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LGH092/152U M3 SEQUENCE OF OPERATION
Power:
1- Line voltage through the S48 unit disconnect, TB2 terminal block, or CB10 circuit breaker energizes the T1 trans
former. T1 provides 24VAC power to A55 Unit Controller which provides 24VAC to the unit cooling, heating and
blower controls.
2- Line voltage is also routed to compressor crankcase heaters, compressor contactors, supply air inverter control,
condenser fan relays and exhaust fan relays.
Blower Operation:
Supply Air Inverter: Refer to Supply Air Inverter or Direct Drive blower diagram and sequence of operation.
Economizer Operation:
3- A55 receives a demand and energizes exhaust fan relay K65 with 24VAC at 50% outside air damper open (ad
justable).
4- N.O. K65-1 and N.O. K65-2 both close, energizing exhaust fan motor B10.
1st Stage Cooling (compressor B1 or B2)
5- A55 receives a Y1 thermostat demand.
6- After A55 proves N.C. low pressure switch S87, N.C. freezestat S49 and N.C. high pressure switch S4, compres
sor contactor K1 or K2 are energized. Note - A55 logic (using input from RT37 and RT38 temperature sensors and
A185 pressure transducer) determines which contactor is energized.
7- N.O. contacts K1-1 or K2-1 close energizing compressor B1 or B2.
At the same time A55 energizes:
All three condenser fans, B4, B5 & B21, on LOW speed.
K191 compressor 1 crankcase heater relay when K1 is energized or K194 compressor 2 crankcase heater relay
when K2 is energized (after A55 proves N.O. low ambient switch S11 is closed).
8- N.C. K191-1 compressor 1 crankcase heater contacts or N.C. K194-1 compressor 2 crankcase heater contacts
open and de-energize compressor crankcase heater HR1 or HR2.
2nd Stage Cooling (compressor B1 and B2 are energized)
9- A55 receives a Y2 thermostat demand.
10-The K1 or K2 compressor contactor which was not energized will close.
11-N.O. K1-1 or K2-1 relay contacts which were not energized will close. The corresponding B1 or B2 compressor will
operate in tandem with the other compressor.
At the same time A55 energizes:
All three condenser fans, B4, B5 & B21, on HIGH speed.
The K191 or K194 crankcase heater relay which was not energized will close. N.C. K191-1 or K194-1 relay con
tacts open and de-energize the corresponding crankcase heater HR1 or HR2.
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