Circuit A
(MBB Relay 6 - OFC1,4). For size 60 ton units with
MCHX condensers, MBB - Relay 6 drives OFC4 and com-
pressor contactor B 1or B2 auxiliary contacts drive OFC 1.
• Outputs-cFANS-cCD.EB - Condenser Fan Circuit B
(MBB Relay 5 - OFC2)
Head Pressure Control Operation -- The following logic de-
scribes the head pressure control routines for the unit sizes out-
lined in Table 64.
For 020 to 035 size units, there are two outdoor fans that are
colrnnon to both refrigerant circuits. The control cycles two
stages of outdoor fans, one fan per stage, to maintain accept-
able head pressure.
For 036 to 050 size units, there are four outdoor fans that are
colrnnon to both refrigerant circuits. The control cycles two
stages of outdoor fans, two fans per stage, to maintain accept-
able head pressure.
For 051 and 060 size units - There are six outdoor fans that
are colrnnon to both refrigerant circuits (size 060 MCHX units
have 4 fans). The control cycles three stages of outdoor fans,
two fans for stage one, four fans for stage two, and six fans for
stage three to maintain acceptable head pressure.
When a compressor has been colrnnanded on, then con-
denser fan A (MBB Relay 6) will be energized (CD.EA =
ON). Condenser fan A will remain on until all compressors
have been cognnanded off. If the highest active circuit SCT is
above the HPSP or if OAT is greater than 75 F then condenser
fan B (MBB Relay 5) will be energized (CD.EB = ON). Con-
denser fan B will remain on until all compressors have been
colrnnanded off, or the highest active circuit SCT drops 40 F
below the HPSP for greater than 2 minutes and OAT is less
than 73 E
NOTE: For size 60 units with RTPF condenser heat exchang-
ers not configured for Motormaster control, the control stages
down differently than the other units. For these units, the con-
trol will first turn off condenser fan relay A. After 2 minutes,
the control will turn offrelay B and turn back on relay A.
49
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