Thermo King 920000, Manual

Page: 38 / 190

Unit Description
38
Electric Operation
During electric operation, the electric motor will
stop when both evaporators are in Null. The
electric motor will not start until one of the
thermostats energizes a speed relay. During single
compartment operation, the Zone 2 evaporator is
controlled by the Zone 1 thermostat. The Zone 1
and Zone 2 evaporators use the same control
algorithm during electric operation.
Defrost Operation
As the RD-MT is operated to cool the cargo box,
frost will gradually build up on the evaporator
coils. Periodically this frost must be melted to
prevent a loss of cooling and airflow.
Defrost is accomplished by passing hot refrigerant
gas through the evaporator coils, thus melting the
frost (or ice). Melted frost drains out of the unit
onto the ground through the drain tubes.
Evaporator fans are off during Defrost to retard
warm air from entering the cargo area. Electric
Standby Defrost also uses hot refrigerant gas.
Defrost can be initiated in ether compartment by
pressing the appropriate Manual Defrost switch,
or automatically by an air switch or a defrost
timer any time the corresponding evaporator coil
temperature is below 42 F (6 C). If the condensing
unit is in Null, manually initiating Defrost will
cause the condensing unit to start and run. The
appropriate evaporator will operate in Defrost and
the other evaporator will operate according to
thermostat demand.
When a Defrost cycle is initiated, the defrost relay
energizes the condenser inlet solenoid and the hot
gas solenoid, and de-energizes the evaporator fan.
The evaporator remains in Defrost until the
evaporator coil temperature rises to 52 F (11 C),
where the defrost termination switch opens to
terminate Defrost. The evaporator then returns to
operation dependent on the thermostat demands.
The Defrost mode can be initiated manually any
time the evaporator coil temperature is below 42 F
(6 C).
0. Setpoint
1. Temperature Drop
2. Temperature Rise
3. Cool—Force Condensing Unit to High Speed
4. Null (Cool if Condensing Unit is Running)
5. Null
6. Heat (Null if Setpoint is Below 15 F [-9.4 C])
Figure 11: Electronic Control Algorithm