Ingersoll-Rand 900829, Maintenance Manual

Page: 28 / 142

General Description
26
• When the discharge pressure is greater than
300 psig (2068 kPa), RY6 and RY10 close and
RY9 opens. CF1 and CF2 become connected
in parallel, receive high voltage, and operate at
high speed.
Model 30 and 50 Units
The following components are added to the
refrigeration system for Model 30 and 50 units:
condenser blocking solenoid, accumulator tank,
CPR (compressor pressure regulator) valve, and
liquid line check valve. Cool in Model 30 and 50
units is basically the same as Cool in Model 10
and 20 units because the condenser blocking
solenoid is open and the defrost solenoid is
closed. See Figure 6 on page 27.
Null Mode - All Units
The unit operates in Null mode when the setpoint
temperature is reached and cooling (or heating) is
not required. All outputs are de-energized. If the
temperature rises a pre-determined number of
degrees, the unit restarts in Cool mode. If the
temperature falls a pre-determined number of
degrees, and a heat option is present, the unit
restarts in Heat mode.
In addition, the evaporator fans (parameter EFc)
operate during Null mode.
Heat Mode - Model 10 and 20 Units
Only
If the Heat option is present, the unit enters Heat
mode when the temperature falls a pre-determined
number of degrees below the setpoint
temperature. When heat is required, the outputs of
the microprocessor energize the evaporator fans.
(For units with Electric Standby, the Compressor
Motor Contactor are energized when the standby
input is high).
The unit operates in Heat mode until the setpoint
temperature is reached. The unit then enters Null
mode.
• If the temperature falls a pre-determined
number of degrees, the unit restarts in Heat
mode.
• If the temperature rises a pre-determined
number of degrees, the unit restarts in Cool
mode.