System Function & Seq. Of Operation; Communications And Status Function Lights; Time Delays; Compressor Operation - Carrier GC Series Installation Instructions Manual

SYSTEM FUNCTIONS AND SEQUENCE
OF OPERATION
GC units are designed for communicating- -only controls.
Communicating Sequence:
functions. See Fig. 27.
Communication and Status Function Lights
A green LED (COMM light) on the UPM (see Fig. 23) indicates
successful communication with the other system products. The
green LED will remain OFF until communication is established.
Once a valid command is received, the green LED will turn ON
continuously. If no communication is received within 2 minutes,
the LED will be turned OFF until the next valid communication.
Amber Status Light - - An amber colored STATUS light is used to
display the operation mode and fault codes as specified in the
troubleshooting section. See Table 14 for codes and definitions.
NOTE: Only one code will be displayed on the UPM board (the
most recent, with the highest priority).

Time Delays

Unit time delays include:
S Five minute time delay to start cooling or heating operation
when there is a call from the thermostat or user interface. To
bypass this feature, momentarily short and release Timer
Speed- -Up pins.
S Five minute compressor re- -cycle delay on return from a
brown- -out condition.
S Two minute time delay to return to standby operation from last
valid communication (with UI only).
S There is no delay between staging from low to high and from
high to low capacity. The compressor will change from low to
high and from high to low capacity "on the fly" to meet the
demand.

Compressor Operation

The basic scroll design has been modified with the addition of an
internal unloading mechanism that opens a by- -pass port in the first
compression pocket, effectively reducing the displacement of the
scroll. The opening and closing of the by- -pass port is controlled by
an internal electrically operated solenoid.
The modulated scroll uses a single step of unloading to go from
full capacity to approximately 67% capacity. A single speed, high
efficiency motor continues to run while the scroll modulates
between the two capacity steps. Modulation is achieved by venting
a portion of the gas in the first suction pocket back to the low side
of the compressor, thereby reducing the effective displacement of
the compressor.
Full capacity is achieved by blocking these vents, thus increasing
the displacement to 100%. A DC solenoid in the compressor
controlled by a rectified 24 volt AC signal in the external solenoid
plug moves the slider ring that covers and uncovers these vents.
The vent covers are arranged in such a manner that the compressor
operates at approximately 67% capacity when the solenoid is not
energized and 100% capacity when the solenoid is energized. The
loading and unloading of the two step scroll is done "on the fly"
without shutting off the motor between steps.
NOTE:
67% compressor capacity translates to approximately
75% cooling or heating capacity at the indoor coil.
compressor will always start unloaded and stay unloaded for five
seconds even when the thermostat is calling for high stage capacity.

Troubleshooting units for proper switching between

low & high stages
Check the suction pressures at the service valves. Suction pressure
should be reduced by 3- -10% when switching from low to high
capacity.
The UPM board controls all
The
Compressor current should increase 20 to 45% when switching
from low to high stage. The compressor solenoid when energized
in high stage, should measure 24VAC across pin numbers PL5- -2
HI and PL5- -5 C . When the compressor is operating in low stage,
the 24V DC compressor solenoid coil is de- -energized. When the
compressor is operating in high stage, the 24V DC solenoid coil is
energized.
The solenoid plug harness that is connected to the compressor has
an internal rectifier that converts the 24V AC signal to 24V DC.
DO NOT INSTALL A PLUG WITHOUT AN INTERNAL
RECTIFIER.

Unloader Test Procedure

The unloader is the compressor internal mechanism, controlled by
the DC solenoid, that modulates between high and low stage. If it
is suspected that the unloader is not working, the following
methods may be used to verify operation.
1. Operate the system and measure compressor amperage. Cy-
cle the unloader on and off at 30 second plus intervals at the
User Interface (from low to high stage and back to low
stage). Wait 5 seconds after staging to high before taking a
reading. The compressor amperage should go up or down
at least 20 percent.
2. If the expected result is not achieved, remove the solenoid
plug from the compressor and with the unit running and the
User Interface or thermostat calling for high stage, test the
voltage output at the plug with a DC voltmeter. The read-
ing should be 24 volts DC.
3. If the correct DC voltage is at the control circuit molded
plug, measure the compressor unloader coil resistance. The
resistance should be approximately 330 or 1640 ohms de-
pending on unloader coil supplier. If the coil resistance is
infinite or is grounded, the compressor must be replaced.

Two Stage Compressor

The two stage compressor contains motor windings that provide
2- -pole (3500 RPM) operation.
Compressor Internal Pressure Relief
The compressor is protected by an internal pressure relief (IPR)
which relieves discharge gas into the compressor shell when
differential between suction and discharge pressure exceeds
550- -625 psi. The compressor is also protected by an internal
overload attached to motor windings.

Compressor Control Contactor

The contactor has a 24volt coil. The electronic control board
controls the operation of the contactor.

Troubleshoot Compressor

If the compressor fails to operate, Table 13 can be used to verify if
there is any damage to the compressor windings causing system
malfunction.
Table 13 – Winding Resistance
Compressor Ohms
Model Start Winding
GC024 1.64
GC036 1.52
GC048 1.86
GC060 1.63
GC072 1.85
Tolerance +/- 7%. All resistance values must be measured with
compressor at room temperature.
33
Run Winding
1.3
0.88
0.52
0.39
0.34