VYPER
™
VARIABLE SPEED DRIVE
OPERATION
100.200-IOM (SEP 13)
Page 49
NOTICE
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 53 - Example 2 VFD and Capacity Control Setpoints
The standard 2:1 Turndown Control Strategy is depicted in
Figure 52. 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 53 provides the
VFD and Capacity Control setpoints typical for the operation
shown in Figure 52.
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 = 21 psig
Capacity Control
[20psig]
Maximum Output
[100%]
Minimum Output
[20%]
VFD Proportional Band [4psig]
VFD Integration Time
[30 sec]
With the actual pressure at 21 psig the proportional com-
ponent would be 20%. The Proportional Component is
calculated as:
[(Actual Value – Setpoint) / PB] x (Maximum output – Mini-
mum Output).
[(21 – 20) / 4] x (100 – 20) = 20%
When this proportional component is added to the mini-
mum outpoint setpoint, the total output is 40% propor-
tionally at an actual value of 21 psig. It is the Integration
Time setpoint which will increase the output over time
without an increase in the actual value.
As long as the actual value (21 psig) does not change, the
Integration Time setpoint, in this case [30 sec], will increase
the output by the proportional component over every 30
seconds to drive the actual value back to the Capacity Control
setpoint. If the actual value remains 21 psig, the Integration
Time Setpoint would increase the drive output to 100% over
1.5 minutes. See Figure 54.
Figure 54 - Capacity Control Setpoints Screen