Carrier 38AU Appendix C, Low Ambient Option - Factory Installed, Operation, Troubleshooting

APPENDIX C

Low Ambient Option — Factory Installed

Units with the factory installed low ambient option are equipped with a MotormasterR 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.

SUse 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 location
	Check speed sensor resistance
	OFM2, OFM4 only:
	Check low ambient switch (LAS)
	Check low ambient relay (LAR)
Cooling --- Center outdoor	Normal operation
fans (OFM2, OFM4) off
below approximately 60_F
(16_C) outdoor ambient.
Cooling --- Center outdoor	Check low ambient switch (LAS)
fans (OFM2, OFM4) not on	Check low ambient relay (LAR)
above approximately 60_F
(16_C) 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		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

47