44
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
REF.P
DP.A
and
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
UNIT
SIZE
), refrigerant type
(
Configuration
UNIT
RFG.T
), and condenser heat ex-
changer type (
Configuration
UNIT
CND.T
) are used to
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
®
V control. But the
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
REF.T
submenu. The equiva-
lent refrigerant pressure values,
DP.A
and
DP.B
, can be viewed
under the
Pressures
REF.P
submenu.
Head Pressure Configurations — There are two configura-
tions provided for head pressure control that can be found at
the local display:
Configuration
COOL
M.M.
(Motormaster Control)
Configuration
COOL
HPSP
(Head Pressure Setpoint)
Head Pressure Outputs — There are two condenser fan relays
used to control head pressure:
Condenser Fan A (
Outputs
FANS
CD.F.A
)
Condenser Fan B (
Outputs
FANS
CD.F.B
)
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
ECON
ACTV
= YES), mechanical
cooling may be delayed or even held off indefinitely.
NOTE: Once mechanical cooling has started, this delay logic
is no longer relevant.
Table 49 — Condenser Fan Staging for Motormaster Option Unit
FAN RELAY
48/50A UNIT SIZE
020-035
040-050
060
060 with MCHX
SCB2 - RELAY 1
OFM1
OFM1, OFM2
OFM1, OFM2
OFM3
OFC2 (MBB - RELAY 5)
OFM2
OFM3, OFM4
OFM3, OFM4, OFM5, OFM6
OFM2
SCB2 - RELAY 2
NA
NA
NA
OFM1
OFC4 (MBB - RELAY 6)
NA
NA
NA
OFM4
Содержание Carrier Weathermaker 48A2
Страница 105: ...105 Fig 20 Typical Main Control Box Wiring Schematic 48 50A2 A3 A4 A5 Units...
Страница 106: ...106 Fig 21 Typical Auxiliary Control Box Wiring Schematic...
Страница 107: ...107 Fig 22 Typical 2 Stage Gas Heat Wiring Schematic Size 060 Units Shown a48 8357...
Страница 108: ...108 TO NEXT PAGE Fig 23 Typical Staged Gas Heat Wiring Schematic Size 060 Units Shown A48 7296...
Страница 109: ...109 Fig 23 Typical Staged Gas Heat Wiring Schematic Size 060 Units Shown cont A48 8358...
Страница 110: ...110 Fig 24 Typical Electric Heat Control Schematic 50 Series Size 060 Units Shown a50 8228...
Страница 111: ...111 Fig 25 Typical Power Schematic 48 50A2 A3 A4 A5 060 Unit Shown...
Страница 112: ...112 Fig 26 Typical Low Ambient Controls Option Wiring...
Страница 113: ...113 Fig 27 Typical Small Chassis Component Location Size 020 035 Units...
Страница 114: ...114 Fig 28 Typical Large Chassis Component Locations Size 040 060 Units...
Страница 118: ...118 Fig 30 Economizer Control Board ECB1 and VAV Control Board ECB2 A48 7706...
Страница 142: ...142 A48 3733 Fig 56 Main Burner Removal...
Страница 176: ...176 APPENDIX C VFD INFORMATION cont Fig F Internal Enclosure Fan Replacement A48 7716...
