ShopTek™ ST4, ST5, ST7, ST11, ST15 Three-phase 60 Hz User Manual 2: Description
88292018-236 R00
Subject to EAR, ECCN EAR99 and related export control restrictions. 15
2.5 Control system—functional
description
Refer to Figure 2-5 and Figure 2-6. The compressor con-
trol system regulates its output to match the demand.
This
system consists of a solenoid valve and an inlet
valve that control the compressor’s operational modes.
The following descriptions of operational modes apply to
a compressor whose rated operating pressure is
125 psig (8.6 bar).
Start mode—0 to 50 psig (0 to 3.5 bar)
There is no load on the compressor at startup, the sole-
noid valve is open and the inlet va
lve is closed. When the
compressor (START) pad is pressed, the separator
tube pressure rises from 0 to 50 psig (0 to 3.4 bar). When
it re
aches its full operating speed (maximum rpm), the
compressor switches to the full load mode.
Full load mode—50 to 127 psig (3.4 to 8.8 bar)
When the compressed air pressure rises above 50 psig
(3.4 bar), the minimum pressure valve opens allowing
comp
ressed air to flow into the service line. From this
point on, the Controller continuously monitors the line air
pressure. The solenoid valve remains closed in this
mode. As long as the compressor is running at 127 psig
(8
.8 bar) or lower, the inlet v
alve is fully open.
Unload mode—greater than 127 psig (8.8 bar)
When there is no, or only a small demand, the service
line pressure will rise. The controller de-energizes the
solenoid valve when the pressure exceeds 127 psig
(8
.8 bar) allowing the separator tube air pressure to be
supplied directly
which closes the inlet valve. At the same
time the solenoid valve exhausts the system pressure to
the atmosphere and lowers the separator tube pressure
to approximately 29 psig (2.0 bar). A check valve in the
air
service line prevents air from back-flowing to the sep-
arator tube.
When the pressure drops to the low setting: (cut-in pres-
sure) usually 112 psig (7.7 bar) for high pressure (9 bar)
compres
sors, 137 psig (9.4 bar) for (10 bar) compres-
sors, 161 psig (11.1 bar) for (12 bar) compressors: the
con
troller energizes the solenoid valve and prevents line
pressure from reaching the inlet control valve.
NOTE
Other pressure settings can be selected
depending on compressor application or rating.
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