6 – 13
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
Section 6
•
Compressor Control
Figure 6-11. Compressor Control Screen - Oil Control & Liquid Injection Control
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 1 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 1. 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
liquid refrigerant to be injected into the compressor.
• PID algorithm can be notoriously hard to tune. As
a result the Vission 20/20 offers a couple of addi-
tional features to help control wild
fl
uctuations in
oil temperatures that could result in the compres-
sor tripping off. The operator can choose to enable
the minimum value position that automatically sets
the liquid injection motorized value to the speci
fi
ed
value whenever the discharge temperature has fallen
below the Liquid inj. Setpoint 1. 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 discharge temperature can vary
quite drastically forcing the PID algorithm to drasti-
cally adjust the motorized value. By averaging the
more stable oil manifold temperature and discharge
temperature, the control variable stabilizes and the
PID is more easily tuned.
Please note that as stated above, PID algorithms can be
dif
fi
cult to tune and there is no one set of PID values that
will work. The work required for a compressor to meet
the requirement of its installation vary greatly and there-
fore the amount of heat transferred to the oil varies just
as greatly. We recommend the operator consult PID tun-
ing guides available from many different sources before
attempting to tune this PID.
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