Low Ambient Option - Factory Installed - Carrier 38AUD Installation, Start-Up And Service Instructions Manual

APPENDIX C
Low Ambient Option — Factory Installed
Units with the factory installed low ambient option are
equipped with a Motormaster
control which regulates fan speed. A temperature sensor,
mounted on circuit 1 of the outdoor coil (see Figs. 35
through 38) 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 - - 28_C
(- - 20_F).
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. 39 and Table 16).
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 38_C (100_F)
at higher outdoor ambient temperature and 27_C (80_F) 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 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 water >49_C
(>120_F), or
b. rewire to bypass control by connecting speed
control input and output power wires.
R
solid- - state head pressure
Troubleshooting —
OBSERVATION POSSIBLE REMEDY
Fans won't start All fans:
OFM1, OFM3 only:
OFM2, OFM4 only:
Cooling --- Center outdoor Normal operation
fans (OFM2, OFM4) off
below approximately
16_C (60_F) outdoor am-
bient.
Cooling --- Center outdoor Check low ambient switch (LAS)
fans (OFM2, OFM4) not on Check low ambient relay (LAR)
above approximately 16_C
(60_F) outdoor ambient
Cooling --- Slow fan speed Normal operation
for outer fans (OFM1,
OFM3) at start or during
low outdoor ambient
Cooling --- Slow fan speed Check speed control sensor location
for outer fans (OFM1, Check speed control sensor
OFM3) above 85_F (29_F) resistance
outdoor ambient (should Check fan motor capacitor
be full speed)
Cooling --- motor current Normal operation
into speed control is Up to 30% higher A at partial speed
greater than motor name- at low ambient
plate FLA
Speed Control Sensor Resistance —
TEMPERATURE
_C +/---1_C _F +/---2_F_
--- 30 --- 22
--- 20 --- 4
--- 10 14
0 32
10 50
20 68
25 77
30 86
40 104
50 122
60 140
70 158
44
Check power & wiring
Check outdoor fan relay (OFR)
Check speed control sensor location
Check speed sensor resistance
Check low ambient switch (LAS)
Check low ambient relay (LAR)
RESISTANCE
Ohms, nominal
88350
48485
27650
16325
9950
6245
5000
4028
2663
1801
1244
876