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22.0 SELECTING A CONTROL STRATEGY
22.1 The control strategy best suited to a particular application varies widely. The following
information is offered as a set of general guidelines and definitions. Each application should
be evaluated by qualified personnel familiar with the actual operating conditions.
22.2 The DE-2500 controller is normally programmed to regulate compressor throughput by
controlling both speed and capacity. In any particular application, it may be desirable to control
the compressor using only one of these. This can be done on an intermittent basis using the
MANUAL override options for speed or capacity control from the keypad to disable the selected
function. If it is desired to permanently disable either speed or capacity control, this can be
done when programming the unit. To use capacity control only, for example, the prime mover
normally runs at a fixed RPM, then the control values for the MINIMUM RPM and the
MAXIMUM RPM are simply programmed as the same value. If the compressor has no means
of bypassing or regulating gas flow to control capacity and only speed control can be used,
then the capacity outputs are left unconnected and the DE-2500 will regulate only the engine
RPM.
22.3 The speed control output of the DE-2500 is a 4-20 ma current loop which can be connected
directly to an electronic governor. The RPM values corresponding to 4 ma and 20 ma for the
governor are entered during the programming of the DE-2500 so that the control output is
prescaled to the correct value for the minimum and maximum RPM setpoints. If a governor
requiring a pneumatic setpoint (3-15 psi ) is used then the 4-20 ma output is connected to an
appropriate I/P transducer.
22.4 The capacity control output of the DE-2500 is available in two formats, to drive the most
common actuating systems. The first output format is an industry standard 4-20 ma current
loop. The 4-20 ma output would be used to interface to pneumatically controlled turn valves
on screw compressors (internal stepless bypass) or to pneumatically controlled external bypass
valves on reciprocating compressors. In both of these cases, the 4-20 ma output is connected
to an appropriate I/P transducer which then connects via tubing to the actuator. See drawing.
The second set of capacity control outputs is brought out of the controller in a digital format
(discrete transistors). The digital or discrete outputs consist of two transistors which switch on
to ground and are designed to drive relay coils or pilot duty solenoid valves. One switch is
labeled DIGITAL OUT 1 and energizes to LOAD the compressor the other is labeled DIGITAL
OUT 2 and energizes to UNLOAD the compressor. When used with the hydraulically positioned
slide valve on a screw compressor application, these outputs are typically connected to a three-
way solenoid valve. On reciprocating compressors, the digital outputs can be used to actuate
a motor controlled bypass valve via relay contacts which energize the motor in the open or
close directions.
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