Emerson Vilter Vission 20/20 Operation And Service Manual page 56

Section 6 • Compressor Control
Capacity Slide Position:
• Defines the minimum and maximum positions for
the capacity slide. While in 1 step control these values
should be 0% for minimum and 100% for maximum.
VFD Speed:
• Defines the minimum and maximum speed for the
motor. While in 1 step control these values should re-
flect the full range of the VFD.
2 Step VFD Control:
• Enables the second step in the VFD control algorithm.
Capacity Slide Position:
• Defines the minimum and maximum position of the
capacity slide in the 2 step VFD control.
VFD Speed:
• Defines the minimum and maximum speed for the
motor in the 2 step VFD control.
P = Proportional (gain) setpoint:
• Used to adjust the motor speed action in direct pro-
portion to the difference between the control set-
point and the process variable (SP - PV error). This is
a unitless quantity and is used for coarse adjustment.
This setpoint should be set to the lowest value that
6 – 6
Figure 6-7. VFD One-Step Control Method
Figure 6-8. VFD Two-Step Control Method
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
gives adequate control system response. Increasing
the proportional setting increases the control sys-
tem's sensitivity to small process fluctuations and the
tendency to hunt.
I = Integral (reset) setpoint:
• Used to adjust the capacity control action, integrating
the error over time, to account for a small error that
has persisted for a long time. This quantity is used for
fine adjustment. This setpoint is used to smooth out
process variations. This setpoint should be set high
enough to prevent hunting but low enough to pre-
vent control system overshoot.
D = Derivative (rate) setpoint:
• Used to adjust the capacity control action, account-
ing for how fast the error is changing, positively or
negatively. A standard PID loop variable, it is not used
for our applications.