Page 43
SCA120 SEQUENCE OF OPERATION
Power:
1. Line voltage from unit disconnect energizes transformer T1. T1 provides 24VAC power to
terminal strip TB34. TB34 provides 24VAC to the unit cooling, heating and blower controls.
2. TB13 is also energized when unit disconnect switch closes. TB13 provides line voltage to
compressors crankcase heaters, compressor contactors, the blower motor contactor
(relay in MSAV units) and condenser fan relay.
Blower Operation:
3. The main control module receives a demand from thermostat terminal G. A55 energizes
blower contactor K3 (relay K203 in MSVAt units See Page 46 and or Page 47) with
24VAC.
4. Blower B3 is energized.
Economizer Operation:
5. The economizer control module A56 receives a demand and energizes exhaust fan relay
K65 with 24VAC at 50% outside air damper open (adjustable).
6. N.O. K65−1 and N.O. K65−2 both close, energizing exhaust fan motor B10.
1st Stage Cooling (compressor B1)
7. First stage cooling demand energizes Y1 and G in the thermostat. G energizes blower.
8. 24VAC is routed through TB34 to the main control module A55. After A55 proves N.C. low
pressure switch S87, N.C. freezestat S49 and N.C. high pressure switch S4, compressor
contactor K1 is energized.
9. N.O. contacts K1−1 close energizing compressor B1. Thermostat S40 opens de−energiz-
ing crankcase heater HR1.
10. N.O. low ambient switch S11 closes to energize condenser fan relay K10.
11. N.O. contacts K10−1 close energizing condenser fan B4.
2nd Stage Cooling (compressor B2 is energized)
12. Second stage cooling demand energizes Y2.
13. 24VAC is routed through TB34 to compressor 2 module A57. After A57 proves N.C. low
pressure switch S88, N.C. freezestat S50 and N.C. high pressure switch S7, compressor
contactor K2 is energized.
14. N.O. contacts K2−1 close energizing compressor B2. Thermostat S162 opens de−energiz-
ing crankcase heater HR2.
15. A57 energizes condenser fan relay K68. N.O. contacts K68−1 and K68−2 close energizing
condenser fans B5 and B21.
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