3.15 Modes Of Operation; Heat/Cool Mode; Defrost Cycle; Continuous Or Start-Stop Operation - Carrier Supra 950MT Operation & Service Manual

3.15 MODES OF OPERATION

3.15.1

Heat/Cool Mode

Once the engine is considered to be running, the unit microprocessor maintains the set point temperature by
switching between heat and cool modes.
3.15.2

Defrost Cycle

Defrost is an independent cycle that overrides cooling and heating functions, and de-ices the evaporator(s) as
required. During Defrost mode, the microprocessor displays DF on the right hand temperature display. The left
hand display continues to display the set point.
There are two ways of initiating a defrost.
• Method one: Defrost may be initiated automatically at preset intervals by the defrost timer in the microprocessor.
• Method two: Press the MANUAL DEFROST key.
a. Initiating Automatic Defrost
When defrost time entered via Functions has elapsed (see
cycle is initiated, the defrost timer is reset to zero. The microprocessor holds the last entered defrost interval in
memory. The defrost timer runs only when a compartment defrost temperature sensor (DTS) is sensed at or below
45°F (7°C). The lowest number compartment will go into null mode, fans only. The highest number compartment
will initiate defrost first by turning off the compartment fans and using both hot gas and electric heat to complete
defrost. After compartment defrost terminates, there will be a one minute operation in cool without fans by the com-
partment that just completed defrost to allow excess moisture to freeze on the coil. A recovery time of 15 minutes
will be utilized by both compartments. The highest number compartment will then go into null mode, fans only. The
lowest compartment will initiate defrost by first turning off compartment fans and using both hot gas and electric
heat to complete defrost. After compartment defrost terminates, there is a one minute operation in cool without fans
by the compartment that just completed defrost to allow excess moisture to freeze on the coil.
b. Manual Defrost
Pressing the manual defrost on the cab command will put the lowest number compartment into null mode, fans
only. The highest number compartment will initiate defrost first by turning off compartment fans and using both hot
gas and electric heat to complete defrost. After compartment defrost terminates, there will be a one minute opera-
tion in cool without fans by the compartment that just completed defrost to allow excess moisture to freeze on the
coil. The highest number compartment will then go into null mode, fans only. The lowest compartment will initiate
defrost by first turning off compartment fans and using both hot gas and electric heat to complete defrost. There is
a one minute operation in cool without fans by the compartment that just completed defrost to allow excess mois-
ture to freeze on the coil.
c. Defrost Termination
After initiation, defrost mode terminates when the Defrost Temperature Sensor (DTS) is sensed at 50°F (10°C) indi-
cating that the evaporator is de-iced and the defrost cycle is considered to be complete. Once a defrost has been
initiated, the defrost interval timer resets to zero time. The compressor operates in four-cylinder operation during
defrost and also uses electric heat. The length of defrost will depend on condition of evaporator coil(s). If a two
compartment system is used and one of the compartments' Defrost Thermostat Sensor (DTS) is above 50°F
(10°C), defrost will terminate and the system will continue to operate in temperature control mode.
c. Fail Safe Defrost Termination
If the defrost cycle does not complete within 45 minutes, the defrost cycle is terminated for that individual compart-
ment by the microprocessor and the defrost cycle will continue within the next compartment. The Manual Defrost
switch overrides this mode and starts a new 45-minute cycle.
When defrost override is active, the appropriate alarm is indicated. If the run relay is de-energized during defrost,
defrost is terminated and/or if a shutdown alarm occurs, defrost will be terminated.
3.15.3

Continuous or Start-Stop Operation

a. Auto Start Failure
If the unit fails to start, shuts down on a safety, or fails to run for the minimum run time three consecutive times, the
Start/Fail alarm is activated.
62-11863
Table 3–1), a defrost is initiated. Whenever a defrost
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