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Section 4 • Operation
Wait a minimum of 20 minutes (to allow the
compressor unit to equalize to suction pressure)
between pre-lubing or pushing the start button.
Emergency Shutdown
Emergency shutdown is initiated by the following:
1.
A shutdown or trip condition of a process variable
while the system is in operation. If a process vari-
able reaches a high-high or low-low shutdown set-
point, the compressor unit will automatically stop.
A shutdown alarm is also generated on the control
panel HMI screen annunciating the specific process
variable trip condition.
2.
The Local Emergency Shutdown push button locat-
ed on the side of the control panel enclosure. When
the Local Emergency Shutdown push button is acti-
vated, the entire unit powers down. Also, the com-
pressor capacity and volume slide valve will stay in
their last position until the unit is powered up. Once
recovery has been accomplished and the unit is to
be re-powered, the Local Emergency Shutdown
push button must be pulled out to power up the
unit and controls.
Slide Valve Actuator Operation
The slide valve actuator is a gear-motor with a position
sensor. The motor is powered in the forward and reverse
directions from the main computer in the control panel.
The position sensor tells the main computer the position
of the slide valve. The main computer uses the position
and process information to decide where to move the
slide valve next.
The position sensors works by optically counting motor
turns. On the shaft of the motor is a small aluminum
"photochopper". It has a 180 degree fence that passes
through the slots of two slotted optocouplers.
optocouplers have an infrared light emitting diode (LED)
on one side of the slot and a phototransistor on the
other. The phototransistor behaves as a light controlled
switch. When the photochopper fence is blocking the
slot, light from the LED is prevented from reaching
the phototransistor and the switch is open. When
photochopper fence is not blocking the slot, the switch
is closed.
As the motor turns, the photochopper fence alternately
blocks and opens the optocoupler slots, generating a
sequence that the position sensor microcontroller can
use to determine motor position by counting. Because
the motor is connected to the slide valve by gears,
knowing the motor position means knowing the slide
valve position.
VSS/VSM/VSH/VSSH • Installation, Operation and Maintenance Manual • Emerson • 35391SD
NOTE
During calibration, the position sensor records the high
and low count of motor turns. The operator tells the
position sensor when the actuator is at the high or low
position with the push button. Refer to the calibration
instructions for the detailed calibration procedure.
The position sensor can get "lost" if the motor is moved
while the position sensor is not powered. To prevent
this, the motor can only be moved electrically while the
position sensor is powered. When the position sensor
loses power, power is cut to the motor. A capacitor
stores enough energy to keep the position sensor
circuitry alive long enough for the motor to come to a
complete stop and then save the motor position to non-
volatile EEPROM memory. When power is restored, the
saved motor position is read from EEPROM memory and
the actuators resumes normal function
This scheme is not foolproof. If the motor is moved
manually while the power is off or the motor brake has
failed, allowing the motor to free wheel for too long
after the position sensor looses power, the actuator will
become lost.
A brake failure can sometimes be detected by the
position sensor. If the motor never stops turning after a
power loss, the position sensor detects this, knows it will
be lost, and goes immediately into calibrate mode when
power is restored.
The
4 – 3
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Table of Contents
Troubleshooting
