39
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. 32 and
33) 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. 35 and Table 13) and mounting brackets (see Fig.
36).
NOTE
: Mounting brackets are for use on 15 ton model
units only.
S
Install the wind baffles as show in Fig. 37, for 15 ton
units and Fig. 38, 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 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 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 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
_
F +/---2
_
F
_
_
C +/---1C
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
38A
U
Q
Summary of Contents for 38AUQ 16 Series
Page 4: ...4 C10675 Fig 2 38AUQ 25 Unit Dimensions 38AUQ...
Page 19: ...19 C10784 Fig 16 38AUQ 16 Charging Chart 38AUQ...
Page 20: ...20 C10785 Fig 17 38AUQ 25 Charging Chart 38AUQ...
Page 21: ...21 C10786 Fig 18 38AUQ 16 Power Schematic 208 230 3 60 shown 38AUQ...
Page 22: ...22 C10787 Fig 19 38AUQ 25 Power Schematic 208 230 3 60 shown 38AUQ...
Page 23: ...23 C10788 Fig 20 38AUQ 16 25 Control Schematic 38AUQ...
Page 41: ...41 C10363 Fig 35 Wind Baffles Fabrication 38AUQ...
Page 42: ...42 C10366 Fig 36 15 Ton Wind Baffle Brackets Fabrication 38AUQ...
Page 44: ...44 RIGHT BACK LEFT FRONT C10368 Fig 38 Wind Baffle Installation 20 Ton Units 38AUQ...
Page 45: ...45 38AUQ...
