CRANKCASE HEATERS
It is important that the crankcase heaters be energized 8 hours
before starting the compressors. To energize the crankcase
heaters, the room thermostat or status panel system switch
must be open to prevent the compressor from starting. If a
disconnect switch is installed outside the unit, turn it to “ON”.
The nonfused disconnect (optional) located in the unit’s main
supply panel must also be turned “ON”.
CAUTION: Do not attempt to start the compressors without at
least 8 hours of crankcase heat or compressor
damage will occur.
SAFETY AND SERVICE FEATURES
The control circuit includes the following safety features:
1. The supply air blower motor is protected with manual reset
starter overload protectors.
2. The condenser fan motors have inherent protection with
automatic reset.
3. The primary winding of transformer 1T is protected by fuse
12FU. The secondary winding and the 115 volt control
circuit are protected by fuse 13FU.
4. All safety controls in the 115 volt circuit have the “return” or
“common” side of the 1T transformer grounded. Fuse 13FU
will “BLOW” whenever a dangerous condition occurs. The
unit casing is also grounded.
5. The wiring to each compressor motor, each condenser fan
motor and the supply air blower motor are individually fused
according to the National Electrical Standards.
6. The compressor(s) are protected by a 100 watt immersion
type crankcase heater(s). Heater(s) are energized when-
ever power is supplied to the unit. When the compressor(s)
are energized, the heater(s) are turned off.
CAUTION: Do not attempt to start the compressors without at
least 8 hours of crankcase heat or compressor
damage will occur.
7. Each refrigeration system has a low pressure cutout (1LP
for system No. 1 and 2LP for system No. 2) to shut down
the compressor due to loss of refrigerant charge or a build
up of frozen condensate on the compressor before the
suction valve and are set to open when the suction pressure
drops to 7 psig. These controls will automatically reset when
the suction pressures rise to 22 psig. The opening of the
low pressure cutouts will activate the lockout circuit.
8. High pressure cutouts (1HP for system No. 1 and 2HP for
system No. 2) are located in each system on the compres-
sor before the discharge valve. Should a system discharge
pressure exceed 398 psig, the control will open and de-en-
ergize the compressor. The pressure cutout will close when
the discharge pressure drops to 310 psig. The opening of
the high pressure cutout will activate the lockout circuit. The
high pressure cutouts may open due to a dirty or restricted
condenser coil, loss of air flow or too high an ambient air
temperature.
9. The lockout circuits mentioned above will not be energized
during normal operation because they have a high resis-
tance. The flow of electricity will normally follow the path of
least resistance through the compressor contactors 1M and
2M. If, however, a low or high pressure cutout opens, the
lockout relay 10R or 11R will be energized. Since the
voltage across 10R or 11R will exceed 100V, the voltage
across 1M or 2M will be too low to pull in the contactors.
The normally closed contacts of 10R or 11R will open the
circuit in series with the low or high pressure cutout. When
these contacts automatically close, the lockout circuit can
be reset by interrupting the control circuit at the room
thermostat. Two advantages are gained by this circuit:
a. Prevents rapid cycling of the compressors which can be
damaging.
b. The alternate use of manual reset cutoffs avoids the
problem above but may require an expensive service
call to reset these controls.
10. Freezestat 3TH senses the suction temperature of the No.
1 system and will shut down compressor(s) when this
temperature drops to 32ÉF. It will reset automatically at
37ÉF
11. The supply air flow must be proven by vacuum switch 5LP.
In the event the belts on the supply air motor break or the
supply air motor should be de-energized by its oveload
protectors, 5 LP will open and interrupt the cooling, heating
and economizer control circuits. This assures that the
various modes of operation do not continue without proper
air flow.
12. The compressor motor protector(s), 1MP, 2MP will interrupt
the compressor control circuit when sensing an overload
condition. Also an anti-recycle timer is part of this device to
prevent the compressor from rapid cycling. The compressor
will stay off for five minutes. It also monitors the voltage of
the 120 volt control circuit and will shut down the
compressor if the voltage drops below 85 æ4.5 volts.
13. Oil pressure control switches are installed in the compres-
sor circuits. These assure that adequate oil pressure is
present to lubricate the moving parts of the compressor. If
pressure is not adequate, the switch opens shutting down
the compressor. It is manually reset.
14. When the pumpdown accessory, Model 2PD04700101, is
installed, the compressor will continue to operate through
contact 12TR-2 after the room thermostat or controller is
satisfied.
Solenoid 3SOL in the liquid line is now closed and the
compressor pumps most of the refrigerant out of the evapo-
rator to the high side of the system. The system suction
pressure drops until the low pressure cutoff (1LP) opens.
This will shut down the compressor and de-energizes the
12TR relay which opens 12TR-2 to put the operation of the
compressor under the control of the 1R relay. Also present
is a 12TR-1 contact which is closed when the system is off
and provides a bypass circuit to allow contactors 1M and
2M to be activated in the event the low pressure cutout has
not reset. Without this pumpdown circuit, it is possible to
have the oil pressure switch (OP/PS) open on light load and
short cycle of the compressor. Refrigerant floods back to
the compressor on each start-up, foams the oil in the
compressor crankcase and pumps the oil to the conden-
sate coil. The run times are too short to bring the oil back
START-UP
530.25-N3Y
Unitary Products Group
23