Lennox LGM Series Manual page 56

Sequence of Operation LGM/LCM092U/150U
Power:
1. Line voltage through the TB13 terminal block the T1 transformer. 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:
Refer to 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 switches S87, S88 and N.C. high pressure switches S4, S7 compressor con­
tactor K1 or K2 are energized. Note - A55 logic (using input from RT42, RT43, RT44, RT45, RT46, RT47 , RT48 and
RT49 temperature sensors) 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:
Both condenser fans, B4, B5 on LOW speed.
8. N.C. K191-1 compressor 1 crankcase heater contacts or N.C. K2 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:
Both condenser fans, B4 and B5 on HIGH speed.
The K191 or K2 crankcase heater relay which was not energized will close, de-energize the corresponding crank­
case heater HR1 or HR2.
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