Ingersoll-Rand 900829, Maintenance Manual

Page: 27 / 142

General Description
25
Unit Operation
Standard Model 10 and 20 units (without heat
options) operate in Cool mode or Null mode, as
required, to maintain the load compartment
temperature at the setpoint temperature.
Model 30 and 50 units and 10 and 20 units with
heat options operate in Cool, Null or Heat, as
required, to maintain the load compartment
temperature at the setpoint temperature.
Defrost cycles occur manually or automatically,
as required.
If power is shut off, the unit comes back in Null
mode when the unit is restarted. There is a
momentary delay at auto start-up for circuit
protection.
NOTE: The compressor will “bump start” five
times while coming out of Null when the unit is
restarted after being turned off. This does not
happen if the unit went into Null because it
reached setpoint. In a “bump start” the
compressor is turned on for about one second
and then goes off for about four seconds.
Figure 5: Thermostat Algorithm
For units with Electric Standby, there are
protective delays for the electric
compressor/compressor motor contactor.
Options
• The Heat options provide heating by hot water
or hot gas. Some Model 20 units also have
electric heaters.
• The Electric Standby options provide a second
electric compressor for standby applications.
• The TC options provide temperature control
for two-compartment systems.
Operation
The vehicle engine must be running and the unit
must be turned on. On units with Electric Standby,
connect the external power cord and the unit
switches to Electric mode operation. Unit
operation can be tailored, as required, using
programmable settings that are shown later is this
section.
Cool Mode
Standard Model 10 and 20 Units
When cooling is required (when there is a
requirement to lower the evaporator return air
temperature in the load compartment), the
controller energizes the compressor clutch (or
compressor contactor in model 20 units) and
evaporator fans. The controller monitors the
discharge pressure through the high pressure
transducer (THPCO). The condenser fans are
energized if necessary and turn on and off as
determined by the controller.
The unit operates in Cool mode until the setpoint
temperature is reached. The unit then enters Null
mode. When the temperature rises to a
pre-determined number of degrees, the unit
restarts in Cool mode.
V-520 units only have a triple-cooling capacity
(TCC) feature. The controller monitors the
discharge pressure through the high pressure
transducer (THPCO) and controls the speed of
condenser fans CF1 and CF2 by opening and
closing relays RY6, RY9, RY10 in the following
manner:
• When the discharge pressure is less than 180
psig (1241 kPa), RY6, RY9, and RY10 are
open. CF1 and CF2 receive no voltage and are
in Null state.
• When the discharge pressure is between 180
psig (1241 kPa) and 300 psig (2068 kPa), RY9
closes. CF1 and CF2 become connected in
series, receive low voltage, and operate at low
speed
Decrease in
Temperature
Temperature Setpoint
Increase in
temperature
X C/F above the setpoint
X C/F below the setpoint
COOL
HEAT
NULL
MODE