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APPENDIX C
Low Ambient Option — Factory Installed
Units with the factory installed low ambient option are
equipped 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. 44 and
45) controls the speed of approved outdoor fan motors in
order to maintain a constant head pressure in the outdoor
coil. The control maintains the appropriate head pressure
at low ambient temperatures down to --20_F(--28_C).
Wind baffles are required to prevent wind cross currents
from causing abnormally low condensing temperatures.
S Use 20--gauge sheet metal to fabricate wind baffles (see
Fig. 46 and Table 14) and mounting brackets (see Fig.
47).
NOTE: Mounting brackets are for use on 15 ton model
units only.
S Install the wind baffles as show in Fig. 48, for 15 ton
units and Fig. 49, for 20 ton units .
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+/--
5_F, close 57_F+/--5_F (open 5.5_C+/--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.
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 loc ation
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 60_F
(16_C) outdoor ambient.
Normal operation
Cooling --- Center outdoor
fans (OFM2, OFM4) not on
above approximately 60_F
(16_C) 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 loc ation
Check speed control sensor
resistance
Check fan m oto r c apacito r
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
_F +/ --- 2 _F _C +/ --- 1_C Ohms, nominal
--- 22 --- 30 88350
--- 4 --- 20 48485
14 --- 10 27650
32 0 16325
50 10 9950
68 20 6245
77 25 5000
86 30 4028
104 40 2663
122 50 1801
140 60 1244
158 70 876
38AU