When the temperature in the conditioned space rises above
the thermostats cooling set point, the thermostat sends a
modulating voltage signal to the logic panel. The thermostat
signal is modified by a signal from the discharge sensor, which
is mounted in the discharge air duct to provide anticipation to
the system. The logic panel responds to the combined
thermostat discharge sensor signal by activating the minimum
amount of cooling to satisfy the thermostat demand.
Normal sequence of operation is as follows:
1. Power is supplied to the unit through the disconnect switch.
The switch can be field-installed or factory-installed as an
option.
2. As soon as power is supplied to the unit the compressor
crankcase heaters 1TH (and 2TH) will be energized.
CAUTION: Do not attempt to start the compressors without at
least 8 hours of crankcase heat or compressor
damage will occur.
3. The system switch on the room thermostat or the status
panel must be in the AUTO or COOL position to complete
the circuit between the terminals 119 and 103/RC.
4. On demand of the room thermostat for cooling the “Cool 1"
contacts on the logic panel close and energizes 1R coil.
5. An internal connection in the thermostat or status panel
energizes the 7R coil. 7R contact closes energizing 3M coil.
3M contacts close, starting the supply air fan motor. After
an adequate air flow is established, 5 LP closes to energize
6R coil. 6R contact closes in the compressor control circuit.
NOTE: The above operation is for intermittent fan operation.
For continuous operation, the fan switch on the room
thermostat or the status panel should be manually
closed to energize 7R.
6. Relay contact 1R closes and energizes 1M coil, which
starts the No. 1 compressor. At the same time, 4M is
energized to start the condenser fans.
7. If the room temperature continues to rise, the “Cool 2"
contacts on the logic panel close energizes 2R coil.
8. On DSC480 units the 2R contacts close, energizing the 2M
coil which starts the No. 2 compressor or on DSC360 units
the 2R contacts close de-energizing the compressor un-
loading solenoid which loads the compressor.
9. The compressor short cycle time and low voltage protection
is part of the solid state compressor protection module. It
prevents a compressor from starting unless it has been off
for five minutes. It also monitors the voltage of the 120 volt
control circuit and will shut down the compressor if the 120
volt control voltage drops below 85 æ4.5 volts.
10. To maintain sufficient head pressure during low ambient
operation, condenser fans No. 1 and No. 2 will be
de-energized by 1TH. Condenser fans No. 3 and 4 will be
de-energized by 2TH (see Table 6).
EXHAUST AIR SYSTEM
The exhaust air fans are energized by manually closing a circuit
between terminals 60 and 64 of terminal block 2TB or
automatically be an end switch on the economizer damper
motor. As the economizer damper opens, it closes the end
switch on the damper motor and turns on the exhaust air fans.
The degree of blade opening at which the exhaust fans are
energized can be adjusted by the following procedure.
1. Use the black scale plate as an indicator of motor position
for switch operation.
2. Loosen the thumb nut and the cam locking screws.
3. Set the operational and differential cams.
4. Check switch adjustment by moving the adjuster 2RH to
move the motor. Switch should click when desired make
and break points are lined up with index mark and the
exhaust fan should start and stop.
WARNING: 120 volt power is connected to the switch.
5. Tighten thumb nut.
SUPPLY AIR BLOWER ADJUSTMENT
Knowing the required CFM, the unit options, and the static
resistances of both the supply and the return air duct systems,
the RPM for the supply air blowers can be determined from the
blower performance. (See Table 11.)
Knowing the required blower RPM and the blower motor HP,
the setting (turns open) for the supply air motor pulley can be
determined from Table 7. The 20 HP drive for the DSC480 has
a fixed motor pulley and a fixed speed.
Each motor pulley has:
1. A threaded barrel with two flats (or notched recesses) 180
degrees apart.
2. Either one or two movable flanges, each with two set screws
180 degrees apart.
UNIT
1TH
2TH
DSC360
55 + 6ÉF
25 + 6ÉF
DSC480
55 + 6ÉF
35 + 6ÉF
TABLE 6 - AMBIENT THERMOSTAT SETTING
530.25-N3Y
16
Unitary Products Group
