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Section 6 • Compressor Control
Volume Slide Position Offset
These setpoints offer the ability to alter the Volume posi-
tion table to take advantage of potential energy savings.
Since the volume position is a function of the capacity po-
sition, the offset to the volume is based on the position
of the capacity slide. The volume offset can be applied to
the entire capacity slide range or just a portion using the
Capacity Range minimum and maximum setpoints.
Volume Slide Adjustment %:
• The value in percentage of the volume slide offset.
Capacity Range:
• Defines the range where the volume position slide off-
set will be applied.
Soft Load
This setpoint is used to slow the loading of the compres-
sor. In some refrigeration systems, a loading compressor
can have dramatic effects on the system parameters. This
setpoint allows an operator to reduce the continuous
load pulse as defined in the proportional control section
to a percent duty cycle.
Soft load %:
• Defines the duty cycle of the continuous load pulse. At
100%, the continuous pulse will truly be continuous.
At 50%, the continuous pulse would be reduced to half
time on and half time off in the time interval defined in
the proportional control section.
6 – 16
Liquid Injection
The setpoints in this section are to control the behavior
of the liquid refrigerant injected into the compressor for
oil cooling purposes. The liquid injection solenoid con-
trol is based off the discharge temperature whether the
compressor uses just an injection solenoid or a motorized
valve in conjunction with the solenoid, see Figure 6-16.
Liquid Injection Solenoid Control ONLY
• When using only the liquid injection solenoid, the so-
lenoid is activated once the value of discharge tem-
perature meets or exceeds the value of "Liquid inj.
Setpoint" and the value of the oil separator temper-
ature meets or exceeds the value of "Oil Sep. Temp.
Override". The injection solenoid will deactivate if ei-
ther setpoints are not met. This will prevent situations
where the discharge temperature may rise quickly, but
the oil temperature is still very cold. By preventing the
liquid injection solenoid from turning on at this point,
the oil separator will not be subjected to additional
liquid refrigerant that would cool the oil even further.
Liquid Injection Control using a 4-20mA motorized valve:
• When a motorized valve is used to control the amount
of liquid being injected into the compressor the pre-
viously mentioned setpoints have a slightly different
function. The Oil Sep. Temp. Override is still used in
controlling the injection solenoid, however, the Liquid
Inj. Setpoint is now used as the target temperature
for the PID Algorithm that controls the position of
the motorized valve. The algorithm compares the
actual discharge temperature against the Liquid Inj.
Setpoint. The difference between these is the error.
The PID algorithm tries to drive the error to "zero" by
moving the positioning valve to allow more or less liq-
uid refrigerant to be injected into the compressor.
• A PID algorithm can be notoriously hard to tune. As a
result the Vission 20/20 offers a couple of additional
features to help control wild fluctuations in oil temper-
atures that could result in the compressor tripping off.
The operator can choose to enable the minimum val-
ue position that automatically sets the liquid injection
motorized valve to the specified value whenever the
discharge temperature has fallen below the Liquid inj.
Setpoint. This feature nearly eliminates the overshoot
of the PID in the downward direction and reduces the
chance of the compressor tripping off due to low oil
temperature. The operator can also choose to use
an average of the discharge temperature and the oil
manifold temperature as the control variable. The dis-
charge temperature can vary quite drastically forcing
the PID algorithm to drastically adjust the motorized
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
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