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If the suction pressure is above the maximum operating pressure set point (MOP), the valve will modulate
to control the pressure at or below the maximum operating pressure set point (MOP). When the operating
suction pressure is lower than MOP, it will go back to superheat control. Suction pressure is used to calculate
saturated temperature (TSAT).
If the suction pressure is below the Minimum pressure set point (MNPR), the valve will close and the
control signal to the external relay will be turned off. It will resume normal operation when the pressure is
above the Minimum pressure set point.
While in COOL MODE, the four-digit display on Master will show ‘REFR’ for three seconds, ‘RMSP’ for two
seconds, and the numerical display of the room temperature for five seconds. The four-digit display on
controllers without an air temperature sensor will show ‘REFR’.
DEFROST MODE (DEFR)
There are two methods to determine if a defrost will be initiated for the Master Controller: scheduled defrost
and demand defrost. If the number of defrosts per day ‘DFPD’ is set to 0, the controller will do the demand
defrost by default. If the number of defrosts per day ‘DFPD’ is set from 1 to 8, the controller will do the
scheduled defrost.
Scheduled Defrost
The following is the description of the scheduled defrost.
The time_of_day is really an elapsed counter that counts the number of minutes that have passed. An elapsed
count of 0 is 12:00 AM. The count goes up to 1439 which corresponds to 11:59 PM. The counter then will reset
to 0.
The time of day will be kept as long as input power is connected. If input power is turned off, then back on,
the time of day will be reset to 0 which corresponds to 12:00 AM.
The first defrost start time is an elapsed time of 0 (12:00 AM). The subsequent defrost start times are
determined by adding the number of minutes between each defrost to the previous start time until there is a
defrost start time for each defrost per day. The number of minutes between each defrost is determined by
taking 1440 / number of defrosts per day as set up by the ‘DFPD’ set point.
When starting an electric defrost, FAN relay is de-energized to turn off the fans. The controller waits for five
seconds, then the DEFROST relay is energized to start a defrost.
When starting a reverse cycle defrost, the FAN relay is de-energized to turn off the fans while, at the same
time, COMPRESSOR relay is de-energized to turn off the compressor. There is a 10 second delay before the
DEFROST relay is energized to switch the four-way reverse valve. Then there is a 30 second waiting period for
pressure equalizing. Afterward, COMPRESSOR relay is energized to turn the compressor. Hot gas will be
reversed to flow to the evaporator while the electric expansion valve is modulated to start a defrost.
The controllers use the coil sensor ‘TS3’ as the defrost termination sensor. When this temperature gets
above the preprogrammed Defrost Termination Set Point (DTSP) before the preprogrammed Maximum
Defrost Time (MXDT), the defrost will terminate. Otherwise, it will be terminated when the Maximum Defrost
Time (MXDT) times out.
While in DEFROST MODE, the four-digit display will be ‘DEFR’ for three seconds, ‘DTTP’ for three
seconds, and the numerical display of the temperature reading from sensor TS3 for three seconds.
Demand Defrost
When ‘DFPD’ is set to ‘0’, the controller will initiate a demand defrost. The controller will not go to ‘DEFROST
MODE’ until a heavy frost accumulation is in the evaporator coil. When frost is built up in the evaporator, it will
block the air flowing through the evaporator coil and reduce the heat transfer area. It will also decrease the
evaporating temperature, which, in turn, increases probability of frosting. A demand defrost scheme to detect
the frost build-up and the criteria to start defrost are programmed in the Master Controller. Unlike scheduled
defrost, the Master Controller 2.0 with demand defrost is really an energy saver. If no selection is made, the
controller will automatically select demand defrost when the power is applied to the controller. The defrost
procedure is the same as described for the scheduled defrost.
After selecting the demand defrost, if the elapsed time since the last defrost has been a selectable time
(INTV) from 8 hours (480) to 72 hours(4320), the Master Controller 2.0 will go into defrost to ensure a proper
oil return.
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