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CMC TECHNICAL REFERENCE MANUAL
1X36003 Version 2.52
1996-1999 Ingersoll-Rand Company
Date of Issue: 18-Oct-1999
Autodual automatically unloads the machine when the bypass valve is opened beyond the
Unload Point for a programmed time period called the Unload Delay Time. The Bypass
Valve Unload Point is selected to correspond with the check valve closing since at this point
the machine is not supplying the system (Figure 1). The Unload Delay Timer should be set
to prevent unloading during short excursions through the Unload Point. The Reload Percent
determines the System Pressure at which the machine will automatically load into the
system.
How does Constant Pressure Modulation Work?
The goal of constant pressure modulation is to maintain a specified discharge pressure in
the compressed air system while the capacity requirements change. Modulate control
provides constant pressure from 100 percent of the compressor's capacity to zero capacity.
Autodual control provides constant pressure from the 100 percent of the compressor's
capacity to the Unload Point.
If all plant air systems were identical in capacity usage requirements, the CMC could be
preprogrammed to respond to those changes; however, all plant air systems are not alike.
The frequency and variability of the capacity changes means that the control logic must be
flexible, so the CMC utilizes proportional plus integral control algorithms to determine the
magnitude of the signal that is sent to the inlet and bypass valves. These algorithms, or
programming logic, allow the CMC control system to be "tuned" to a specific plant air
system.
Measuring the Discharge Pressure
In order to maintain constant pressure, the system discharge air pressure must be
measured. A pressure transducer is mounted in the control panel and tubed to the
compressor discharge downstream of the check valve as shown in Figure 4.
Compressor
Motor
Starter
CT
Bypass
Valve
Inlet
Valve
Check
Valve
Base
Control
Module
PT
4-20 mA
4-20 mA
CMC
Pneumatic Tubing
Figure 4: Performance Control
This transducer sends a 4-20 mA signal to the CMC board. The CMC compares the
measured discharge pressure to the system pressure set point entered into the CMC by the
user through the Operator User Interface (OUI). Depending upon the difference between
these two values the CMC will send a 4-20 mA signal to "Modulate", open or close, the inlet
and/or bypass valve to maintain the specified system pressure set point.
