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Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 1.8.5153
NOTE
VFD installation is not covered in this manual. A VFD
that is not properly installed and confi gured has the
potential of causing intermittent and dangerous
problems. Please consult your VFD manual.
1 Step VFD Control:
• Enables the fi rst step in the VFD control algorithm.
This check box is not deselectable by the operator.
Capacity Slide Position:
• Defi nes 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:
• Defi nes the minimum and maximum speed for the
motor speed. While in 1 step control these values
should refl ect the full range of the VFD.
2 Step VFD Control:
• Enables the second step in the VFD control algorithm.
Capacity Slide Position:
• Defi nes the minimum and maximum position of the
capacity slide in the 2 step VFD control.
VFD Speed:
• Defi nes 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
unit-less quantity and is used for coarse adjustment.
This setpoint should be set to the lowest value that
gives adequate control system response. Increasing
the proportional setting increases the control sys-
tem’s sensitivity to small process fl uctuations 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
fi ne 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, accounting
for how fast the error is changing, positively or nega-
tively. A standard PID loop variable, it is not used for
our applications.
Pumpdown Control
The Pumpdown Control defi nes a method of “pumping”
down a chiller, which is to draw off refrigerant from the
chiller. This feature can be enabled or disabled from this
menu which is the part of Compressor Control Screen.
If Pumpdown is enabled, this feature will only function
when the compressor is running in local Auto Mode and
Control Mode Confi gured is Suction Pressure.
If Pumpdown Feature is enabled, then;
• The Auto-cycle functionality is ignored. Pumpdown
mode will cause the compressor to cycle off via the
Pumpdown Stop Pressure setpoint. Normally, the
Pumpdown Stop Pressure setpoint will be set lower
than the Auto-cycle Stop setpoint. Therefore, as the
suction pressure is pulled down, the compressor is
prevented from shutting down prematurely via the
Auto-cycle Stop setpoint by automatically ignoring
the Auto-cycle feature.
• The compressor will be placed into “Stop” mode af-
ter the suction pressure is equal to, or goes below the
Pumpdown Stop Pressure.
Pumpdown:
• This checkbox enables the Pumpdown feature. If
this box is unchecked, Pumpdown setpoints are ig-
nored and the user is not allowed to edit Pumpdown
setpoints.
Stop Pressure:
• This setpoint defi nes the suction pressure value at
which the compressor will cycle off. Normally, this
setpoint is set below the Suction Pressure Auto-cycle
Stop Pressure setpoint.
Stop Delay:
• This setpoint delays the compressor from stopping
when the suction pressure is equal to or less than the
Stop Pressure.
Min Slide Position:
• The minimum capacity slide is the setpoint that
the compressor is allowed to run at. By forcing the
compressor capacity to operate at a value above
minimum, we insure that the suction pressure will be
pulled down to the Stop Pressure setpoint.
Pumpdown Operation (Run/Stop):
• This button starts/stops the Pumpdown operation.
Section 6 • Compressor Control