Carrier CAS072 Installation, Start-up And Service Instructions Manual

50 Specifications subject to change without notice. CAS-072-153-01SI Rev. AAPPENDIX C — LOW AMBIENT OPTIONUnits with the factory-installed low ambient control optionare equipped with a solid-state head pressure control whichregulates fan speed. A temperature sensor, mounted oneither circuit 1 (CAS072/091/121/151 units) or both circuits1 and 2 (CAS123/153 units) of the outdoor coil controls thespeed of approved outdoor fan motors in order to maintain aconstant head pressure in the outdoor coil (see Fig. O andFig. P). The control maintains the appropriate head pres-sure at low ambient temperatures down to –20°F (–28°C).Wind baffles are required to prevent wind cross currentsfrom causing abnormally low condensing temperatures.Use 20-gauge sheet metal to fabricate wind baffles (seeFig. Q and Table A).OPERATIONFan on/off control in cooling-only units is provided by anoutdoor fan relay (OFR). In cooling mode, fan motor speedof outdoor motors OFM1 and OFM2 is regulated by thespeed control temperature sensor on outdoor coil 1 for aminimum coil condensing temperature of approximately100°F (38°C) at higher outdoor ambient temperature and80°F (27°C) at lower ambient.To override the speed control for full fan speed operationduring service or maintenance, either:a. remove sensor and place in hot water >120°F(>49°C).orb. rewire to bypass control by connecting speed con-trol input and output power wires.TroubleshootingSpeed Control Sensor ResistanceIMPORTANT: Wind baffles are field-fabricated; theyare not included with the factory-installed Motormastercontrol option.OBSERVATION POSSIBLE REMEDYFans won’t start.All fans:Check power and wiringCheck outdoor fan relay (OFR)OFM1, OFM3 only:Check speed control sensor locationCheck speed sensor resistanceOFM2, OFM4 only:Check low ambient switch (LAS)Check low ambient relay (LAR)Cooling — Slow fan speedfor outer fans (OFM1, OFM2)at start or during low outdoorambientNormal operationCooling — Slow fan speedfor outer fans (OFM1, OFM2)above 85°F (29°F) outdoorambient (should be fullspeed)Check speed control sensor locationCheck speed control sensor resistanceCheck fan motor capacitorCooling — Motor current intospeed control is greater thanmotor nameplate FLANormal operationUp to 30% higher A at partial speed atlow ambientTEMPERATURE RESISTANCE°F ± –2°F °C ± –1°C Ohms, nominal–22 –30 88,350–4 –20 48,48514 –10 27,65032 0 16,32550 12 9,95068 20 6,24577 25 5,00086 30 4,028104 40 2,663122 50 1,801140 60 1,244158 70 876