CSR-20 & CSR-40, September 1999
µ
P-D Controller
General Theory of Operation
4-9
Frozen Loads (Setpoint at -10 C [14 F] and Below)
At setpoints of -10 C (14 F) and below, the controller controls
unit operation based on the return air sensor temperature and
setpoint temperature.
The system operates on Full Cool to provide accurate con-
trol of frozen cargo. If the return air sensor becomes discon-
nected or fails while it is being used to control unit operation,
the controller will automatically switch and control unit opera-
tion from the supply air sensor plus 4.4 C (7.9 F). If both the
supply and return air sensors fails, the controller will operate
the unit continuously on Cool (frozen load operation only).
Automatic Phase Selection
The
µ
P-D controller monitors each phase of the power supply
to ensure proper rotation of the condenser and evaporator fans,
and the compressor. During unit start-up, “PAUSE CHECK”
appears in the controller display while the controller deter-
mines the correct phase sequence. The controller energizes
phase select contactor ABC and then the electric heaters, con-
denser fan and evaporator fans. The controller senses the
incoming power phase for 10 seconds and then de-energizes
phase select contactor ABC. 30 seconds later, phase select
contactor CBA is energized with the selectric heaters, con-
denser fan and evaporator fans. The controller senses the
incoming power phase for another 10 seconds to select the cor-
rect phase sequence. The controller then de-energizes all unit
loads and energizes either the ABC or CBA phase select con-
tactor to assure correct compressor, condenser fan and evapo-
rator fan rotation.
Compressor Liquid Injection
During compressor operation, a liquid injection system injects
refrigerant into the suction line to protect the compressor for
excessively high operating temperatures. The controller acti-
vates liquid injection when the:
• Compressor starts. The controller turns on liquid injection
for 5 minutes after each compressor start-up. Compressor
start-ups include initial unit start, start after Defrost and start
after Null.
• Modulation valve is energized. The controller turns on liquid
injection continuously during the Modulation Cool and
Power Limit modes.
• Compressor discharge temperature exceeds 138 C (280 F).
Liquid injection stops when the compressor discharge tem-
perature decreases to 132 C (270 F).
When liquid injection is active, the controller energizes
the liquid injection valve continuously. The liquid injection
line injects refrigerant into the center scroll of the compressor.
Modulation Valve Setting (PCVAL)
The modulation valve controls refrigerant return to the com-
pressor during Modulation Cool. The modulation valve set-
ting is displayed in the View menu under PCVAL. The
PCVAL reading indicates the percent the modulation valve is
closed: 100 =100% closed.
Evaporator Fan Control
The controller determines evaporator fan motor speed based
on the return air temperature and the Economy mode setting.
• Evaporator fans operate on HIGH speed at return air temper-
atures of -9.9 C (14.1 F) and above. If the Economy mode is
ON and temperature is In-range, the controller shifts the
evaporator fans to LOW speed.
• Evaporator fans operate on LOW speed at return air tempera-
tures of -10 C (14 F) and below. If the Economy mode is ON
and the unit is in the Null mode, the controller STOPS the
evaporator fans. The controller then operates the evaporator
fans on LOW speed for 5 minutes every 45 minutes as long
as the unit remains in the Null mode.
Condenser Fan Control
The controller cycles the condenser fan between ON and OFF
based the compressor discharge temperature and/or the con-
denser coil temperature. The controller uses a software algo-
rithm to monitor these temperature sensors and their rate of
temperature change. Therefore, when the condenser fan will
start and stop can not be exactly predicted. However, the con-
denser fan will typically be ON when:
• The compressor temperature is above 50 C (122 F) and
increasing, or
• The condenser temperature is above 35 C (95F).
NOTE: The condenser fan operates continuously if
either the compressor or condenser temperature
sensor is defective.
