VYPER
™
VARIABLE SPEED DRIVE
OPERATION
100-210 IOM (
JUL
09)
Page 52
Example 2 - Suggested Control Strategy for Standard 2 to 1 (2:1) Turndown
NOTE: The Variable Speed Minimum Slide Valve Position
of 25% is a factory default value based on the minimum
slide valve position at 50% speed.
VFD
Maximum Drive Output
100%
Minimum Drive Output
50 %
Delay
Rate of Increase
3 %
5 SEC
Rate of Decrease
3 %
5 SEC
Capacity Control
The Drive Speed will increase and decrease proportionally
with the Slide Valve* until the Slide Valve* reaches
95 %
and the Drive Speed reaches
50 %
Above these setpoints, the Drive Speed will be controlled
by the Rate Of Increase and Decrease while the
Slide Valve* will operate independently.
*Slide Valve or other mechanical unloader
Variable Speed Minimum Slide Valve Position
25.0%
NOTE: Slide Valve Range limited to protect compressor
Figure 63 - Example 2 VFD and Capacity Control Setpoints
Figure 62 - 2:1 Turndown Suggested Control Strategy
The standard 2:1 Turndown Control Strategy is depicted in
Figure 62. This control strategy is a variation on the generic
control strategy described previously. Not all applications
require or are suitable for the very low speeds obtainable
by the Vyper
™
drive. Many times the motor is the limiting
factor and is not suitable for 20% speed but is capable of
50% speed. For these applications, the settings within the
Quantum
™
LX can assure that the motor never runs slower
than the allowable minimum speed.
Figure 63 provides the VFD and Capacity Control setpoints
typical for the operation shown in Figure 62.
VFD Proportional Band –
This is the value above the Capac-
ity Control setpoint for forward action at which the output to
the drive will match the maximum output setpoint. Typically,
proportional control alone will not permit running at the control
setpoint. If the pressure is within the proportional band but
not increasing the output will no increase with proportional
control only.
VFD Integration Time –
This is the value that sets the in-
crease of the proportional component over time.
Example: with the following setpoints, Actual Value = 21psig
Capacity Control
[20psig]
Maximum Output
[100%]
Minimum Output
[20%]
VFD Proportional Band [4psig]
VFD Integration Time
[30 sec]
