United Technologies Carrier TRANSICOLD 68RM35-604-20 Operation And Service page 15

rises, the switch will re-open. Refer to Table 1-2 for
switch settings.
Unloader pressure switch UPS3 closes on
pressure rise to energize unloader valve UV2.
Energizing UV2 will place the compressor in four
cylinder operation. As pressure drops, the switch
will re-open. Refer to Table 1-2 for switch settings.
Energizing of unloader valve UV1 can also be
accomplished through
LOAD/UNLOAD switch.
e. Relays
Fault Relay (FR)
The fault relay (FR) is plugged into the relay board
on the electrical control panel located in the rear
compartment of the unit. (See Figure 1-3.) The FR
relay is energized during initial start-up of the unit.
If the high or low pressure switch (HPS or LPS)
opens due to unsafe operating conditions, the
fault relay (FR) de-energizes and closes an
internal set of FR contacts to energize the ACSR.
A/C Stop Relays (ACSR1 and ACSR2)
The A/C stop relays (ACSR1 and ACSR2) are
plugged into the relay board on the electrical
control panel located in the rear compartment of
the unit. (See Figure 1-3.). The ACSR relay
energizes the A/C stop light when activated by the
fault relay (FR).
To restart the unit and turn the A/C stop light off,
the A/C switch must be toggled off and on.
Clutch Relay (CR)
The clutch relay (CR) is plugged into the relay
board on the electrical control panel located in the
rear compartment of the unit. (See Figure 1-3.)
When the CR relay is energized, a set of internal
contacts will close to activate the compressor
clutch (CL) to start the refrigerant cycle.
Cool Disable Relay (CDR)
The cool disable relay (CDR) coil is de-energized
opening a set of normally open CR contacts in the
clutch relay circuit whenever the evaporator fans
are shut down; this prevents the compressor from
operating when the fans are not operating.
Evaporator Fan Relays (EFR1 and EFR2)
Evaporator fan relays (EFR1 and EFR2) are
installed on the electrical control panel located in
the rear compartment of the unit. (See Figure 1-3.)
When the A/C Mode Switch (ACMS) is placed in
T-288
the controller
1-8
the ON position, EFR1 and EFR2 relays are
energized. When energized, a set of internal
EFR1 and EFR2 contacts are closed to start the
evaporator fan motors.
Evaporator Overload Relays (OR1 and OR2)
Evaporator overload relays (OR1 and OR2) are
plugged into the relay board on the electrical
control panel located in the rear compartment of
the unit. (See Figure 1-3.) These relays are
energized at the same moment the evaporator fan
relays are. The OR1 and OR2 relays, when
energized, will close a set of internal contacts to
energize the evaporator speed relay (ESR).
Evaporator Speed Relay (ESR)
The evaporator speed relay (ESR) is installed on
the electrical control panel located in the rear
compartment of the unit. (See Figure 1-3.) This
relay will energize when the thermostat calls for
low speed evaporator fan operation. When
energized, the internal contacts will open
removing a third ground path, which puts the fans
in low speed operation. When the ESR coil is
de-energized, the internal contacts close, which
adds the third ground path and puts the fans in
high speed operation.
Condenser Fan Relays (CFR1 and CFR2)
The condenser fan relays (CFR1 and CFR2) are
installed on the electrical control panel located in
the rear compartment of the unit. (See Figure 1-3.)
When the A/C Mode Switch (ACMS) is placed in
the ON position and the thermostat is calling for
cooling, CFR1 and CFR2 are energized. When
energized, a set of internal CFR1 and CFR2
contacts are closed to start the condenser fan
motors. The low ambient thermostat (LATH) must
be closed to energize the condenser fan relays.
Condenser Speed Relay (CSR1)
The condenser speed relay (CSR1) is installed on
the electrical control panel located in the rear
compartment of the unit. (See Figure 1-3.) This
relay is energized at the same moment that the
A/C Mode Switch (ACMS) is placed in the ON
position, provided that the condenser fan switch
(CFS) is in the closed position and the thermostat
is calling for cooling. If the condenser coil pressure
reaches the CFS switch cutout setting, the CFS
switch will open to de-energize condenser speed
relay CSR1. (Refer to section 1.2 for switch
settings.); this will cause the condenser fan motors
(CM1 and CM2) to run at high speed.