Thermo King 098916, Maintenance Manual

Page: 113 / 196

Operating Theory
112
the time interval each timed defrost
interval. Defrost synchronization creates
step intervals of 3, 4, 4, 5, 5, 6, 6 and 7
hours. Maximum time interval is 7 hours.
• On Frozen Loads, the initial time interval is 8
hours. Two (2) hours are added to the time
interval each timed defrost interval. Maximum
accumulated time interval is 24 hours.
• Defrost timer resets if the unit is off more than
12 hours, setpoint is changed more than 5 C
(8.9 F) or PTI (pretrip) test occurs.
NOTE: If unit operating conditions do not
allow the unit to enter a defrost cycle,
“Defrost Not Activated” appears on LCD
display when the
D
EFROST
key is pressed.
When the Defrost mode is initiated:
• The controller stops the compressor,
condenser fan and evaporator fans.
• When the compressor stops, the controller
turns on the Defrost LED, Heat LED and
energizes the heater contactor, turning on the
electric heaters.
The controller terminates the Defrost
mode when:
• Evaporator temperature:
• Chill mode: Evaporator coil sensor
temperature reaches 30 C (86 F); or
exceeds 18 C (65 F) for 35 minutes/45
minutes if voltage is less than 440 volts.
• Frozen mode: Evaporator coil sensor
temperature reaches 18 C (65 F); or
exceeds 8 C (46 F) for 35 minutes/45
minutes if voltage is less than 440 volts.
• Interval timer: Controller terminates defrost
after 90 minutes on 60 Hz power (120 on
50 Hz power). Alarm Code 20 will be
generated if this occurs.
• Power off: Turning
U
NIT
O
N
/O
FF
switch
O
FF
terminates defrost.
When the defrost mode is terminated:
• The Heat and Defrost LEDs turn off and the
heater contactor is de-energized. The
controller starts the compressor to pre-cool the
evaporator coil. The condenser fan starts if
required.
The controller pre-cools the evaporator coil to the
supply air temperature (or for 3 minutes
maximum) to minimize heat energy release into
the container. The controller then starts the
evaporator fans.
Compressor Digital Control
Valve
The MP-3000a controller pulses the Compressor
Digital Control solenoid valve between open and
closed positions. This provides precise cooling
capacity control. No pump down function or
warm gas bypass control is used in conjunction
with the Compressor Digital Control valve.
Figure 59: Compressor Digital Control Solenoid
Valve
The Compressor Digital Control valve is normally
closed. The normally closed position provides full
cooling capacity. When the controller energizes, it
opens the Compressor Digital Control valve.
Refrigerant gas flows from the digital port of the
compressor back to the suction line. This
disengages the compressor 100 percent and
temporarily reduces the compressor pumping
capability.
The controller uses a proportional-integral
derivative (PID) algorithm to provide accurate
temperature control. This is in direct response to
load demand. However, instead of generating a
voltage signal to position a suction line
modulation valve to regulate cooling capacity, the
algorithm establishes a pulse width signal to cycle
the Compressor Digital Control valve open and
closed on a duty cycle. The percent ON time
AXA0428