57
Sizes 088 and 104: The SCT is used to control the condenser
(outdoor) fan motor(s) (OFM) controlled by the head pressure
control relay (MMR) for each circuit. See Table 52 for fan
control points. If outdoor fan is energized in addition to
Motormaster® control, then Motormaster control will be
locked in the energized mode.
All Sizes: The 2 other stages of head pressure control are con-
trolled by the SCT on standard units, or the SCT and suction
transducers on units equipped with suction pressure transduc-
ers and suction sensors. Table 52 shows the fan configurations
and lists the on and off points for OFC1 and OFC2 (OFA and
OFB on sizes 088 and 104).
Table 52 also describes the fan sequence of operation and
defines the particular fans controlled by stage.
Table 51A — Head Pressure/Fan Cycling Control (034-078 Sizes)
CONTROL LOGIC
CONTROL OUTPUTS
LEGEND
UNIT CONFIGURATION STAGE OFM ON OFM OFF
STANDARD (with standard SCT sensors)
(MMAS = Yes)
(TRNS = No)
1 SCT > (HPSP – 15 F)
SCT < (HPSP – 37 F) for 90 secs
AND Stage 2 motors OFF
2
SCT > HPSP
(start delayed 60 secs after start of
compressor, unless SCT > 143 F)
SCT < (HPSP – 35 F) for 120 secs
With Accessory Sensors
(Pressure Transducers)
(MMAS = Yes)
(TRNS = Yes)
1 SCT > 138 F
SCT < (HPSP – 37 F) for 90 secs
AND Stage 2 motors OFF
2
SCT > HPSP
(start delayed 60 secs after start of
compressor, unless SCT > 143 F)
SCT < (HPSP – 35 F) for 120 secs
Motormaster Control Disabled
(MMAS = No)
1
On with compressor Off with compressor
2
SCT > HPSP SCT < (HPSP – 35 F) for 120 secs
UNIT SIZES
FAN STAGE/CIRCUIT
NO.
DEVICE/CHANNEL RELAY CONTACTOR
034-038
Stage 1/Common PSIO-1/13 MM MMC
Stage 2/Common DSIO-1/29 — OFC1
044,048
Stage 1/Common PSIO-1/13 MM MMC
Stage 2/Circuit 1 DSIO-2/29 — OFC1
Stage 2/Circuit 2 DSIO-2/30 — OFC2
054-078
Stage 1/Common PSIO-1/13 MM MMC1, MMC2
Stage 2/Circuit 1 DSIO-2/29 — OFC1
Stage 2/Circuit 2 DSIO-2/30 — OFC2
DSIO
—
Relay Module
HPSP
—
Head Pressure Set Point
MM
—
Motormaster Device
MMC
—
Motormaster Contactor
OFC
—
Outdoor-Fan Contactor
OFM
—
Outdoor-Fan Motor
PSIO
—
Processor Module
SCT
—
Saturated Condensing Temperature
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