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APPENDIX C — LOW AMBIENT OPTION, FACTORY INSTALLED
Units with the factory installed low ambient option are
equipped with a Motormaster
trol which regulates fan speed. A temperature sensor, mount-
ed on either circuit 1 (38AUZ units) or circuit 2 (38AUD
units) of the outdoor coil controls the speed of approved out-
door fan motors in order to maintain a constant head pressure
in the outdoor coil (see Fig. A and Fig. B). The control main-
tains the appropriate head pressure at low ambient tempera-
tures down to –20°F (–28°C).
Wind baffles are required to prevent wind cross currents
from causing abnormally low condensing temperatures.
Use 20-gauge sheet metal to fabricate wind baffles (see
Fig. C and Table A).
OPERATION
Fan on/off control in cooling-only units (38AUZ, 38AUD) is
provided by an outdoor fan relay (OFR). In cooling mode, fan
motor speed of outdoor motors OFM1 and OFM3 is regulated
by the speed control temperature sensor on outdoor coil 1 for a
minimum coil condensing temperature of approximately 100°F
(38°C) at higher outdoor ambient temperature and 80°F (27°C)
at lower ambient. Additionally, outdoor fan motor OFM2 and
OFM4 are turned on/off by the low ambient temperature switch
(LAS), operating the low ambient relay (LAR). The LAS control
temperatures are open 42°F
2.8°C, close 13.9°C
2.8°C).
To override the speed control for full fan speed operation
during service or maintenance, either:
a.
remove sensor and place in hot water >120°F (>49°C).
or
b.
rewire to bypass control by connecting speed control
input and output power wires.
®
solid-state head pressure con-
5 °F, close 57°F
5°F (open 5.5°C
Troubleshooting
OBSERVATION
Fans won't start.
Cooling — Center outdoor
fans (OFM2, OFM4) off
below approximately 60°F
(16°C) outdoor ambient.
Cooling — Center outdoor
fans (OFM2, OFM4) not on
above approximately 60°F
(16°C) outdoor ambient.
Cooling — Slow fan speed
for outer fans (OFM1,
OFM3) at start or during low
outdoor ambient
Cooling — Slow fan speed
for outer fans (OFM1,
OFM3) above 85°F (29°F)
outdoor ambient (should be
full speed)
Cooling — motor current
into speed control is greater
than motor nameplate FLA
Speed Control Sensor Resistance
TEMPERATURE
°F ± –2°F
–22
–4
14
32
50
68
77
86
104
122
140
158
43
POSSIBLE REMEDY
All fans:
Check power and wiring
Check outdoor fan relay (OFR)
OFM1, OFM3 only:
Check speed control sensor location
Check speed sensor resistance
OFM2, OFM4 only:
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check speed control sensor location
Check speed control sensor
resistance
Check fan motor capacitor
Normal operation
Up to 30% higher A at partial speed at
low ambient
RESISTANCE
°C ± –1°C
Ohms, nominal
–30
88,350
–20
48,485
–10
27,650
0
16,325
12
9,950
20
6,245
25
5,000
30
4,028
40
2,663
50
1,801
60
1,244
70
876
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