44
APPENDIX C
Low Ambient Option — Factory Installed
Units with the factory installed low ambient option are
equippe d with a Motormaster
R
solid--state head pressure
control which regulates fan speed. A temperature sensor,
mounted on circuit 1 of the outdoor coil (see Figs. 35
through 38) c ontrols the speed of a pproved outdoor fan
motors in order to maintain a constant head pressure in the
outdoor coi l. T he control maintains the appropriate head
pressure at low ambient temperatures down to --28_C
(--20_F).
Wind baffles are required t o prevent wind cross currents
from causing abnormall y low condensing temperatures.
Use 20--gauge sheet metal to fabricate wind baffles (see
Fig. 39 and Table 16).
Operation —
Fan on/off control in c ooling--only units (38AUZ,
38AUD) is provided by an outdoor fan relay (OFR).
In cooli ng 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 38_C (100_F)
at higher outdoor ambient temperature and 27_C(80_ F) at
lower ambient. Additionall y, 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 open
5.5_C+/--2.8_C, close 13.9_C+/--2.8_C(42_F+/--5_F,
close 57_F+/--5_F).
To override the speed control for full fan speed operation
during service or maintenance, either:
a. remove sensor and place in hot wat er >49_C
(>120_F), or
b. rewire to bypass control by connecting speed
control input and output power wires.
Troubleshooting —
OBSERVATION POSSIBLE REMEDY
Fans won’t start All fans:
Check power & 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)
Cooling --- Center outdoor
fans (OFM2, OFM4) off
below approximately
16_C(60_F) outdoor am-
bient.
Normal operation
Cooling --- Center outdoor
fans (OFM2, OFM4) not on
above approximately 16_C
(60_F) outdoor ambient
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Cooling --- Slow fan speed
for outer fans (OFM1,
OFM3) at start or during
low outdoor ambient
Normal operation
Cooling --- Slow fan speed
for outer fans (OFM1,
OFM3) above 85_F(29_F)
outdoor ambient (should
be full speed)
Check speed control sensor location
Check speed control sensor
resistance
Check fan motor capacitor
Cooling --- motor current
into speed control is
greater than motor name-
plate FLA
Normal operation
Up to 30% higher A at partial speed
at low ambient
Speed Control Sensor Resistance —
TEMPERATURE RESISTANCE
_C + / --- 1 _C _F + / --- 2 _F_ Ohms, nominal
--- 3 0 --- 2 2 88350
--- 2 0 --- 4 48485
--- 1 0 14 27650
0 32 16325
10 50 9950
20 68 6245
25 77 5000
30 86 4028
40 104 2663
50 122 1801
60 140 1244
70 158 876
38AU
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