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Head Pressure Control Operation — Condenser head pres-
sure control for the 48/50A Series rooftops is controlled direct-
ly by the unit, except when the unit is equipped and configured
for Motormaster
®
V control. The control is able to cycle up to
three stages of outdoor fans (see Table 49) to maintain accept-
able head pressure.
For 48/50A2,A3,A4,A5 units, fan stages react to discharge
pressure transducers (DPT) (Pressures
DP.B) 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
REF.T
SCT.A and SCT.B).
Unit size (Configuration
(Configuration
UNIT
RFG.T), and condenser heat ex-
changer type (Configuration
determine if the second stage fans are configured to respond to
a particular refrigerant circuit (independent control) or both re-
frigerant circuits (common control). The 48/50A2,A3,A4,A5
060 units with microchannel (MCHX) condenser heat ex-
changers are the only units that utilize independent fan
controls.
If the unit is equipped with the accessory Motormaster V
control, the Motormaster installed configuration (Configura-
tion
COOL
M.M.) must be set to YES, if the unit size
(SIZE) = 60 tons. This is because "Condenser fan relay A"
must be energized to enable Motormaster
60 ton offers 3 stages of head pressure 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.
The SCT.A and SCT.B sensors, which are connected to the
condenser coils in circuit A and B, will be used to measure the
saturated condensing temperature and may be used to control
head pressure. The saturated condensing temperatures can be
viewed in the Temperatures
lent refrigerant pressure values, DP.A and DP.B, can be viewed
under the Pressures
REF.P submenu.
Table 49 — Condenser Fan Staging for Motormaster Option Unit
FAN RELAY
SCB2 - RELAY 1
OFC2 (MBB - RELAY 5)
SCB2 - RELAY 2
OFC4 (MBB - RELAY 6)
REF.P
DP.A and
UNIT
SIZE), refrigerant type
UNIT
CND.T) are used to
®
V control. But the
REF.T submenu. The equiva-
020-035
OFM1
OFM2
NA
NA
Head Pressure Configurations — There are two configura-
tions provided for head pressure control that can be found at
the local display:
Configuration
Configuration
Head Pressure Outputs — There are two condenser fan relays
used to control head pressure:
Condenser Fan A (Outputs
Condenser Fan B (Outputs
Head Pressure Algorithm — The following logic will de-
scribe the head pressure control routine when any compressor
has been commanded on:
• CD.F.A = ON
• If the highest active circuit SCT is above the HPSP:
CD.F.B = ON
• If OAT is above 75 F: CD.F.A = ON and CD.F.B = ON
(until OAT temperature drops below 73 F or the compres-
sors are turned off)
• If the SCT on an active circuit drops 40 F below the HPSP
for 2 minutes: CD.FB = OFF*
* For 60 ton size units not configured for Motormaster control,
the control stages down differently than the other units.
Because the condenser fan relays each turn on a different num-
ber of outdoor fans, the control, when staging down will first
turn off condenser fan relay A and then in 2 more minutes will
turn off relay B and turn back on relay A.
The details of fan staging are summarized in Fig. 9 for each
scenario.
Failure Mode Operation — If either of the SCT sensors has
failed, then the control defaults to control based on the OAT
sensor and turns on CD.F.B when the ambient is above 65 F
and off when the ambient temperature is below 50 F. If the
SCT and OAT sensors have all failed then the control turns on
CD.F.B when compressors are on.
ECONOMIZER INTEGRATION WITH MECHANICAL
COOLING — When the economizer is able to provide free
cooling (Run Status
cooling may be delayed or even held off indefinitely.
NOTE: Once mechanical cooling has started, this delay logic
is no longer relevant.
48/50A UNIT SIZE
040-050
OFM1, OFM2
OFM3, OFM4
OFM3, OFM4, OFM5, OFM6
NA
NA
44
COOL
M.M. (Motormaster Control)
COOL
HPSP (Head Pressure Setpoint)
FANS
CD.F.A)
FANS
CD.F.B)
ECON
ACTV = YES), mechanical
060
OFM1, OFM2
NA
NA
060 with MCHX
OFM3
OFM2
OFM1
OFM4
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