Page 45
Sequence of Operation SCA240/SCB288
Power:
1. Line voltage from TB2 or CB10, energizes transformer
T1 and T18. Transformer T1 provides 24VAC power to
terminal strip TB34 and T18 provides 24VAC power to
terminal strip TB35. The two terminal strips provide
24VAC power to the unit cooling, heating and blower
controls and thermostat.
2. Terminal strip TB13 is also energized when the unit dis-
connect closes. TB13 supplies line voltage to com-
pressor crankcase heaters, compressors, blower
motors, and fan motors.
Blower Operation (OCP input must be on):
3. The main control module A55 receives a demand from
thermostat terminal G. A55 energizes blower contactor
K3 (relay K203 in MSAVt 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% (travel) outside air damper open (adjustable).
6. N.O. K65-1 and K65-2 both close, energizing exhaust
fan motors B10, B11 and B12.
1st Stage Cooling
7. First stage cooling demand energizes Y1 and G in the
thermostat. G energizes blower, if blower is not already
running (see step 3).
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, com-
pressor contactor K1 is energized.
9. N.O. contacts K1−1 close energizing compressor B1. Ther-
mostat S40 opens de−energizing crankcase heaters HR1
and HR2.
10. Main control A55 energizes condenser fan contactor K10.
11. N.O. contacts K10−1 close energizing condenser fan B4
12. Simultaneous with step 8, 24VAC is routed through the
compressor 2 control 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.
13. N.O. contacts K2−1 close energizing compressor B2.
14. Compressor 2 control module A57 energizes condenser
fan 2 relay K68.
15. N.O. contacts K68−1 close energizing condenser fan B21.
2nd Stage Cooling
16. Second stage cooling demand energizes Y2.
17.24VAC is routed through TB35 to compressor 3 and 4
module A59. After A59 proves N.C. low pressure
switches S98 and S97, N.C. freezestats S53 and S95,
and N.C. high pressure switches S28 and S96, com-
pressor contactors K14 and K146 are energized.
18. N.O. contacts K14−1 close energizing compressor B13.
Thermostat S162 opens de−energizing crankcase heaters
HR5 and HR11.
19. N.O. contacts K146−1 close energizing compressor B20 .
20. N.O. low ambient pressure switches S11 and S84 close to
energize condenser fan relay K149.
21.N.O. contacts K149−1 close energizing condenser fan
B21.
22. N.O. low ambient switches S85 and S94 close energizing
condenser fan relay K150.
23.N.O. contacts K150−1 close energizing condenser fan
B22.
24.A59 energizes condenser fan relays K152 and K153.
25.K152−1 and K153−1 closes energizing condenser fans
B23 and B24.
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