Emerson Vission 20/20, Operation And Service Manual

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6 – 2
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
Section 6 • Compressor Control
Figure 6-2. Compressor Control Screen - Process Temperature Control
the interval time given a near continuous movement of
the capacity slide. Adjusting these setpoints can be use-
ful in slowing down the reaction time of the compressor
if large thermal time contents are present in the refrig-
eration cycle. As mentioned in the above paragraph, the
distance of the process variable from the control set-
point determines the size of the pulse used to move the
capacity slide. This is called the proportional band and is
set by the Proportional Setpoint. When the process vari-
able is outside the proportional band, the slide will move
in the direction of the error continuously. Increasing the
size of the proportional band can help slow the compres-
sors reaction by varying loads if desired, see Figure 6-3.
The Dead Band Setpoint de
fi nes area around the con-
trol setpoint where the algorithm stops adjusting the
capacity slide. This area is a percentage of the propor-
tional band. By default the proportional band is set to 4
Psig and the dead band is set to 10% of 4 Psig. Making
the dead band +/- 0.4 Psig of the control setpoint. Once
the process variable is within the dead band, the algo-
rithm considers the compressor to be on setpoint. If the
operator wishes the compressor to operate closer, the
setpoint can be set to a smaller percentage. However
this will result in the capacity slide excessively moving to
maintain the setpoint and could over heat the actuator
or shorten the actuators operational life.
Auto-Cycle
The auto-cycle setpoints de fi ne the control points in
which the compressor will automatically cycle on and off
when the compressor has been placed into “Auto” run
mode. These setpoints can be “enabled” or “disabled”
using the check box. A delay can be entered to momen-
tarily delay the start or stop from immediately occurring
when the setpoint is met. If a compressor shutdown is
desired on a suction pressure drop and a manual reset
is required, set the OFF value below the Low Suction
Pressure safety trip value. This will shut down the com-
pressor and a Reset will be required to restart it.
The auto-cycle function will operate only in local “Auto”
mode and Direct I/O “Remote Auto” mode. If the