Lennox LGM Series Manual page 21

5-Filter Drier
LGM units have a filter drier located in the liquid line of each
refrigerant circuit. See figure 6. The drier removes contami­
nants and moisture from the system.
6-Low Pressure Switches S87, S88
The low pressure switch is an auto‐reset SPST N.O. switch
(held N.C. by refrigerant pressure) which opens on a pres­
sure drop. All units are equipped with this switch. The
switch is located in the compressor suction line and wired to
A55 unit controller.
A55 governs the low pressure switches by shunting the
switches during start up until pressure is stabilized. After
the shunt period, the control has a three‐strike counter, dur­
ing first thermostat demand, before the compressor is
locked out. The control is reset by breaking and remaking
the thermostat demand or manually resetting the control.
When suction pressure drops to 40 + 5 psig (276 ± 34 kPa),
(indicating low pressure), the switch opens and the com­
pressor(s) is(are) de-energized. The switch automatically
resets when pressure in the suction line rises to 90 + 5 psig
(620 ± 34 kPa) due to many causes such as refrigerant be­
ing added.
7-Condenser Fans B4 and B5
See SPECIFICATIONS tables at the front of this manual for
specifications of condenser fans used in all units. All con­
denser fans have single-phase motors. The fan assembly
may be removed for servicing and cleaning.
Units are equipped with electronically commutated con­
denser fan motors (ECM). The ECM motors are wired di­
rectly to 230VAC power but do not operate until a pulse
width modulated (PWM) control signal is sent from the A55
Unit Controller. The PWM signal determines the condenser
fan speed. Both fans will operate in low speed with a Y1 de­
mand and both will will operate in high speed with a Y2 de­
mand.
Transformer T5 and Fuse F57 460VAC & 575VAC only
460VAC and 575VAC units will use a Transformer T5 to
step-down the line voltage to the correct 230VAC. There
are two fuses F57 located next to the T5 transformer. The
location of the T5 transformer is behind the disconnect box
just above the bottom power entry cover.
Both low and high voltage plugs are located at the top of the
blower compartment in the indoor section of the unit. Con­
denser fan motors B4 and B5 high voltage plugs are J86,
and J87. Low voltage plugs are J336 and and J338 respec­
tively. Refer to wiring markings to identify plugs.
If an ECM fan is not operating:
1- Check to make sure high voltage is present before
checking low voltage.
2- Read the voltage at the appropriate high voltage fan
motor plug (J86 or J87) using the VAC meter setting.
3- If high voltage is present, check the low voltage plug
(J336 or J337) for a signal from the Unit Controller. Use
either the duty cycle (%) or a VDC meter setting.
Note - The VDC reading may fluctuate and is normal for a
PWM signal.
8-Temperature Sensors RT42, RT43, RT44,
RT45, RT46, RT47RT48 and RT49
Units are equipped with eight factory-installed thermistors
(RT42 - RT49) located on different points on the refrigerant
circuit.
The thermistors provide the Unit Controller with constant
temperature readings of four specific locations on the re­
frigeration circuit. These temperatures are used as feed­
back in certain modes of unit operation. In addition, the Unit
Controller uses these temperatures to initiate alarms such
as loss of condenser or evaporator airflow and loss of
charge. See figures 10, 11 and 12 for location.
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