Service manual

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 deﬁ 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 deﬁ 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